ic engines 2012 edition theory & questions

1. Gas Power CyclesTheory at a Glance (For IES, GATE & PSUs)

Carnot CycleA Carnot gas cycle operating in a given temperature range is shown in the p-v and T-s diagram in Fig. below. The isentropic expansion process 4-1 and the isentropic compression process 2-3 can be simulated quite well by a well-designed turbine and compressor respectively, but the isothermal expansion process 3-4 and the isothermal compression process 1-2 are most difficult to achieve. Because of these difficulties, a steady-flow Carnot gas cycle is not practical. We know that the Carnot cycle efficiency is given by the expression.

Carnot cycle on p-v diagrams Carnot cycle on T-s diagrams

Fig. Working of Carnot engine

Gas Power CyclesS K Mondal’s Chapter 1Since the working fluid is an ideal gas with constant specific heats we have, for the isentropic process,

Now, T1 = T2 and T4 = T3, therefore

compression or expansion ratioCarnot cycle efficiency may be written as,

From the above equation, it can be observed that the Carnot cycle efficiency increases as ‘r’ increases. This implies that the high thermal efficiency of a Carnot cycle is obtained at the expence of large piston displacement. Also, for isentropic processes we have,

Since, T1 = T2 and T4 = T3, We have

= pressure ratio

Therefore, Carnot cycle efficiency may be written as,

From the above equation, it can be observed that, the Carnot cycle efficiency can be increased by increasing the pressure ratio. This means that Carnot cycle should be operated at high peak pressure to obtain large efficiency.

Note: For Carnot cycle only

Stirling Cycle (Regenerative Cycle)The Carnot cycle has a low mean effective pressure because of its very low work output. Hence, one of the modified forms of the cycle to produce higher mean effective pressure while theoretically achieving full Carnot cycle efficiency is the Stirling cycle. It consists of two isothermal and two constant volume processes. The heat rejection and addition take place at constant temperature. The p-v and T-s diagrams for the Stirling cycle are shown in figure below.

Gas Power CyclesS K Mondal’s Chapter 1

Stirling cycle processes on p-v diagrams

Stirling cycle processes on T-S diagrams

Stirling Cycle Processes:(a) The air is compressed isothermally from state 1 to 2.(b) The air at state-2 is passed into the regenerator from the top at a temperature T 1.

The air passing through the regenerator matrix gets heated from TL to TH.(c) The air at state-3 expands isothermally in the cylinder until it reaches state-4.(d) The air coming out of the engine at temperature TH (condition 4) enters into

regenerator from the bottom and gets cooled while passing through the regenerator matrix at constant volume and it comes out at a temperature TL, at condition 1 and the cycle is repeated.

(e) It can be shown that the heat absorbed by the air from the regenerator matrix during the process 2-3 is equal to the heat given by the air to the regenerator matrix during the process 4-1, then the exchange of heat with external source will be only during the isothermal processes.

Now we can write, Net work done = W = QS - QRHeat supplied = QS = heat supplied during the isothermal process 3-4.

Heat rejected = QR =Heat during the isothermal compression process, 1-2.

Now,

and

Gas Power CyclesS K Mondal’s Chapter 1Thus the efficiency of Stirling cycle is equal to that of Carnot cycle efficiency when both are working with the same temperature limits. It is not possible to obtain 100% efficient regenerator and hence there will be always 10 by 20 % loss of heat in the regenerator, which decreases the cycle efficiency, Considering regenerator efficiency, the efficiency of the cycle can be written as,

Where, is the regenerator efficiency.

Efficiency of Stirling cycle without regeneration

Ericsson cycle The Ericsson cycle consists of two isothermal and two constant pressure processes.The processes are:Process 1-2: Reversible isothermal compression.Process 2-3: Constant pressure heat addition.Process 3-4: Reversible isothermal expansion.Process 4-1: Constant pressure heat rejection.

The heat addition and rejection take place at constant pressure as well as isothermal processes. Since the process 2-3 and 3-4 are parallel to each other on the T-s diagram, the net effect is that the heat need to be added only at constant temperature T 3 = T4 and rejected at the constant temperature T1 = T2. The cycle is shown on p-v and T-s diagrams in fig. The advantage of the Ericsson cycle over the Carnot and Stirling cycles is its smaller pressure ratio for a given ratio of maximum to minimum specific volume with higher mean effective pressure.

Fig. Ericsson cycle on p-v diagrams Fig. Ericsson cycle on T-s diagram

The thermal efficiency of Ericsson cycle is given by, (derivation is same as that of Stirling cycle),

The Ericsson cycle does not find practical application in piston engines but is approached by a gas turbine employing a large number of stages with heat exchangers, insulators and reheaters.

Gas Power CyclesS K Mondal’s Chapter 1Lenoir cycle The Lenoir cycle consists of the following processes. Constant volume heat addition (1-2); isentropic expansion (2-3); constant pressure heat rejection (3-1). The Lenoir cycle is used for pulse jet engines.

Fig. Lenoir Cycle

Taking P2 / P1 = rp, we have T2 = T1rp and

Thus the efficiency of the Lenoir cycle depends upon the pressure ratio as well as the ratio of specific heats, viz.

Otto cycle (The constant volume) The main drawback of the Carnot cycle is its impracticability due to high pressure and high volume ratios employed with comparatively low mean effective pressure. Nicolaus Otto (1876), proposed a constant-volume heat addition cycle which forms the

Gas Power CyclesS K Mondal’s Chapter 1basis for the working of today's spark ignition engines. The cycle is shown on p-V and T-s diagrams in figures below respectively.

Fig. Otto Cycle

When the engine is working on full throttle, the processes 0 1 and 1 0 on the p - V diagram represents suction and exhaust processes and their effect is nullified. The process 1 2 represents isentropic compression of the air when the piston moves from bottom dead centre to top dead centre. During the process 2 3 heat is supplied reversibly at constant volume. This process corresponds to spark-ignition and combustion in the actual engine. The processes 3 4 and 4 1 represent isentropic expansion and constant volume heat rejection respectively.

The thermal efficiency of Otto cycle can be written as

Considering constant volume processes 2-3 and 4-1, the heat supplied and rejected of air can be written as

Considering isentropic processes 1 2 and 3 4, we have

and

But the volume ratios V1/V2 and V4/V3 are equal to the compression ratio, r. Therefore,


Therefore,

From Eq.3.21, it can be easily shown that

Note that the thermal efficiency of Otto cycle is a function of compression ratio r and the ratio of specific heats, . As is assumed to be a constant for any working fluid, the efficiency is increased by increasing the compression ratio. Further, the efficiency is independent of heat supplied and pressure ratio. The use of gases with higher values would increase efficiency of Otto cycle. Fig. below shows the effect of and r on the efficiency.

Effect of r and on Efficiency for Otto Cycle

Work OutputThe net work output for an Otto cycle can be expressed as

Also

Therefore,


Mean Effective PressureThe mean effective pressure of the cycle is given by

Thus, it can be seen that the work output is directly proportional to pressure ratio, . The mean effective pressure which is an indication of the internal work output increases with a pressure ratio at a fixed value of compression ratio and ratio of specific heats. For an Otto cycle, an increase in the compression ratio leads to an increase in the mean effective pressure as well as the thermal efficiency.

Diesel cycleIn actual spark-ignition engines, the upper limit of compression ratio is limited by the self-ignition temperature of the fuel. This limitation on the compression ratio can be circumvented if air and fuel are compressed separately and brought together at the time of combustion. In such an arrangement fuel can be injected into the cylinder which contains compressed air at a higher temperature than the self-ignition temperature of the fuel. Hence the fuel ignites on its own and requires no special device like an ignition system in a spark-ignition engine. Such engines work on heavy liquid fuels. These engines are called compression-ignition engines and they work on a ideal cycle known as Diesel cycle. The difference between Otto and Diesel cycles is in the process of heat addition. In Otto cycle the heat addition takes place at constant volume whereas in the Diesel cycle it is at constant pressure. For this reason, the Diesel cycle is often referred to as the constant-pressure cycle. It is better to avoid this term as it creates confusion with Joules cycle. The Diesel cycle is shown on p-V and T-s diagrams in fig. respectively.


Figure: Diesel Cycle

To analyze the diesel cycle the suction and exhaust strokes represented by 01 and l

0, are neglected as in the case of the Otto cycle. Here, the volume ratio is the

compression ratio, r, the volume ratio is called the cut-off ratio, rc.

Thermal EfficiencyThe thermal efficiency of the Diesel cycle is given by

Considering the process 1 2

Considering the constant pressure process 2 3, we have

Considering process 3 4, we have


It may be noted that the efficiency of the Diesel cycle is different from that of the Otto cycle only in the bracketed factor. This factor is always greater than unity. Hence for a given compression ratio, the Otto cycle is more efficient. In diesel engines the fuel cut-off ratio, rc, depends on output, being maximum for maximum output. Therefore, unlike the Otto cycle the air-standard efficiency of the Diesel cycle depends on output. The higher efficiency of the Otto cycle as compared to the Diesel cycle for the same compression ratio is of no practical importance. In practice the operating compression ratios of diesel engines are much higher compared to spark ignition engines working on Otto cycle. The normal range of compression ratio for diesel engine is 16 to 20 whereas for spark-ignition engines it is 6 to 10. Due to the higher compression ratios used in diesel engines the efficiency of a diesel engine is more than that of the gasoline engine.

Work OutputThe net work output for a diesel cycle is given by


Mean Effective PressureThe expression for mean effective pressure can be shown to be

The dual or mixed or limited pressure cycle In the Otto cycle, combustion is assumed at constant volume while in Diesel cycle combustion is at constant pressure. In practice they are far from real. Since, some time interval is required for the chemical reactions during combustion process, the combustion cannot take place at constant volume. Similarly, due to rapid uncontrolled combustion in diesel engines, combustion does not occur at constant pressure. The Dual cycle, also called a mixed cycle or limited pressure cycle, is a compromise between Otto and Diesel cycles. Figures show the Dual cycle on p-V and T-s diagrams respectively.In a Dual cycle a part of the heat is first supplied to the system at constant volume and then the remaining part at constant pressure.

Fig. Dual CycleThermal Efficiency

The efficiency of the cycle may be written as


It can be seen from the above equation that a value of rp > 1 results in an increased efficiency for a given value of rc and . Thus the efficiency of Dual cycle lies between that of the Otto cycle and the Diesel cycle having same compression ratio.

With = 1, it becomes an Otto cycle, and with = 1, it becomes a Diesel cycle.

Work Output

The net work output of the cycle is given by

Mean Effective Pressure

The effective pressure is given by

Atkinson cycleAtkinson cycle is an ideal cycle for an Otto engine exhausting to a gas turbine. In this cycle the isentropic expansion (3-4) of an Otto cycle is allowed to further expand to the lowest cycle pressure (3-5) so as to increase the work output.

Fig. Atkinson Cycle


For 1 kg gas,

Let r compression ratio =

re, expansion ratio =

Substituting ,

Comparison of Otto, Diesel, and Dual cycles for the same compression ratio and heat rejection


Fig. Comparison of Otto, Diesel and Dual Cycles for the same Compression Ratio

1- 2- 6- 5 - Otto cycle1-2-7-5 - Diesel cycle1-2-3-4-5 - Duel cycle

Comparison of Otto, Diesel, and Dual cycles for the same Maximum pressure and temperature and the heat rejection being also the same.

Fig. Comparison of Otto, Diesel and Dual Cycles for the Same Maximum Pressure and Temperature

1-6-4-5 -Otto cycle1-7-4-5 - Diesel cycle1-2-3-4-5 - Duel cycle

Same Peak Pressure, Peak Temperature and Heat Rejection:Figure shows the Otto cycle 1-2-3-4 and Diesel cycle 1-2’-3-4 on p-V and T-s coordinates, where the peak pressure and temperature and the amount of heat rejected are the same.The efficiency of the Otto cycle,

Gas Power CyclesS K Mondal’s Chapter 1Where, Qs in the area under the curve 2-3 in Fig. (b). The efficiency of the Diesel cycle, 1-2-3’-3-4 is,

Same Maximum Pressure and Heat Input:

For same maximum pressure and heat input, the Otto cycle (1-2-3-4-1) and Diesel cycle (1-2'-3'-4'-1) are shown on p-V and T-s diagrams in Fig. It is evident from the figure that the heat rejection for Otto cycle (area 1-5-6-4 on T-s diagram) is more than the heat rejected in Diesel cycle (1-5-6'-4'). Hence Diesel cycle is more efficient than Otto cycle for the condition of same maximum pressure and heat input. One can make a note that with these conditions, the Diesel cycle has higher compression ratio than that of Otto cycle. One should also note that the cycle which is having higher efficiency allows maximum expansion. The Dual cycle efficiency will be between these two.

For Same Maximum Pressure and Work OutputFor same work output the area 1-2-3-4 (work output of Otto cycle) and area 1-2'-3'-4' (work output of Diesel cycle) are same. To achieve this, the entropy at 3 should be greater than entropy at 3' .It is clear that the heat rejection for Otto cycle is more than that of diesel cycle. Hence, for these conditions, the Diesel cycle is more efficient than the Otto cycle. The efficiency of Dual cycle lies between the two cycles.


OBJECTIVE QUESTIONS (GATE, IES, IAS)Previous Years GATE Questions

Carnot cycleQ1. A cyclic heat engine does 50 kJ of work per cycle. If the efficiency of

the heat engine is 75%, the heat rejected per cycle is [GATE-2001]

Q2. A Carnot cycle is having an efficiency of 0.75. If the temperature of the high temperature reservoir is 727° C/ what is the temperature of low temperature reservoir? [GATE-2002](a) 23°C (b) -23°C (c) 0°C (d) 250°C

Q3. A heat transformer is a device that transfers a part of the heat, supplied to it at an intermediate temperature, to a high temperature reservoir while rejecting the remaining part to a low temperature heat sink. In such a heat transformer, 100 kJ of heat is supplied at 350 K. The maximum amount of heat in kJ that can be transferred to 400 K, when the rest is rejected to that can be transferred to 400 K, when rest is rejected to a heat sink at 300 K is (a) 12.50 (b) 14.29 (c) 33.33 (d) 57.14 [GATE-

2007]

Q4. A solar energy based heat engine which receives 80 kJ of heat at 100 deg C and rejects 70 kJ of heat to the ambient at 30 deg C is to be designed. The thermal efficiency of the heat engine is

[GATE-1996](a) 70% (b) 1.88% (c) 12.5% (d) indeterminate

Stirling cycleQ5. A Stirling cycle and a Carnot cycle operate between 50°C and 350°C.

Their efficiencies are ηs and ηc respectively. In this case, which of the following statements is true? [GATE-1999](a) ηs > ηc (b) ηs = ηc(c) ηs < ηc(d) The sign of (ηs - ηc) depends on the working fluids used

Ericsson cycle Q6. A cycle consisting of two reversible isothermal processes and two

reversible isobaric processes is known as [GATE-1996](a) Atkinson cycle (b) Stirling cycle(c) Brayton cycle (d) Ericsson cycle

Gas Power CyclesS K Mondal’s Chapter 1Q7. A gas turbine cycle with infinitely large number of stages during

compression and expansion leads to [GATE-1994]

(a) Stirling cycle (b) Atkinson cycle(c) Ericsson cycle (d) Brayton cycle

The constant volume or Otto cycle Q8. Which one of the following is NOT a necessary assumption for the air-

standard Otto cycle?(a) All processes are both internally as well as externally reversible. (b) Intake and exhaust processes are constant volume heat rejection processes.(c) The combustion process is a constant volume heat addition process. (d) The working fluid is an ideal gas with constant specific heats. [GATE-

2008]

Q9. An engine working on air standard Otto cycle has a cylinder diameter of 10 cm and stroke length of 15 cm. The ratio of specific heats for air is 1.4. If the clearance volume is 196.3 cc and the heat supplied per kg of air per cycle is 1800kJ/kg, then work output per cycle per kg of air is

[GATE-2004](a) 879.1 kJ (b) 890.2 kJ (c) 895.3 kJ (d) 973.5 kJ

Q10. For an engine operating on air standard Otto cycle, the clearance volume is 10% of the swept volume. The specific heat ratio of air is 1.4. The air standard cycle efficiency is

[GATE-2003](a) 38.3% (b) 39.8% (c) 60.2% (d) 61.7%

Q11. An ideal air standard Otto cycle has a compression ratio of 8.5. If the ratio of the specific heats of (y) is 1.4/ then what is the thermal efficiency (in percentage) of the Otto cycle?

[GATE-2002](a) 57.5 (b) 45.7 (c) 52.5 (d) 95

Q12. In an air-standard Otto cycle, the compression ratio is 10. The condition at the beginning of the compression process is 100 kPa and 270C. Heat added at constant volume is 1500 kJ/kg, while 700 kJ/kg of heat is rejected during the other constant volume process in the cycle. Specific gas constant for air = 0.287 kJ/kgK. The mean effective pressure (in kPa) of the cycle is [GATE -2009](a) 103 (b) 310 (c) 515 (d) 1032

Q13. In a spark ignition engine working on the ideal Otto cycle, the compression ratio is 5.5. The work output per cycle (i.e., area of the P-V diagram) is equal to 23.625 x 105 x Vc J, where Vc is the clearance volume in m3. The indicated mean effective pressure is

[GATE-2001](a) 4.295 bar (b) 5.250 bar(c) 86.870 bar (d) 106.300 bar

Gas Power CyclesS K Mondal’s Chapter 1Comparison of Otto, diesel, and dual (limited-pressure) cyclesQ14. List l List II [GATE-

1995](Heat Engines) (Cycles)

(A) Gas Turbine 1. Constant volume heat addition and constant volume heat rejection

(B) Petrol Engine 2. Constant pressure heat addition and constant volume heat rejection

(C) Stirling Engine 3. Constant pressure heat addition and constant pressure heat rejection

(D) Diesel Engine 4. Heat addition at constant volume followed by heat addition at constant temperature5. Heat rejection at constant volume followed by heat rejection at constant temperature

A B C D A B C D(a) 3 1 4 2 (b) 1 4 2 3(c) 4 2 3 1 (d) 2 3 1 4

Common Data for Questions 15 & 16:In two air standard cycles - one operating on the Otto and the other on the Brayton cycle-air is isentropic ally compressed from 300 to 450 K. Heat is added to raise the temperature to 600 K in the Otto cycle and to 550 K in the Brayton cycle.

Q15. If and are the efficiencies of the Otto and Brayton cycles, then (a) [GATE-

2005](b) (c) (d) It is not possible to calculate the efficiencies unless the temperature after the expansion is given

Q16. If Wo and WB are work outputs per unit mass, then [GATE-2005]

(a) Wo> WB(b) WO<WB(c) WO=WB(d) it is not possible to calculate the work outputs unless the temperature after expansion is Given.

Q17. A diesel engine is usually more efficient than a spark ignition engine because

(a) diesel being a heavier hydrocarbon, releases more heat per kg than gasoline(b) The air standard efficiency of diesel cycle is higher than the Otto cycle, at a fixed compression ratio [GATE-2003](c) The compression ratio of a diesel engine is higher than that of an SI engine(d) Self ignition temperature of diesel is higher than that of gasoline

Gas Power CyclesS K Mondal’s Chapter 1Q18. Consider air standard Otto and Diesel cycles, both having the same

state of air at the start of compression. If the maximum pressure in both the cycles is the same, then compression ratio 'r' and the efficiency 'η' are related by (a) rDiesel > rOtto (b) rDiesel < rOtto(c) ηOtto > ηDiesel (d) ηOtto < ηDiesel [GATE-

2000]

Previous Years IES QuestionsCarnot cycleQ1. Which gas power cycle consists of four processes during which work

alone is transferred and heat alone is transferred during the other two processes? (a) Atkinson cycle (b) Carnot cycle [IES-2008](c) Diesel cycle (d) Otto cycle

Q2. Three engines A, B and C operating on Carnot cycle use working substances as Argon, Oxygen and Air respectively. Which engine will have higher efficiency?

[IES-2009](a) Engine A (b) Engine B(c) Engine C (d) All engines have same efficiency

Q3. Which one of the following cycles has the highest thermal efficiency for given maximum and minimum cycle temperatures? [IES-2005](a) Brayton cycle (b) Otto cycle(c) Diesel cycle (d) Stirling cycle

Q4. For a heat engine operating on the Carnot cycle, the work output is ¼ th of the heat transferred to the sink. The efficiency of the engine is [IES-2003] (a) 20 % (b) 33.3 % (c) 40 % (d) 50 %

Q5. The data given in the table refers to an engine based on Carnot cycle,

where Q1 = Heat received (kJ/min), Q2 = Heat rejected (kJ/s), W = Work output

(kW)S. No. Q1 Q2 W1.2.3.4.

1500160017001800

16.8017.9219.0320.15

8.208.759.309.85

If heat received by the engine is 2000 kJ/minute the work output will be, nearly, [IES-2001](a) 9.98 (b) 10.39 (c) 11.54 (d) 10.95

Q6. A Carnot engine uses nitrogen as the working fluid (γ = 1.4). The heat supplied is 52 kJ and adiabatic expansion ratio 32:1. The receiver


temperature is 295 K. What is the amount of heat rejected? [IES 2007](a) 11 kJ (b) 13 kJ (c) 26 kJ (d) 28 kJ

Q7. In a heat engine operating in a cycle between a source temperature of 606°C and a sink temperature of 20°C, what will be the least rate of heat rejection per kW net output of the engine?

[IES-2004](a) 0.50 kW (b) 0.667 kW (c) 1.5 kW (d) 0.0341 kW

Q8. Which one of the following changes/sets of changes in the source and sink temperatures (T1 and T2 respectively) of a reversible engine will result in the maximum improvement in efficiency?(a) (b) (c) and (d) and [IES-

1994]

Q9. A heat engine using lake water at 12oC as source and the surrounding atmosphere at 2 oC as sink executes 1080 cycles per min. If the amount of heat drawn per cycle is 57 J, then the output of the engine will be

[IES-1993](a) 66W (b) 56W (c) 46 W (d) 36 W

Stirling cycleQ10. What does the reversed ideal Stirling cycle consist of?

[IES-2005](a) Tow reversible isothermal processes and two reversible adiabatic processes(b) Two reversible isothermal processes and two reversible isochoric processes(c) Two reversible isobaric processes and two reversible adiabatic processes(d) Two reversible adiabatic processes and two reversible isochoric processes

Q11.

Thermodynamic cycle shown above on the temperature – entropy diagram pertains to which one of the following? (a) Stirling cycle (b) Ericsson cycle(c) Vapour compression cycle (d) Brayton cycle [IES

2007]

Ericsson cycle Q12. Which cycle consists of two reversible isotherms and two reversible

isobars?(a) Carnot cycle (b) Stirling cycle(c) Ericsson cycle (d) Brayton cycle [IES-

2009]

Gas Power CyclesS K Mondal’s Chapter 1Q13. Which one of the following parameters is significant to ascertain

chemical equilibrium of a system? [IES-2009](a) Clapeyron relation (b) Maxwell relation(c) Gibbs function (d) Helmholtz function

Q14. Brayton cycle with infinite inter-cooling and reheating stages would approximate a [IES-2002](a) Stirling cycle (b) Ericsson cycle(c) Otto cycle (d) Atkinson cycle

The constant volume or Otto cycle Q15. Which one of the following p-T diagrams illustrates the Otto cycle of an

ideal gas? [IES-1996]

Q16. Which one of the following diagrams represents Otto cycle on temperature (T) - entropy (s) plane?

[IES-1993]


Q17. An Otto cycle on internal energy (U) and entropy(s) diagram is shown in [IES-1992]

Q18. Consider the following statements regarding Otto cycle: [IES-1998]

1. It is not a reversible cycle.2. Its efficiency can be improved by using a working fluid of higher value of ratio of specific heats.3. The practical way of increasing its efficiency is to increase the compression ratio.4. Carburetted gasoline engines working on Otto cycle can work with compression ratios more than Of these statements(a) 1, 3 and 4 are correct (b) 1, 2 and 3 are correct(c) 1, 2 and 4 are correct (d) 2, 3 and 4 are correct

Q19. For maximum specific output of a constant volume cycle (Otto cycle) (a) The working fluid should be air

[IES-1997](b) The speed should be high(c) Suction temperature should be high


(d) Temperature of the working fluid at the end of compression and expansion should be equal

Q20. In an air standard Otto cycle, r is the volume compression ratio and y is

an adiabatic index (Cp/ Cv), the air standard p v efficiency is given by [IES-2002]

(a) (b)

(c) (d)

Q21. For the same maximum pressure and heat input, the most efficient cycle is (a) Otto cycle (b) Diesel cycle

[IES-2000](c) Brayton cycle (d) Dual combustion cycle

Q22. Assertion (A): Power generated by a four stroke engine working on Otto cycle is higher than the power generated by a two stroke engine for the same swept volume, speed, temperature and pressure conditions.

[IES-2003]Reason (R): In a four stroke engine one cycle is completed in two revolutions.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

The diesel cycle Q23. Consider the following statements:

[IES-2006]1. For a Diesel cycle, the thermal efficiency decreases as the cut off

ratio increases.2. In a petrol engine the high voltage for spark is in the order of 1000 V3. The material for centre electrode in spark plug is carbon.Which of the statements given above is/are correct?(a) Only 1 (b) Only 1 and 2(c) Only 2 and 3 (d) 1, 2 and 3

Q24. In an air-standard Diesel cycle, r is the compression ratio, p is the fuel cut - off ratio and y is the adiabatic index (Cp/ Cv). Its air standard efficiency is given by [IES-2002]

(a) (b)

(c) (d)

Gas Power CyclesS K Mondal’s Chapter 1Q25. Assertion (A): The air standard efficiency of the diesel cycle decreases as the

load is increasedReason (R): With increase of load, cut-off ratio increases.

[IES-2001](a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

The dual or mixed or limited pressure cycle Q26. In a standard dual air cycle, for a fixed amount of heat supplied and a

fixed value of compression ratio, the mean effective pressure [IES-2003](a) Shall increase with increase in rp (pressure ratio for constant volume heating) and decrease in rc (constant pressure cut-off ratio)(b) Shall increase with decrease in rp and increase in rc(c) Shall remain independent of rp(d) Shall remain independent of rc

Comparison of Otto, diesel, and dual (limited-pressure) cyclesQ27.

The correct sequence of the cycles given in the above T-S diagrams is (a) Vapour compression refrigeration, Rankine, Diesel, Otto

[IES-2003](b) Rankine, Vapour compression refrigeration, Diesel, Brayton(c) Rankine, Carnot, Otto, Brayton(d) Vapour compression refrigeration, Carnot, Diesel, Otto

Q28.


The correct sequence of the given four cycles on T-s plane in Figure (1), (2), (3), (4) is [IES-2002](a) Rankine, Otto, Carnot and Diesel(b) Rankine, Otto, Diesel and Carnot(c) Otto, Rankine, Diesel and Carnot(d) Otto, Rankine, Carnot and Diesel

Q29. For the same maximum pressure and heat input [IES-1992]

(a) The exhaust temperature of patrol is more than that of diesel engine(b) The exhaust temperature of diesel engine is more than that of patrol engine(c) The exhaust temperature of dual cycle engine is less than that of diesel

engine(d) The exhaust temperature of dual cycle engine is more than that of patrol

engine

Q30. Match List I with II and select the correct answer using the code given below the [IES 2007]Lists:

List I List II (Prime Mover) (Air Standard Cycle) A. High Speed diesel engine 1. Atkinson CycleB. IC engine having expansion 2. Dual combustion limited

pressure ratio greater than compression ratio cycleC. Pulse jet engine 3. Erickson CycleD. Gas turbine with multistage 4. Lenoir cyclecompression and multistage expansionCode:

A B C D A B C D(a) 3 1 4 2 (b) 2 4 1 3(c) 3 4 1 2 (d) 2 1 4 3

Q31. The order of values of thermal efficiency of Otto, Diesel and Dual cycle, when they have equal compression ratio and heat rejection, is given by [IES-2002](a) (b) (c) (d)

Q32. Match List-I with List-II and select the correct answer using the codes given below the lists: [IES-2001]

List-I List-II(Cycles operating between (Characteristic of cycle efficiency ɳ)fixed temperature limits) A. Otto cycle 1. ɳ depends only upon temperature limitsB. Diesel cycle 2. ɳ depends only on pressure limitsC. Carnot cycle 3. ɳ depends on volume compression ratioD. Brayton cycle 4. ɳ depends on cut-off ratio and volume

compression ratioA B C D A B C D

(a) 3 4 1 2 (b) 1 4 3 2(c) 3 2 1 4 (d) 1 2 3 4

Gas Power CyclesS K Mondal’s Chapter 1Q33. Match List-I with List-II and select the correct answer using the codes

given below the lists: [IES-2001]

List-I List-II A. Air standard efficiency of Otto cycle 1. Mechanical efficiencyB. Morse test 2. Diesel cycleC. Constant volume cycle 3. Brake thermal

efficiencyD. Constant pressure heat addition 4. Otto cycle

5. Codes:

A B C D A B C D(a) 5 1 4 2 (b) 3 5 2 4(c) 3 5 4 2 (c) 5 1 2 4

Q34. Assertion (A): The C.I. engine is found to be more efficient than an S.I. engine. Reason (R): Modern C.I. engines operate on a dual-cycle, which has efficiency greater than the Otto cycle. [IES-2001](a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q35. Match List-I (name of cycles) with List-II (pv diagrams) and select the correct answer using the codes given below the lists: [IES-1999]

List I List IIA. Stirling cycle

B. Diesel cycle

C. Otto cycle

D. Atkinson cycle


Code:A B C D A B C D

(a) 2 3 1 5 (b) 1 3 2 5(c) 2 3 1 4 (d) 5 3 2 1

Q36. Match List-I (details of the processes of the cycle) with List-II (name of the cycle) and select correct answer using the codes given below the Lists: List-I List-II [IES-

1997]A. Two isothermals and two adiabatic 1. OttoB. Two isothermals and two constant volumes 2. JouleC. Two adiabatic and two constant volumes 3. CarnotD. Two adiabatic and two constant pressures 4. StirlingCode:

A B C D A B C D(a) 4 3 1 2 (b) 4 3 2 1(c) 3 4 1 2 (d) 3 4 2 1

Q37. Assertion (A): In practice, the efficiency of diesel engines is higher than that of petrol engines. [IES-1997]Reason (R): For the same compression ratio, the efficiency of diesel cycle is higher than that of Otto cycle.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q38. For constant maximum pressure and heat input, the air standard efficiency of gas power cycles is in the order [IES-1993](a) Diesel cycle, dual cycle, Otto cycle (b) Otto cycle, Diesel cycle, dual cycle(c) Dual cycle, Otto cycle, Diesel cycle (d) Diesel. cycle, Otto cycle, dual cycle

Q39. For the same maximum pressure and temperature

[IES-1992](a) Otto cycle is more efficient than diesel cycle (b) Diesel cycle is more efficient than Otto cycle (c) Dual cycle is more efficient than Otto and diesel cycles(d) Dual cycle is less efficient than Otto diesel cycles

Q40. Match List I (Cycles) with List II (Processes) and select the correct answer using the codes given below the Lists: [IES-2003]

List I List II (Cycles) (Processes)


A. Bell Coleman cycle 1. One constant pressure, one constant volume and two is entropic

B. Stirling cycle 2. Two constant pressure and two is entropicC. Ericsson cycle 3. Two constant pressure and two isothermalD. Diesel cycle 4. Two constant volume and two isothermalCodes:

A B C D A B C D(a) 2 3 4 1 (b) 1 4 3 2(c) 2 4 3 1 (d) 1 3 4 2

Q41. Match List-l (Gas Cycles) with List-ll (Thermodynamic co-ordinates) and select the correct answer using the codes given below the lists: [IES-2009]

List-IA. Carnot cycleB. Brayton cycleC. Ericsson cycleD. Stirling cycle

List-II1. Pressure-Entropy2. Pressure-

Temperature3. Temperature-Volume4. Temperature-Entropy

Codes:A B C D A B C D

(a) 4 2 1 3 (b) 3 2 1 4(c) 4 1 2 3 (d) 3 1 2 4

Previous Years IAS QuestionsCarnot cycleQ1. Assertion (A): Two engines A and B work on the Carnot cycle. Engine A uses

air as the working substance and B uses steam as the working substance. Both engines are having same efficiency.

[IAS-2007]Reason (R): Carnot cycle efficiency is independent of working substance.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q2. A heat engine working on Carnot cycle receives heat at the rate of 40 kW from a source at 1200 K and rejects it to a sink at 300 K. The heat rejected is (a) 30 kW (b) 20 kW (c) 10 kW (d) 5 kW [IAS-

2001]

Q3. A heat engine using lake water at 12oC as source and the surrounding atmosphere at 2oC as sink executes 1080 cycles per min. If amount of heat supplied per cycle is 57 J, what is the output of the engine? [IAS-2004](a) 66 W (b) 56 W (c) 46 W (d) 36 W

Gas Power CyclesS K Mondal’s Chapter 1Stirling cycleQ4. Which one of the following pairs of air standard cycles has the same

efficiency as that of Carnot cycle when operating between the same maximum (source) and minimum (sink) temperatures together with ideal regeneration?(a) Otto and Brayton (b) Brayton and Ericsson(c) Ericsson and Stirling (d) Stirling and Otto

[IAS-1997]

Ericsson cycle Q5. A gas turbine cycle with infinitely large number of stages during

compression and expansion approaches [IAS-2003]

(a) Stirling cycle (b) Atkinson cycle(c) Ericsson cycle (d) Brayton cycle

Lenoir cycle Q6. Pulse jet engine operation can be idealized/ approximated to follow

which one of the following? [IAS-2007](a) Brayton cycle (b) Ericsson cycle(c) Lenoir cycle (d) Stirling cycle

The diesel cycle Q7. In the case of a Diesel cycle, increasing the cut-off ratio will increase

(a) Efficiency (b) mean effective pressure [IAS-1996]

(c) The maximum weight (d) the engine weight

Comparison of Otto, diesel, and dual (limited-pressure) cycles Q8. Match List - I (Thermodynamic system/prime mover) with List - II

(Thermodynamic Cycle) and select the correct answer using the codes given below the list: [IAS-1998]List – I List - IIA. Air liquefaction plant 1. Atkinson cycleB. Gas turbine with multistage compression 2. Brayton cycleand multistage expansion C. Free piston engine/compressor with 3. Ericsson cycle a gas turbine D. Pulse jet 4. Reversed striling cycle

5. Lenoir cycleCodes:

A B C D A B C D(a) 1 2 4 3 (b) 1 2 3 4(c) 4 3 1 5 (d) 4 3 5 1

Q9. Assertion (A): For a given compression ratio, the thermal efficiency of the

Diesel cycle will be higher than that of the Otto cycle. [IAS-2000]


Reason(R): In the Diesel cycle, work is also delivered during heat addition.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q10. Match List I (Cycle) with List II (Process) and select the correct answer:List I List IIA. Otto 1. Two isothermal and two constant volumes

[IAS-2000]B. Stirling 2. Two isothermal and two isobarsC. Ericsson 3 Two isentropic and two isobarD. Brayton 4.Two isentropic and two constant volume

5. Two isentropic and two isothermalA B C D A B C D

(a) 2 3 4 1 (b) 3 2 1 4(c) 3 2 4 1 (d) 2 3 1 4

Q11. The air standard efficiency of diesel cycle will be less than that of Otto cycle in the case of [IAS-1999](a) Same compression ratio and same heat input(b) Same maximum pressure and same heat input(c) Same maximum pressure and same output(d) Same maximum pressure and S8.me maximum temperature

Q12. Which one of the following cycles working within the same temperature limits has the highest work ratio?

[IAS-1998](a) Carnot cycle (b) Joule cycle(c) Otto cycle (d) Rankine cycle

Q13. Match List I with List II and select the correct answer using the codes given below the lists: [IAS-1996]

List I List IIA. Compression ratio 1. Brayton cycleB. Pressure ratio 2. Diesel cycleC. Cut-off ratio 3. Dual combustion cycleD. Explosion ratio 4. Otto cycleCodes: A B C D A B C D

(a) 1 4 3 2 (b) 1 4 2 3(c) 4 1 2 3 (d) 4 1 3 2

Q14. A Diesel and Otto cycle have the same compression ratio 'r'. The cut-off ratio of the cycle is‘s’. The air standard efficiency of these cycles will be equal when (a) sk - k (s - 1) - 1= 0 (b) sk - k (s - 1)+ 1 = 0

[IAS-1996](c) sk - k (s - 1)+ 1 = 0 (d) sk - (s - 1)- k = 0

Q15. For the same compression ratio and the same heat input, the correct sequence of the increasing order of the thermal efficiencies of the given cycles is [IAS-1996]


(a) Otto, Diesel, dual (b) Diesel, dual Otto (c) Dual, Diesel, Otto (d) Dual, Otto, Diesel

Q16. Match List I with List II and select the correct answer using the codes given below the lists: [IAS-1995]

List-I List-IIA. Twoconstant volumes and two adiabatics 1. EricssonB. Twoconstant pressures and two adiabatics 2. StirlingC.Twoconstant volumes and two isothermals 3. JouleD. Twoconstant pressure and two isothermals 4. OttoCodes: A B C D A B C D

(a) 3 4 2 1 (b) 3 4 1 2(c) 4 3 1 2 (d) 4 3 2 1

Q17. Otto cycle efficiency is higher than Diesel cycle efficiency for the same compression ratio and heat input because, in Otto cycle [IAS 1994](a) Combustion is at constant volume(b) Expansion and compression are isentropic(c) Maximum temperature is higher(d) Heat rejection is lower

Q18. Which one of the following hypothetical heat engine cycle represents maximum efficiency? [IAS-1999]


ANSWER WITH EXPLANATION

Previous Years GATE Answers

1. Ans. (a)

2. Ans. (b) 3. Ans. (d)

4. Ans. (c) 5. Ans. (c) Note:

(i) Since part of the heat is transfers at constant volume process, the efficiency of the Stirling cycle is less than that of the Carnot cycle.(ii) The regenerative Stirling cycle has the same efficiency as that of Carnot

cycle(iii) Efficiency of Stirling cycle without regeneration

6 Ans. (d)7. Ans. (c) Brayton cycle with many stages of intercooling and reheating

approximates to Ericsson cycle.

8. Ans. (b) Intake process isn’t constant volume heat rejection processes. it is constant pressure process.9. Ans. (d) Initial volume (v1)

=

Final Volume (v2) = 196.3 cc = 0.1963 x 10-3 m3 = vc


10. Ans. (d)

11. Ans. (a)12. Ans.(d)

13. Ans. (b)

14. Ans. (a)15. Ans. (b)

Otto cycle Brayton cycle


16. Ans. (a)

17. Ans. (c)18. Ans. (a, d)

Previous Years IES Answers1. Ans. (b)

2. Ans. (d) We know that

Since Efficiency of the Engine purely depends upon source and sink temperatures and independent of the working substances. All the engines have same efficiency

3. Ans. (d) Don’t confuse with Diesel cycle. As stirling cycle’s efficiency is equal to Carnot cycle.4. Ans. (a)5. Ans. (d)

6. Ans. (b) T2

T1 = ( p2

p1)γ−1γ

=( v1

v2)γ−1

Given v2

v1 =32 or T2

T1 = (1

32 )1.4−1

= 14

Q1

T 1 =Q2

T 2 or Q2=Q1xT2

T1 =52x14 = 13 kJ


7. Ans. (a) Least rate of heat rejection per kW net output = it will occur when reversible process will occur.

8. Ans. (c) For maximum improvement in efficiency source temperature should be raised and sink temperature lowered.9. Ans. (d) Here T1 = 273 + 12 = 285°K

T2 = 273 + 2 = 275°K.Heat drawn per cycle = 57J and no. of cycles per m = 1080 i.e. 1080/60 = 18

cycles/sec.

10. Ans. (b)11. Ans. (a) 1-2 and 3-4 are isothermal process

2-3 and 4-1 may be isobaric or isochoric processSo this cycle may be Starling cycle of Ericsson cycle but steepness of the curve 2-3 and 1-4 is very high. Therefore we may say it is Starling cycle.

12. Ans. (c)

13. Ans. (c)14. Ans. (b)15. Ans. (a)16. Ans. (c) Otto cycle involves two isentropic and two constant volume processes.17. Ans. (c)18. Ans. (d)19. Ans. (d)20. Ans. (a)21. Ans. (b)22. Ans. (d)23. Ans. (a) 2 is false, the voltage for spark is in the order of 25 KV

3 is false, best material platinum but mostly used nickel alloy.24. Ans. (c)25. Ans. (a)26. Ans. (a)


27. Ans. (b)28. Ans. (b)29. Ans. (a)30. Ans. (d)31. Ans. (d)32. Ans. (a)33. Ans. (a)34. Ans. (b)35. Ans. (b)36. Ans. (c)37. Ans. (c) 38. Ans. (a)39. Ans. (b)

Following figures shows cycles with same maximum pressure and same maximum temperature. In this case, Otto cycle has to be limited to lower compression ratio to fulfil the condition that point 3 is to be a common state for both cycles. T-s diagram shows that both cycles will reject the same amount of heat.

40. Ans. (c)41. Ans. (c)

Previous Years IAS Answers

1. Ans. (d) efficiency of cannot cycle ( ) =1- it only depends on reservoirs temperature nothing else.

2. Ans. (c)

3. Ans. (d)

4. Ans. (c)5. Ans. (c)6. Ans. (c)7. Ans. (b)8. Ans. (c)

Gas Power CyclesS K Mondal’s Chapter 19. Ans. (d) For same compression ratio Otto cycle has maximum efficiency.10. Ans. (a)11. Ans. (a)12. Ans. (d) Wc is pump work in liquid state i.e. minimum13. Ans. (c)14. Ans. (a)

15. Ans. (b)16. Ans. (d)17. Ans. (d) For same heat input and same compression ratio, in case of Otto cycle, efficiency in higher because the heat rejection is lower.18. Ans. (b)

Student’s Notes

2. SI and CI EnginesTheory at a Glance (For IES, GATE & PSUs)

Engine It is a device which transforms one form of energy into another form, especially converting other forms of energy into mechanical energy.

However, while transforming energy from one form to another the efficiency of conversion plays an important role. Normally, most of the engines convert thermal energy into mechanical work and therefore they are called heat engines.

Heat Engine It is a device which transforms the chemical energy of a fuel into thermal energy and utilizes this thermal energy to perform useful work.

Thus thermal energy is converted to mechanical energy in a heat engine. Heat engines can be broadly classified into two categories:(i) Internal Combustion Engine(ii) External Combustion Engine

Engines whether I.C or E.C are of two types, viz,(a) Rotary Engines(b) Reciprocating Engines.

H E AT E N G IN E

I.C E N G IN E E .C E N G IN E

R o ta ry R e c ip ro c a tin g R o ta ry R e c ip ro c a tin g

W a n k e d E n g in e

O p en c y c leG a s T u rb in e

G a s o n lin e o r p e tro len g in e

D ie s e len g in e

S te a mT u rb in e

C lo s ed c y c leg as tu r b in e

S te a mE n g in e

S tir lin gE n g in e

IC Engine vs E.C EngineInternal combustion engine is a type of heat engine in which the products of combustion is the working fluid of the cycle.

SI and CI EnginesS K Mondal’s Chapter 2Whereas external combustion engine is a type of heat engine in which the products of combustion transfer heat energy to another fluid which is the working fluid of the cycle.IC Engine is not self starting Jet Engines and Rocket are also IC Engines.

The main advantages of I.C engines over E.C Engine like steam plants are greater mechanical simplicity, lower ratio absence of auxiliary apparatus due to absence of auxiliary apparatus like boiler and condenser and hence lower first cost (except in the case of very large units), higher overall efficiency, and lesser requirement of water for dissipation of energy through coding system.

The advantages of external combustion engine are - use of cheaper fuels including solid fuels, and high starting torque.

Engine Components

Spark-Ignition Engine

(i) Cylinder: - It is a cylindrical vessel or space in which the piston makes a reciprocating motion.The varying volume created in the cylinder during the operation of the engine is filled with the working fluid and subjected to different thermodynamic processes. The cylinder is supported in the cylinder block.Material - Cast Iron or Aluminium Alloy or Alloy Steel.

(ii) Piston - It is a cylindrical component filled into the cylinder forming the moving boundary of the combustion system.It fits perfectly (snugly) into the cylinder providing a gas-tight space with the piston rings and the lubricant. It forms the first link in transmitting the gas forces to the output shaft.Material - Cast Iron or Aluminium Alloy.

(iii) Combustion Chamber: - It is the space enclosed in the upper part of the cylinder, by the cylinder head and piston top during the combustion process head and the piston top during the combustion process.The combustion of fuel and the consequent release of thermal energy results in the building up of pressure in this part of the cylinder.

SI and CI EnginesS K Mondal’s Chapter 2(iv) Inlet Manifold - It is the passage which connects the intake system to the inlet valve of the engine and through which air or air-fuel mixture is drawn into the cylinder.

(v) Exhaust Manifold - It is the passage which connects the exhaust system to the exhaust valve of the engine and through which the products of combustion escape into the atmosphere.

(vi) Inlet and Exhaust Valves - Valves are commonly mushroom shaped poppet type. They are provided either on the cylinder head on the side of cylinder for regulating the charge coming into the cylinder (inlet valve) and for discharging the products of combustion (exhaust valve) from the cylinder.

(vii) Spark plug - It is a component to initiate the combustion process in S.I Engines and is usually located on the cylinder head.

(viii) Connecting rod - It is a component which interconnects the piston and the crankshaft and transmits the gas forces from the piston to the crankshaft.Material - Steel or Aluminium alloys. (Forced)

(ix) Crank shaft - It is a component which along with connecting rod converts the reciprocating motion of the piston into useful rotary motion of the output shaft. In the crankshaft of a single cylinder engine there is a pair of crank arms and balance weights. The balance weights are provided for static and dynamic balancing of the rotating system. The crankshaft is enclosed in a crankcase.Material - Cast steel or forged steel or Alloy steel.

(x) Piston rings - These are the rings filled into the slots around the piston to provide a tight seal between the piston and the cylinder wall and thus proventing leakage of combustion gases.Material - Cast Iron or Aluminium Alloy.

(xi) Gudgeon or Wrist or Piston pin - It is the link between the small end of the connecting rod and the piston.Material - Steel

(xii) Camshaft - It is a shaft and along with its associated parts control the opening and closing of the two valves.The associated parts are push rods, rocker arms, valve springs and tapets. This shaft also provides the drive to the ignition system. The camshaft is driven by the crankshaft through timing gears.

(xiii) Cams - It is a mechanical member used to import desired motion of a follower by direct physical contact.These are made as integral parts of the camshaft and are designed in such a way to open the valves at the correct timing and to keep them open for the necessary duration.

(xiv) Flywheel - It is a device used to limit the fluctuation of speed during each cycle of the an engine.It is a heavy wheel used in engines and machinery where reciprocating motion is converted into rotary motion.

The net torque imported to the crankshaft during one complete cycle of operation of the engine fluctuates causing a change in the angular velocity of the shaft. In order to

SI and CI EnginesS K Mondal’s Chapter 2achieve a uniform torque an inertia mass in the form of a wheel is attached to the output shaft and this wheel is called flywheel.

The variation of net torque decreases with increase in the number of cylinders in the engine and thereby the size of the flywheel also becomes smaller. This means that a single cylinder engine will have a larger flywheel whereas a multi-cylinder engine will have a smaller flywheel.

Nomenclature(i) Cylinder bore (d) - It is the nominal dia of the working cylinder. It is usually expressed in mm.(ii) Piston Area (A) - It is the area of a circle of dia equal to the cylinder bore. It is usually expressed in cm2.

(iii) Stroke (L) - It is the nominal distance through which a working piston moves between two successive reversals of its direction of motion. It is usually expressed in mm.It is the nominal distance between dead centers.

(iv) Dead centre - It is the position at which the direction of the piston motion is reversed at either end of the stroke. There are two dead centres.-(a) TDC or IDC(b) BDC or ODC

(a) Top dead centre or Inner dead centre - It is the dead centre when the piston is farthest from the crankshaft.

(b) Bottom dead centre or Outer dead centre - It is the dead centre when the piston is nearest to the crankshaft.

(v) Displacement or Swept or Stroke Volume (Vs) - It is the nominal volume swept by the working piston when traveling from one dead centre to the outer. It is generally expressed in cm3.

(vi) Clearance volume (Vc) - It is the nominal volume of the combustion chamber above the piston when it is at TDC.

(vii) Compression ratio (r) - It is the ratio of the total cylinder volume, when the piston is at bottom dead centre (VT) to the clearance volume (Vc) For S.I Engine : r = 6 to 11For C.I Engine : r = 12 - 22

4 Stroke S.I Engine In this engine, the cycle of operations is completed in four strokes of the piston or two revolutions of the crankshaft. During the four strokes, there are five events to be completed, viz.

Suction.Compression. Combustion.Expansion.Exhaust.

SI and CI EnginesS K Mondal’s Chapter 2

Each stroke consists of 1800 of crankshaft rotation and hence a four stroke cycle is completed through 720° of crank rotation.

Comparison of S.I and C.I engines - (4 Strokes)In four - stroke engines, there is one power stroke for every two revolutions of the crankshaft. There are two non-productive strokes of exhaust and suction which are necessary for flushing the products of combustion from the cylinder and filling it with the fresh charge.Since both SI and CI engines have much in common, it is worthwhile to compare them based on important parameters.

Description SI Engine CI EngineBasic cycle Works on Otto cycle or constant

volume heat addition cycle.Works on Diesel cycle or constant pressure heat addition cycle.

Fuel Gasoline, a highly volatile fuel. Self-ignition temperature is high.

Diesel oil, a non-volatile fuel. Self-ignition temperature is comparatively low.

Introduction of fuel

A Gaseous mixture of fuel-air is introduced during the suction stroke. A carburettor and an ignition system are necessary. Modern engines have gasoline injection.

Fuel is injected directly into the combustion chamber at high pressure at the end of the compression stroke. A fuel pump and injector are necessary.

Load control Throttle controls the quantity of fuel of fuel-air mixture introduced.

The quantity of fuel is regulated. Air quantity is not controlled.

Compression ratio

6 to 10. Upper limit is fixed by antiknock quality of the fuel.

16 to 20. Upper limit is limited by weight increase of the engine.

Speed Due to light weight and also due to homogeneous combustion, they are high speed engines.

Due to heavy weight and also due to heterogeneous combustion, they are low speed engines.

Thermal efficiency

Because of the lower CR, the maximum value of thermal efficiency that can be obtained is lower.

Because of higher CR, the maximum value of thermal efficiency that can be obtained is higher.

Weight Lighter due to lower peak pressures.

Heavier due to higher peak pressures.

Four Stroke EngineThe four stroke engine was first demonstrated by Nikolaus Otto in 18761 . hence it is also known as the Otto cycle. The technically correct term is actually four stroke cycle. The four stroke engine is probable the most common engine type nowadays. It powers almost all cars and tracks.


The four stroke of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston. Therefore the complete cycle requires two revolutions of the crankshaft to complete.

Intake- During the intake stroke, the piston moves downward, drawing a fresh charge of vaporized fuel/air mixture. The illustrated engine features a ‘popper’ intake valve which is drawn open by the vacuum produced by the intake stroke. Some early engines worked this way, however most modern engines incorporate an extra cam/lifter arrangement as seen on the exhaust valve. The exhaust valve is held shut by a spring (not illustrated here).

Compression- As the piston rises the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture.

Power- At the top of the compression stroke the spark plug fires, igniting the compressed fuel. As the fuel burns it expands, driving the piston downward.

Exhaust- At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder

Two Stroke Engine

SI and CI EnginesS K Mondal’s Chapter 2The two stroke engine employs the crankcase as well as the cylinder to achieve all the elements of the Otto cycle in only two strokes of the piston.

Intake- The fuel/air mixture is first drawn into the crankcase by the vacuum created during the upward stroke of the piston. The illustrated engine features a poppet intake valve incorporated into the crankshaft.

During the downward stroke the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.

Transfer/Exhaust- Toward the end of the stroke. The piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port. Usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well.

Compression- The piston then rises, driven by flywheel momentum, and compresses the fuel mixture.

SI and CI EnginesS K Mondal’s Chapter 2Power- At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete.

Pre-combustion chamberThis chamber is located at the cylinder head and is connected to the engine cylinder by small holes. It occupies 40% of the total cylinder volume. During the compression stroke, air from the main cylinder enters the pre-combustion chamber. At this moment, fuel is injected into the pre-combustion chamber and combustion begins. Pressure increases and the fuel droplets are forced through the small holes into the main cylinder, resulting in a very good mix of the fuel and air. The bulk of the combustion actually takes place in the main cylinder. This type of combustion chamber has multi-fuel capability because the temperature of the pre-chamber vaporizes the fuel before the main combustion event occurs.

Stages of Combustion in S.I EnginesA typical theoretical pressure -crank angle diagram, during the process of compression (a-b) combustion (b - c) and expansion (c -d) in an ideal 4-s S.I engine is shown below.

Fig.

In an ideal engine, as can be seen from the diagram, the entire pressure rise during combustion takes place at constant volume i.e at TDC.However, in an actual engine this does not happen.

Sir, Ricardo known as the father of engine research. Describes the combustion process in a S.I engine as consisting of these stages - I - Ignition Lag or Preparation phase II - Propagation of flame III - After burningThe pressure variation due to combustion in a practical engine is shown below.


Fig. Point A: The point of passage of sparkPoint B: The point of which the beginning of pressure rise can be detected (say 80 b TDC)Point C: The point at which peak pressure is attained. (Say at 120 a TDC)

Ist Stage: - Ignition Lag or Preparation phase in which growth and development of a self propagating nucleus of flame takes place. This is a chemical process depending upon both temperature and pressure, the nature of the fuel and the preparation of the exhaust residual gas. Further, it also depends upon the relationship between the temperature and the rate of reaction.

IInd Stage: - Propagation of flame is a physical process and is covered with the spread of the flame throughout the combustion chamber. The starting point is the point where the line of combustion deposits from the motoring curve in the compression time.

During this stage the flame propagates practically at a small part of the burning mixture comes in contact with the cylinder wall during this period. The rate of heat release depends largely on the turbulence intensity and also on the reaction rate which is dependent on the mixture composition.

The rate of pressure rise is proportional to the rate of heat release because during this stage, the combustion chamber volume remains practically constant (since piston is near the top dead centre).

IIIrd Stage: - The starting point of the third stage i.e, after burning is the instant at which the maximum pressure is reached on the indicator diagram. The flame velocity decreases during this stage. The rate of combustion becomes low due to lower flame velocity and reduced flame front surface.

Since the expansion stroke starts before this stage of combustion with the piston moving away from the top dead centre there can be no pressure rise during this stage.

Flame front propagation: - For efficient combustion the rate of propagation of the flame front within the cylinder is quite critical.

The two important factors which determine the rate of movement of the flame front across the combustion chamber are -(i) Reaction rate (chemical process)(ii) Transposition rate (Physical process)The reaction rate is the result of a purely chemical combination process in which the flame eats its way into the unburned charge.


The transposition rate is due to the physical movement of the flame front relative to the cylinder wall and is also the result of the pressure difference setup between the burning gases and the unburned gases in the combustion chamber.

In area I, (A > B) the flame front progresses relatively slowly due to a low transposition rate and low turbulence. The transposition of the flame front is very little since there is a comparatively small mass of charge burned at the start. The low reaction rate plays a dominant role resulting in a slow advance of the flame. Also, since the spark plug is to be necessarily located in a quiescent layer of gas that is close to the cylinder wall, the lack of turbulence reaches the reaction rate and further lowers the flame speed.

As the flame front leaves the quiescent zone and produces into more turbulent areas (area II) where it consumes a greater mass of mixture, it progresses more rapidly and at a consistent rate (B - C).

The volume of unburned charge is reduced appreciably towards the end of flame travel and the transposition rate again becomes negligible thereby reducing the flame speed. The reaction rate is also reduced again since the flame is entering a zone (area III) of relatively low turbulence (C - D).

Factors influencing the flame speedThe flame velocity influences the rate of pressure rise in the cylinder and it is related to certain types of abnormal combustion that occur in S.I engines.

Important factors which influence the flame speed are: - (i) Turbulence(ii) Fuel - air ratio(iii) Temperate and pressure at inlet(iv) Compression ratio(v) Engine output(vi) Engine speed(vii) Engine size.

Abnormal Combustion In normal combustion, the flame initiated by the spark travels across the combustion chamber in a fairly uniform manner. Under certain operating conditions the combustion gets deviated from its normal course leading to loss of performance and possible damage to the engine. This type of combustion may be termed as an

SI and CI EnginesS K Mondal’s Chapter 2abnormal or knocking combustion. This type of combustion may termed as an abnormal combustion or knocking combustion. The onsequences of this abnormal combustion process are the loss of power, recurring pre-ignition and echanical damage to the engine.

The important abnormal combustions are detonation or knock, pre-ignition run of which knock is most important because it puts a limit on the compression ratio at and to some extent, power output.

(a) (b)

Normal Combustion Abnormal Combustion

In the normal combustion a normal flame front travels across the combustion chamber from A towards D. The speed of the flame front is 15 to 30 m/s. As the flame front advances it compresses the unburned charges BB,D, raising its temperature. The temperature is also increased by radiation from the advancing flame front and due to reaction taking place in the unburned mixture itself. It this unburned charge does not reach its critical temperature for auto-ignition, it will not auto ignite, and the flame front BB’ will move across through unburned change to farthest point of the chamber D in the normal manner.

In the abnormal combustion called detonation or knocking, the end charge auto ignites before the flame front reaches it. In order to auto ignite, the last unburned portion of the charge must reach above a certain critical temperature and remain at this temperature for a certain length of time. During this period certain chemical reactions take place which prepare the charge for auto ignition. The time required in this preparation phase is called ignition delay.

Fig. (b) shows combustion with detonation. The flame front has reaches BB’ and the unburned charge BB’D has reached the critical conditions for auto ignition. If the flame front can proceed from BB’ to D and consume the unburned charge in a normal manner, prior to completion of the delay period, there will be no detonation. If, however, the flame front is able to proceed only as say CC’ during the ignition delay period, then the remaining portion of the unburned charge CC’D will auto ignite and cause extreme pressure fluctuation called detonation, detonation in SI engines occur near the end of combustion where as in diesel engine it occur in the beginning if the combustion.

We have seen that if the unburned charge does not reach its critical temperature there will be no detonation. Further, if the ignition delay period is longer than the time required for the fame front to burn through the unburned charge, there again will be no detonation. Only when the critical temperature is reached and maintained, and the ignition delay is shorter than the time it takes for the flame front to burn through the unburned charge, there will be detonation. Hence in order to avoid or suppress the detonation, a high auto ignition temperature and a long ignition delay are desirable qualities for SI engine fuel.

SI and CI EnginesS K Mondal’s Chapter 2Effect of detonation1. Noise and roughness2. Mechanical damage3. Carbon deposits4. Increase in heat transfer5. Decrease in power output and efficiency6. Pre-ignition

To reduce detonation in SI engineIncrease:1. Engine speed2. Turbulence3. Humidity

Reduce1. C.R.2. Spark timing3. Inlet air temperature & pressure4. Coolant temperature5. Distance of flame travel6. Throttle (load)7. Cylinder and combustion chamber wall temperature.

Use1. High octane fuel2. (A/F) ratio too lean or too rich

Diesel knockThe long delay period will allow a large amount of fuel injection. Hence, large amount of fuel is accumulated in the chamber. The auto ignition of this large amount of fuel may cause high rate of pressure rise and high maximum pressure which may knocking in diesel engines. A long delay period not only increases the amount of fuel injected by the moment of ignition, but also improves the homogeneity of the fuel air mixture and its chemical preparedness for explosion-type self ignition similar to detonation in SI engine. The detonation occurs near the beginning of combustion in diesel engine.

Delay period or ignition lagThe first stage of combustion in the CI engine, i.e. the delay period, exerts a very great influence on both engine design and performance. Ignition delay period during which some fuel has been admitted but has not been ignition. The ignition delay is counted from the start of injection to the point where the P- curves separates from the pure air compression curve.


Stage of Combustion in a CI Engine

The period of physical delay is the time between the beginning of injection and the attainment of chemical reaction conditions. In the physical delay period the fuel is atomized, vaporized, mixed with air, and raised in temperature.

Chemical Delay: The second part of the delay is called chemical delay in which preflame reaction start slowly and then accelerate until local inflammation or ignition take place.

To reduce diesel knockIncrease:1. Injection pressure2. C.R.3. Intake temperature4. Jacket water temperature5. Intake pressure (super changing)6. Load (fuel air ratio)7. Fuel temperature

Reduce:1. Turbulence2. Engine speed3. Injection advance angle

Use:1. High centane fuel.2. Delay angle is reduced by using chemical dopes called ignition accelerator i.e. (i) Ethyl nitrate (ii) Amyl nitrate.3. Small amount of fuel is injected at first.

Effect of Variables on the Delay Period


Increases in variable Effect on Delay Period ReasonCetane number of fuel Reduces Reduces the self-ignition

temperatureInjection pressure Reduces Reduces physical delay due

to greater surface-volume ratio

Injection timing advance Reduces Reduces pressures and temperatures when the injection begins

Compression ratio Reduces Increases air temperature and pressure and reduces auto ignition begins

Intake temperature Reduces Increases air temperatureJacket water temperature Reduces Increases wall and hence

air temperatureFuel temperature Reduces Increases chemical

reaction due to better vaporization

Intake Pressure (supercharging)

Reduces Increases density and also reduces auto ignition temperature

Speed Increases in terms of crank angle. Reduces in terms of milliseconds

Reduces loss of heat

Load (fuel-air-ratio) Decreases Increases the operating temperature

Engine size Decreases in terms of crank angle. Little effect in terms of milliseconds

Larger engines operate normally at low speeds

Type of combustion chamber

Lower for engines with pre combustion chamber

Due to compactness of the chamber

Combustion Chambers for SI EnginesThe design of the combustion chamber for an SI engine has an important influence on the engine performance and its knocking tendencies. The design involves the shape of the combustion chamber, the location of spark plug involves the shape of the plug and the location of inlet and exhaust valves. Because of this importance, the combustion chamber design has been a subject of considerable amount of research and development in the last, fifty years. It has resulted in the raising of the compression ratio of the engine from 4 before the first world war period to 11 in the present times with special combustion chamber designs and suitable antiknock fuels. The important requirements of an SI engine combustion chamber are to provide high power output with minimum octane requirement, high thermal efficiency and smooth engine operation.Combustion chambers must be designed carefully. Keeping in mind the following general objectives.

Smooth Engine OperationThe aim of any engine design is to have a smooth operation and a good economy. These can be achieved by the following:Moderate Rate of Pressure Rise: The rate of pressure rise can be regulated such that the greatest force is applied to the piston as closely after TDC on the power stroke as

SI and CI EnginesS K Mondal’s Chapter 2possible, with a gradual decrease in the force on the piston during the power stroke. The forces must be applied to the piston smoothly, thus limiting the rate of pressure rise as well as the position of the peak pressure with respect to TDC.Reducing the possibility of Knocking: Reduction in the possibility of knocking in an engine can be achieved by,

(i) Reducing the distance of the flame travel by centrally locating the spark plug also by avoiding pockets of stagnant charge.

(ii) Satisfactory cooling of the spark plug and of exhaust valve area which are the source of hot spots in the majority of the combustion chambers.

(iii) Reducing the temperature of the last portion of the charge, through application of a high surface to volume ratio in that part where the last portion of the charge, burns. Heat transfer to the combustion chamber walls can be increased by using high surface to volume ratio thereby reducing the temperature.

Combustion Chambers for CI EnginesThe most important function of the CI engine combustion chamber is to provide proper mixing of fuel and air in a short time. In order to achieve this, an organized air movement called the air swirl is provided to produce high relative velocity between the fuel droplets and the air. The fuel is injected into the combustion chamber by an injector having a single or multihole orifices. The increase in the number if jets reduces the intensity of air swirl needed.

(i) Direct-Injection (DI) Type: This type of combustion chamber is also called an open combustion chamber is located in the main cylinder and the fuel is injected into this volume.

(ii) Indirect-Injection (IDI) Type: In this type of combustion chambers, the combustion space is divided into two parts, one part in the main cylinder and the other part in the cylinder head. The fuel-injection is effected usually into that part of the chamber located in the cylinder head. These chambers are classified further into:

(a) Swirl chamber in which compression swirl is generated.(b) Pre combustion chamber in which combustion swirl is induced. (c) Air cell chamber in which both compression and combustion swirl are induced.

SI and CI EnginesS K Mondal’s Chapter 2Knock limited parameters: - It should be the aim of the designer of reduces the tendency of knock in the engine.In this connection certain knock limited parameters are: -(a) Knock limited compression ratio(b) Knock limited inlet pressure.(c) Knock limited indicated mean effective pressure.

Knock Limited Compression ratio: The knock limited compression ratio is obtained by increasing the compression on a variable compression ratio engine until incipient knocking is observed. Any change in operating condition such as fuel-air ratio or in the engine design that increases the knock limited compression ratio is said to reduce the tendency towards knocking.

Knock Limited Inlet Pressure: The inlet pressure can be increased by opening the throttle or increasing delivery pressure until incipient knock is observed. An increase in knock limited inlet pressure indicates a reduction in the knocking tendency.

Knock limited Indicated Mean Effective Pressure:An useful measure of knocking tendency called the performance number, has been developed from the concept of knock limited indicated mean effective pressure. This number is defined as the ratio of Klimep with the fuel in question to Klimep with iso-octane when the inlet pressure is kept constant. This performance number is related to octane number and one of the advantages of this is that it can be applied to fuels whose knocking characteristics are superior to that of iso-octane, i.e., it extends the octane scale beyond 100.Further simplification on the use of performance number requirements is done by introducing the concept of relative performance number, rpm, which is defined as:

Effect of engine variables on knockFour major factors are involved in either producing or preventing knock. These are -

(i) Temperature(ii) Pressure(iii) Density of the unburned charge(iv) Time factors.

Since, the effect of temperature Pressure and density are closely interrelated, these can be consolidated into one group, and thus there are three factors - (a) Density factor(b) Time factor(c) Composition factor


Characteristics Tending to Reduce Detonation or Knock

S.No. Characteristics SI Engines CI Engines1. Ignition temperature of fuel High Low2. Ignition delay Long Short3. Compression ratio Low High4. Inlet temperature Low High5. Inlet pressure Low High6. Combustion wall temperature Low High7. Speed, rpm High Low8. Cylinder size Small Large

Fuel-Air (F/A) or Air-Fuel Ratio (A/F)

The relative proportions of the fuel and air in the engine are very important from the standpoint of combustion and the efficiency of the engine. This is expressed either as a ratio of the mass of the fuel to that of the air or vice versa.

In the SI engine the fuel-air ratio practically remains a constant over a wide range of operation. In CI engines at a given speed the air flow does not vary with load: it is the fuel flow that varies directly with load. Therefore, the term fuel-air ratio is generally used instead of air-fuel ratio.

A mixture that contains just enough air for complete combustion of all the fuel in the mixture is called a chemically correct or stoichiometric fuel-air ratio. A mixture having more fuel than that in a chemically correct mixture is termed as rich mixture and a mixture that contains less fuel (orexcess air) is called a lean mixture. The ratio of actual fuel-air ratio to stoichiometric fuel-air ratio is called equivalence ratio and is denoted by .

Accordingly, = 1 means stoichiometric (chemically correct) mixture, < 1 means lean mixture and > 1 means rich mixture.

Air-Fuel Mixtures: - An engine is generally operated at different loads and speeds. For this, proper air-fuel mixture should be supplied to the engine cylinder. Fuel and air are mixed to form three different types of mixtures.(i) Chemically correct mixture(ii) Rich mixture and (iii) Lean mixture

SI and CI EnginesS K Mondal’s Chapter 2Chemically correct or stoichiometric mixture is one in which these is just enough air for complete combustion of the fuel.For example, to burn one kg of octane (C8H18) completely 15.12 kg of air is required. Hence chemically correct A/F ratio for C8H18 is 15.12 :1, usually approximated to 15 : 1.

This chemically correct mixture will vary only slightly in numerical value between equations for complete combustion for a particular fuel.

A mixture which contains less air than the stoichiometric requirements are called a rich mixture and these contain more air is called a lean mixture.

These are, however, a limited range of A/F ratios in a homogenous mixture, only within which combustion in an SI engine will occur.

E x c e ss fu el E x c e s s A ir

F ig : U sefu l a ir- fue l m ix tu re ra nge o f gas o lin e (k g a ir /k g fu el)

Outside this range, the ratio is either too rich or too lean to sustain flame propagation.This range of useful A/F ratio rums from approximately 19:1 (lean) to 9 : 1 (rich).

The carburetor should provide an A/F ratio in accordance with engine operating requirements and this must within the combustible range.Mixture requirements at different loads and speedsThe air-fuel ratio at which an engine operates has a considerable influence on its performance. Let us consider an engine operating at full throttle and constant speed with varying A/F ratio. Under these conditions, the A/F ratio will affect both the power output and the brake specific fuel consumption as indicated by the typical curve shown below.


Fig.

The mixture corresponding to the maximum output on the curve is called the best power mixture with an A/F ratio of approximately 12:1. The mixture corresponding to the minimum point on the bsfc curve is called the best economy mixture. The A/F ratio is approximately 16 : 1.The best power mixture is much richer than the chemically correct mixture and the best economy mixture is slightly leaner than the chemically correct.

Above figure is based on full throttle operation. The A/F ratios for the best power and best economy at part throttle are not strictly the same as at full throttle. If the A/F ratios for best power and best economy are constant over the full range of throttle operation and if the influence of other factors is disregarded, the ideal fuel metering device would be merely a two position carburetor. Such a carburetor could be set for the best power mixture when maximum performance is desired and for the best economy mixture when the primary consideration is the fuel economy.These two settings are indicated below by the solid horizontal lines X-X’ and Z - Z’ respectively.

Fig : Carburetor Performance necessary to fulfill Engine Requirements .

The CI engine combustion chambersDiesel engines can have divided and undivided combustion chambers (indirect and direct injection engines). Direct injection engines have a greater efficiency and operate more economically than indirect injection engines using a pre- or swirl chamber. Direct injection engines are used for all commercial vehicle applications. Because of lower engine noise, indirect injection engines are more suitable for cars. In addition, they are significantly less polluting (HC and NOx) and cheaper to produce. For these reasons their higher fuel consumption in comparison with direct injection engines is generally accepted as a compromise. With the pre-chamber method for car diesel engines, fuel is injected into a hot pre-chamber in which pre-combustion initiates a good mixture preparation for the main combustion. Fuel is injected by means of a throttling pintle nozzle at relatively low pressure (up to 300 bar). The partially combusted air-fuel mixture then passes to the main combustion chamber. A variation of this method is the swirl chamber method. With a direct injection method used primarily for commercial vehicles and stationary engines, fuel is injected directly into the combustion chamber without the use of a pre-chamber.

SI and CI EnginesS K Mondal’s Chapter 2Pre-combustion chamberThis chamber is located at the cylinder head and is connected to the engine cylinder by small holes. It occupies 40% of the total cylinder volume. During the compression stroke, air from the main cylinder enters the pre-combustion chamber. At this moment, fuel is injected into the pre-combustion chamber and combustion begins. Pressure increases and the fuel droplets are forced through the small holes into the main cylinder, resulting in a very good mix of the fuel and air. The bulk of the combustion actually takes place in the main cylinder. This type of combustion chamber has multi-fuel capability because the temperature of the pre-chamber vaporizes the fuel before the main combustion event occurs.


OBJECTIVE QUESTIONS (GATE & IES)Previous Years GATE Questions

Ignition limitsQ1. A fuel represented by the formula C8H16 is used in an I.E. Engine. Given

that the molecular weight of air is 29 and that 4.76 kmols of air contain 1 kmol of oxygen and 3.76 kmol of nitrogen, the Air /Fuel ratio by mass is [GATE-2000](a) 11.47 (b) 12.78 (c) 14.79 (d) 19.52

Q2. For a spark ignition engine, the equivalence ratio ( ) of mixture entering the combustion chamber has values

[GATE-2003](a) < 1 for idling and > 1 for peak power conditions(b) > 1 for both idling and peak power conditions(c) > 1 for idling and < 1 for peak power conditions(d) < 1 for both idling and peak power conditions

Combustion in SI engineQ3. Knocking tendency in a S.l. engine reduces with increasing [GATE-1993]

(a) Compression ratio (b) wall temperature (c) Supercharging (d) engine speed

Diesel knockQ4. An IC engine has a bore and stroke of 2 units each. The area to

calculate heat loss can be taken as [GATE-1998]

Previous Years IES QuestionsIgnition limitsQ1. For a conventional S.I. engine, what is the value of fuel-air ratio in the

normal operating range?(a) 0.056 – 0.083 (b) 0.083 – 0.56(c) 0.0056 – 0.83 (d) 0.056 – 0.83 [IES

2007]

Q2. The stoichiometric air/fuel ratio for petrol is 15: 1. [IES-2004]

What is the air/fuel ratio required for maximum power?(a) 16 : 1 - 18 : 1 (b) 15 : 1 (c) 12 : 1 - 18 : 1 (d) 9: 1 - 11 : 1

SI and CI EnginesS K Mondal’s Chapter 2Q3. The air fuel ratio for idling speed of an automobile patrol engine is

closer to (a) 10: 1 (b)15:1 (c) 17 : 1 (c) 21: 1 [IES-

1992]

Q4. Match List I with List II and select the correct answer using the given code given below the lists: [IES 2007]List I List I(SI Engine Operational mode) (A/F Ratio Supplied by the

Carburetor)A. Idling 1. 3B. Cruising 2. 10C. Maximum Power 3. 13D. Cold starting 4. 16

5. 20Codes:

A B C D A B C D(a) 2 4 5 1 (b) 4 5 3 2(c) 2 4 3 1 (d) 4 5 3 1

Q5. Match List I (S.I. Engine Operational Mode) with List II (Air fuel Ratio by Mass) and select the correct answer: [IES-2004]List I List IIA. Idling 1. 4: 1B. Cruising 2. 10: 1C. Maximum power 3. 12.5: 1D. Cold starting 4. 16: 1

5. 14.8: 1A B C D A B C D

(a) 2 4 3 1 (b) 5 4 1 3(c) 2 3 5 1 (d) 5 3 1 4

Q6. The air fuel ration for idling speed of an automobile patrol engine is closer to

(a) 10: 1 (b)15:1 (c) 17 : 1 (c) 21: 1 [IES-1992]

Q7. Match List I with List II and select the correct answer [IES-1996]

List I (SI engine operating mode) List II (Desired air-fuel ratio)A. Idling 1. 13.0B. Cold starting 2. 4.0.C. Cruising 3. 16.0D. Full throttle 4. 9.0Codes: A B C D A B C D

(a) 4 2 3 1 (b) 2 4 1 3(c) 1 2 1 3 (d) 2 4 3 1

Q8. Match List I (Air-fuel ratio by mass) with List II (Engine operation

mode) and select the correct answer using the codes given below the Lists [IES-2000]List I List IIA. 10: 1 1. CI engine part load


B. 16: 1 2. SI engine part loadC. 35: 1 3. SI engine idlingD. 12.5: 1 4. CI full load

5. SI full loadCodes: A B C D A B C D

(a) 3 2 1 5 (b) 4 2 1 5(c) 3 1 2 4 (d) 4 1 2 3

Q9. Match List I with List II and select the correct answer using the codes given below the lists:List I (Operating mode of SI engine) List II (Appropriate Air-Fuel

ratio)A. Idling 1. 12.5

[IES-1995]B. Cold starting 2. 9.0C. Cruising 3. 16.0D. Maximum power 4. 22.0

5. 3.0Codes: A B C D A B C D(a) 2 4 5 1 (b) 1 3 4 2(c) 5 2 1 3 (d) 2 5 3 1

Q10. Match List I (S.I. Engine Operating Mode) with List II (Approximate A/F

Ratio) and select the correct answer using the code given below the Lists: List I List II [IES-

2005]A Cold Start 1. 10: 1B. Idling 2. 16:1C. Cruising 3. 13 : 1D. Maximum Power 4. 4 : 1

5. 20: 1A B C D A B C D

(a) 4 3 2 1 (b) 2 1 5 3(c) 4 1 2 3 (d) 2 3 5 1

Q11. Match List I with List II and select the correct answer. [IES-1994]

List I (Operating condition) List II (Approximate air fuel ratio)A. Idling 1. 16B. Part load operation 2. 10C. Full load 3. 12.5D. Cold start 4. 3Codes: A B C D A B C D(a) 2 1 3 4 (b) 1 2 4 3(c) 2 1 4 3 (d) 1 2 3 4

Q12. If methane undergoes combustion with the stoichiometric quantity of

air, the air-fuel ratio on molar basis would be: [IES-1997](a) 15.22: 1 (b) 12.30: 1 (c) 14.56: 1 (d) 9.52: 1

Q13. Stoichiometric air-fuel ratio by volume for combustion of methane in air is:


(a) 15: 1 (b)17.16: 1 (c) 9.52: 1 (d) 10.58: 1 [IES-2002]

Q14. Auto -ignition time for petrol- air mixture is minimum when the ratio of actual fuel-air ratio and chemically correct fuel air ratio is [IES-2002](a) 0.8 (b) 1.0 (c) 1.2 (d) 1.5

Combustion in SI engineQ15. Velocity of flame propagation in the SI engine is maximum for a fuel-air

mixture which is [IES-1999](a) 10% richer than stoichiometric (b) Equal to stoichiometric (c) More than 10% richer than stoichiometric (d) 10% leaner than stoichiometric

Q16. Assertion (A): In SI engines, as the engine speed increases, spark is required to be advanced.Reason (R): As the engine speed increases, flame velocity increases.


Q17. Which of the following cannot be caused by a hot spark plug? [IES-2005]

1. Pre-ignition 2. Post-ignition 3. Detonation 4. Run-on-ignitionSelect the correct answer using the code given below:(a) 1 and 4 (b) 2 only (c) 2 and 3 (d) 3 only

Q18. By higher octane number of SI fuel, it is meant that the fuel has [IES-1995]

(a) Higher heating value (b) higher flash point (c) Lower volatility (d) longer ignition delay

Q19. Match List I with List IT and select the correct answer using the codes given below the lists: [IES-1993]List I List II (S.l. Engine problem) (Characteristic of fuel responsible for the

problem)A. Cold starting 1. Front end volatilityB. Carburetor icing 2. Mid-range volatilityC. Crankcase dilution 3. Tail end volatilityCode:A B C A B C(a) 1 2 3 (b) 1 3 2(c) 2 3 1 (d) 3 1 2

Q20. List I gives the different terms related to combustion while List II gives

the outcome of the events that follow. Match List I with List II and select the correct answer. [IES-1996]

SI and CI EnginesS K Mondal’s Chapter 2 List I List II

A. Association 1. Pseudo shockB. Dissociation 2. KnockC. Flame front 3. EndothermicD. Abnormal combustion 4. ExothermicCode:A B C D A B C D(a) 3 4 1 2 (b) 4 3 1 2(c) 3 4 2 1 (d) 4 3 2 1

Detonation or knockingQ21. Which of the following factors increase detonation in the SI engine?

1. Increased spark advance. [IES-1993]

2. Increased speed.3. Increased air-fuel ratio beyond stoichiometric strength4. Increased compression ratio.Select the correct answer using the codes given below:(a) 1 and 3 (b) 2 and 4 (c) 1, 2 and 4 (d) 1 and 4

Q22. Consider the following statements:

[IES-2006]1. In the SI engines detonation occurs near the end of combustion whereas in CI engines knocking occurs near the beginning of combustion.2. In SI engines no problems are encountered on account of pre-

ignition.3. Low inlet pressure and temperature reduce knocking tendency in SI engines but increase the knocking tendency in CI engines.Which of the statements given above are correct?(a) 1, 2 and 3 (b) Only 1 and 2 (c) Only 2 and 3 (d) Only 1 and

3

Q23. The tendency of petrol to detonate in terms of octane number is determined by comparison of fuel with which of the following?

[IES-2006](a) Iso-octane (b) Mixture of normal heptane and

iso-octane(c) Alpha methyl naphthalene (d) Mixture of methane and ethane

Q24. Consider the following statements: [IES-2006]

In order to prevent detonation in a spark-ignition engine, the charge away from the spark plug should have1. Low temperature 2. Low density 3. Long ignition delayWhich of the statements given above is/are correct?(a) Only 1 (b) Only 2 (c) Only 3 (d) 1, 2 and 3

Q25. For minimizing knocking tendency is SI engine, where should the space plug be located? [IES-2008](a) Near inlet valve(b) Away from both the valves(c) Near exhaust valve(d) Midway between inlet and exhaust valves


Q26. Which of the following action(s) increaser(s) the knocking tendency in the S I. engine?(a) Increasing mixture strength beyond equivalence ratio () =14

[IES-2004](b) Retarding the spark and increasing the compression ratio(c) Increasing the compression ratio and reducing engine speed(d) Increasing both mixture strength beyond equivalence ratio ()= 14 and the compression ratio

Q27. In spark ignition engines knocking can be reduced by: [IES-2002]

(a) Increasing the compression ratio(b) Increasing the cooling water temperature(c) Retarding the spark advance(d) Increasing the inlet air temperature

Q28. In a SI engine, combustion stage I takes 1 ms and combustion stage II takes 1·5 ms when the engine runs at 1000 rpm. If stage I time duration is independent of engine speed what will be the additional spark advance necessary when the engine speed is doubled?

[IES-2008](a) 0° (b) 6° (c) 12° (d) 24°

Q29. Which of the following are the assumptions involved in the auto-ignition theory put forth for the onset of knock in SI engines? [IES-1998]1. Flame velocity is normal before the onset of auto ignition.2. A number of end-gas elements auto ignite simultaneously.3. Preflame reactions are responsible for preparing the end-gas to

ignite.Select the correct answer using the codes given below:(a) 1 and 2 (b) 1 and 3 (c) 2 and 3 (d) 1, 2 and 3


Knock in the SI engine can be reduced by1. Supercharging 2. Retarding the spark.3. Using a fuel of long straight chain structure 4. Increasing the engine speed.Of these correct statements are(a) 1 and 2 (b) 2 and 3 (c) 1, 3 and 4 (d) 2 and 4

Q31. Assertion (A): Self-ignition temperature of the end charge must be higher to

prevent knocking of an SI engine. [IES-1994]Reason (R): Higher compression ration increases the temperature of the air-fuel mixture.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

SI and CI EnginesS K Mondal’s Chapter 2Q32. Match List I with List II, in respect of SI engines, and select the correct

answer by using the codes given below the lists: [IES-1995]List I List IIA. Highest useful compression ratio 1. Ignitable mixtureB. Dopes 2. Knock rating of fuelsC. Limiting mixture strength 3. DetonationD. Delay period 4. Chain of chemical reactions in

combustion chamberCodes: A B C D A B C D

(a) 2 3 1 4 (b) 3 2 1 4(c) 2 3 4 1 (d) 3 4 2 1


Detonation in the S.I. engine can be suppressed by1. Retarding the spark timing. 2. Increasing the engine speed.3. Using 10% rich mixture.Of these statements(a) 1 and 3 are correct (b) 2 and 3 are correct (c) 1, 2 and 3 are correct (d) 1 and 2 are correct

Q34. Consider the following statements [IES-2000]

1. Octane rating of gasoline is based on iso-octane and iso-heptane fuels which are paraffin

2. Tetraethyl lead is added to gasoline to increase octane number.3. Ethylene di-bromide is added as scavenging agent to remove lead

deposits on spark plugs.4. Surface ignition need not necessarily cause knocking.Which of these statements are correct?(a) 1, 2, 3 and 4 (b) 2, 3 and 4 (c) 1 and 4 (d) 1,2 and 3

SI engine combustion chamber designsQ35. The volumetric efficiency of a well designed SI engine is in the range of

(a) 40% - 50% (b) 50% - 60%(c) 60% - 70% (d) 70% - 90%

[IES-2002]

Q36. Consider the following statements relevant to the ignition system of SI engine:1. Too small a dwell angle will lead to the burning of condenser and

contact points.2. Too small a dwell angle will result in misfiring. [IES-

2002]3. Too large a dwell angle will result in burning of condenser and

contact points.4. Too large a dwell angle will result in misfiring.Which of the above statements are correct?(a) 1 and 2 (b) 2 and 3 (c) 3 and 4 (d) 4 and 1

SI and CI EnginesS K Mondal’s Chapter 2Q37. Knocking in the SI engine decreases in which one of the following

orders of combustion chamber designs? [IES-1995](a) F head, L head, I head (b) T head, L head, F head.(c) I head, T head, F head (d) F head, I head, T head.

Q38. Which of the following feature(s) is/are used in the combustion chamber design to reduce S.I engine knock? [IES-2004](a) Spark plug located away from exhaust value, wedge shaped combustion chamber and short flame travel distance(b) Wedge shaped combustion chamber(c) Wedge shaped combustion chamber and short flame travel distance(d) Spark plug located away from exhaust value, short flame travel distance and side value design

Q39. Which of the following combustion chamber design features reduce(s) knocking in S.I. engines? [IES-2005]1. Spark plug located near the inlet valve. 2. T-head.3. Wedge shaped combustion chamber 4. Short flame travel distance.Select the correct answer using the code given below:(a) 1 and 3 (b) 3 only (c) 3 and 4 (d) 1 and 2

Q40. A two-stroke engine has a speed of 750 rpm. A four-stroke engine having an identical cylinder size runs at 1500 rpm. The theoretical output of the two-stroke engine will(a) Be twice that of the four-stroke engine (b) Be half that of the four-stroke engine(c) Be the same as that of the four-stroke

[IES-1997](d) Depend upon whether it is a C.I. or S.I. engine

Q41. For same power output and same compression ratio, as compared to two-stroke engines, four-stroke S.I. engines have: [IES-1997]

(a) Higher fuel consumption (b) lower thermal efficiency(c) Higher exhaust temperatures (d) higher thermal efficiency

Q42. Match List I with List 11and select the correct answer using the codes given below the lists:List I List II [IES-

1993]A. Pre-combustion chamber 1. Compression swirlB. Turbulent chamber 2. Masked inlet valveC. Open combustion chamber 3. Spark ignitionD. F-head combustion chamber 4. Combustion induced swirl

5. M-chamberCode:A B C D A B C D(a) 4 5 3 2 (b) 1 3 5 2(c) 2 3 1 5 (d) 4 1 2 3

Q43. If the approximate average mean pressures during induction,

compression, power and exhaust strokes of an internal combustion


engine are respectively 15 kN/m2 below atmosphere, 200 kN/m2 above atmosphere, 1000 kN/m2 above atmosphere and 20 kN/m2 above atmosphere, then the resultant mean effective pressure, in kN/m2 , is

[IES-1994](a) 765 (b) 795 (c) 800 (d) 805

Combustion in the CI engineQ44. The delay period in CI engine depends upon which of the following?

(a) Temperature and pressure in the cylinder at the time of injection(b) Nature of the fuel mixture strength(c) Relative velocity between the fuel injection and air turbulence pressure of residual gases(d) All of the above [IES

2007]

Diesel knockQ45. Which of the following statements is "true"?

[IES-1992](a) The term "KNOCK" is used for on identical phenomenon in a spark ignition and compression ignition engine(b) "KNOCK" is a term associated with a phenomenon taking place in the early part of combustion in a spark ignition engine and the later part of combustion in a spark ignition engine(c) "KNOCK" is a term associated with a phenomenon taking place in the early part of combustion in a spark ignition engine and the later part of combustion in a compression ignition engine(d) None of the above

Q46. The knocking tendency in compression ignition engines increases with: (a) Increase of coolant water temperature (b) Increase of temperature of inlet

air(c) Decrease of compression ratio (d) Increase of compression ratio

[IES-2005]

Q47. Assertion (A): A very high compression ratio is favoured for a CI engine, in order to attain high mechanical efficiency without knocking. [IES-1996]Reason (R): The delay period in CI combustion affects rate of pressure rise and hence knocking.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q48. Which of the following factors would increase the probability of knock in the CI engines?1. Long ignition delay of fuel 2. High self ignition temperature of fuel3. Low volatility of fuel [IES-

1995]Select the correct answer using the codes given below:(a) 1, 2 and 3 (b) 1 and 2 (c) 1 and 3 (d) 2 and 3.

SI and CI EnginesS K Mondal’s Chapter 2Q49. Consider the following measures: [IES-

1994]1. Increasing the compression ratio 2. Increasing the intake air temperature.3. Increasing the length to diameter ratio of the cylinder.4. Increasing the engine speed.The measures necessary to reduce the tendency to knock in CI engines would include(a) 1, 2 and 3 (b) 1, 2 and 4 (c) 1, 3 and 4 (d) 2, 3 and 4

Q50. Hypothetical pressure diagram

for a compression ignition engine is shown in the given figure. The diesel knock is generated during the period

(a) AB (b) BC (c) CD (d) after D

[IES-1998]

Q51. The tendency of knocking in C I engine reduces by [IES-2002]

(a) High self - ignition temperature of fuel(b) Decrease in jacket water temperature(c) Injection of fuel just before TDC(d) Decrease in injection pressure

Q52. Which of the following factor(s) increase(s) the tendency for knocking in the C.I. engine? (a) Increasing both the compression ratio and the coolant temperature

[IES-2004](b) Increasing both the speed and the injection advance(c) Increasing the speed, injection advance and coolant temperature(d) Increasing the compression ratio

Q53. Consider the following statements: Diesel knock can be reduced by1. Increasing the compression ratio.2. Increasing the engine speed.3. Increasing the injection retard.4. Decreasing the inlet air temperatureWhich of the statements given above are correct?(a) 2 and 4 only (b) 1, 2 and 3 only(c) 1 and 3 only (d) 1, 2, 3 and 4 [IES

2007]

The CI engine combustion chambersQ54. The object of providing masked inlet valve in the air passage of

compression- ignition engines is to [IES-1994]


(a) Enhance flow rate (b) control air flow.(c) Induce primary swirl (d) induce secondary turbulence.

Q55. Which one of the following events would reduce the volumetric efficiency of a vertical compression - ignition engine?

[IES-1994](a) Inlet valve closing after bottom dead centre.(b) Inlet valve closing before bottom dead centre.(c) Inlet valve opening before top dead centre.(d) Exhaust valve closing after top dead centre.

Q56. If the performance of diesel engines of different sizes, cylinder dimensions and power ratings are to be compared, which of the following parameters can be used for such comparison?

[IES-2003](a) Swept volume (b) Air fuel ratio (c) Specific brake fuel consumption (d) Volumetric efficiency

Q57. A 4-stroke diesel engine, when running at 2000 rpm has injection duration of 1.5 ms. What is the corresponding duration of the crank angle in degrees? (a) 18° (b) 9o (c) 36° (d) 315°

[IES-2005]

Q58. In the operation of four-stroke diesel engines, the term ‘squish’ refers to the:

(a) Injection of fuel in the pre-combustion chamber [IES-2001]

(b) Discharge of gases from the pre-combustion chamber(c) Entry of air into the combustion chamber(d) Stripping of fuel from the core

Q59. Divided chamber diesel engines use lower injection pressures compared to open chamber engines because

[IES-1999](a) Pintle nozzles cannot withstand high injection pressures(b) High air swirl does not require high injection pressures for atomization(c) High injection pressures may cause over-penetration(d) High injection pressure causes leakage of the fuel at the pintle

Q60. For which of the following reasons, do the indirect injection diesel engines have higher specific output compared to direct injection diesel engines? 1. They have lower surface to volume ratio.

[IES-2005]2. They run at higher speeds.3. They have higher air utilization factor.4. They have lower relative heat loss.Select the correct answer using the code given below:(a) 1 and 2 (b) 2 only (c) 2 and 3 (d) 3 and 4

Q61. Assertion (A): Pre-chamber diesel engines use higher injection pressures when compared to open combustion chamber engines. [IES-1997]Reason (R): Pre-chamber engines have higher compression pressures.


(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q62. Assertion (A): A pintle nozzle is employed to mix the fuel properly even with the slow air movement available with many open combustion chambers in C I engines. Reason (R): The mixing of fuel and air is greatly affected by the nature of the air movement in the combustion chamber of C I engines. [IES-2002](a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Comparison of SI and CI EnginesQ63. Which of the following pairs of engine and performance/characteristics

is/are correctly matched? 1. Turbojet - Efficiency increases with flight speed2. SI engine - Lowest specific fuel consumption3. Turboprop - Suitable for low flight speedsSelect the correct answer using the codes given below:

[IES-1998](a) 1 and 2 (b) 2 and 3 (c) 1 and 3 (d) 2 alone


1. Volumetric efficiency of diesel engines is higher than that of SI engines

2. When a SI engine is throttled; its mechanical efficiency decreases.3. Specific fuel consumption increases as the power capacity of the engine increases.4. In spite of higher compression ratios, the exhaust temperature in diesel engines is much lower than that in SI engines.Of these statements(a) 1, 2, 3 and 4 are correct (b) 1, 2 and 3 are correct(c) 3 and 4 are correct (d) 1, 2 and 4 are correct

Q65. The correct sequence of the decreasing order of brake thermal efficiency of the three given basic type of IC engines is(a) 4 stroke CI engine, 4 stroke SI engine, 2 stroke SI engine

[IES-1995](b) 4 stroke SI engine, 4 stroke CI engine, 2 stroke SI engine(c) 4 stroke CI engine, 2 stroke SI engine, 4 stroke SI engine(d) 2 stroke SI engine, 4 stroke SI engine, 4 stroke CI engine.

Q66. Assertion (A): Specific output of a diesel engine is higher than that of the SI engine.

Reason (R): Diesel engine is built stronger and heavier with higher compression ratio.

(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A


(c) A is true but R is false [IES-1998]

(d) A is false but R is trueQ67. Assertion (A): Knocking in S.I. engines is due to auto-ignition of the end charge

while knocking, in C.I. engines are due to auto-ignition of the first charge.Reason (R): Spark ignition engines employ lower compression ratio than diesel engines and the fuel used has a calorific value lower than that of diesel oil. [IES-2001](a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q68. With reference to Turbojet and Rocket engines, consider the following statements:1. Efficiency of Rocket engines is higher than that of Jet engines

[IES-2001]2. Exit velocities of exhaust gases in Rocket engines are much higher

than those in Jet engines3. Stagnation conditions exist at the combustion chamber in Rocket

engines4. Rocket engines are air-breathing enginesWhich of these statements are correct?(a) 1 and 2 (b) 1, 3 and 4 (c) 2, 3 and 4 (d) 1, 2

and 3



Previous Years GATE Answers1 Ans. (c)

2. Ans. (b)3. Ans. (d)4. Ans. (c)

Previous Years IES Answers1. Ans. (a) (A/F) for maximum power = 12 and (A/F) for Stoichiometric = 14.5 (A/F) for maximum fuel economy = 16. 2. Ans. (d) For Maximum power Air/Fuel ratio=12:1

For Maximum economy Air/Fuel ratio=16:1For Maximum power we need rich mixture.

3. Ans. (a)4. Ans. (c)5. Ans. (a)6. Ans. (a)7. Ans. (a) Cold starting requires maximum fuel, idling requires little less fuel, full throttle requires still less fuel and idling requires minimum fuel.8. Ans. (a)9. Ans. (d)10. Ans. (c)11. Ans. (a)12. Ans. (d)

13. Ans. (c)14. Ans. (c)15. Ans. (a) When the mixture is nearly 10% richer than stoichiometric (fuel-air ratio = 0.08 i.e. air-fuel ratio 12.5:1) the velocity of flame propagation is maximum and ignition lag of the end gas is minimum.16. Ans. (c)17. Ans. (b)18. Ans. (d) A fuel with higher octane number has longer ignition delay.19. Ans. (a)20. Ans. (b)

SI and CI EnginesS K Mondal’s Chapter 221. Ans. (d) Detonation in the S.I. engines is increased by increasing spark advance and increased compression. The increased speed and lean mixtures do not have much influence.22. Ans. (d)23. Ans. (b)24. Ans. (d)25. Ans. (b)26. Ans. (c)27. Ans. (c)28. Ans. (b)29. Ans. (d)30. Ans. (d) Knock in SI engine can be reduced by retarding spark and increasing the engine speed.31. Ans. (d) Self ignition temperature of end of charge must be lower to prevent knocking. Higher compression ratio increases temperature of air fuel mixture. Thus A is false and R is correct.32. Ans. (b)33. Ans. (d) Decreasing the engine speed increases possibility of detonation34. Ans. (b) Octane rating of gasoline is based on iso-octane and n-heptane fuels which are paraffin so 1 is wrong.35. Ans. (d)36. Ans. (b)37. Ans. (c)38. Ans. (c)39. Ans. (c)40. Ans. (c) In two-stroke engine there is one power stroke in 2 strokes, but in four-stroke engine there is one power stroke in 4 strokes.41. Ans. (d) Efficiency of 4 stroke engine is higher because of better utilization compared to 2 stroke engine.42. Ans. (a)43. Ans. (a) Resultant mean effective pressure = 1000 - 200 - (15 + 20) = 800 - 35 = 765 kN/m2

44. Ans. (d)45. Ans. (a)46. Ans. (c)47. Ans. (d) A very high compression ratio is favoured for a CI engine, in order to attain high THERMAL efficiency without knocking. 48. Ans. (a) 49. Ans. (a)50. Ans. (b)51. Ans. (c)52. Ans. (b) Increasing coolant temperature will reduce knocking in C I engine.53. Ans. (c) Increasing the engine speed reduces knock in SI engine. Decreasing the engine speed reduces knock in CI engine.54. Ans. (a)55. Ans. (b) Volumetric efficiency will reduce if fresh charge filled is reduced. i.e. inlet valve closes before bottom dead centre.56. Ans. (c)57. Ans. (a)

58. Ans. (c) Mixing of the fuel more thoroughly. Improved mixing had to be accomplished by imparting additional motion to the air, most commonly by induction-

SI and CI EnginesS K Mondal’s Chapter 2produced air swirls or a radial movement of the air, called squish, or both, from the outer edge of the piston toward the centre. Various methods have been employed to create this swirl and squish.59. Ans. (b) In divided combustion chamber, the restrictions or throat between chambers results in high velocities which helps in rapid mixing of fuel with air. Thus high injection pressure is not required for atomization.60. Ans. (d)61. Ans. (d) Pre-combustion diesel engines use lower injection pressures62. Ans. (b)63. Ans. (c)64. Ans. (d) Specific fuel consumption decreases as the power capacity of the engine increases.65. Ans. (a)66. Ans. (d) Due to higher compression ratio. Specific power output (means power /weight) of a diesel engine is lower than that of the SI engine. That so why we use SI engine for light vehicles. 67. Ans. (b)68. Ans. (d)


Conventional Questions with AnswersQ1. The spark plug is fixed at 18o before top dead centre (TDC) in an SI

engine running at 1800 r.p.m. It takes 8o of rotation to start combustion and get into flame propagation mode. Flame termination occurs at 12o after TDC. Flame front can be approximated as a sphere moving out from the spark plug which is offset 8 mm from the centre line of the cylinder whose bore diameter is 8.4 cm. Calculate the effective flame front speed during flame propagation. The engine speed is increased to 3000 r.p.m. and subsequently as a result of which the effective flame front speed increases at a rate such that it is directly proportional to 0.85 times of engine speed. Flame development after spark plug firing still takes 8o of engine rotation. Calculate how much engine rotation must be advanced such that the flame termination again occurs at 12o after TDC. [IES-2010, 10-Marks]

Ans. Insufficient data.

3. Carburetion and Fuel Injection

Theory at a Glance (For IES, GATE & PSUs)Definition of CarburetionThe process of formation of a combustible fuel-air mixture by mixing the proper amount of fuel with air before admission to engine cylinder is called carburetion and the device which does this job is called a carburettor.

Induction system showing typical pressure at the start of intake stroke under idling condition.

Factors Affecting CarburetionOf the various factors, the process of carburetion is influenced by (i) The engine speed (ii) The vapourization characteristics of the fuel (iii) The temperature of the incoming air and (iv) The design of the carburetor

Therefore, in order to have high quality carburetion (that is mixture with high vapour vapour content) the velocity of the air stream at the point where the fuel is injected has to be increased. This is achieved by introducing a venturi section in the path of the air. The fuel is discharged by introducing a venturi section in the path of the air. The fuel is discharged from the main metering jet at the minimum cross section of the venturi (called throat).

Other factors which ensure high quality carburetion within a short period are the presence of highly volatile hydrocarbons in the fuel. Therefore, suitable evaporation characteristics of the fuel, indicated by its distillation curve, are necessary for efficient carburetion especially at high engine speeds.

The design of the carburator, the intake, system and the combustion chamber have considerable influence on uniform distribution of mixture to the various cylinders of the engine. Proper design of carburetor element alone ensures the supply of desired composition of the mixture under different operating conditions of the engine.

Carburetion and Fuel InjectionS K Mondal’s Chapter 3

Performance Curve of an Automobile Carburettor

Principle of CarburetionInto the chamber. In the carburettor, air passing into the combustion chamber picks up fuel discharged from a tube. This tube has a fine orifice called carburettor jet which is exposed to the air path. The rate at which fuel is discharged into the air depends on the pressure difference or pressure head between the float chamber and the throat of the venturi and on the area of the output of the tube. In order that the fuel drawn from the nozzle may be thoroughly atomized, the suction effect must be strong and the nozzle outlet comparatively small. In order to produce a strong suction, the pipe in the carburettor carrying air to the engine is made to have a restriction. At this restriction called throat due to increase in velocity of flow, a suction effect is created. The restriction is made in the form of a venture as shown in Fig. to minimize throttling losses. The end of the fuel jet is located at the venture or throat of the carburettor.

Operation of the Venturi Tube

Simple Carburetor The simple carburetor mainly consist of a float chamber, fuel discharge nozzle and a metering orifice, a venturi, a throttle valve and a choke. The float and needle valve system maintains a constant level of gasoline in the float chamber. If the amount of fuel in the float chamber falls below the designed level, the float goes down, thereby opening the fuel supply valve and admitting fuel. When the designed level has been recharged, the float closes the fuel supply valve thus stopping additional fuel flow from the supply system. Float chamber is vented either to the atmosphere or to the upstream side of the venture.During suction stroke air is drawn through the venture. As already described, venture is a tube of decreasing cross-section with a minimum area at the throat. Venturi tube is also known as the choke tube and is so shaped that it offers minimum resistance to the air flow. As the air passes through the venture the velocity increase reaching a maximum at the venture throat. Correspondingly, the pressure decreases reaching a

Carburetion and Fuel InjectionS K Mondal’s Chapter 3minimum. From the float chamber, the fuel is fed to a discharge jet, the tip if which is located in the throat of the venture. Because of the differential pressure between the float chamber and the throat of the venture, known as carburetor depression, fuel is discharged into the air stream. The fuel discharge is affected by the size of the discharge jet it is chosen to give the required air-fuel ratio. The pressure at the throat at the fully open throttle condition lies between 4 to 5 cm of Hg, below atmospheric and seldom exceeds 8cm Hg below atmospheric. To avoid overflow of fuel through the jet, the level of the liquid in the float chamber is maintained at a level slightly below the tip of the discharge jet. This is called the tip of the nozzle. The difference in the height between the top of the nozzle and the float chamber level is marked h in Fig. The gasoline engine is quantity, which means that when power output is to be varied at a particular speed, the amount of the charge delivered to the cylinder is varied. This is achieved by means of a throttle valve usually of the butterfly type which is situated after the venture tube. As the throttle is closed less air flows through the venturi tube and less is the quantity of air fuel mixture delivered to the cylinder and hence power output is reduced. As the throttle is opened, more air flow through the chock tube resulting in increased quantity of mixing being delivered to the engine. This increase the engine power output.

A simple carburetor of the type described above suffers from a fundamental drawback in that it provides the requires A/F ratio only at one throttle position. At the other throttle is opened less or more. As the throttle opening is varied, the air flows of fuel through the nozzle. Therefore, the velocity of flow of air and fuel very in a similar manner. At the same time, the density of air decrease at the pressure at the venturi throat decrease with increasing air flow whereas that of the fuel remains unchanged. This result in a simple carburetor producing a progressively rich mixture with increasing throttle opening.

The Simple Carburettor

ESSENTIAL PARTS OF A CARBORATOR (i) Float chamber(ii) Nozzle (iii) Metering orifice (iv) Venturi

Carburetion and Fuel InjectionS K Mondal’s Chapter 3 (v) Throttle valve.

The float and a needle valve system maintains a constant level of gasoline in the float chamber falls below the designed level, the float goes down , thereby opening the fuel supply valve and admitting fuel.

When the designed level has been reached the float closes the fuel supply valve thus stopping additional fuel flow from the supply system. Float chamber is vented either to the atmosphere or to the upstream of the venturi.

During suction stroke air is drawn through the venturi. Venturi tube is also known as the choke tube and is so shaped that it offers minimum resistance to the air flow. As the air passes through the venturi the velocity increases reaching a maximum at the venturi increases reaching a maximum at the venturi throat. Correspondingly , the pressure decreases reaching a minimum. From the float chamber, the fuel is fed to a discharge jet, the tip of which is located in the throat of the venturi. Because of the differential pressure between the float chamber and the throat of the venturi known as carburettor depression, fuel is discharged into the air stream. The fuel discharge is affected by the size of the discharge jet and it is chosen to give the required A/F ratio.

The pressure at the throat at the fully open throlle condition lies between 4 to 5 cm of Hg, below atmospheric and seldom exceeds 8 cm of Hg below atmospheric.

To avoid overflow of fuel through the jet, the level of the liquid in the float chamber is maintain at a level slightly below the tip of the discharge jet.

The gasoline engine is quantity governed, which means that when power output is to be varied at a particular speed, the amount of charge delivered to the cylinder is varied. This is achieved by means of a throttle valve usually of the butterfly type which is situated after the venturi tube. As the throttle is closed less air flows through the venturi tube and less is the flows through the venturi tube and less is the quantity of air-fuel mixture delivered to the cylinder and hence power output is reduced. As the throttle is opened, more air flows through the choke tube resulting in increased quantity of mixture being delivered to the engine. This increases the engine power output.

A simple carburetor of the type shown above suffers from a fundamental drawbacks in that it provides the required A/F ratio only at one throttle position. At the other throttle positions the mixture is either leaner or richer depending on whether the throttle is opened less or more. As the throttle opening is varied, the air flow varies and creates a certain pressure differential between the float chamber and the venturi throat. The same pressure differential regulates the flow of fuel through the nozzle. Therefore, the velocity of flow of air and fuel vary in a similar manner. At the same time, the density of air decreases as the pressure at the venturi throat decreases as the pressure at the venturi throat decreases with increasing air flow whereas that of the fuel remains unchanged. This results in a simple carburetor producing a progressively rich mixture with increasing throttle opening.

Size of the CarburettorThe size of a carburettor is generally given in terms of the diameter of the venture tube in mm and the jet size in hundredths of a millimetre. The calibrated jets have a stamped number which gives the flow in ml/min under a head of 500 mm of pure benzol.

Carburetion and Fuel InjectionS K Mondal’s Chapter 3For a venture of 30 to 35 mm size (having a jet size which is one sixteenth of venture size) the pressure difference (P1-P2) is about 50 mm of Hg. The velocity at throat is about 90 – 100 m/s and the coefficient of discharge for venture Cda is usually 0.85.

The Choke and the ThrottleWhen the vehicle is kept stationary for a long period during cool winter seasons, may be overnight, starting becomes more difficult. As already explained, at low cranking speeds and intake temperature a very rich mixture is required to initiate combustion. Some times air-fuel ratio as rich as 9:1 is required. The main reason is that very large fraction of the fuel may remain as liquid suspended in air even in the cylinder. For initiating combustion, fuel-vapour and air in the form of mixture at a ratio that can sustain combustion is required. It may be noted that at very low temperature vapour fraction of the fuel is also very small and this forms combustible mixture to initiate combustion. Hence, a very rich mixture must be supplied. The most popular method of providing such mixture is by the use of choke valve. This is simple butterfly valve located between the entrances.

Complete CarburettorIn order to satisfy the demands of an engine under all conditions of operation the following additional systems are added to the simple carburetor.(i) Main metering system(ii) Idling system(iii) Power enrichment by economiser system(iv) Acceleration pump system(v) Choke.

(i) Main Metering System- The main metering system of a carburetor is designed to supply a nearly constant basis fuel-air ratio over a wide range of speeds and loads. This mixture correspondence approximately to best economy at full throttle (A/F ratio 15.6 or F/A ratio 0.064). Since a simple or elementary carburetor tends to enrich the mixture at higher speeds automatic compensating device are incorporated in the main metering system to correct this tendency. These devices are:

(a) Use of a compensating jet that allows an increasing flow of air through a fuel passage as the mixture flow increases.

(b) Use of emulsion tube for air bleeding. In this device the emphasis is on air bleeding alone.(c) Use of a tapered metering pin that is arranged to be moved in and out of the main

or auxiliary fuel orifice either manually or by means of some automatic mechanism changing the quantity of fuel drawn into the air charge.

(d) Back-suction control or pressure reduction in the float chamber.(e) Changing the position or jet in the venturi. The suction action is highest at the

venturi throat, therefore by raising the venturi the nozzle relatively moves to points with smaller suction and the flow of fuel is decreased.

(f) Use of an auxiliary air valve or port that automatically admits additional air as mixture flow increases.

(ii) Idling system- It has already been shown that at idling and low load the engine requires a rich mixture. However, the main metering system not only fails to enrich the mixture at low air flows but also supplies no fuel at all at idling. For this reason, a separate idling jet must be added to the basic carburetor. It consists of a small fuel line from the float chamber to a point a little on the engine side of the throttle. This line contains a fixed fuel orifice. When the throttle is practically closed, the full manifold suction operates on the outlet to this jet. In addition, the very high velocity

Carburetion and Fuel InjectionS K Mondal’s Chapter 3past the throttle plate increases the suction locally. Fuel can therefore be lifted by the additional height up to the discharge point, but this occurs only at very low rates of air flow. As he throttle is opened, the main jet gradually takes over while the idle jet becomes ineffective

The desired air fuel ratio foe the idling jet is regulated manually be idle adjust, which is a needle valve controlling the air bleed.

(iii) Power enrichment or economizer system- As the maximum power range of operation (75% to 100% load) is approaches; some device must allow richer mixture to be supplied despite the compensating leanness. Such a device is the meter rod economizer. The name economizer is rather misleading. It stems from the fact that such a device provides a rich uneconomical mixture at high load demand without interfering with economical operation in the normal power range. The meter rod economizer simply provides a large orifice opening to the main jet as the throttle is opened beyond a certain point. The rod may be tapered or stepped. Other examples provide for the opening of auxiliary jets through some linkage to the throttle movement or through a spring action when manifold vacuum is lost as the throttle is opened.

(iv) Acceleration pump system- It has already been shown that when it is desired to accelerate the engine rapidly, a simple carburetor will not provide the required rich mixture. Rapid opening of the throttle will be immediately followed by an increased airflow, but the inertia of the liquid fuel will cause at least a momentarily lean mixture just when richness is desired for power. To overcome this deficiency an acceleration pump is provided. The pump consists of a spring-loaded plunger. A linkage mechanism is provided so that when the throttle is rapidly opened to the plunger moves into the cylinder and forces an additional jet of fuel into the venturi. The plunger is raised again against the spring force when the throttle is partly closed. Arrangement is provided so that when the throttle is opened slowly, the fuel in the pump cylinder is not forced into the venturi but leaks past plunger or some holes into the float chamber.

Instead of the mechanical linkage shown some carburetors have a pump plunger held up by manifold vacuum. Whenever that vacuum is reduced by the rapid opening of the throttle, a spring forces the plunger down in a pumping action identical to that of the pump illustrated.

(v) choke- During cold starting period, at low cranking speed and before the engine has warmed up, a mixture much richer than usual mixtures (almost 5 to 10 times more fuel) must be supplied simply because a large fraction of the fuel will remain liquid even in the cylinder, and only the vapour fraction can provide a combustible mixture with the air. The most common means of obtaining this rich mixture is by the use of a choke, which is a butterfly type of valve placed between the entrance to the carburettor and the venture throat. By partially closing the choke, a large pressure drop can be produced at the venture throat that would normally result from the amount of air flowing through the venture. This strong section at the throat will draw large quantity of fuel from the main nozzle and supply a sufficiently rich mixture so that the ratio of evaporated fuel to air in the cylinder is within combustible limit. Choke valves are sometimes made with a spring loaded by pass so that high pressure drop and excessive choking will not result after the engine had started and has attain a higher speed. Some manufacture make the choke operates automatically by the means of a thermostats such that when the engine is coal the choke is closed by a bimetallic element. After starting and the engine warms of the bimetallic element gradually open the choke to its fully open position.

Carburetion and Fuel InjectionS K Mondal’s Chapter 3Aircraft Carburettor1. The important difference between an aircraft and auto mobile carburettor is that

the former operates at varying altitudes whereas the latter operates mostly at ground level conditions. As the altitude increases the air density reduces and the air/fuel ratio, which is proportional to

reduces, i.e. the fuel mixture becomes richer with altitude. At about 7000 metres the air density is nearly one-half that at ground level and hence the A/F ratio about 0.7 times the value at ground level, i.e. the mixture will be about 40 per cent richer.

It is, therefore, necessary to provide an altitude mixture correction device in aircraft carburettors to progressively reduces the amount of fuel with altitude. The various methods used for this purpose are, (i) back suction control, (ii) air bleeding, and (iii) providing a metering pin. The other special features of aircraft carburettors are:

2. Special float chamber is needed for the level system. The ordinary hydrostatic float method is unsatisfactory for conditions of engine tilt at more than small angles and when the aircraft is doing acrobatics it is necessary to employ a special system of fuel supply to ensure that the float chamber or its equivalent reservoir (and diaphragm) always contains sufficient fuel.

3. Yet another problem in aircraft carburettors is the formation of ice in the choke tube and on the throttle valve due to the low temperature of the air. For eliminating the ice either an automatic de-icing unit is fitted or a supply of hot engine oil is arranged to flow around the carburettor barrel and through the hollow throttle valve.

Petrol InjectionThe fuel-injection system is the most vital component in the working of CI engines. The engine performance viz., power output, economy etc. is greatly dependent on the effectiveness of the fuel-injection system. The injection system has to perform the important duty of initiating and controlling the combustion process.

Basically, the purpose of carburetion and fuel-injection is the same viz., preparation of the combustible charge. But in case of carburetion fuel is atomized by processes relying on the air speed greater than fuel speed at the fuel nozzle, whereas, in fuel-injection the fuel speed at the point of delivery is greater than the air speed to atomize the fuel. in carburettors.

When the fuel is injected into the combustion chamber towards the end of compression stroke, it is atomized into very fine droplets. These droplets vaporize due to heat transfer from the compressed air and form a fuel-air mixture. Due to continued heat transfer from hot air to the fuel, the temperature reaches a value higher than its self-ignition temperature. This causes the fuel to ignite spontaneously initiating the combustion process.

Functional requirement of an injection system: - For a proper running and good performance from the engine, the following requirements must be met by the injection system-

Carburetion and Fuel InjectionS K Mondal’s Chapter 3(i) Accurate metering(ii) Timing the injection(iii) Rate of injection(iv) Proper atomization(v) Proper spray pattern(vi) Uniform distribution of fuel droplets(vii) Equal quantities of metered fuel to all cylinders for multi cylinder engines.(viii) No lag thus eliminating dribbling.

Classification of Injection SystemIn a constant-pressure cycle or diesel engine, only air is compressed in the cylinder and then fuel is injected into the cylinder by means of a fuel-injection system. For producing the required pressure for atomizing the fuel either air or a mechanical means is used. Accordingly the injection systems can be classified as:(i) Air injection systems(ii) Solid injection systems

Air Injection systemIn this system, fuel is forced into the cylinder by means of compressed air. This system is little used nowadays, because it requires a bulky multi-stage air compressor, This cause an increase in engine weight and reduces the brake power output further. One advantage that is claimed for the air injection system is good mixing of fuel with the air with resultant higher mean effective pressure. Another is the ability to utilize fuels of high viscosity. These advantages are off-set by the requirement of a multistage compressor thereby making the air-injection system obsolete.

Advantages1. The main advantage of the air injection system is the good atomization obtained. A

high mep can be attained as rapid combustion results due to good mixing of fuel and air.

2. Heavy and viscous fuels, which are cheaper, can also be injected. 3. The fuel pump is required to develop only a small pressure.

Disadvantages1. It requires a high pressure multi-stage compressor. The large number of parts the

inter-cooler, etc., make the system complicated and expensive.2. A separate mechanical linkage is required to time the operation of the fuel value.

Schematic diagram of an air injection systemSolid Injection System

Carburetion and Fuel InjectionS K Mondal’s Chapter 3In this system the liquid fuel is injected directly into the combustion chamber without the aid of compressed air. Hence, it is also called airless mechanical injection or solid injection system. Solid injection systems can be classified into four types.

(i) Individual pump and nozzle system(ii) Unit injector system(iii) Common rail system(iv) Distributor system

All the above systems comprise mainly of the following components -(i) Fuel tank(ii) Fuel feed pump(iii) Injection pump(iv) Governor(v) Injector(vi) Fuel fitters

Typical Fuel Feed System for a CI Engine

Injection PumpThe main objectives of fuel-injection pump is to deliver accurately metered quantity of fuel under high pressure (in the range from 120 to 200 bar) at the correct instant to the injector fitted on each cylinder. Injection pumps are of two types, viz. (i) Jerk type pumps (ii) Distributor type pumps

Fuel InjectorQuick and complete combustion is ensured by a well designed fuel injector. By atomizing the fuel into very fine droplets, it increases the surface area of the fuel droplets resulting in better mixing and subsequent combustion. Atomization is done by forcing the fuel through a small orifice under high pressure.

The injector assembly consists of(i) A needle valve(ii) A compression spring(iii)A nozzle(iv) An injector bodyTypes & Fuel InjectorsThere are three main types of fuel injectors. These are:1. Blast injectors.2. Mechanically operated injectors.

Carburetion and Fuel InjectionS K Mondal’s Chapter 33. Automatic injectors.

Quantity of Fuel per Cycle, Size of Nozzle OrificThe quantity of the fuel injected per cycle depends upon the amount of air available (displacement volume) and the load of the engine. The fuel is supplied into the combustion chamber through the nozzle holes. The velocity of the fuel for good atomization is of the order of 400m/sec. Roughly, the velocity of the fuel through nozzle orifice can be given by

Where Cf = velocity of fuel,Cd = flow coefficient of orificep = the pressure difference between injection and cylinder

pressureThe volume of the fuel injected per second, Q, is given byQ = Area of all orifices velocity time of one injection No. of injections per second for one orifice

Q = Where d is the diameter of the orifice, n number of holes, duration of injection in crankangle and N number of injections per minute.Usually the rate of fuel injection is expressed is mm3/degrees crank angle/litre cylinder displacement volume to avoid effect of engine size.

Fuel InjectorInjection in SI EngineFuel-injection systems are commonly used in CI engines. Presently gasoline injection system is coming into vogue in SI engines because of the following drawback of the carburetion.(a) Non uniform distribution of mixture in multicylinder engines.

Carburetion and Fuel InjectionS K Mondal’s Chapter 3(b) Loss of volumetric efficiency due to restriction for the mixture flow and the

possibility of back firing.A gasoline injection system eliminates all these drawbacks. The injection of fuel in SI engine can be employing any of the following methods (a) Direct injection of fuel into the cylinder(b) Injection of fuel close to the inlet valve(c) Injection of fuel into the inlet manifold

There are two types of gasoline injection systems, viz.(i) Continuous Injection: Fuel is continuously injected. It is adopted when manifold

injection is contemplated.(ii) Timed Injection: Fuel is injected only during induction stroke over a limited period.

Injection timing is not a critical factor in SI engines.

Major advantages of fuel-injection in an SI engine are:(i) Increased volumetric efficiency(ii) Better thermal efficiency(iii) Lower exhaust emissions(iv) High quality fuel distribution

Advantages and Disadvantages of Petrol Injection(1) The main advantages of petrol injection are:

Increases volumetric efficiency and hence increased power and torque, due to the absence of any restriction such as ventures and other metering elements in the air passage.

(2) Better distribution of mixture to each cylinder and hence lower specific fuel consumption.

(3) Lower mixture temperature in the engine cylinders, despite the increase in power developed and hence possibility of employing higher compression ratios (higher by 1 to 1.5).

(4) Freedom from blowbacks and icing.(5) Better starting and acceleration.(6) Engine fitted with petrol injection system can be used in any till position which

will cause trouble in carburettors.

The disadvantages of fuel injection as against conventional carburetion are as follows:(1) The greatest disadvantage of fuel injection system is against conventional

carburetion is its considerably higher initial cost due to prices and complicated component assemblies. The number of individual parts in petrol injection system may be 3 to 4 times that in a carburetion system. In Junkers engine fuel injection system has 1576 parts compared to Mercedes Carburettor system 433 parts (both for 12 cylinder engine).

(2) Increased service problem. Injection system has many wearing parts such as camshaft, rotor, shuttle, etc.

(3) Weight and bulk of petrol injection system is more than that part of a carburettor.(4) Injection system generates more noise.

Requirements of a Diesel Injection SystemThe above analysis indicates that the following requirements should be fulfilled by the diesel injection equipment:(1) The fuel should be introduced into the combustion chamber within a precisely

defined period of the cycle.

Carburetion and Fuel InjectionS K Mondal’s Chapter 3(2) The amount of the fuel injected per cycle should be metered very accurately. The

clearances between the working parts of a fuel pumps as well as the size of the orifice are vary small. The working clearance is as small as 0.001mm and the nozzle orifice size of even a big engine is as small as 0.625mm in diameter. If it is enlarged by output may result in imbalance, overheating or smoky exhaust.

(3) The rate of injection should be such that it results in the desired heat release pattern.

(4) The quantities of the fuel metered should very to meet changing speed and load requirements.

(5) The injected fuel must be broken into very fine droplets, i.e., good atomisation should be obtained.

(6) The spray-pattern must be such that it results in rapid mixing of fuel and air.(7) The beginning and the end of injection should bee sharp, i.e., there should not be

any dribbling or after-injection.(8) The injection timing, if desired, should change to suit the engine speed and load

requirements.(9) In the case of multicylinder engines, the distribution of the meter fuel among

various cylinders should be uniform.(10) In addition to the above requirements, the weight and the size of the fuel

injection system must be minimum. It should not be costly to manufacture and expensive to, adjust and repair.

To accomplish the objectives of precise metering, distributing, timing and atomising the following functional elements are required in a fuel injection system.(i) Pumping elements to move the fuel from fuel tank to cylinder (plus piping, etc.).(ii) Metering elements to measure and supply the fuel according to the requirement

of speed dand load.Metering controls to adjust the rate of the metering elements for changes in load and speed of the engine.Distributing elements to divide the metered fuel equally among the cylinders.Timing controls to adjust the start and the stop injection.Mixing elements to automize and distribute the fuel within the combustion chamber. There could be various designs of injection systems incorporating the above functional elements as described below.

Spray FormationThe various phases of spray formation as the fuel is injected through the nozzle are shown in fig. At the start of the fuel injection the pressure difference across the orifice is low. Therefore single droplets are formed as in Fig. (a). As the pressure difference increases the following process occur one after the other.A stream of fuel emerges from the nozzle, with further and further increasing the pressure difference, the break-up distance decrease and the cone angle increases until

Carburetion and Fuel InjectionS K Mondal’s Chapter 3the apex of the cone practically coincides with the orifice At the exit of the orifice the fuel jet velocity, Vf, is of the order of 400 m/s. It is given by the following equation

Where Cd = coefficient of discharge for the orificePinj = fuel pressure at the inlet to injector, N/m2

Pcyl = pressure of charge inside the cylinder, N/m2

= fuel density, kg/m3

The spray form circular orifice has a denser and compact core, surrounded by a droplets of various sizes and vaporized liquid. Larger droplets provide a higher penetration into the chamber but smaller droplets are required for quick mixing and evaporation of the fuel. The diameter of most of the droplets in a fuel spray is less than 5 microns. The droplet sizes depends on various factors which are listed below:Mean droplet size decreases with increase in injection pressure.Mean droplet size decreases with increase in air density.Mean droplet size increases with increase in fuel viscosity.Size of droplets increases with increase in the size of the orifice.

Injection TimingConsider a cylinder of a four cylinder engine. The fuel is injected into the inlet manifold of each cylinder at different timings. The timing at which the injection of the fuel takes place inside the inlet manifold is called injection timing.The effect of injection advance to the pressure variation is shown in Fig., for three injection advance timing of 9o, 18o and 27o before TDC. The injected quantity of fuel per cycle is constant. As the pressure and temperature at the beginning of injection are lower for higher ignition advance, the delay period increases with increase in injection advance. The optimum angle of injection advance depends on many factors but generally it is about 20obTDC.


OBJECTIVE QUESTIONS (GATE, IES & IAS)

Previous Years GATE QuestionsComplete carburettorQ1. At the time of starting, idling and low speed operation, the carburretor

supplies a mixture which can be termed as [GATE-2004](a) Lean (b) slightly leaner than stoichiometric(c) stoichiometric (d) rich

Previous Years IES QuestionsA simple or elementary carburettor Q1. The essential function of the carburettor in a spark ignition engine is

to: (a) Meter the fuel into air stream and amount dictated by the load and speed(b) Bring about mixing of air and fuel to get a homogeneous mixture

[IES-1992](c) Vaporise the fuel(d) Distribute fuel uniformly to all cylinders in a multi cylinder engine and also vaporise it.

Q2. Which one of the following curves is a proper representation of pressure differential (y-axis) vs velocity of air (x-axis) at the throat of a carburettor? [IES-1993]

Q3. Assertion (A): A simple or elementary carburettor provides progressively rich mixture with increasing air flow [IES-2004]


Reason (R): The density of the air tends to increase as the rate of air flow increases.


Q4. Assertion (A): A simple carburettor which is set to give a correct mixture at low air speeds will give a progressively rich mixture as the air speed is increased. [IES-1995]Reason (R): As the pressure difference over the jet of a simple carburettor increases the weight of petrol discharge increases at a greater rate than does the air supply.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Complete carburettorQ5. Consider the following statements:

[IES-2005]1. In a carburettor the throttle valve is used to control the fuel supply.2. The fuel level in the float chambers is to be about 4 to 5 mm below

the orifice level of main jet.3. An idle jet provides extra fuel during sudden acceleration.4. A choke valve restricts the air supply to make the gas richer with

fuel.Which of the statements given above are correct?(a) 2 and 4 (b) 1 and 3 (c) 1, 2 and 3 (d) 2, 3 and 4

Q6. Match List I with List II and select the correct answer using the codes given below the lists :

List I List II [IES-1993]

(Elements of a complete carburetor) (Rich-mixture requirement)A. Idling system 1. To compensate for dilution of

chargeB. Economizer 2. For cold startingC. Acceleration pump 3. For meeting maximum power

range of operationD. Choke 4. For meeting rapid opening of

throttleCode:A B C D A B C D(a) 1 2 3 4 (b) 1 3 4 2(c) 2 3 4 1 (d) 4 1 2 3

Q7. Compensating jet in a carburettor supplies almost constant amount of petrol at all speeds because [IES-1996](a) The jet area is automatically varied depending on the suction.(b) The flow from the main jet is diverted to the compensating jet with increase in speed.(c) The diameter of the jet is constant and the discharge coefficient is invariant.(d) The flow is produced due to the static head in the float chamber.


Q8. Assertion (A): One of the important requirements of a carburettor is to supply lean mixture at starting. Reason (R): A rather lean mixture is required at No-load and low-load operation of a SI engine. [IES-1997](a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q9. Consider the following statements for a carburettor: [IES-2004]

1. Acceleration jet is located just behind the throttle value2. Idle jet is located close to the choke3. Main jet alone supplies petrol at normal engine speedsWhich of the statements given above are correct?(a) 1, 2 and 3 (b) 1 and 2 (c) 2 and 3 (d) 1 and 3

Q10. In some carburettor, meter rod and economiser device is used for [IES-1998]

(a) Cold starting (b) idling (c) Power enrichment (d) acceleration

Q11. Oxides of nitrogen in the engine exhaust can be reduced by which of

the following methods?1. Decrease in compression ratio2. Exhaust gas recirculation3. Use of 5% lean mixture4. Use of oxidation catalysts in exhaust manifoldSelect the correct answer using the code given below:

[IES-2008](a) 1 and 2 only(b) 1 and 3 only(c) 1, 2 and 4(d) 2 and 4 only


1. Recycling exhaust gases with intake increases emission of oxides of nitrogen from the engine.

2. When the carburettor throttle is suddenly opened, the fuel air mixture leans out temporarily causing engine stall.

3. The effect of increase in altitude on carburettor is to enrich the entire part-throttle operation.

4. Use of multiple venturi system makes it possible to obtain a high velocity air stream when the fuel is introduced at the main venturi throat.

Which of these statements are correct?(a) 1 and 3 (b) 1 and 2 (c) 2 and 3 (d) 2 and 4


In down draft carburettor, a hot spot is formed at the bottom wall which is common for intake and exhaust manifolds. This helps to1. Improve evaporation of liquid fuel


2. Provide higher thermal efficiency3. Reduce fuel consumption4. Lower the exhaust gas temperatureWhich of these statements are correct?(a) 1, 2 and 4 (b) 1, 2 and 3 (c) 1, 3 and 4(d) 2, 3 and 4

Petrol injectionQ14. Consider the following statements regarding the advantages of fuel

injection over carburetion in S.I. engines:1. Higher power output and increased volumetric efficiency.

[IES-2001]2. Simple and inexpensive injection equipment.3. Longer life of injection equipment.4. Less knocking and reduced tendency for back-fire.Select the correct answer using the codes given below:(a) 1, 2 and 3 (b) 1, 2 and 4 (c) 2 and 3 (d) 1 and

4

Requirements of a diesel injection system Q15. Where does mixing of fuel and air take place in case of diesel engine?

(a) Injection pump (b) Injector [IES-2006]

(c) Engine cylinder (d) Inlet manifold

Q16. Assertion (A): Air injection system finds wide application in modern diesel engines.

Reason (R): Very good atomization of fuels is attained by the air injection system.

(a) Both A and R are individually true and R is the correct explanation of A [IES-1994]

(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true


The injector nozzle of a CI engine is required to inject fuel at a sufficiently high pressure in order to1. be able to inject fuel in a chamber of high pressure at the end of the compression stroke.2. Inject fuel at high velocity to facilitate atomization.3. Ensure that penetration is not high.Of the above statements(a) 1 and 2 (b) 1 and 3 (c) 2 and 3 (d) 1, 2 and 3

Fuel pump Q18. Generally, in Bosch type fuel injection pumps, the quantity of fuel is

increased or decreased with change in load, due to change in [IES-1994](a) Timing of start of fuel injection. (b) Timing of end of fuel injection(c) Injection pressure of fuel (d) velocity of flow of fuel


Previous Years IAS QuestionsA simple or elementary carburettor Q1. Assertion (A): The carburetor in a petrol engine is categorized as a closed loop

control system. [IAS-2001]Reason (R): There is no method by which the air-fuel ratio is measured in the carburetor.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true



Previous Years GATE Answers1. Ans. (d)

Previous Years IES Answers1. Ans. (a)2. Ans. (a) The relationship between [parabolic x2 =4Ay] which is represented by curve (a).3. Ans. (c)4. Ans. (a) Both A and R are true and R provides correct explanation for A5. Ans. (a) 1 is false: In a carburetor the throttle valve is used to control the air-fuel

mixture supply.3 is false: An Idle jet provides extra-fuel during idling and low speed.

6. Ans. (b) The idling system is used to compensate for dilution of charge, economizer is for meeting maximum power range of operation, acceleration pump for meeting rapid opening of throttle valve, and choke is used for cold starting.7. Ans. (b)8. Ans. (d) At starting rich mixture is required9. Ans. (d)10. Ans. (c) An economizer is a valve which remains closed at normal speed and gets opened to supply enriched mixture at full throttle operation.11. Ans. (c) Decrease in compression ratio will decrease the temperature therefore decrease NOx emission.12. Ans. (c)13. Ans. (a)14. Ans. (d)15. Ans. (c)16. Ans. (a) Both A and R are correct and R is correct explanation of A.17. Ans. (a)18. Ans. (b) The quantity of fuel is varied by rotating helix on pump plunger, i.e. timing the end of fuel injection.

Previous Years IAS Answers1. Ans. (c) There are several methods by which the air-fuel ratio is measured in the carburettor.


Conventional Questions with AnswersQ1. Derive an expression for air/fuel ratio of a carburettor by

(i) Neglecting compressibility of air(ii) Taking compressibility effects into account. [IES-2009, 8 + 7 = 15-Marks]

Ans. (i) Neglecting compressibility effect, we have

(ii) Let carburetor with the tip of the fuel nozzle meters above the fuel level in the float chamber, then applying energy equation

Where q = heat w = workh = enthalpyc = velocity

Assuming, adiabatic flow, we get Q = 0, w = 0 and c1 = 0

Hence Assuming gas behaves ideally so we have

As the flow process from inlet to the venturing throat, we can assume isentropic process, so we have

Substituting in equation (i), we get

21 2 2

a a

1 22

a

a 2 2 a

2 a 1 2

n 1 2a 2ff f 1 2 f

a a2

ff f

P P C2P PC 2

m A CA 2 P P

air / fuel ratioP Pm A

m A P P gzm AIf z 0, m A

2 22 1 2 1

1q - w = h h c c2

2 1 2c 2 h h

p

2 p 1 2

h c T

c 2c T T ... (i)

12 2

1 11

21 2 1

1

T PT P

PT T T 1 P

12

2 p 11

Pc 2c T 1 P


Now, mass flow of air,

Where are cross – sectional areas at the air inlet and outlet. To calculate mass flow rate of air, we have

In order to calculate air fuel ratio, fuel flow rate is to be calculated. As the fuel is in incompressible, applying Bernoulli’s equation.

Where = density of fuel= fuel velocity at the nozzle exit

Z = height of the nozzle exit above the level of fuel in the float bowl

Mass flow rate of fuel,

Where = area of the cross–section of the nozzle = density of the fuel.

A/F ratio

a 1 1 1 2 2 2m p A c p A c

1 2A , A

1 2

1 2

P Pp p

1/2

2 11

Pp pP

1/2

a1

Pm P

1

12

p 11

Pp 2c T 1 P

1 1a

1

A pmR T

12/2 2

p1 1

P P2c .....(ii)P P

21 2 fff

p p c gz2

f

fc

1 2f

f

p pc 2 gz

ff ff

ff 1 2 f

m A cA 2 p p gz ...... (iii)

fAf

2 1

1 f 1 2 f

A p ........ (iv)2T p p gz

12/

2 21 1

p pwhere p p


So equation (iv) is the required equation.

Q2. Drive an expression for the diameter of the injector orifice to spray

fuel /cycle/cylinder in terms of injection pressure ,

combustion chamber pressure , density of fuel

and period of injection seconds.Calculate the diameter of the injectgor orifice of a six-cylinder, 4-stroke CI engine using the following data:Brake power = 250 kW; Engine speed = 1500 r.p.m; BSFC = 0.3 kg/kW Cylinder pressure = 35 bar; Injection pressure = 200 bar; Specific gravity of fuel = 0.88; Coefficient of discharge of the fuel orifice = 0.92; Duration of injection = 36ºC of crank angle. [IES-2007, 15-Marks]

Ans. Given: For CI Engine,Quantity of fuel sprayed

Injection pressure Combustion chamber’s pressure

Density of fuel, t = Period of injection, second.Now velocity of fuel through injector orifice

where h = differential pressure head between injection and cylinder pressure

Now volume of fuel injected/cylinder/sec = Area of orifice × fuel jet velocity × time of one injection × number of injection/sec for one orifice

For one cycle,

P

1.43 1.712 2

1 1

But C 1.005 J / kg.R 287 J /kg K and = 1.4

p p= p p

3Q cm 2kN /injp m

21 kN /cyp m

f3kg /cm

32 cm /cycle/cylinder

injp 2kN /mcylp 2kN /mf 3kg /cm

f dv C 2gh

3inj cyl

6f

p p 10h

g 10

inj cyl3

f

p pm

10 g

inj cyl2 p pf d 3

fv C m /s

10

2 if

N60d v4 360 N 60

360

6 3Q 10 m /cycle/cylinder

2 if

N60d v4 N 60


i.e.

i.e.

Now, given for 4 stroke CI engine, number of cylinders = 6B.P. = 250 kW.Engine speed = N = 1500 r.p.mBSFC = 0.3 kg/kW

Specific gravity of fuel = 0.88

= coefficient of discharge for fuel orifice = 0.92Duration of injection = = 36º of crankNow, time deviation (period) of injection

Velocity of injection

Fuel consumed/hour = BSFC × power output= 0.3 × 250 = 75 kgFuel consumption/cylinder

Fuel consumption/cycle

Where for 4 stroke engine = 750 Fuel consumption/cycle

.Volume of fuel injected/cycle

.Now, injection orifice area,

6 2fQ 10 d v t4

inj cyl

62

2 p pd 3

f

4Q 10d

t C10

11/2 43

2fd inj cyl

104Qd 10t c 2 p p

14

inj cyl

132 f

d 2 p p

104Q cmtc

5 2cylp 35 bar 35 10 N /m

5 2cylp 200 bar 200 10 N /cm

31 880 kg /m

dC

236 0.4 10 sec1500360 60

injv

inj cyl2 p pd

fC

52 200 35 100.92 880

178.16 m /sec

75 12.5 kg /hr6

Fuel consumption /minuten

Nn 2

312.5/60 0.277 10 kg750

30.277 10880

0.3148 cc /cycle 6 30.3148 10 m /cycle


Now, area of orifice Diameter of injector orifice,

m

fVolume off uel injected /cycleA Injection velocity Injection time

6

f 30.3148 10A

178.16 4 10

6 20.4417 10 m

2d4

6 20.4417 10 m

64 0.4417 10d

30.75 10 0.75 mm

Student’s Notes

4. FuelsTheory at a Glance (For IES, GATE & PSUs)

FuelsInternal combustion engines can be operated on different types of fuels such as liquid, gaseous and even solid fuels. Depending upon the type of fuel to be used the engines has to be designed accordingly.

Solid FuelThe solid fuels find little practical application at present at because of the problems in handling the fuel as well as in disposing off, the solid rsidue or ash after combustion. However in the initial stages of the engine development, solid fuels such as finely powdered coal was attempted. Compared to gaseous and liquid fuels, solid fuels are quite difficult to handle and storage and feeding are quite cumbersome. Because of the complications in the design of the fuel feed systems these fuels have become unsuitable in solid form. Attempts are being made to generate gaseous of liquid fuels from charcoal for use in IC engines.

Liquid FuelsIn most of the modern internal combustion engines, liquid fuels which are the derivatives of liquid petroleum are being used. The three principal commercial types of liquid fuels are benzyl, alcohol and petroleum products, However, petroleum products from the main fuel for internal combustion engines as on today.

Chemical Structure of PetroleumPetroleum as obtained from the oil wells, is predominantly a mixture of many hydrocarbons with differing molecular structure. It also contains small amounts of sulphur, oxygen, nitrogen and impurities such as water and sand. The molecular and hydrogen atoms may be linked in different ways in a hydrocarbon molecule and this linking influences the chemical and physical properties of different hydrocarbon groups. Most petroleum fuels tend to exhibit the characteristics of that type of hydrocarbon which forms a major constituent of the fuel.The carbon and hydrogen combine in different proportions and molecular structures to form a variety of hydrocarbons. The carbon to hydrogen ratio which is one of the important parameters and their nature of bonding determine the energy characteristics of the hydrocarbon fuels. Depending upon the number of carbon and hydrogen atoms the petroleum products are classified into different groups.The differences in physical and chemical properties between the different types of hydrocarbon depend on their chemical composition and affect mainly the combustion processes and hence, the proportion of fuel and air required in the engine. The basic families of hydrocarbons, their general formulae and their molecular arrangement are shown in Table 6.1

Family of hydrocarbons

General formula

Molecular structure

Saturated/ Unsaturated

Stability

ParaffinOlefinNaphtheneAromatic

CnH2n+2CnH2nCnH2nCnH2n-6

ChainChainRingRing

SaturatedUnsaturatedSaturated HighlyUnsaturated

StableUnstableStableMost unstable

FuelsS K Mondal’s Chapter 4Paraffin SeriesThe normal paraffin hydrocarbons are straight and chain molecular structure. They are represented by a general chemical formula, CnH2n+2. The molecular structures of the first few members of the paraffin family of hydrocarbons are shown below.

In these hydrocarbons the valency of all the carbon atoms is fully utilized by single bonds with hydrogen atoms. Therefore, the paraffin hydrocarbons are saturated compounds and are characteristically vary stable.

A variation of the paraffin family consist of an open chain structure with an attached branch and is usually termed a branched paraffin. The hydrocarbons which have the same chemical formulate but different structural formulae are known as isomers.Isobutane shown above has the same general chemical formula and molecular weight as butane but a different molecular structure and physical characteristics. It is called an isomer of butane and is known as isobutane. Isoparaffins are also stable compounds.

Olefin SeriesOlefins are also straight chain compounds similar to paraffins but are unsaturated because they contain one or more double bonds between carbon atoms. Their chemical formula is CnH2n. Mono-olefins have one double bond whereas diolefin have two in their structure.

Olefins are not as stable as the single bond paraffins because of the presence of the double bonds in their structure. Consequenlty, these are readily oxidized in storage to form gummy deposits. Hence, olefin content in certain petroleum products is kept low by specification.

Naphthene SeriesThe nephthenes have the same chemical formula as the olefin series of hydrocarbons but have a ring structure and therefore, often they are called as cyclo-paraffins. They

FuelsS K Mondal’s Chapter 4are structured and tend to be stable. The naphthenes are saturated compounds whereas olefins are unsaturated. Cyclopentane is one of the compounds in the naphthene series (CnH2n).

Aromatic SeriesAromatic compounds are ring structured having a benzene molecule as their central structure and have a general chemical formula CnH2n-6. Though the presence of double bonds indicates that they are unsaturated, a peculiar nature of these double bonds causes them to be more stable then the other unsaturated compounds. Various aromatic compounds are formed by replacing one or more of the hydrogen atoms of the benzene molecules with an organic radical such as paraffins, nephthenes and olefins. By adding a methyl group (CH3), benzene is converted to toluene (C6H5CH3), the base for the preparation of Trinitrotiluene (TNT) which is a highly explosive compound.

The above families of hydrocarbons exhibit some general characteristics due to their molecular structure which are summarized below(i) Normal paraffins exhibit the poorest antiknock quality when used in an SI

engine. But the antiknock quality improves with the increasing number of carbon atoms and the compactness of the molecular structure. The aromatics offer the best resistance to knocking in SI Engine.

(ii) For CI engines, the order is reversed i.e., the normal paraffins are the best fuel and aromatics are the best desirable.

(iii) As the number of atoms in the molecular structure increase, the boiling temperature increases. Thus fuels with fewer atoms in the molecule tend to be more volatile.

(iv) The heating value generally increases as the proportion of hydrogen atoms to carbon atoms in the molecule increases due to the higher heating value and the aromatics the least.

FuelsS K Mondal’s Chapter 4In an internal combustion engine the heat energy is released by burning fuel in the engine cylinder. The chemical reactions which permit the release of heat energy are quite fast but the time taken in preparing a proper mixture of fuel and air preparing a proper mixture of fuel and air depends mainly upon the nature of the fuel and the method of introducing it into the combustion chamber. Thus certain requirements are imposed on the fuel used for internal combustion engines. In other words, the fuel used in the IC engines are designed to satisfy the performance requirements of the engine system in which they are used. Thus, the fuel must have certain physical, chemical and combustion properties, such as - (i) High energy density(ii) Good thermal stability(iii) Good combustion qualities(iv) Low deposit forming tendencies(v) Compatibility with the engine hardware.(vi) Good fire safety(vii) Low toxicity(viii) Low pollution(ix) Easy transferability and on board vehicle always.

Important Qualities of Engine FuelsFuels used in IC engines should possess certain basic qualities which are important for the smooth running of the engines. In this section, the important qualities of fuels for both SI and CI engines are reviewed.

(i) Volatility Volatility is one of the main characteristic properties of gasoline which determines its suitability for use in an S.I engine .Since gasoline is a mixture of different hydrocarbons, volatility depends on the fractional composition of the fuel volatility is the distillation of the fuel in a special device of its own vapour. The fraction that boils off at a definite temperature is measured. The characteristic points are the temperature at which 10 , 40,50 and 90% of the volume evaporates as well as the temperature at which boiling of the fuel terminates.

The following fig shows the fractional distillation curve of gasoline far both winter and summer grade gasoline.

Typical Distillation Curves of Gasoline

FuelsS K Mondal’s Chapter 4Volume evaporated (%) :- The method for meaning volatility has been standardized by the Amercian Society for Testing materials (ASTM) and the graphical representation of the result of the tests is generally referred to as ASTM distillation curve. The more important aspects of volatility related to engine fuels are discussed in detail in conjunction with the distillation curve.

(a) Starting and Warm up - A certain part of the gasoline should vapourize at the room temp for easy starting of the engine. Hence, the portion of the distrillation curve between about 0 to 10% boiled off having relatively low boiling temperatures. As the engine warms up, the temp. will gradually increase to the operating temp. Low distillation temps are desirable throughout the range of the distillation curve far best warm-up.

(b) Operating range performance - In order to obtain good vaporization of the gasoline, low distillation temps are preferable in the engine operating range. Better vapourization tends to produce both more uniform distribution of fuel to the cylinder as well as better acceleration characteristics by reducing the quantity liquid droplets in the intake mainfold.

(c) Crankcase dilution - Liquid fuel in the cylinder causes loss of lubricating oil (washing away oil from cylinder walls) which deteriorates the quality of lubrication and tends to cause damage to the engine through increased friction. The liquid gasoline may also dilute the lubricating oil and weaken the oil film between rubbing surfaces. To prevent these possibilities, the upper portion of the distillation temps. to measure that all gasoline in the cylinder is vapourised by the time of combustion.

(d) Vapour lock characteristics - High rate of vapourisation of gasoline can upset the carburetter metering or even stop the fuel flow to the engine by setting up a vapour lock in the fuel passages. This characteristic, demands the pressure of relatively high boiling temp. Hydrocarbons throughout the distribution range. Since this requirement is not consistent with the other requirement is not consistent with the other requirement desired in a, b, & c a compromise must be made for the desired distribution temps.

(ii) Antiknock QualityAbnormal burning or detonation in an S.I engine combustion chamber causes a very high rate of energy release, excessive temp and pressure inside the cylinder and adversely affects its thermal efficiency. Therefore, the characteristics of the fuel used should be such that it resists the tendency to produce detornation and this property is called is antiknock properly.

The antiknock property of a fuel depends on the self-ignition characteristics of the mixture and varies largely with chemical composition and molecular structure of the fuel. In general, the best S.I engine fuel will be that having the highest antiknock property, since this permits the use of higher compression rates and thus the engine thermal efficiency and the power output can be greatly increases.

(iii) Gum deposits Reactive hydrocarbons and the impurities in the fuel have a tendency to oxidise upon storage and form liquid and solid gummy substance. The gasoline containing hydrocations of the paraffin, naphthalene and aromatic families forms little gum while cracked gasoline containing unsaturated hydrocarbons is the worst offender. A gasoline with high gum content will cause operating difficulties such as sticking values and piston rings carbon deposits in the engine, gum deposits in

FuelsS K Mondal’s Chapter 4the manifold, dagging of carburettor jets and enlarging of the valve stems, cylinders and pistons. The amount of gum increases with increased concentrations of oxygen, with rise in temp, with exposure to sunlight and also on contact with metals. Gasoline specifications therefore limit both the gum content and its tendency to form gum on storage.

(iv) Sulphur Content Hydrocarbon fuels may contain free sulphur, hydrogen sulphide and other sulphur compounds which are objectionable for serval reasons. The sulphur is a corrosive element of the fuel that can corrode fuel lines, carburettors and injection pumps and it will write with oxygen to form sulphur diozide that, in the presence of water at low temps, may form sulphurous acid, Since sulphur has a low ignition temperature the presence of sulphur can reduce the self-ignition temp, then promoting knock in the S.I engine.

Important qualities of C.I engine fuels Diesel fuels are petroleum fractions that lie between kerosene and the lubricating oils. Diesel fuel properties are influenced by the crude fuel source and the method of refining. Some of the important qualities of diesel fuel are -

(i) Knock characteristics - Knock in the C.I engine occurs because of an ignition lag in the combustion of the fuel between the time of injection and the time of actual burning. As the ignition lag increases. The amount of fuel accumulated in the combustion chamber increases and when combustion actually takes place, abnormal amount of energy is suddenly released causing an excessive rate of pressure rise which results in an audible knock. Hence, a good CI engine fuel should have a short ignition lag and will ignite more rapidly.

Furthermore, ignition lag affects the starting, warm up; and leads to the production of exhaust smoke in CI engines.

(ii) Volatility - The fuel should be sufficiently volatile in the operating range of temp. to produce good mixing and combustion. The following fig shows a representative curve of a typical diesel fuel.

Typical Distillation Curve for Diesel

(iii) Starting characteristic - The fuel should help in starting the engine easily. This requirement demands high enough volatility to form a combustible mixture reading and a high cetane rating in order that the self-ignition temp. is low.

FuelsS K Mondal’s Chapter 4(iv) Smoking and odour - The fuel should not promote either smoke or odour in the engine exhaust. Generally, good volatility is the first prerequisite to ensure good mixing and therefore complete combustion.(V) Viscosity - C.I engine fuels should be able to flow through the fuel system and the strainers under the lowest operating temps. to which the engine is subjected to.

(vi) Corrosion and wear - The fuel should not cause corrosion and wear of the engine components before or after combustion. These requirements are directly related to the presence of sulphur, ash and residue in the fuel. (vii) Handling case - The fuel should be a liquid that will readily flow under will conditions that are encountered in actual use. This requirements is measured by the power point and the viscosity of the fuel. The fuel should also have a high flash point and a high fire point.

Note:-(a) Pour Point - It is the temp. below which the entire mass of fuel, solid, and liquid together freeze and thus cause flow of fuel impossible.

(b) Flash Point - It is the temp. at which a flammable liquid will produce, with a standardized apparatus and procedure a mixture of its vapour and air which will ignite to give a visible flash by contact with an open flame.

(c) Fire Point - It is the temp. at which the flash will sustain itself as a steady flame for at least fire seconds.

The flash and fire point indicate the temp. Below which oil can be handled without danger of fire. Fire point is usually 110C higher than flash point.

Knock rating of SI engine fuels

Rating of FuelsNormally fuels are rated for their antiknock qualities. The rating of fuels is done by defining two parameters called Octane number and Cetane number for gasoline and diesel oil respectively.

Rating of S.I engine fuels - Resistance to knocking is an extremely important characteristic of fuel for S.I engines. These fuels differ widely in their ability to resist knock depending on their chemical composition. A satisfactory quality of the various fuels has been established. In addition to the chemical characteristics of hydrocarbons such as F/A ratio, ignition timing, dilution, engine speed, shape of the combustion ratio etc. affect the tendency to knock in the engine cylinder. Therefore, in order to determine the knock resistance characteristic of the fuel, the engine and its operating variables must be fixed at standard values.

According to a standard practice, the antiknock value of an SI engine fuel is determined by comparing. Its antiknock property with a mixture of two reference fuels, iso-octane chemically being a very good antiknock fuel, is arbitrarily assigned a rating of 100 octane number. Normal heptane (C7H16) on the other hand has very poor antiknock quantities and is given a rating of 0 octane number.

FuelsS K Mondal’s Chapter 4The octane number of a fuel is defined as the percentage by volume of iso-octane in a mixture of iso-octane and normal heptane, which exactly matches the knocking intensity of the fuel in a standard engine under a set of standard operating conditions.The addition of certain compounds (e.g tetraethyl lead) to iso-octane produces fuels of greater antiknock quality (above 100 ON) The antiknock effectiveness of tetraethyl load, for the same quantity of lead added, decreases as the total content of lead in the fuel increases.

Furthermore, each octane number at the higher range of the octane scale will produce greater antiknock effect compared to the same unit at the lower and of the scale. For instance, octane number increase from 92 to 93 produce greater from 32 to 33 octane number. Because of this non-linear variation a new scale was derived which expresses the approximate relative engine performance and the units of this scale are known as the performance number PN. octane number above 100 can be computed by-

Where A is TEL in ml/gal of fuel, or from the performance number, PN,

Octane Value of the Fuel: A higher self-ignition temperature of the fuel and a low preflame reactivity would reduce the tendency of knocking. In general, paraffin series of hydrocarbon have the maximum and aromatic series the minimum tendency to knock. The napthene series comes in between the two. Usually, compounds with more compact molecular structure are less prone to knock. In aliphatic hydrocarbons, unsaturated compounds show lesser knocking tendency than saturated hydrocarbons, the exception being ethylene, acetylene and propylene.

The advantages of high-octane fuel are summarised below:(1) The engine can be operated at high compression ratio and, therefore, with high

efficiency without detonation.(2) The engine can be supercharged to the high output without detonation.

Diesel FuelsDiesel fuels are petroleum fractions that lie between kerosene and the lubricating oils. Diesel fuel properties are influenced by the crude source and the method of refining. The properties important for a good diesel fuel can be discussed under three main headings:1. Satisfactory handling and storage.2. Smooth and efficient burning.3. Continued cleanliness during uses.

Diesel IndexAn alternative method of expressing the quality of diesel oils is ‘Diesel index’ which is defined as

The main desirable characteristics of diesel fuels, somewhat in order of importance are:1. Cleanliness – Carbon residue, contamination, sulphur, etc.2. Ignition quality – Cetane number of diesel index, API gravity.

FuelsS K Mondal’s Chapter 43. Fluidity – Viscocity, pour point, etc.4. Volatility – Flash point, carbon residue.

Non-Petroleum FuelsNon-petroleum liquid fuels find use when petroleum fuels are scarce or costly. They are also used for blending purposes for improving the quality of petroleum fuels.1. Benzol.2. Alcohol. Methanol (CH3OH) and ethanol (C2H5OH) are also used as fuels in SI

Engines.3. Gaseous Fuels.4. Ammonia.

Alternative Fuels for C.I. Engines

Alcohols as Diesel FuelsWhile S.I. engines can use alcohol fuel with minimal modifications to their fuel delivering systems the diesel engine engine has not been a good candidate for alcohols. Basically alcohols are unsuitable as diesel fuels for the following reasons:The cetane number of alcohol fuels is very low (of the order of zero to eight), which prevents their ignitions by compression. Alcohol fuels have low lubricating qualities causing trouble in injection pumps and nozzles.There are material problems caused by the harsh reaction of methanol towards various plastics and metals.

AdditivesSome compounds called additives or dopes are used to improve combustion performance of fuels.

Cetane NumberThe cetane rating of a diesel fuel is a measure of its ability to autoignite quickly when it is injected into the compressed and heated air in the engine. Though ignition delay is affected by several engine design parameters such as compression ratio, injection rate, injection time, inlet air, temperature etc., it is also dependent on hydrocarbon composition of the fuel and to some extend on its volatility characteristic. The cetane number is a numerical measure of the influence the diesel fuel has in determining the ignition delay. Higher the cetane rating of the fuel lesser is the propensity for diesel knock.The cetane number of a fuel is the percentage by volume of cetane in a mixture of cetane and - methylnaphthalene (C10H7CH3) that relationship between octane number and cetane number is

Normal paraffins (CnH2n+2) which are straight chain compounds have highest cetane number and lowest specific gravity.

Fuel for Gas Turbine and Jet Engines In gas turbine fuel system continuous type of burner is used which can burn satisfactorily a wide range of fuels including gaseous fuels and liquid fuels including gasoline, diesel fuel and kerosene.

FuelsS K Mondal’s Chapter 4Since burning is continuous, properties involving ignition delay (Octane and cetane numbers) are not important. Hence gas turbine fuels can be made cheaper.

The major requirement of fuels for stationary gas turbine is that the fuel and the products of combustion should not cause corrosion and erosion of nozzles and turbine blades and should not deposit in the combustion chamber and the fuel system.

Aircraft gas turbine have special problems of cold starting, flame blow down at high attitude, evaporation loss at low pressures and in supersonic aircrafts boiling due to high stagnation temp.

The advantages of kerosene - type over gasoline are as follows:-(i) It reduces vapour lock(ii) It is better lubricant for the fuel pumps because of its high viscosity.(iii) It has about 10% higher calorific value on volume basis.(iv) It has lesser fire hazard when refilling or when accidents occur.

Disadvantages(i) It has about 3% lower colorific value on mass basis.(ii) It has been starting and relighting characteristics.(iii) Due to higher viscosity it carries more dist into the fuel pump.(iv) It requires more time for refilling.

The choice of fuel far gas turbine depends upon the application. Industrial and marine gas turbines use very cheap fuels, such as residual oil or surplus gas as they have no compete with diesel engines. Residual oils create the problem of exhaust smoke and ash. For civil aircraft the main requirements are safety, reliability low cost and case of handling. For military aircrafts fuel cost is of secondary importance compared with availability, supply logistics and the need for trouble-free operation over a wide rang of conditions.

FuelsS K Mondal’s Chapter 4

OBJECTIVE QUESTIONS (GATE, IES & IAS)

Previous Years GATE QuestionsDiesel fuelsQ1. Alcohols are unsuitable as diesel engine fuels because

[GATE-1992](a) The cetane number of alcohol fuels is very low which prevents their ignition by compression(b) The cetane number of alcohol fuels is very high which prevents their ignition by compression(c) The cetane number of alcohol fuels is very low which prevents their ignition by compression(d) None of the above

Cetane NumberQ2. List I List II [GATE-

1996](A) Cetane number 1. Ideal gas(B) Approach and range 2. Van der Waals gas

(C) 3. S.l. engine(D) dh = cp dT, even when pressure varies 4. C.l. engine p

5. Cooling towers6. Heat exchangers

Code:A B C D A B C D

(a) 4 5 2 1 (b) 3 4 2 1(c) 2 1 3 4 (d) 5 4 3 2

Previous Years IES QuestionsQ1. What is the flash point of a liquid fuel?

[IES-2006](a) The temperature at which the fuel ignites spontaneously with a bang(b) The temperature at which the fuel emits vapours at a rate which produces an inflammable mixture with air(c) The temperature at which the fuel ignites with a clearly(d) The temperature at which the fuel ignites without a spark

Fuels for spark-ignition enginesQ2. In a petrol engine car, which one of the following performance

characteristics is affected by the front-end volatility of the gasoline used? [IES-2000](a) Hot starting and vapour lock(b) Engine warm-up and spark plug fouling


(c) Spark plug fouling and hot starting(d) Vapour lock, engine warm-up and spark plug fouling

Q3.

Which one of the following is represented by the molecular structure of the paraffin family of hydrocarbon fuel given above, for an IC engine?(a) Ethane (b) Propane(c) Butane (d) Hexane [IES

2007]

Q4. Which one of the following fuels can be obtained by fermentation of vegetable matter? (a) Benzene (b) Diesel (c) Gasoline (d) Alcohol [IES-

2005]

Knock rating of SI engine fuelsQ5. In a SI Engine, which one of the following is the correct order of the

fuels with increasing detonation tendency? [IES-1997](a) Paraffins, Olefins, Naphthenes, Aromatics(b) Aromatics, Naphthenes, Paraffins, Olefins(c) Naphthenes, Olefins, Aromatics, Paraffins(d) Aromatics, Naphthenes, Olefins, Paraffins

Octane number requirement (ONR)Q6. Consider the following statements regarding knock rating of SI engine

fuels: 1. Iso-octane is assigned a rating of zero octane number.

[IES-2002]2. Normal heptane is assigned a rating of hundred octane number3. Iso-octane is assigned a rating of hundred octane number4. Normal heptane is assigned a rating of zero octane numberWhich of the above statements are correct?(a) 1 and 2 (b) 2 and 3 (c) 3 and 4 (d) 4 and 1

Q7. Assertion (A): Octane number is used for rating of fuels in spark ignition engine.

Reason (R): Octane number of a fuel is defined as percentage by volume, of iso-octane in a mixture of iso-octane and ex-methylnaphthalene.


Q8. Reference fuels for knock rating of SI engine fuels would include [IES-1994]


(a) iso- octane and alpha-methyl naphthalene(b) Normal octane and aniline.(c) iso-octane and n-hexane (d) n-heptane and iso - octane.

Q9. Assertion (A): Iso-octane has been chosen as the reference for S.I. engine fuels and has been assigned a value of octane number 100.

[IES-1993]Reason (R): Among the fuels, iso-octane ensures the highest compression ratio at which an S.I. engine can be operated without knocking.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Cetane NumberQ10. The two reference fuels used for cetane rating are

[IES-1995](a) cetane and iso-octane (b) cetane and tetraethyl lead(c) cetane and n-heptane (d) cetane and -methyl naphthalene.

Q11. The Cetane number of automotive diesel fuel used in India is in which one of the following ranges? [IES 2007](a) 30 – 40 (b) 41 – 50 (c) 51 – 60 (d) 61 – 70

Q12. Consider the following statements [IES-2001]

1. Motor gasoline is a mixture of various hydrocarbons with a major proportion being aromatic hydrocarbons.2. Compressed natural gas is mainly composed of methane.3. Producer gas has a predominant component of hydrogen with lesser proportion of carbon monoxide.4. Cetane number of fuel used in diesel engines in India is in the range of 80 to 90.Which of these statements are correct?(a) 1 and 2 (b) 1 and 3 (c) 2, 3 and 4 (d) 1, 2, 3 and

4

Q13. Consider the following statements regarding n-Cetane: [IES-1996]

1. It is a standard fuel used for knock rating of diesel engines.2. Its chemical name is n-hexadecane 3. It is a saturated hydrocarbon of paraffin series.4. It has long carbon chain structure.Of the above correct statements are (a) 1, 3 and 4 (b) 1, 2 and 3 (c) 1, 2 and 4 (d) 2, 3 and 4



Previous Years GATE Answers1. Ans. (a)2. Ans. (a)

Previous Years IES Answers1. Ans. (b)2. Ans. (a) 3. Ans. (c)4. Ans. (d)5. Ans. (d)6. Ans. (c)7. Ans. (c) Octane number of a fuel is defined as percentage by volume, of iso-octane

in a mixture of iso-octane and n-heptane.8. Ans. (d) Reference fuel for knock rating of SI engine fuels would include n-heptane

and iso-octane.9. Ans. (a) Both assertion and reason given are true. Also the reason R is the correct

explanation of the assertion.10. Ans. (d) Two reference fuels for cetane rating are cetane and -methyl

naphthalene.11. Ans. (b) Indian Diesel: Year 2000: CN48, Sulfur 0.25%

Year 2010: CN48, sulfur 350PPM will be Euro 3 Note: sulfur 50PPM will be Euro412. Ans. (a) Cetane number of fuel used in diesel engines in India is in the range of

55 to 6513. Ans. (a)


Conventional Questions with AnswersQ1. What are the advantages and disadvantages of using hydrogen as an

I.C. Engine fuel?Explain one method by which hydrogen can be used in C.I. Engine.

[IES-2006, 10-Marks]

Ans. Advantages of using hydrogen as I.C. engine fuel(i) Low emission(ii) Fuel availability; There are a number of different ways of making

hydrogen, including electrolysis of water.(iii) Fuel leakage to environment is not a pollutant.(iv) High energy content per volume when stored as a liquid. This would give a

large vehicle range for a given fuel tank capacity.

Disadvantages(i) Storage problem; Hydrogen can be stored as a cryogenic liquid or as a

compressed gas. This will required a thermally super insulated fuel tank or high pressure vessel.(ii) Difficult to refuel & the possibility of detonation.(iii) Poor engine volumetric efficiency.(iv) Fuel cost would be high at present day technology and availability.(v) High emission because of high flame temperature.(vi) Can detonate.

Use of hydrogen in C.I. engineHydrogen is introduced directly into the cylinder at the end of compression. Since the self ignition temperature of hydrogen is very high, the gas spray is made to impinge on a hot glow plug in the combustion chamber, that is by surface ignition. It is also possible to feed a very lean hydrogen air mixture during the entrance into an engine and then inject the bulk of the hydrogen towards the end of hydrogen stroke.

XNo

Student’s Notes

5. SuperchargingTheory at a Glance (For IES, GATE & PSUs)

The method of increasing the inlet air density, called supercharging is usually employed to increase the power output of the engine. This is done by supplying air at a pressure higher than the pressure at which the engine naturally aspirates air from the atmosphere by using a pressure boosting devices called a supercharger.

The power output can also be increased by increasing the thermal efficiency of the engine, say by increasing the compression ratio. However, this increases the maximum cylinder pressure. The rate of increase of maximum cylinder pressure is less than the rate of increase of break mean effective pressure in case of a supercharged engine. This means that for a given maximum cylinder pressure more power can be obtained by supercharging as compared to that obtained by increase in compression ratio. The rate of increase of maximum temp is also low in case of supercharging. This results in lower thermal loads.

Objective of supercharging The increase in the amount of air inducted per unit time by supercharging is obtained mainly to burn a greater amount of fuel in a given engine and thus increase its power output. The objects of supercharging include one or more of the following:(1) To increase the power output for a given weight and bulk of the engine. This is

important for aircraft, marine and automotive engines where weight and space are important.

(ii) To compensate for the loss of power due to altitude. This mainly relates to aircraft engines which lose power at an approximate rate of one percent per 100 metres altitude. This is also relevant for other engines which are used at high attitudes.

(iii) To obtain more power from an existing engine.

Thermodynamic cycle with supercharging The following fig shows the difference between the p - v diagrams of an unsupercharged and supercharged engines.

SuperchargingS K Mondal’s Chapter 5

Two important differences are -(i) Increase in pressure over the unsupercharged cycle.(ii) The pumping loop of a supercharged engine is positive instead of negative. Hence to get the net IP the power represented by pumping loop is to be added instead of being substracted.

The gain in the output of a supercharged engine is mainly due to increase in the amount of air inducted for the same swept volume. An additional amount of air is also inducted due to compression of residual volume to a higher pressure.

Supercharging also results in an increase in mechanical efficiency, and in better gas-exchanger process. An engine should be designed from the start as a supercharged engine to obtain performance with the desired life.

Effects of Supercharging(I) Higher power output(II) Greater induction of charge mass(III) Better atomization of fuel(IV) Better mixing of fuel and air(V) Better scavenging of products(VI) Better torque characteristic over the whole speed range(VII) Quicker acceleration of vehicle(VIII) More complete and smoother combustion(IX) Inferior or poor ignition quality fuel usage(X) Smoother operation and reduction in diesel knock tendency

Supercharing of S.I enginesAs for as S. I engines are concerned, supercharging is employed only for aircraft and racing car engines. This is because the increase in supercharging pressure increases the tendency to detonate and pre-ignite.Aprat from increasing the volumetric efficiency of the engine supercharging volumetric efficiency of the engine supercharging results in increase in the intake temp of the engine. Increased intake pressure and temp reduces ignition delay and increase flame speed. Both these effects result in a greater tendency to detonate or pre-ignite. For this reason, the supercharged petrol engines employ lower compression ratios. The use of lower compression ratios and increased heat losses due to higher value of

SuperchargingS K Mondal’s Chapter 5specific heats and dissociation losses at higher temp results in lower thermal efficencies for such engine. Thus supercharged petrol engines have greater fuel consumption than naturally aspirated engines.

Increased flame speeds make the petrol engine more sensitive to fuel air ratio and the engine cannot run on weak mixtures without knock. Rich mixtures are used to control detonation. This futher increases the specific fuel consumption of the engine.Knocking can be controlled in highly supercharged engine by injection of water in the combustion chamber. However, large amount of liquid needed for this purpose becomes prohibitive. Another alternative is to use intercooling of the charge before it is fed to the engine.Because of poor fuel economy supercharged S I engine is not very popular. Supercharging of CI EnginesUnlike S I engines supercharging does not result in any combustion problem, rather it improves combustion, in a diesel engine. Increase in pressure and temp of the intake air reduces ignition delay and hence the rate of pressure rise resulting in a better, quieter and smoother combustion. This improvement in combustion allows a poor quality fuel to be used in a diesel engine and it is also not sensitive to the type of fuel used. The increase in intake air temp. Reduces volumetric and thermal efficiency but the increase in the density due to pressure compensates for this and intercooling is not necessary except for highly supercharged engines.

However, mechanical and thermal loading increases with an increase in supercharging. But this increase in mechanical and thermal loading is only moderate because of the use of lower compression ratios and the effect of supercharged engine. It is possible to use lower fuel-air ratios in a supercharged engine as the increase in fuel flow is less than the increase in air flow. This results in lower temp over the full engine cycle and reduced smoke from the engine. Low fuel-air ratio and high expansion ratio results in lower exhaust temp. This results in increased life of the exhaust valves.

If an unsupercharged engine is supercharged it will increase the reliability and durability of the engine due to smoother combustion and lower exhaust temp. The degree of supercharging is limited by the thermal and mechanical load on the engine and strongly depends on the type of supercharger used and the design of the engines.

Effect of supercharging on the performance of the engine (1) Power output - The power output of a supercharged engine is higher than its naturally aspirated counterpart due to the following reasons.(a) The amount of air inducted per cycle for a given swept volume is increased.(b) The mechanical efficiency is slightly improved.(c) During the gas excharge process some of the work done on the supercharger is recovered.(d) Supercharging results in better seavenging and reduced exhaust gas temp in the engine.

The reduced residual gas fraction helps in better combustion and reduced temp improve volumetric efficiency.The above-mentioned effects are common in both petrol and diesel engine. However, in petrol engines supercharging results in tendency to knock. In contrast to this, the diesel engine runs smoother and is able to utilise low F/A ratio if supercharged.

SuperchargingS K Mondal’s Chapter 5Increased reliability, durability and better fuel consumption are some other benefits of supercharging a diesel engines.

(ii) Mechanical efficiency - An increase in the supercharging pressure increases the gas load and hence large bearing areas and heavier components are needed. This increases the frictional forces. However, the increase in bmep is much more than increase in frictional forces. Typical values are 11% and 7.5% increase in frictional forces for petrol and diesel engines as compared to 40% increase in bmep for 60% supercharging.Thus the mechanical effiences of supercharged engine are slightly better than the naturally aspirated engines. (iii) Fuel consumption - The power required to run the superchrger varies with different arrangements of supercharging. If the supercharger is directly driven by the engine some of the power developed by the engine will be used in running the supercharger. Moreover, at part loads the compression of the supercharger is not fully utilized. This will result in greater loss, hence the specific fuel consumption for mechanically driven superchargers will be more at port loads.

In addition to this, the fact that highly supercharged Otto engines use very high rich mixtures to avoid knock and pre-ignition will give rise to higher specific fuel consumption.Thus in spite of better mixing and combustion due to reduced delay a mechanically supercharged Otto engine will almost always have specific fuel consumption higher than a naturally aspirated engine.The specific fuel consumption for C.I engines is somewhat less than that for naturally aspirated engines due to better combustion and increased mechanical efficiency. Exhaust driven supercharges do not require any power from the engine. Moreover, a part of the exhaust energy is utilized, thus giving about 5% better thermal efficiency at full load. This increase in efficiency result in improved fuel consumption. All turbocharged engines, if properly designed, give a specific fuel consumption lower than the naturally aspirated engines.

Types of SuperchargersSupercharger is a pressure-boosting device which supplies air (or mixture) at a higher pressure. A centrifugal or axial flow or displacement type compressor is normally used. If the supercharger is driven by the engine crankshaft, then it is called mechanically driven supercharger. Some superchargers are driven by a gas turbine, which derives its power from the engine exhaust gases. Such a supercharger is called turbocharger. There are types of superchargers(i) Centrifugal type(ii) Root’s type(iii) Vane type

SuperchargingS K Mondal’s Chapter 5TurbochargersTurbochargers are centrifugal compressors driven by the exhaust gas turbines. These are nowadays extensively used for supercharging almost all types of two-stroke engines. By utilizing the exhaust energy of the engine it recovers a substantial part of energy which would otherwise goes waste. Thus the turbocharger will not draw upon the engine power.


OBJECTIVE QUESTIONS (IES)Previous Years IES Questions

Objectives of superchargingQ1. What is the main objective of supercharging of the engine?

[IES 2007](a) To reduce the mass of the engine per brake power(b) To reduce space occupied by engine(c) To increase the power output of engine(d) All of the above

Q2. What is the purpose of employing supercharging for an engine? [IES-2006]

(a) To provide forced cooling air(b) To raise exhaust pressure(c) To inject excess fuel for coping with higher load(d) To supply an intake of air at a density greater than the density of the surrounding atmosphere


1. Supercharging increases the power output of an engine.2. Supercharging increases the brake thermal efficiency considerably.3. Supercharging helps scavenging of cylinders.Which of the statements given above are correct?(a) Only 1 and 2 (b) Only 2 and 3(c) Only 1 and 3 (d) 1,2 and 3

Supercharging of CI EngineQ4. Consider the following statements with reference to supercharging of

I.C. engines:1. Reciprocating compressors are invariably used for high degree of

supercharging2. Rotary compressors like roots blowers are quite suitable for low

degree of supercharging [IES-2004]

3. Axial flow compressors are most commonly employed for supercharging diesel engines used in heavy duty transport vehicles

4. Centrifugal compressors are used for turbo -chargingWhich of the statements given above are correct?(a) 1 and 2 (b) 2 and 3 (c) 1 and 4 (d) 2 and 4

Q5. Surging basically implies [IES-

1996](a) Unsteady, periodic and reversed flow.(b) Forward motion of air at a speed above sonic velocity.(c) The surging action due to the blast of air produced in a compressor.(d) Forward movement of aircraft.

SuperchargingS K Mondal’s Chapter 5Q6. Which one of the following types of compressors is mostly used for

supercharging of I.C. engines? [IES-1996](a) Radial flow compressor (b) Axial flow compressor(c) Roots blower (d) Reciprocating compressor

Effect of supercharging on performance of the engineQ7. Assertion (A): The CI engine is basically more suitable for supercharging than

the SI engine. [IES-2000]Reason (R): In the CI engine supercharging tends to prevent diesel knocking. (a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q8. Consider the following statements:1. Supercharging increases the power output and increases the volumetric efficiency.2. Supercharging is more suitable for S.I. engines than C.I. engines.

[IES-2005]3. The limit of supercharging for an S.I. engine is set by knock while that for a C.I. engine is set by thermal loading.Which of the statements given above are correct?(a) 2 and 3 (b) 1, 2 and 3 (c) 1 and 3 (d) 1 and 2



Previous Years IES Answers1. Ans. (d) All of the above are objective of supercharging. But main objective is to

increase the power output of engine.2. Ans. (d)3. Ans. (d)4. Ans. (d)5. Ans. (a)6. Ans. (c) Roots blower is mostly used for supercharging of I.C. engines7. Ans. (a)8. Ans. (c) Supercharging is more suitable for C.I. engines than S.I. engines.

6. Jet PropulsionTheory at a Glance (For IES, GATE & PSUs)

Jet PropulsionThe principle of jet propulsion is obtained from the application of Newton’s laws of motion. We know that when a fluid is to be accelerated, a force is required to produce this acceleration in the fluid. At the same time, there is an equal and opposite reaction force of the fluid on the engine which is known as the thrust. Hence, it may stated that the working of jet propulsion is based on the reaction principle. Thus all devices that move through fluids must follow this basic principle.

Newton’s Second Law: Rate of change of the momentum in direction is proportional to the force acting in that direction.Newton’s Third Law: For every action there is equal and opposite reaction. Experience shows that only two types of fluids are particularly suitable for jet propulsion.

(i) A heated and compressed atmospheric air- mixed with the products of combustion produced by burning fuel in that air can be used for jet propulsion. The thermo chemical energy of the fuel is utilize for increasing the temperature of the air to the desired value. The jet of this character is called a thermal jet and the jet propulsion engine using atmospheric air is called air breathing engines.

(ii) Another class of jet-propulsion engines use a jet of gas produced by the chemical reactions of fuel and oxidizer. Each of them is carried with the system itself. The fuel-oxidant mixture is called the propellant. No atmospheric air is used for the formation of the jet. But the oxidant in the propellant is used for generating the thermal jet. A jet produced in this way is known as rocket jet and the equipment wherein the chemical reaction takes place is called a rocket motor. The complete unit including the propellant is called a rocket engine.

The open cycle gas turbine is best suited far the jet propulsion. If the working fluid is expanded in the turbine such that the power developed is only sufficient to drive the compressor and accessories and rest expansion is achieved in a nozzle which is placed just after the turbine. This unit will serve as a jet propulsion.

It is clear that jet-propulsion engines may be classified broadly into two groups.(i) Air breathing engines and(ii) Rocket engines

Gas Turbine EnginesAll modern aircrafts are fitted with gas turbines. Gas turbine engines can be classified into

(i) ramjet engine,(ii) pulse jet engine,(iii) turbo jet engine,(iv) Turboprop engine.

Jet PropulsionS K Mondal’s Chapter 6The Turbojet EngineThe two pilot less air breathing engines, viz. ramjet and pulsejet are simple in construction. However, their application is limited and, to date, they have not been used very extensively. The most common type of air breathing engine apart from turboprop is the turbojet engine. The important features are shown in Fig.

This engine is consists of the following components:(i) a diffuser,(ii) a mechanical compressor,(iii) a mechanical turbine and (iv) an exhaust nozzle.

The function of the diffuser is to convert the kinetic energy of the entering air into a static pressure rise which is achieved by the ram effect. After this air enters the mechanical compressor. The compressor used in a turbojet can be either centrifugal type or axial flow type. The use of a particular type of compressor gives the turbojet typical characteristics. The centrifugal compressor produces a high pressure ratio of about 4:1 to 5:1 in a single stage and usually a double-sided rotor is used to reduce the engine diameter.

Basic cycle for turbo-jet engine

Figures show the basic thermodynamic cycle of a turbojet engine of p-V and T-s diagrams. This is Joule of Brayton cycle. The turbojet is almost a constant thrust

Jet PropulsionS K Mondal’s Chapter 6engine. The specific fuel consumption based on thrust power reduces because with almost constant thrust, the thrust power increases as shown in Fig. As the altitude increases, the thrust decreases due to decrease in density, pressure and temperature of the air. However, the rate of decrease of thrust is less than the rate of decrease of density with altitude because some loss due to reduced density is compensated by lesser drag. The thrust is maximum at sea level.

The operational range of turbojet engine is about 800 to 1100 km/h and the specific fuel consumption is about 1.0 to 1.5kg/thrust h at cruising speeds and are still greater at lower speeds. The altitude limit is about 10000m. Let us consider the control volume of a schematic propulsive device shown in Fig. 7.16. A mass of air enters the control volume with a velocity ci and pressure pi and the products of combustion of mass leaves the control volume with a velocity cj and pressure pj. The flow is assumed to be steady and reversible outside the control volume, the pressure and velocity being constant over the entire control volume except that at the exhaust area Aj. Force F is the force necessary to balance the thrust produced due to change in momentum of the fluid as it passes through the control volume.

Thrust power (TP) :- It is the rate at which work must be developed by the engine if the aircraft is to be kept moving at a constant velocity (Va) against friction force or drag.

Ram effect in jet engines : - In a jet engine or turbojet the KE of the incoming air can be used to obtain a ram compression the intake duct , thus raising overall efficiency. Compression of the air by the conversion of the KE of the atmospheric air relative to the aircraft is called “ram effect”.

Thrust, thrust power, propulsive efficiency and thermal efficiency Thermal efficiency of a propulsive device is an indication of the degree of utilization of energy in fuel in accelerating the fluid flow and is defines as the ratio of propulsive power furnished to exhaust nozzle to the heat supplied and is given by

Where Qi = f CV = heat supplied to the engine per kg of air and is the fuel-air ratio and CV is the calorific value of the fuel.

Jet PropulsionS K Mondal’s Chapter 6The propeller produces thrust power by accelerating the air. The propeller itself is driven by the engine. The efficiency of the propeller is defined as the ratio of the thrust power to the shaft power.

In the case of turboprop engine the thrust power developed by the exhaust is also considered.

As the turbojet engines operate at very low fuel-air ratios, f, very high air-fuel ratios), f may be neglected, then

Overall Efficiency of a Propulsive Systemo = Thermal efficiency Transmission efficiency Propulsive efficiency

o = th tr p

Energy flow through a jet EngineThe flow of energy through a jet engine has certain similarities to that of a reciprocating engine. However, the manifestation of the energy differs, and also the terminology employed. In jet engine the heat energy supplied is converted into K. E. That portion of the heat supplied to the combustion chamber which was utilised to effect the change in KE of the air is termed the propulsive power. The difference between the heat supplied and the propulsive power comprises the heat loss, and the thermal efficiency of the engine is expressed as the ratio of the propulsive power to the heat supplied. That power which is the rate of the actual useful work developed by the engine, termed thrust power, is equal to the propulsive power less the KE losses. From this it can be deduced that the propulsive efficiency of the engine is the ratio of the thrust power to propulsive power and the overall efficiency becomes the ratio of the thrust power to the heat supplied. The propulsive and overall effencies of the jet engine are then comparable to the mechanical and brake thermal effencies respectively of the reciprocating engine. The turboprop is similar to the turbojet, except that most of the nozzle gas pressure drives the turbine shaft – by the time the gas gets past the turbine, there’s very little pressure left to create trust. Instead, the shaft is geared to a propeller which creates the majority of the thrust. ‘Jet’ helicopters work the same way, except that their engines are connected to the main rotor shaft instead of a propeller.Turboprops are more fuel efficient then turbojets at low altitudes, where the thicker air gives a propeller a lot more ‘traction.’ This makes them popular on planes used for short flights, where the time spent at low altitudes represents a greater percentage of

Jet PropulsionS K Mondal’s Chapter 6the overall flight time. It is a known fact that an higher thrust per unit mass flow of fuel can be obtained by increasing the mass flow of air which results in better fuel economy. This fact is utilized in a turboprop engine which is an intermediate between a pure jet engine and a propeller engine. Turboprop engine attempts to increase the air flow by using a propeller drives by the turbine I addition to a small thrust produces by the exhaust nozzle.

The engine consist of the following components:(i) a diffuser,(ii) a compressor,(iii) a combustion chamber,(iv) a turbine,(v) an exhaust nozzle,(vi) a reduction gear and(vii) a propeller.

Turboprop Performance(i) The turboprop has good take off characteristics and good climb characteristics in the lower subsonic speed range due to the large thrust and power available.

(ii) The variation in the propulsive efficiency of the turboprop engine with flight velocity will be a little lower than the propeller engine.

(iii) Since the thermal efficiency of turbojet and the turboprop are the same, the overall efficiency of the turboprop i.e. the fuel economy will be superior to the turbojet engine at speeds below 800 km /hr.The Ramjet Engine

Jet PropulsionS K Mondal’s Chapter 6

The fact of obtaining very high pressure ratios of about 8 to 10 by ram compression has made it possible to design a jet engine without a mechanical compressor. A deceleration of the air from Mach number 3 at diffuser inlet to Mach number 0.3 in combustion chamber would cause pressure ratio of more than 30. As the ram jet has no compressor hence the entire compression depends upon the ram compression. The ram pressure ratio increases very slowly in the subsonic speed range. That is why ram jet is boosted up to a speed of 290 km/hr by a suitable means such as a turbojet or a rocket before the ram jet will produce any thrust and must be boosted to even higher speeds before the thrust produced exceeds the drag. After the boasting of ram jet, the velocity of air passing through the diffuser decreases and hence pressure increases. This is called ram compression and a “pressure barrier” is created after the end of the diffuser. The fuel is injected through injection nozzle into the combustion chamber where is ignited by means of a spark plug. The expansion of gases toward the diffuser entrance is restricted by the pressure barrier at the after end of the diffuser and as a result the gases are constrauied to expand through the tail pipe and out of the exit nozzle at a high velocity. As the ram jet engine has no turbine , the temp. of the gases of combustion is not limited to a relatively low value as in the turbojet engine. The air fuel ratio is 15:1. The exhaust temperature ranges 1800° C to 2100°C. The jet action gives the necessary foward thrust to the engine.

The basic characteristics of the Ram jet are(i) Simple in construction.(ii) No moving parts and hence free from unbalancing.(iii) Greater thrust per unit engine weight than any other ropulsion engine at

supersonic speed except rockets.(iv) The thrust per unit frontal area increases both with the efficiency and the air flow

through the engine, therefore, much greater thrust per unit area is obtainable at high supersoinc speeds.

(v) The best performance of ram jet engine is obtained at flight speed of 1700 -2200 km/hr.

(vi) For successful operation the diffuser has to be carefully designed so that the KE associated with high entrance velocity is efficiently converted into pressure.

(vii) At low and moderate speeds the fuel consumption is too large. However, the fuel consumption decreases with flight speed.

(viii) The performance is independent of fuel technology and a wide range of fuels and a wide range of fuels can be burned.

Pulse jet engine It is somewhat simular to a ramjet engine with exception that a mechanical value (V - shaped non return value) is used to present the hot gases of combustion from going

Jet PropulsionS K Mondal’s Chapter 6out of the diffuser. Thus pulse jet like ram jet develops thrust by a high velocity jet of exhust gases without the aid of compressor or turbine. It was invented by the German inventor “Paul Schmidt” in 1930.

Turbojet and Ram jet engine are continuous in operation and are based on the cnstant pressure heat addition Brayton cycle. The pulse jet is an intermittent conbustion engine and it operates on a cycle simular to a reciprocating engine and may be better compared.

Advantages :- (i) Pulse jet engine is very inexpensive compased to turbojet engine.(ii) The pulse jet produces static thrust and produces thrust in excess of drag at uch lower speeds than a ram jet.

Disadvantages :-(i) Apparent noise.(ii) Serious limitations to mechanical valve arrangements.(iii) High rate of fuel consumption and low thermodynamic efficiency.(iv) The operating attitude is limited by air density considerations. (v) Severe vibrations.

Rocket EnginesOne very important use for the nozzle is as a means of propulsion. Since the fluid flowing through the nozzle is accelerated relative to the nozzle, then by Newton’s thrid low of motion the fluid exerts a thrust on the nozzle in the opposite direction to the fluid flow.

In jet planes the atmospheric air is drawn in, compressed, heated and allowed to expand through a nozzle at a very high velocity resulting in forward thrust. In order to achieve jet propelled flight in space where there is no atmosphere to be drawn into the vehicle, it is necessary that the fuel plus its axident should be carried. This is known as rocket propulsion.The two substance i.e. fuel and oxidant together is termed as propellants. The propellants may be of two types - solid and liquid.

Rockets are of two types(i) Solid rockets and

Jet PropulsionS K Mondal’s Chapter 6(ii) Liquid rockets.This classification is based on the types of fuel usd.

Solid rocket consists ofPropellant,Casing or chamberNozzle and Ignition system.

Liquid rockets consist of Liquid tanks Pumps or pressure bottleCombustion chamberNozzle.

Liquid fuel can be fed to the combustion chamber by means of a pump or by pressurised N2, Rocket engines are best suited for high speed and altitudes. At any given throttle, the thrust and efficiency of a rocket motor improves slightly with increasing altitude ((because the back-pressure falls thus increasing net thrust at the nozzle exit plane), whereas with a turbojet(or turbofan) the falling density of the air entering the intake (and the hot gases leaving the nozzle) causes the net thrust to increase with increasing altitude. Rocket engines are more efficient than even scramjet above roughly Mach no 15.



Applications of rocketsQ1. An air breathing aircraft is flying where the air density is half the value

at ground level. With reference to the ground level, the air-fuel ratio at this altitude will be [GATE-1998]

Previous Years IES Questions

Turbo-jetQ1. Consider the following statements:

[IES-1996]In open cycle turbo-jet engines used in military aircraft, reheating the exhaust gas from the turbine by burning more fuel is used to increase.1. Thrust 2. The efficiency of engine 3. The range of aircraft.Of these correct statements are(a) 1 and 3 (b) 1 and 2 (c) 2 and 3 (d) 1, 2 and 3

Q2. Which one of the following is correct?The turbine of the turbo-prop engine as compared to that of the turbojet engine is(a) Similar (b) Smaller(c) Bigger (d) Unpredictable [IES

2007]

Q3. The propulsive efficiency of a turbojet aircraft approaches 100% when the thrust approaches [IES-2003](a) Maximum (b) 50% of the maximum(c) 25% of the maximum (d) Zero

Q4. Assertion (A): Compared to a turbo-jet engine, a turbo-prop engine has a higher power for take-off and higher propulsive efficiency at low speeds.

[IES-1997]Reason (R): By mounting the propeller on the turbine shaft, the propeller can be run at a very high speed to obtain higher efficiency.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q5. In a turbojet engine, subsequent to heat addition to compressed air, to get the power output, the working substance is expanded in [IES-1996]


(a) Turbine blades, which is essentially an isentropic process.(b) Turbine blades, which is a polytropic process.(c) Exit nozzle, which is essentially an isentropic process.(d) Exit nozzle, which is a constant volume process.

Q6. Assertion (A): In the subsonic range the propulsive efficiency of a rocket is less

than that of a turbojet. [IES-1995]Reason (R): The jet velocity of rocket is independent of forward motion.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q7. The absolute jet exit velocity from a jet engine is 2800 m/s and the forward flight velocity is 1400 m/s. The propulsive efficiency is

[IES-2003](a) 33.33 % (b) 40 % (c) 66.67 % (d) 90 %

Thrust, thrust power, propulsive efficiency and thermal efficiency Q8. Propulsion efficiency of a jet engine is given by (where u is flight

velocity and V is jet velocity relative to aircraft). [IES-1995]

(a) 2u/(V – u) (b) (V + u)/2u (c) 2u/(V + u) (d) (V-u)/2u

Q9. For a jet propulsion unit, ideally the ratio of compressor work and

turbine work is [IES-2002](a) 2 (b) 1(c) not related to each other (d) unpredictable

Q10. Consider the following statements:In a turbojet engine, thrust may be increased by

[IES-1998]1. Increasing the jet velocity 2. Increasing the mass flow a rate of air 3. After burning of the fuel.Of these statements(a) 1 and 2 are correct (b) 2 and 3 are correct (c) 1 and 3 are correct (d) 1, 2 and 3 are correct

Q11. Assertion (A): After burning increases the thrust of a jet engine.Reason (R): The air fuel ratio of jet engine is high. [IES-2009](a) Both A and R are individually true and R is the correct explanation of A (b) Both A and R are individually true but R is not the correct explanation of A(c) A is true but R is false.(d) A is false but R is true.

Q12. If Vj is the jet velocity and Vo is the vehicle velocity, the propulsive efficiency of a rocket is given by [IES-1994]


(a) (b) (c) (d) 13. Which one of the following is the correct expression for the propulsion

efficiency of a jet plane (neglecting the mass of fuel)? [IES-2005]

(a) (b) (c) (d) (Where Vj = velocity of jet relative to plane, Va = velocity of the plane)

Q14. Consider the following statements regarding performance of turbojet engines:

1. The thrust decreases at higher altitude due to reduced density of air and consequently lower mass flow of air. [IES-2004]

2 At subsonic speeds, the effect of increased velocity is to increase the air flow and the thrust increases.

3. The relative velocity of jet with respect to the medium decreases at higher speeds which tends to reduce the thrust.

4 For turbojet engine the thrust of jet at subsonic speeds remains relatively constants.

Which of the statements given above are correct?(a) 1, 2, 3 and 4 (b) 1 and 3 (c) 1, 2 and 4 (d) 2, 3 and 4

Q15. The theoretical mechanical efficiency of a jet engine (neglecting frictional and thermal losses), when driving a vehicle, has its maximum [GATE-1992](a) Only when the vehicle moves at sonic velocity(b) When outlet gases approach zero absolute velocity(c) When the vehicle speed approaches the magnitude of the relative velocity of gases at nozzle exit(d) Only when the relative velocity at nozzle exit is at its maximum.

Q16. An aircraft flying horizontally at a speed of 900 km/h is propelled by a jet leaving the nozzle at a speed of 500 m/s. The propulsive efficiency is (a) 0.334 (b) 0.426 (c) 0.556 (d) 0.667 [IES-

1999]

Q17. The efficiency of jet engine is [IES-2003]

(a) Higher at high speeds (b) lower at low speeds(c) Higher at high altitudes (d) same at all altitudes

Q18. Assertion (A): Propulsion efficiency of propeller driven aircraft is low at very high speeds.Reason (R): At high speeds, shock waves are formed over propeller blades.

[IES-1998](a) Both A and R are individually true and R is the correct explanation of A



Q19. Assertion (A): A bypass jet engine gives a better propulsive efficiency and better fuel economy than a straight jet engine. [IES-1998]Reason (R): A bypass jet engine gives lower velocity of jet efflux than a straight jet engine.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q20. The thrust of a jet propulsion power unit can be increased by [IES-1993]

(a) Injecting water into the compressor(b) Burning fuel after gas turbine(c) Injecting ammonia into the combustion chamber(d) all of the above

Turbo-propQ21. Assertion (A): The thrust of a turboprop engine increases with the increase in

flight speed.Reason (R): With the increase in flight speed, there is an increase in the pressure and density of the air at the compression inlet due to the ram effect.



As compared to a turboprop, a turbojet1. Can operate at higher altitudes2. Can operate at higher flight velocities3. Is more fuel efficient at lower speedsOf these statements(a) 1, 2 and 3 are correct (b) 1 and 2 are correct (c) 2 and 3 are correct (d) 1 and 3 are correct.

Q23. In turbo prop, the expansion of gases takes place approximately

[IES-2000](a) 100% in the turbine (b) 80% in the turbine and 20% in the nozzle(c) 50% in the turbine and 50% in the nozzle (d) 100% in the nozzle

Q24. Which one of the following is the correct sequence of the position of the given components in a turboprop? [IES-1998](a) Propeller, Compressor, Turbine, Burner(b) Compressor, Propeller, Burner, Turbine


(c) Propeller, Compressor, Burner, Turbine(d) Compressor, Propeller, Turbine, Burner

Q25. Assertion (A): The use of turboprop engine is limited to medium speed applications.

Reason (R): The efficiency of a turboprop engine decreases at higher speed.[IES-1999]


Q26. Assertion (A): Turbojet engine is superior to turboprop engine at all operating conditions. [IES-1994]Reason (R): Efficiency of the propeller is low at high altitude and at high

speeds. (a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q27. A turbo prop is preferred to turbo-jet because [IES-1992]

(a) It has high propulsive efficiency at high speeds(b) It can fly at supersonic speeds(c) It can fly at high elevations(d) It has high power for take off

Rocket enginesQ28. Consider the following statements about a rocket engine: [IES-

1997]1. It is very simple in construction and operation. 2. It can attain very high vehicle velocity.3. It can operate for very long duration.Of these statements(a) 1 and 3 are correct (b) 1 and 2 are correct(c) 2 and 3 are correct (d) 1, 2 and 3 are correct

Q29. Consider the following statements relating to rocket engines: [IES-1996]

1. The combustion chamber in a rocket engine is directly analogous to the reservoir of a supersonic wind tunnel.

2. Stagnation conditions exist at the combustion chamber.3. The exit velocities of exhaust gases are much higher than those in

jet engines.4. Efficiency of rocket engines is higher than that of jet engines.Of these correct statements are(a) 1, 3 and 4 (b) 2, 3 and 4 (c) 1, 2 and 3 (d) 1, 2 and 4

Q30. Only rocket engines can be propelled to 'SPACE' because [IES-1996]

(a) They can generate very high thrust.


(b) They have high propulsion efficiency.(c) These engines can work on several fuels. (d) They are not air-beating engines.

Q31. Which of the following performance advantages does a rocket engine have as compared to a turbojet engine? [IES-2009]1. No altitude limitation 2. Higher efficiency3. Longer flight duration 4. No ram dragSelect the correct answer from the code given below:(a) 1 and 2 only (b) 1 and 4 only (c) 1, 2 and 3 (d) 2, 3 and 4

Requirements of an ideal rocket propellantQ32. Assertion (A): Liquid oxygen-liquid hydrogen propellant system has a higher

specific impulse relative to the liquid oxygen-hydrocarbon system. [IES-1993]Reason (R): Hydrogen has a higher burning velocity than hydrocarbons.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q33. Match List I with List II in respect of chemical rocket engine and select

the correct answerList I List IIA. Ethyl alcohol 1. Liquid oxidizer

[IES-1994]B. Nitrocellulose 2. Liquid fuelC. Ammonium per chlorate 3. Solid oxidizerD. Hydrogen peroxide 4. Solid fuelCodes: A B C D A B C D(a) 1 3 2 4 (b) 2 3 4 1(c) 2 4 3 1 (d) 4 1 2 3

Q34. Match List I with List II and select the correct answer using the codes

given below the lists:List I List II [IES-

1993](Name of Propellant) (Type of propellant)A. Nitric acid 1. FuelB. Hydrogen 2. MonopropellantC. Fuming nitric acid-hydrazine 3. OxidizerD. Methyl nitrate methyl alcohol 4. Compounded liquid

monopropellant5. Hypergolic propellant

Codes: A B C D A B C D

(a) 2 1 4 5 (b) 1 2 5 4(c) 3 1 5 4 (d) 3 1 4 5

Q35. In solid propellants rockets, ammonium picrate is usually added as: (a) An additive (b) an inhibitor [IES-

1992](c) A darkening agent (d) a plasticizer


Thrust work, propulsive work and propulsive efficiencyQ36. The relative jet exit velocity from a rocket is 2700 m/s. The forward

flight velocity is 1350 m/s. What is the propulsive efficiency of the unit? (a) 90% (b) 66.66%(c) 50% (d) 33.33% [IES-1998;

2004]

Q37. Consider the following statements indicating a comparison between rocket and jet propulsion systems:1. Both rocket and jet engines carry the fuel and oxidant.

[IES-2006]2. Rockets do not employ compressor or propeller.3. Rockets can operate in vacuum also.4. Rockets can use solid fuels and oxidants.Which of the statements given above are correct?(a) 1, 2, 3 and 4 (b) Only 1 and 2 (c) Only 2, 3 and 4 (d) Only 1, 3 and 4

Q38. Consider the following statements:The thrust of a rocket engine depends upon [IES-1998]1. Effective jet velocity 2. Weight of the rocket 3. Rate of propellant consumptionOf these statements(a) 1 and 2 are correct (b) 1 and 3 are correct (c) 2 and 3 are correct (d) 1, 2 and 3 are correct

Q39. Assertion (A): A rocket engine can operate even in vacuum and in any fluid medium.

Reason (R): Rocket engine is a pure reaction engine, wherein a propulsive thrust is obtained as a reaction of momentum of ejected matter.




Previous Years GATE Answers1. Ans. (c)

Previous Years IES Answers1. Ans. (b)2. Ans. (c) Though turbo-prop engine used in small aircraft due to its large number of stages its size is Big for same power.

3. Ans. (d) and Thrust power (TP) = (Cj - Ca) CaPropulsive efficiency increases with increase in aircraft velocity (Ca), i.e. efficiency is maximum when thrust approach is zero.

4. Ans. (c)5. Ans. (b)6. Ans. (b) Both A and R are true but R is not correct explanation of A.

7. Ans. (c) 8. Ans. (c)9. Ans. (b)10. Ans. (d)11. Ans. (b)12. Ans. (a)13. Ans. (b)14 Ans. (d) The turbojet is almost a constant thrust engine.15. Ans. (c)

16. Ans. (d)

Jet PropulsionS K Mondal’s Chapter 617. Ans. (a)18. Ans. (a)19. Ans. (a20. Ans. (a) The thrust of a jet propulsion power unit can be increased by injecting water into the compressor.21. Ans. (d) The thrust of turboprop engine is proportional to

(Vj = velocity of jet relative to engine and Va = velocity of approach of air)

Further propulsive efficiency, Therefore with increase in Va, ηp increases but thrust decreases. Thus assertion A is false.However reason R is true.

22. Ans. (b)23. Ans. (b)24. Ans. (c)25. Ans. (c) In fact the efficiency increases with speed but thrust keeps on decreasing.26. Ans. (d) A is false but R is true.27. Ans. (d)28. Ans. (d) All statements are correct.29. Ans. (c) 30. Ans. (d) Rocket engines can be propelled to space because they are not air breathing engines31. Ans. (b)32. Ans. (a) Both assertion and reason are true and also R provides correct explanation for A.33. Ans. (c)34. Ans. (c)35. Ans. (a)

36. Ans. (b)37. Ans. (c) 1 is false, jet engines absorb oxygen from atmosphere.38. Ans. (b)39. Ans. (a) Both A and R are true and R provides correct explanation for A.

Student’s Notes

7. IC Engine PerformancesTheory at a Glance (For IES, GATE & PSUs)

Engine Performance ParametersThe engine performance is indicated by the term efficiency, . Five important engine efficiencies and other related engine performance parameters are given below:(i) Indicated thermal efficiency (ith)(ii) Break thermal efficiency (bth)(iii) Mechanical efficiency (m)(iv) Volumetric efficiency (v)(v) Relative efficiency of Efficiency ratio (rel)(vi) Mean effective pressure (Pm)(vii) Mean piston speed(viii) Specific power output (Ps)(ix) Specific fuel consumption (sfc)(x) Inlet-valve Mach Index (Z)(xi) Fuel-air or air-fuel ratio (F/A or A/F)(xii) Calorific value of the fuel (CV)

Energy distribution in an IC engine

Indicated Thermal Efficiency (ith)Indicated thermal efficiency is the ratio of energy in the indicated power, ip, to the input fuel energy in appropriate units.

IC Engine PerformancesS K Mondal’s Chapter 7Friction PowerThe difference between the indicated and the brake power of an engine is known as friction power. The internal losses in an engine are essentially of two kinds, viz., pumping losses and friction losses. Following methods are used to find the friction power to estimate the performance of the engine.

(i) Willian’s line method(ii) Morse test(iii) Motoring test(iv) From the measurement of indicated and brake power(v) Retardation test

(a) Willain’s Line Method (Mainly to C I Engine)This method is also known as fuel rate extrapolation method. A graph connecting fuel consumption (Y-axis) and brake power (X-axis) at constant speed is drawn and it is extrapolated on the negative axis of brake power. The interoept of the negative axis is taken as the friction power of the engine at that speed. The method of extrapolation is shown in figure (dotted lines)

Willan’s Line As seen from the fig, since, in most of the power range the relation between the fuel consumption and brake power is linear which permits extrapolation.

Further, when the engine does not develop any power, i.e bp = 0, it consumes a certain amount of fuel. The energy would have been spent in overcoming the friction. Hence, the extrapolated negative intercept of the x-axis will be the work representing the combined losses due to mechanical friction, pumping and blowing and as a whole it is termed the frictional loss of the engine.The measured frictional power by this method will hold good only for a particular speed and is applicable mainly to C.I engines.

IC Engine PerformancesS K Mondal’s Chapter 7The main drawback of this method is the long distance to be extrapolated from data obtained between 5 and 40% load towards the zero line of fuel input. The directional margin of error is rather wide because the graph is not exactly linear.

The changing slope along the curve indicates the effect of part load efficiency of the engine. The pronounced change in the slope of this line near full load reflects the limiting influence of the air-fuel ratio and of the quality of combustion. Similarly, there may be slightly curvature at light loads. This is perhaps due to the diffulty in injecting accurately and consistently very small quantities of fuel per cycle.Therefore, it is essential that great core should be taken in extrapolating the line and as many readings as possible should be taken at light loads to establish the true nature of the curve.The accuracy obtained in this method is reasonably good and compares with other methods if extrapolation is carefully done.

(b) Morse Test (for Multi cylinder engines only)In Morse test consists of obtaining IP of the engine without any elaborate equipment. The test consists of making imperative in turn, each cylinder of the engine and noting the reduction in B.P developed.With a gasoline engine each shorting the spark plug of the cylinder; with a diesel engine by cutting off the supply of fuel to each cylinder.This test is applicable only to multi cylinder engines.

Referring to the above figure, the unshaded area of the indicator diagram is a measure of the gross power (gp), develop by the engine, the dotted area being the pumping power (pp).Net IP/ cylinder = gp - pp

(c) Motoring TestIn motoring test the engine is steadily operated at the rated speed by its own power and allowed to remain under the given speed and load conditions for sufficient time so that the temp of the engine components lubricating oil and cooling water reaches a steady stable.

A swinging field type electric dynamometer is used to absorb the power during this period which is most suitable for this test. The ignition is then cut-off and by suitable electric switching devices the dynamometer is converted to run as a motor so as to crank the engine at the same speed at which it was previously operating. The

IC Engine PerformancesS K Mondal’s Chapter 7power supply from the above dynamometer is measured which is a measure of the F.P of the engine at that speed. The water supply is also cut-off during the motoring test so that the actual operating temps are maintained to the extent possible.

This method though determines the F.P at conditions very near to the actual operating temps at the test speed and load, it does not give the true losses occurring under firing conditions due to following reasons -(1) The temps. in the motored engine are different from those in a firing engine.(2) The pressure on bearings and piston rings is lower than in the firing engine.(3) The clearance between piston and cylinder wall is more (due to cooling) and this

reduces the piston friction.(4) The air is drawn at a temp. much lower than when the engine is firing because it

does not get heat from the cylinder (rather losses heat to the cylinder)Motoring method, however, gives reasonably good results and is very suitable

for finding the losses imparted by various engine components. This insight of the losses caused by various components and other parameters is obtained by progressive stripping off of the engine.

(d) From the measurement of IP and BPFP = IP - BP

This is an ideal method by which FP is obtained by computing the difference between IP obtained from an indicator diagram and BP obtained from a dynamotor. This method is mostly used in research laboratories at it is necessary to have elaborate equipment to obtain accurate indicator diagrams at high speeds.

(e) Retardation Test This test involves the method of retarding the engine by cutting the fuel supply. The engine is made to run at no load and rated speed taking into all usual steady operating conditions the supply of fuel is cut-off and simultaneously the time of fall in speeds by say 20%, 40%, 60% and 80% of the rated speed is recorded. The tests are repeated once again with 50% load on the engine.

Comparison of Various methodsThe Willan’s line method and Morse tests are comparatively easy to conduct. However, both these tests give only an overall idea of the losses whereas motoring test gives a very good insight into the various causes of losses and is a much more powerful tool.

IC Engine PerformancesS K Mondal’s Chapter 7As for as accuracy is concerned, the IP-BP method is the most accurate if carefully done. Motoring method usually gives a higher value of FP as compared to that given by the Willian’s line method. Retardation method, though simple, requires accurate determination of the load torque and the time for the fall in speed for the same range.

Brake PowerMeasurement of BP is one of the most important measurement is the test schedule of an engine. It involves the determination of the torque and the angular speed of the engine output shaft. The torque measuring device is called a dynamometer.

The above fig shows the basic principle of a dynamometer. A rotor driven by the engine under test, is mechanically, hydraulically or electromagnetically coupled to a stator. For every revolution of the shaft, the rotor periphery moves through a distance 2R against the coupling force, F. Hence the work done per revolution isW = 2RF

The external moment or torque is equal to S x L, where S is the scale reading and L is the arm length. This moment balanced the turning moment R x F, i.e., S x L = R x F Therefore

Work done/revolution = 2 SLWork done/minute = 2 SLN

Hence brake power is given bybp = 2 NT Watts

where T is the torque and N is rpm.

Dynamometers can be broadly classified into two main types.

Absorption DynamometerThese dynamometers measure and absorb the power output of the engine to which they are coupled. The power absorbed is usually dissipated as heat by some means.Examples - Porny brake, rope brake, hydraulic, eddy current dynamometers etc.Transmission dynamometer:- In transmission the power is transmitted to the load coupled to the engine after it is indicated on some type of scale. These are also called torque meters.

The terms brake and dynamometer mean the same. A dynamometer is also a brake except the measuring devise are including to indicate the amount of force required in attempting to stop the engine.

IC Engine PerformancesS K Mondal’s Chapter 7Fuel ConsumptionThere are two ways of expressing fuel consumption viz by volume or by weight during a specified time. For automobiles it is expressed in terms of kilometers per liter.Accurate measurement of fuel consumption is very important in engine testing work. Though this seems to be a simple matter, it is by no means so as apparent from the occurrence of the following phenomena: -

(a) Due to engine heat, vapor bubbles are formed in the fuel line. When the bubble grows the fuel volume increases and back flow of fuel take place. Some fuel flow meters measure this backflow as it if was forward flow. Some meters do not count backward flow but when place which is counted.

(b) If bubbles are formed before or inside the flow meter the measured flow can be much higher than actual.

(c) If there is any swirl in the fuel flow especially in the case of turbine type flow meter is may register a high flow rate.

(d) The density of the fuel is dependent on temp which can vary over a wide range(- 100C to 700C) giving rise to a error in measurement.

(e) Some flow meters which use a light beam, the measurements may be affected by the colour of the fuel.

(f) The needle valve in the float bowl of the carburetor opens and closes periodically allowing fuel to surge into the float bowl. This way cause water hammer type effect making the turbine type flow meter to continue to rotate even when fuel flow has stopped, thereby producing errors in flow measurements.

Two basic type of measurement methods are - (a) Volumetric type(b) Gravimetric type

Air Consumption(A/F ratio) The diet of an engine consists of air and fuel. For finding out the performance of the engine accurate measurement of both the quantities is essential.In I.C engines the satisfactory measurement of air consumption is quite difficult because the flow is pulsating is quite difficult because the flow is pulsating due to the cycle nature of the engine and because the air is a compressible fluid. Therefore, the simple method of using an orifice in the induction pipe is not satisfactory since the reading will be pulsating and unreliable.Methods used are - (i) Air Box method (ii) Viscous-flow air water.

SpeedSpeed measurement is an art Speed of the engine is widely used in the computation of power design and development. Measurement of speed is accomplished by instruments like mechanical counters and timers, mechanical tachometers, stroboscope, electric counters tachometers, electric generators, electronic pulse counters, etc.

The best method of measuring speed is to count the number of revolution in a given time. This gives an accurate measurement of speed. Many engines are fitted with such revolution counters. A mechanical or electrical tachometer can also be used for measuring speed. Both these types are affected by temp. Variations and are not very accurate.

For accurate and continuous measurements of speed a magnetic pick-up placed near a toothed wheel coupled to the engine shaft can be used. The magnetic pick-up will produce a pulse for every revolution and a pulse counter will accurately measure the sped.

IC Engine PerformancesS K Mondal’s Chapter 7

Exhaust and coolant temperatureSimplest way of measuring the exhaust temp is by means by a thermocouple. Nowadays electronic temp sensitive transducer and ultra-violet radiation analyzes are also in use.Coolant temperature are normally measured using suitable thermometers.

Break Thermal Efficiency (bth)Break thermal efficiency is the ratio of energy in the brake power, bp, to the iput fuel energy in appropriate units.

Mechanical Efficiency (m)Mechanical efficiency is defined as the ratio of break power (delivered power) to the indicated power (power provided to the piston).

fp = ip – bp It can also be defined as the ratio of the brake thermal efficiency of the indicated thermal efficiency.

Volumetric Efficiency (v)This is one of the very important parameters which decided the performance of four-stroke engines. Four-stroke engines have distinct suction strike and therefore the volumetric efficiency indicates the breathing ability of the engine. It is to be noted that the utilization of the air is what going to determine the power output of the engine. Hence, an engine must be able to take in as much air as possible.Volumetric efficiency is defined as the volume flow rate if air into the intake system divided by the rate at which the volume is displaced by the system.

Where pa is the inlet densityThe normal range of volumetric efficiency at full throttle for SI engines is between 80 to 85% where as for CI engine it is between 85 to 90%. Gas engines have much lower volumetric since gaseous fuel displaces air and therefore the breathing capacity of the engine is reduced.

Relative Efficiency or Efficiency Ratio (rel)Relative efficiency or efficiency ratio is the ratio of thermal efficiency of an actual cycle to that of the ideal cycle. The efficiency ratio is a very useful criterion which indicates the degree of development of the engine.

Mean Effective Pressure (Pm)

IC Engine PerformancesS K Mondal’s Chapter 7Mean affective pressure is the average pressure inside the cylinders of an internal combustion engine based on the calculated or measured power output. It increases as manifold pressure increases. For any particular engine,

Where ip = indicated power (kW)pim = indicated mean effective pressure (N/m2)L = length of the stroke (m)A = area of the piston (m2)N = speed in revolutions per minute (rpm)n = Number of power stroke N/2 for 4-stroke and N for 2-stroke

engines K = number of cylindersAnother way of specifying the indicated mean effective pressure p im is from the knowledge of engine indicator diagram (p-V diagram). In this case, p im, may be defined as

Mean Piston Speed ( )An important parameter in engine applications is the mean piston speed, . It is defined as

= 2LNWhere L is the strike and N is the rotational speed of the crankshaft in rpm.

Specific Power Output (Ps)Specific power output of an engine is defined as the power output per unit piston area and is a measure of the engine designer’s success in using the available piston area regardless of cylinder size. The specific power can be shown to be proportional to the product of the mean effective pressure and mean piston speed.

Specific power output, Ps = bp/A

Specific Fuel Consumption (sfc)The fuel consumption characteristics of an engine are generally expressed in team of specific fuel consumption in kilograms of fuel per kilowatt.-hour. It is an important parameter that reflects how good the engine performance is. It is inversely proportional to the thermal efficiency of the engine.

Inlet-Valve Mach Index (Z)In a reciprocating engine the flow of intake charge takes place through the intake valve opening which is varying during the induction operation. Also, the maximum gas velocity through this area is limited by local sonic velocity. Thus gas velocity is finally chosen by the following equation,


When = gas velocity through the inlet valve at smallest flow areaAp = piston areaAi = nominal intake valve opening areaCi = inlet valve flow co-efficient

Fuel-Air (F/A) or Air-Fuel Ratio (A/F)The relative proportions of the fuel and air in the engine are very important from the standpoint of combustion and the efficiency of the engine. This is expressed either as a ratio of the mass of the fuel of that of the air or vice versa.Air ratio. A mixture having more fuel than that in a chemically correct mixture is termed as rich mixture and a mixture that contains less fuel (or excess air) is called a lean mixture. The ratio of actual fuel-air ratio to stoichiometric fuel-air ratio is called equivalence ratio and is denoted by

Accordingly, = 1 means stoichiometric (chemically correct) mixture, <1 means lean mixture and >1 means rich mixture.

Calorific Value (CV)Calorific value of a fuel is the thermal energy released per unit quantity of the fuel when the fuel is burned completely and the products of combustion are cooled back to the initial temperature of the combustible mixture. Other terms used for the calorific value are heating value and heat of combustion.

Engine Performance CharacteristicsEngine performance characteristics are a convenient graphical presentation of an engine performance. They are constructed from the data obtained during actual test runs of the engine and are particularly useful in comparing the performance of one engine with that of another. In this section some of the important performance characteristics of the SI engines are discussed.It is to be noted that there is a certain speed, within the speed range of a particular engine, at which the change inducted per cylinder per cycle will be the maximum. At this point, the maximum force can therefore be exerted on the piston. For all practical purposes, the torque, or engine capacity to do work, will also be maximum at this point. Thus, there is a particular engine speed at which the charge per cylinder per cycle is a maximum, and at approximately this same speed, the torque of the engine will be a maximum.As the speed of the engine is increased above this speed the quantity of the indicated charge will decrease. However, the power output of the engine increases with speed due to more number of cycles are executed per unit time. It should be noted that the air consumption will continue to increase with increased engine speed until some point is reached where the charge per cylinder per stroke decreases very rapidly than the number of strokes per unit time is increasing. Engine are so designed that the maximum air consumption point is not reached within the operation speed of the engine. Increase in air consumption means that increased quantities of fuel can be added per unit time increasing the power output. In fact the ip produced in the cylinder is almost directly proportional to the engine air consumption.The relationship between air charge per cylinder per cycle and torque, as well as air consumption and ip is illustrated in Fig. Note that the maximum torque occurs at a lower speed than the maximum ip. Figure shows some of the other important performance characteristics for a typical SI engine. In this figure, torque, ip,bp and fp

IC Engine PerformancesS K Mondal’s Chapter 7are plotted against engine speed throughout the operating range of the engine, at full throttle and variable load.

The difference between the ip produced in the cylinder, and the bp realized at the drive-shaft, is the fp. At low engine speed, the fp is relatively low, and bp is nearly as large as ip. As engine speed increases, however, fp increases at a greater rate. At engine speed above the usual operating range, fp increases very rapidly. Also, at these higher speeds, Ip will reach a maximum and then fall off. At some point, ip and fp will be equal, and bp will then drop to zero. Note that the torque reaches a maximum at approximately 60% of the rated rpm of the engine, while the ip has not reached maximum even at the rated speed.Figure shows fuel consumption and bsfc plotted against the engine speed, for the same engine operating under the same conditions. The quantity of fuel consumed increases with engine speed. The bsfc, on the other hand, drops as the speed is increased in the high speed range. At low speeds, the heat loss to the consumption chamber walls is greater and combustion efficiency is poor, resulting in higher fuel consumption for the power produced. At the high speeds, the fp is increasing at a rapid rate, resulting in a slower increase in bp than in fuel consumption, with a consequent increase in bpfc. The bsfc curve is for full throttle, variable speed operation. At any one speed, it represents the bsfc which will result when the engine is carrying its maximum load. By reducing throttle opening and load, that same speed may be obtained, but at loads less than the maximum. A family of curves for various speeds can be obtained, each showing the effect on bsfc of varying the load at constant speed. Under these conditions of constant speed and variable load, and at a constant air-fuel ratio, the bsfc will rise consistently and rapidly as the load (and throttle opening) is decreased. The reason for the rapid increase in bsfc with the reduction in throttle opening is that the fp remains essentially constant, while the ip is being reduced. The bp drops more rapidly than fuel consumption, and there by the bsfc rises.



Q1. During a Morse test on a 4 cylinder engine, the following measurements of brake power were taken at constant speed.

[GATE-2004]All cylinders firing 3037 kWNumber 1 cylinder not firing 2102 kWNumber 2 cylinder not firing 2102 kWNumber 3 cylinder not firing 2100 kWNumber 4 cylinder not firing 2098 kWThe mechanical efficiency of the engine is(a) 91.53% (b) 85.07% (c) 81.07% (d) 61.22%

Q2. With increasing temperature of intake air, IC engine efficiency [GATE-1998]

(a) Decreases (b) increases (c) Remains same (d) depends on other factors

Q3. Brake thermal efficiency of the three types of reciprocating engines commonly used in road vehicles are given in the increasing order as (a) 2 stroke Sl engine, 4 stroke Sl engine, 4 stroke Cl engine

[GATE-1992](b) 2 stroke Sl engine, 4 stroke Cl engine, 4 stroke Sl engine(c) 4 stroke Sl engine, 2 stroke SI engine, 4 stroke Cl engine(d) 4 stroke Cl engine, 4 stroke Sl engine, 2 stroke Sl engine

Q4. An automobile engine operates at a fuel air ratio of 0.05, volumetric efficiency of 90% and indicated thermal efficiency of 30%. Given that the calorific value of the fuel is 45 MJ/kg and the density of air at intake is 1 kg/m3, the indicated mean effective pressure for the engine is [GATE-2003](a) 6.075 bar (b) 6.75 bar (c) 67.5 bar (d) 243

bar

Previous Years IES QuestionsQ1. In a variable speed S.I. engine, the maximum torque occurs at the

maximum(a) Speed (b) brake power [IES-

1999](c) Indicated power (d) volumetric efficiency

Q2. In a Morse test for a 2-cylinder, 2-stroke, spark ignition engine, the brake power was 9 kW whereas the brake powers of individual cylinders with spark cut of were 4.25 kW and 3.75 kW respectively. The mechanical efficiency of the engine is

[IES-1999](a) 90% (b) 80% (c) 45.5% (d) 52.5%



I. The performance of an S.I. engine can be improved by increasing the compression ratio.II. Fuels of higher octane number can be employed at higher compression ratio.Of these statements(a) Both I and II are true (b) both I and II are false(c) I is true but II is false (d) I is false but II is true

Q4. Besides mean effective pressure, the data needed for determining the indicated power of an engine would include [IES-1993](a) Piston diameter, length of stroke and calorific value of fuel(b) Piston diameter, specific fuel consumption and calorific value of fuel(c) Piston diameter, length of stroke and speed of rotation(d) Specific fuel consumption, speed of rotation and torque

Q5. The method of determination of indicated power of multi-cylinder SI engine is by the use of [IES-1995](a) Morse test (b) Prony brake test (c) Motorint test (d) Heat balance test.

Q6. Match List-I with List-II and select the correct answer using the codes given below the lists:List-I List-II [IES-

2001]A. Supercharging 1. Multi-cylinder engineB. Morse test 2. C.I. engineC. Heterogeneous combustion 3. Calorific valueD. Ignition quality of petrol 4. Aircraft engine

5. Octane number6. Single cylinder S.I. engine

A B C D A B C D(a) 4 1 2 5 (b) 1 3 4 2(c) 5 4 2 3 (d) 1 4 2 3

Q7. Which one of the following figures correctly represents the variation of thermal efficiency (y-axis) with mixture strength (x-axis)? [IES-1997]

IC Engine PerformancesS K Mondal’s Chapter 7Q8. For a typical automobile C.I. engine, for conditions of increasing engine

speed match List I with List II and select the correct answer using the codes given below the lists :

[IES-1993]

List I List II(Performance parameter) (Tendency, qualitatively)A. Power output 1. Increasing and then decreasing,B. Torque 2. Decreasing and then increasingC. Brake specific fuel consumption 3. Increasing throughout the

range4. Decreasing throughout the range

Codes: A B C A B C(a) 1 2 3 (b) 1 4 3(c) 2 3 4 (d) 3 1 2

Q9. Match List-I (Performance Parameter Y) with List-II (Curves labelled 1,

2, 3, 4 and 5 BHP vs. Y) regarding a C.I engine run at constant speed and select the correct answer using the codes given below the lists:

[IES-2001]List-I List-II(Performance Parameter Y) (Curves BHP vs. Y)A. Total fuel consumption rateB. Mechanical efficiencyC. Indicated powerD. Brake specific fuel consumption

A B C D A B C D(a) 5 3 4 2 (b) 1 3 4 2(c) 5 4 2 3 (d) 1 4 2 3

Q10. The curve show in the given figure is characteristic of diesel engines.

What does the Y-axis represent?

(a) Efficiency (b) Specific fuel consumption (c) Air-fuel ratio (d) Total fuel consumption

[IES-1995]

Q11. Keeping other parameters constant brake power of a diesel engine can be increased by [IES-1995]


(a) decreasing the density of intake air (b) increasing the temperature of intake air

(c) Increasing the pressure of intake air (d) decreasing the pressure of intake air.

Q12. In the context of performance evaluation of I.C. Engine, match List I

with List II and select the correct answer. [IES-1996]

List I (Parameter) List II (Equipment for measurement)A. Brake power (B.H.P.) 1. Bomb calorimeterB. Engine speed 2. Electrical tachometerC. Calorific value of fuel 3. Hydraulic dynamometerD. Exhaust emissions 4. Flame lonization detectorCode:A B C D A B C D(a) 3 1 2 4 (b) 4 2 1 3(c) 3 2 1 4 (d) 2 3 4 1

Q13. Match List I (performance curves, labelled A, B, C and D, for a constant speed diesel engine) with List II (performance parameter) and select the correct answer. [IES-1994] List I List II

1. Smoke level2. Brake specific fuel

consumption.3. Brake thermal efficiency4. Brake power

Codes: A B C D A B C D(a) 3 4 1 2 (b) 3 4 2 1(c) 4 3 1 2 (d) 4 3 2 1

Q14. Which one of the following quantities is assumed constant for an

internal combustion engine while estimating its friction power by extrapolation through Willan's line?

[IES-1994](a) Brake thermal efficiency (b) Indicated thermal efficiency.(c) Mechanical efficiency (d) Volumetric efficiency.

Q15. An engine produces 10 kW brake power while working with a brake

thermal efficiency of 30%. If the calorific value of the fuel used is 40, 000 kJ/Kg, then what is the fuel consumption?

[IES-2005](a) 1.5 kg/hour (b) 3.0 kg/hour (c) 0.3 kg/hour (d) 1.0 kg/hour

Q16. A 40 kW engine has a mechanical efficiency of 80%. If the frictional power is assumed to be constant with load, what is the approximate value of the mechanical efficiency at 50% of the rated load?

[IES-2005](a) 45% (b) 55% (c) 65% (d) 75%


ANSWER WITH EXPLANATIONPrevious Years GATE Answers

1. Ans. (c)

2. Ans. (a)3. Ans. (a)4. Ans. (a)

Previous Years IES Answers1. Ans. (c) The torque developed by an engine is directly proportional to the indicated power. Thus maximum torque will occur corresponding to maximum indicated power.2. Ans. (a) Indicated power of second cylinder is 9 - 4.25 = 4.75 kW and of first

engine is 9 - 3.75 = 5.25 kW.Thus total indicated power of engine is 4.75 + 5.25 = 10 kW.

:. Mechanical efficiency of engine = = 90%3. Ans. (d) The performance of S.I. engine can't be improved by increasing the

compression ratio because of pre-ignition and detonation.Since high octane number tends to suppress detonation, t6 some extent fuels of higher octane number will be helpful at higher compression ratio.

4. Ans. (c) Indicated power is concerned with PmLAN, i.e. mean effective pressure, length of stroke, piston diameters and speed of rotation.

IC Engine PerformancesS K Mondal’s Chapter 75. Ans. (a) Morse test is used to determine indicated power of multi-cylinder S.I. engine6. Ans. (a)7. Ans. (a) For higher thermal efficiency, the mixture strength should be little leaner than stoichiometric8. Ans. (d)9. Ans. (a)10. Ans. (d) Y-axis represents total fuel consumption.11. Ans. (c) Brake power of diesel engine can be increased by increasing pressure of intake air. Supercharging.12. Ans. (c)13. Ans. (b)14. Ans. (b)

15. Ans. (b) 16. Ans. (c)


Conventional Questions with AnswersQ1. A four stroke diesel engine of 3000 cc capacity develops 14 kW per m3

of free air induced per minute. When running at 3500 rev/min it has a volumetric efficiency of 85 per cent referred to free air-conditions of 1.013 bar and . It is proposed to boost the power of the engine by supercharging by a blower (driven mechanically from the engine) of pressure ratio 1.7 and isentropic efficiency of 80 per cent. Assuming that at the end of induction the cylinders contain a volume of charge equal to the swept volume, at the pressure and temperature of the delivery from the blower, estimate the increase in bp to be expected from the engine. Take overall mechanical efficiency as 80 per cent. r for air = 1.4, R = 0.287 kJ/kg K.

[IES-2009, -13-Marks]Ans. Swept volume,

Actual volume of air Inducted Volumetric efficiency

Delivered pressure from the compressor,

Assuming isentropic compressor, we have

Isentropic efficiency of the compressor

Actual volume of air induced corresponding to swept volume at atmospheric condition by the engine

So increase intake volume of air

27 C

SV6 35003000 10 2

35.25 m /min

a SV V v

S v

3

V5.25 0.854.4625 m /min4.4625 1462.475 kW

ap 1.013 1.7 1.7221 bar

10.4

d d 1.4a a

0.4 0.2871.4d a

T p 1.7T p

T T 1.7 300 1.7349.349 K

c

S

ac

ac

ac

Isentropic temperature riseActual temperature rise

TT

349.349 300 0.8 = T 300 T 361.68 K

5.25 1.7221 3001.013 361.68

37.403 m /min


Increase in input pressure due to supercharging= 2.153 × 14 = 30.14 kW

Increase in input pressure due to increase in intake pressure because of supercharging

Total increase in input pressure = 30.14 + 6.204= 36.344 kW

Increase in bp= Mechanical efficiency × input pressure = 0.8 × 36.344 = 29.0752 kW

Q2. The following data are known for a four cylinder four stroke petrol engine: cylinder dimensions: 11 cm bore, 13 cm stroke; engine speed : 2250 rpm; brake power: 50 kW; friction power : 15 kW; fuel consumption rate: 10.5 kg/h; calorific value of fuel: 50,000 kJ/kg; air inhalation rate: 300 kg/h; ambient condition : 15ºC, 1.03 bar. Estimate (i) brake mean effective pressure (ii) Volumetric efficiency (iii) Brake thermal efficiency, and (iv) Mechanical efficiency. [IES-2008, 10-Marks]

Ans. Given, D = Cylinder bore = 11 cm L = Length of stroke = 13 cm N = rpm of engine = 2250 B.P. = 50 kW Friction power =

Fuel consumption rate = Colorific value of fuel = CV = 50000 kJ/kg

Air inhalation rate = Ambient condition, P = 1.03 bar,

(i) Now, Swept volume,

Total swept volume for 4 cylinder = 4 × 0.001235

37.403 5.25 2.153m /min

S

5

p V60 10001.7 1 1.013 10 5.25

60 10006.204 kW

pf 15 kW

fm 10.5 kg/hr

am 300 kg/hr

ambT 15ºC 288 K

sV

2

2 6 3

3

D L411 13 10 m4

0.001235 m

3m


(ii)

volume flow rate of air at intake condition,

(iii) Brake thermal efficiency,

bm

bm

P LAnKBut b.p. = 60000where,P brake mean effective pressure

Nn = for 4- stroke engine2K = number of cylinder = 4(given)

bm

222 4

4 6 3

5 2

bp 60000 P LAnK50 60000 N/m225013 10 11 10 44 2

5.396 10 10 105.396 10 N/m5.396 bar

swept volume per minute,2

s

3

a

V D LnK422504 0.001235 2

5.558 m /min

Again, m 300 kg/hr300= kg /min 5kg /min.60

aa 5

3

v

a

m RI 5 287 288V P 1.03 104.0124 m /min

volumetric efficiency=

Vswept volume/min

4.0124 72.19%5.558


(iv) Since bp = 50 kWfp = 15 kW

ip = bp + fp = 65 kW mechanical efficiency,

mbp 50 76.92%ip 65

Student’s Notes

8. Engine CoolingTheory at a Glance (For IES, GATE & PSUs)

Necessity of Engine CoolingThe question remains as to why cooling is required at all? The cooling of the engine is necessary for the following reasons:1. The lubricating oil used determines the maximum engine temperature that can

be used. Depending upon the type of lubricating oil used, this temperature ranges from 160oC to 200oC. Above these temperatures the lubricating oil deteriorates very rapidly with temperature increase, and it might even evaporate and burn, injuring piston and cylinder surface. Piston seizure due to overheating resulting from the failure of the lubrication is quite common.

2. The strength of the materials used for various engine parts usually decreases with an increase in temperature, and thus established an upper limit for the temperatures at various points of the engine. For example, for water- cooled engines the temperature of the cylinder head should not exceed about exceed about 270oC while for air- cooled engines, which use light alloys, this limit is as low as 200oC. The high local temperatures, in addition to decreasing the strength of the materials, may result in excessive thermal stress due to uneven expansion of various engine parts and may result in cracking .

3. High engine temperatures may result in very hot exhaust valve, which in turn, may give rise to pre-ignition and detonation.

4. If the cylinder head temperature is high, the volumetric efficiency and hence the power output of the engine is reduced.

Thus, it is clear that some form of the cooling must be provided to keep the temperature of the engine low in order to avoid the loss of volumetric efficiency, and, hence, power, engine seizure and to avoid danger of engine failure.

Disadvantages of overcoolingShould we cool the engine as much as, we can? No. The engine must never be overcooled. The engine must always be kept sufficient hot to assure smooth and efficient operation. At too low engine temperatures starting may be difficult and above all, the low temperature corrosion assumes such a significant magnitude (see Fig. 15.2) that the engine life is greatly reduced. At low temperatures, the sulphurous and sulphuric acid resulting from combustion of fuel (fuel always contains some sulphure) attack the cylinder barrel. The due points of these acids vary with pressure and hence the critical temperature, at which corrosion assumes significant proportions, varies along the cylinder barrel. To avoid condensation of acid the coolant temperature should be greater than 70oC. Thus, the cooling system should not only cool but must also keep the cylinder liner temperature above a minimum level to avoid corrosion and ensure good warm up performance of the engine.

Cooling SystemsAll the heat rejected from the engine ultimately goes to air. Nevertheless, two basic systems are used to cool the engine. These are :

1. Air cooling2. Water cooling or indirect air cooling using water as a transfer medium.

Engine CoolingS K Mondal’s Chapter 8Application of air coolingAir cooling is usually used for small engines and for engines whose application gives extreme areas for importance to weight such as aircraft engine. Other areas for air-cooled engines are industrial and agricultural engines where there can be a strong objection to use of water as coolant.For air cooling the cylinder heat transfer area is increased by finning and air is passed over these fins to affect cooling.

Application of water coolingIn case of water-cooling engines the cylinder and the cylinder-heat are enclosed in a water jacket. This water jacket is connected to a radiator (heat exchange). Water is caused to flow in the jacket where it cools the engine, then it gives up this heat to air in the radiator and is again circulated in the water jacket.

Air cooling As already stated, for air cooling of the engine the heat transfer area is increased by finning the cylinder and cylinder head and air is passed over them. Since the heat is directly transferred to air, this result in a vary large temperature gradient between the gases and air for heat transfer, thereby reducing the surface area required. Fig. shows the two fins used to cover the cylinder. The length of the fins and the spacing between them is quite important for efficient cooling. Larger inter-spacing between the fins offers larger area for cooling air but the heating of the air is less, so more cooling air is required. Smaller interspacing between the fins result in smaller flow area for cooling air and hence air is heated more and input to the cooling air is less. However, the pressure drop require for flow is more. If the distance between two fins is quite small the laminar layers of the opposite areas come in contact with each other and the efficiency of the fins is reduced very much. Therefore, a spacing less than 2.5mm is not used. The height of the fins controls the distance between the two cylinders and hence, the size of the engine, Usually the fin height varies from 15 to 25mm. While fixing the fins care must be taken that the most heavily thermally stressed parts such as exhaust valve and exhaust manifold, etc. are adequately finned. Air cooling results in higher engine temperature. This necessitates the provision of bigger clearance between the various parts, especially between piston and cylinder, and between valve and the rocker arm. The clamping of the cylinder heat to cylinder requires great care to avoid distortion. The upper part of the cylinder barrel is most critical part of the air-cooled engine and usually limits the engine rating. Fig. 15.25 shows the relationship between the specific output and the cylinder barrel fin spacing for cast iron and light alloys fin. It can be seen that with light alloys higher specific outputs can be obtained.

Water cooling

Engine CoolingS K Mondal’s Chapter 8

In case of water-cooled engines the cylinder heat and the cylinder are closed in a water jacket. Water is circulated through this jacket to affect cooling. The local velocity of water as well as the bulk rate of water passed through the jacket are important. The flow path as well as the local velocity should be carefully planned to cool highly thermally-stressed areas. Local velocities greater than 3 to 4 m/sec are used to provide a reasonable heat transfer. The water passage should be designed to bring the water as close as possible to the gas side. The cast surfaces are usually machined to improve the heat transfer coefficient. Sometimes drilled coolant passages are provided to cool highly stressed parts.For ease of cold starting many types and anti-freeze solutions are added to the water. Commonly used anti-freeze materials are:

(i) Keroserne,(ii) Wood alcohol,(iii) Denatured alcohol,(iv) Glycerine,(v) Sugar solution,(vi) Calcium or magnesium chloride,(vii) Ethylene glycol and propylene glycol.

In additional some oils are also used as anti-freeze solutions form the sticky deposits on the water side of the cylinder and reduce the heat transfer coefficient drastically and may result in overheating of the prevent deposits.Various methods are used for circulating the water around the cylinder and cylinder head. Thses are:

1. Thermo-syphon cooling.2. Forced or pump cooling.3. Cooling with thermostatic regulator.4. Pressurised water cooling.5. Evaporative cooling.

Thermo-syphon cooling


The fact that the water becomes light on heating is used as the basis of this type of cooling. Fig. 15.26 show the thermo-syphon cooling arrangement. The top of the radiator is connected to the top of the water jacket by a pipe and bottom of the radiator to the bottom of the water jacket. Water travels down the radiator across which air is passed to cool it. The sir flow can take place due to vehicle motion or a fan can be provided for the purpose.The main drawback of this system is that cooling depends only on the temperature and is independent of the engine speed. The rate of the circulation is slow and insufficient. The circulation of water starts only water the starts only after the engine has become hot enough to cause thermo-syphon action. This system requires that the radiator be above the engine for gravity flow of water to engine.

Forced or pump system

In the forced or pump system a pump is used to cause positive circulation of water in the water jacket. This is shown in Fig. 15.27. Usually the pump is belt driven from the engine. The advantages of forced system are that the cooling is ensured under all conditions of operation. The main drawback of the system are:

(i) The cooling is independents of temperature. This may, under certain circumstances, result in overcooling the engine.

(ii) While moving uphill the cooling requirements is increased because more fuel is burned. However, the coolant circulation is reduced which may result in overheating the engine.

(iii) As soon as the engine is stopped the cooling also ceases. This is undesirable because cooling must continue till the temperatures are reduced to normal values.

Thermostat cooling


As already discussed, too low a cylinder barrel temperature, may result in serve corrosion damage due to condensation of acid on the barrel wall. To avoid such a situation it is customary to use a thermostat (a temperature controlling device) to stop flow of coolant below a preset cylinder barrel temperature.Most modern cooling system embody a thermostatic device which prevents the water in the engine jackets from circulating through the radiator for cooling until its temperature has reached to a value suitable for efficient engine operation.

a schematic diagram of a thermostatically controlled cooling system. Also shown is a typical car thermostat. The thermostat bypass valve is fitted between the engine and the radiator top. The thermostat valve is operated by a small element filled with wax. As the temperature increases, the wax melts and valve is opened to allow coolant to go to the radiator. Under normal operating temperatures the valve remains open. When the temperature falls, the valve is closed bypassing the water and, thus , cooling is automatically ceased. Fig. shows the two positions of a typical thermostat.

The main advantages of this system is that engine warms up quickly after starting.However, if the engine becomes very hot there is no method to increase cooling action. The only alternative is to stop the engine and let it cool. To increase the rate of cooling under such situations another arrangement, shown in Fig. is used. In this system shutters are provided on the radiator. These shutters may be closed or opened automatically by a thermostat. When shutters are opened more air is allowed to flow and, thus, more cooling can be obtained.

Pressurized water coolingThe boiling point of the coolant can be increased by increasing its pressure- for two bars, it is 121oC, for 5bars it is 153oC and for 10 bars it is 180oC. This allows a greater heat transfer to occur in the radiator due to a large temperature differential.

Usually the water pressure is kept between 1.5 bars to 2.0 bars bars. Use of pressurized water cooling requires an additional valve, called vaccum valve, to avoid formation of vaccum when the water is cooled after engine has been stopped. This is shown in Fig. 15.30.A safety valve in the form of pressure relief valve, is provided so that whenever the filler cap is opened the pressure is immediately relieved.


Evaporative coolingIn evaporative cooling, also called steam of vapour cooling, the temperature of the cooling water is allowed to reach a temperature of 100oC. This method of cooling utilizes the high latent heat of vaporization of water to obtain cooling with minimum of water. Fig.15.31 shows such a system. The cooling circuit is such that the coolant is always liquid but the steam formed is flashed off in a separate vessel. The make-up water so formed is sent back for cooling. This system is used for cooling of many types of industrial engines.

Comparison of air and water cooling systemsThe allowing is a brief comparison of air and water cooling systems.

Advantages of air cooling1. The direct transfer of heat from engine to air eliminates the use of water as a

coolant. No water jacket, radiator and water pump are required. This many mean a reduction in weight by as much as 20%. The size of the engine is also small.

2. The engine design becomes much simpler.3. The air-cooled engine is less sensitive to climatic conditions. No anti-freeze

solution is needed. Due to greater temperature difference between the cooling air and cylinder, the cooling is hot weather does not deteriorate.

4. Due to high average cylinder temperature is the air-cooled engine thermal losses are small. This results in lower specific fuel consumption.


5. The warm-up performance to the air-cooled engine is better. This results is low wear to cylinders.

6. Since the temperature difference between cooling air and cylinder is more, less amount of cooling air is required.

7. Higher mean cylinder temperature mean reduced carbon deposits on combustion chamber wall. This gives sustained engine performance.

8. An air-cooled engine can take up some degree of damage. A broken fin does not affect much while a hole in the radiator may stop a water-cooled engine.

9. The control of cooling system is much easier than a water-cooled engines.

Disadvantages of air cooling1. Due to the absence of the water passage the combustion noise is not

attenuated. Rather, the air fan is an additional source of noise.2. The volumetric efficiency of an air-cooled engine is lower due to higher cylinder

head temperatures.3. High specific output engines cannot be air-cooled due to the complex nature of

the fins that are required.

Advantages of water-cooled engines1. High specific output engines pose no problem with water cooling. The heat

transfer coefficient of water is about 350 times that of air. This results in compact design.

2. Due to the high latent heat of water the water-cooling system allows greater amount of heat from any local hot spot. This acts as a useful safety valve for overheating troubles.

3. The water-cooled engine can be installed anywhere in the vehicle.4. The volumetric efficiency of water-cooled engines is higher than that of air-

cooled engines.

Disadvantages of water-cooled engines1. The need for a radiator and the pump increase the weight and the dimensions of

the engine. Due to the presence of radiator the frontal area of the vehicle is increased resulting in greater air resistance.

2. Water-cooling system requires more maintenance. A slight leakage of the radiator may result in breakdown of the engine.

3. The engine performance becomes more sensitive requires use of anti-freeze solutions which may, sometimes, result in the deposits on the water side of the cylinder and in reduced heat transfer.

4. The warm-up performance is poor. This result in greater cylinder wear.5. The power absorbed by the pump is slightly higher than that necessary from

air-cooled engines.


OBJECTIVE QUESTIONS (IES)Previous Years IES Questions

Q1. Assertion (A): Cooling system in an IC engine must be such that there is no excessive cooling.Reason (R): Overcooling would result in increased viscosity of the lubricant due to which the overall efficiency of the engine will decrease. [IES-1998](a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Q2. Consider the following statements in respect of automobile engine with thermo-syphon cooling: [IES-2003]1. Heat transfer from gases to cylinder walls takes place by convection and radiation.2. Most of the heat transfer from radiator to atmosphere takes place by radiation.3. Most amount of heat transfer from radiator to atmosphere takes place by convection.4. Heat transfer from cylinder walls takes place by conduction and convection.Which of the above statements are correct?(a) 1, 2 and 4 (b) 1, 3 and 4 (c) 2, 3 and 4 (d) 1 and 2


ANSWER WITH EXPLANATIONPrevious Years IES Answers

1. Ans. (c) Disadvantages of overcooling: At too low engine temperatures starting may be difficult and above all, The low temperature corrosion assumes such a significant magnitude that the

engine life is greatly reduced. At low temperatures, the sulphurous and sulphuric acids resulting from

combustion of fuel (fuel always contains some sulphur) attack the cylinder barrel.

The dew points of these acids vary with pressure and hence the critical temperature, at which corrosion. Assumes significant proportions, varies along the cylinder barrel.

To avoid condensation of acids the coolant temperature should be greater than 70°C.

The cooling system should not only cool but must also keep the cylinder liner temperature above a minimum level to avoid corrosion and ensure good warm up performance of the engine

2. Ans. (c)

Student’s Notes

9. Emission & ControlTheory at a Glance (For IES, GATE & PSUs)

Emission & ControlI C engine generates undesirable emission during the combustion process. In this ,both SI and CI engines are equally responsible. The major cause for these emission are non-stoichiometric combustion, dissociation of nitrogen, and impurities in the fuel and air. The emission of concern are: unburnt hydrocarbon(HC), oxides of nitrogen(NOx), oxide of carbon(COx), oxide of sulphur(SOx), and solid carbon particulates.

Pollutions from Gasoline Engines

SI Engine Emission

CI Engine Emission

Emission & ControlS K Mondal’s Chapter 9There are four possible sources of atmospheric pollution form a petrol engine powered vehicle: the fuel tank, the carburetor, the crankcase and the exhaust pipe.

The evaporative losses are the direct losses of raw gasoline from the engine fuel system; the blowby gases are the vapours and the gases leaking into the crackcase from the combustion chamber and the pollutants from the exhaust pipe are due to incomplete combustion.

Evaporative LossesEvaporative emissions account for 15 to 25 percent of total hydrocarbon emission from a gasoline engine. The two main sources of evaporative emissions are the fuel tank and the carburetor.

(i)Fuel tank lossesFuel tank losses occur by displacement of vapour during filling of petrol tank, or by vaporization of fuel in the tank, forcing the vapour through a breather went to the atmosphere. Where the temperature is low the fuel tank breaths in air. When the temperature goes high it ‘breathes out’ air loaded with petrol vapour. Fuel tank losses occur because the tank temperature is increased during the vehicle operation which cause an increase in the vapour pressure and thermal expansion of tank vapour.

(ii) Carburetor lossesCarburetor losses result from (a) external venting of the float bowl relieving the internal pressure as the carburetor heats, and (b) ‘hot soak’ losses which occur after the engine has been stopped, as a result of evaporation of petrol stored in the bowl, loss being through vent pipe or through the air cleaner. Most of the loss from the carburetor occurs due to direct boiling of the fuel in the carburetor bowl during hot soak.

Crankcase BlowbyThe blowby is the phenomenon of the leakage pat the piston and piston rings from the cylinder to the crankcase. The blowby HC emissions are about 20 percent of the total HC emission from the engine. This is increased to about 30 percent if the rings are worn.

Exhaust EmissionTail pipe exhaust emissions are the major source of automotive emissions. Petrol consists of a mixture of various hydrocarbons and if we could get perfect combustion then the exhaust would consist only of carbon dioxide and water vapours plus air that did not enter into the combustion process. However, for several reasons combustion is incomplete and hence we also get carbon monoxide, a deadly poison-ous gas, and unburnt hydrocarbons (UBHC) in exhaust. Hydrocarbons paly an active part in the formation of smog.

In additional to CO and HC, the third main pollutant is oxides of nitrogen (NOx). The air supplied for combustion contains about 77 percent of nitrogen. At lower temperature is inert but at temperature higher than 1100oC nitrogen reacts with oxygen. During the combustion process some of the nitrogen in the fuel-air mixture due to the high temperatures in the combustion chamber, unites with oxygen to form various oxides of nitrogen. Some of the oxides of nitrogen are very toxic and harmful. The different oxides of nitrogen are referred by the chemical symbol NOx , with x standing for the varying amount of oxygen.

Emission & ControlS K Mondal’s Chapter 9

(a) Carbon monoxide (CO)Carbon monoxide occurs only in engine exhaust. It is a product of incomplete combustion due to the insufficient amount of air in the air-fuel mixture or insufficient time in the cycle for completion of combustion.

(b) HydrocarbonsUnburnt hydrocarbon emissions are the direct result of incomplete combustion. The pattern of hydrocarbon emission is closely related to many design and operating variables. Two of the important design variables are induction system design and combustion chamber design, while main operating variables are air-fuel ratio, speed, land and mode of operation. Maintenance is also an important factor.

(c) Particular matter and partial oxidation productOrganic and inorganic compounds of higher molecular weights and lead compounds resulting from the use of TEL are exhausted in the form of very small size particles of the order of 0.02 to 0.06. About 75 percent of the lead burned in the engine is exhaustion into the atmosphere in this form and rest is deposited on engine parts.

(d) Oxides of Nitrogen (NOx)Oxides of nitrogen which also occur only in the engine exhaust are a combination of nitric oxide (NO) and nitrogen dioxide (NO2). Nitrogen and oxygen react at relatively high temperature s. Therefore, high temperatures and availability of oxygen are the two main reasons for the formation of NOx.

Gasoline Engine Emission Control

Engine design modificationTable 20.2 shows the effect of design and operating variables on exhaust emissions. Table 20.3 shows the desirable design characteristics of a petrol engine for reduced exhaust emissions.

(1)Use of leaner air-fuel ratios:- The carburetor may be modified to provide relatively lean and stable air-fuel mixtures during idling and cruise operation. With this modification idle speed needs to be increased to prevent stalling and rough idle associated with leaner fuel-air ratios. Fuel distribution is improved by better manifold design, inlet air heating, raising of coolant temperature and use of electronic fuel injection system.

(2)Retarding ignition timing:- Retarding ignition timing allows increased time for fuel burning. The controls are designed to retard the spark timing at idle while providing normal spark advance during acceleration and cruising. Retarding the spark reduces NOx emission by decreasing the maximum temperatures. It also reduces HC emission by causing higher exhaust temperatures. However, retarding the ignition timing results in greater cooling requirement and three is some loss in power and fuel economy.

(3)Modification of combustion chamber configuration to reduce quench areas:- Modification of combustion chamber using attempts to avoid flame quenching zones where combustion might otherwise be incomplete and resulting in high HC emission. This includes reducing surface to volume ratio, reduced squish


area, reduced deal space around piston ring and reduced distance of the top piston ring fro the top of the piston. Fig. shows an example of a modified combustion chamber where the surface to volume ratio has been reduced.

(4)Lower compression ratio:- The lower compression ratio reduces the quenching effect by reducing the quenching area, thus reducing HC. Lower compression ratio also reduces NOx emissions due to lower maximum temperature. However, reducing the compression ratio results in some loss in power and fuel economy. But there is advantage of reduced octane number which will make it easier to phase the lead out of petrol, i.e., use of unleaded gasoline.

(5)Reduced valve overlap:- Increased valve overlap allows some mixture to escape directly and increase emission level. This can be controlled by reducing valve overlap.

(6)Alternation in induction system:- The supply of designed air-fuel ratio to all cylinders under all operating conditions can be affected by alterations in induction system which includes inlet air heating use of carburetors which have closer carburetion tolerance and special type of carburetors, e.g., high velocity carburetors or multi-choke carburetors. This also includes the fuel injection in manifold.

Exhaustion gas oxidationExhaust gas from the engine manifold is treated to reduce HC/CO emissions. A number of devices have been used. They are given below:

(a)After burner:- An addition of an after-burner to the exhaust system as shown in Fig. can completely burn the partially burned HC in the exhaust gases. After-burners have not been successful in curbing the emission due to the difficulty in sustaining the combustion during low HC emission because of high heat losses over a large area.

(b)Exhaust manifold reactor:- All these reactors work on the fact that it air is mixed into the high temperature HC they will react to complete the oxidation of the HC. This is actually development of the after-burner concept. The changes in design of the after-burner to minimize the heat loss have lead to the treatment of exhaust gases just after the manifold. And the need of providing sufficient time for oxidation and mixing through different shapes have been evolved. In an earlier type of reactor developed by Du Pont, the entry of exhaust gases was radial and the air flow peripheral.

(c)Catalytic converter:- Catalytic converter is a honeycomb structure. It consist of two separate elements one for NOx and other for HC/CO emission. Frist comes reducing bed because it require the presence of CO to convert NOx into N2and O2 and the O2 produce will be utilize in the oxydising bed to convert HC and CO into CO2 and H2O. the catalyst material most commonly used are platinum, palladium and rhodium. Palladium and platinum promots the oidation of CO and HC with platinum especially in hydrocarbon reaction. Rhodium promotesthe reaction of NOx.


Catalytic ConverterBlowby control The basic principle of all type of crankcase blowby control is recirculation of the vapours back to the in take air cleaner. There are a large number of different systems in use. In the PCV system the draft tube is eliminated and the blowby gases are reduced back in to the intake manifold or inlet of the carburetor. The blowby gases are consequently reintroduced into the combustion chamber where they are burned along with fresh incoming air and fuel. Since the blowby headling devices place the crankcase under a slight vacuum, they quickly became known as positive crankcase ventilation (PCV) systems.


OBJECTIVE QUESTIONS (GATE, IES & IAS)Previous Years GATE Questions

Q1. Global warming is caused by [GATE-2000]

(a) Ozone (b) carbon dioxide(c) Nitrogen (d) carbon monoxide

Q2. The silencer of an internal combustion engine

[GATE-1999](a) Reduces noise (b) decrease brake specific fuel consumption (BSFC)(c) Increase BSFC (d) has no effect on its efficiency

Previous Years IES QuestionsQ1. Which one of the following set of materials is most commonly used in

catalytic converters for CI engines? [IES-2008](a) Platinum, palladium and rhodium(b) Palladium, rhodium and ruthenium(c) Rhodium, ruthenium and platinum(d) Ruthenium, platinum and palladium

Q2. The three way catalytic converter cannot control which one of the

following? (a) HC emission (b) CO emission [IES-

2008](c) NOx emission (d) PM emission

Q3. Which one of the following cannot be controlled by a three-way catalytic converter?(a) HC emission (b) CO emission(c) NOx emission (d) SPM emission [IES-

2005]

Q4. Which of the following symptoms shows that the combustion is necessarily complete? [IES-2009](a) Presence of free carbon in exhaust (b) Presence of CO in exhaust(c) Presence of oxygen in exhaust (d) Presence of nitrogen in exhaust

Emission & ControlS K Mondal’s Chapter 9Q5. The graph shown in the

given figure represents the emission of a pollutant from an SI engine for different fuel/air ratios. The pollutant in question is

(a) CO (b) CO2 (c) hydrocarbons (d) NOx

[IES-1998]

Q6. With respect to I.C. engine emissions, consider the following statements:

1. Evaporative emissions have no carbon monoxide and oxides of nitrogen.

2. Blow by emissions is essentially carbon monoxide and suspended particulate matter. [IES-2001]3. Exhaust emissions contain 100% of carbon monoxide, 100% of oxides of nitrogen and around 50-55% of hydrocarbons emitted by the engine.4. There are no suspended particulates in the exhaust.Of these statements are correct(a) 1 and 4 (b) 1 and 3 (c) 2 and 3 (d) 1, 2, 3, and 4

Q7. Which one of the following automobile exhaust gas pollutants is a

major cause of photochemical smog? (a) CO (b) HC (c) NOx (d) SOx

[IES 2007]

Q8. Consider the following emissions of an IC engine: [IES-1999]

1. CO2 2. HC 3. NOx 4. Particulates

Which of these emissions cause photochemical smog?(a) 1 and 4 (b) 1 and 2 (c) 2 and 3 (d) 3 and 4

Q9. The presence of nitrogen in the products of combustion ensures that:

(a) Complete combustion of fuel takes place [IES-1997]

(b) Incomplete combustion of fuel occurs(c) Dry products of combustion are analyzed(d) Air is used for the combustion


Exhaust emissions of carbon monoxide from spark ignition engine is1. mainly fuel-air mixture strength dependent2. in the range of zero to 10%3. measured with the help of an instrument working on the principle of

non-dispersive infra-red analysis4. controlled by the use of a two way catalytic convertor


Which of the statements given above are correct?(a) 1 and 4 (b) 2 and 3 (c) 1 and 3 (d) 1, 2, 3 and 4

Q11. An engine using octane-air mixture has N2, O2, CO2, CO and H2O as constituents in the exhaust gas. Which one of the following can be concluded? (a) Supply mixture is stoichiometric [IES-

2004](b) Supply mixture has incomplete combustion(c) Supply mixture is rich(d) Supply mixture is lean

Q12. Which of the following factors are responsible formation of NO, in spark ignition engine combustion: [IES-1996]1. Incomplete combustion 2. High temperature3. Availability of oxygen.Select the correct answer.(a) 2 and 3 (b) 1 and 2 (c) 1 and 3 (d) 1, 2 and 3

Q13. Exhaust emissions vs air fuel ratio curves for a petrol engine are shown in the given figure. The curve C represents

(a) Hydro carbon (b) Carbon dioxide (c) Carbon monoxide (d) Oxides of nitrogen

[IES-2003]

Q14. The discharge of hydrocarbons from petrol automobile exhaust is minimum when the vehicle is: [IES-2005](a) Idling (b) Cruising (c) Accelerating (d) Decelerating

Q15. Consider the following statements for NOx emissions from I.C. engines:1. Formation of NOx depends upon combustion temperature

[IES-2004]2. Formation of NOx depends upon type of coolant used3. Exhaust gas recirculation is an effective means for control of NOx4. Activated Platinum is used for reduction of NOx Which of the statements given above are correct?(a) 1 and 2 (b) 1, 2 and 3 (b) 2 and 4 (d) 1 and 3

Q16. Assertion (A): Catalytic converters for reduction of oxides of nitrogen in engine exhaust cannot be used with leaded fuels. [IES-2000]Reason (R): Catalyst will be removed due to chemical corrosion by lead salts.(a) Both A and R are individually true and R is the correct explanation of A



Q17. Assertion (A): In Infrared gas analyser, the amount of absorption is the

function of concentration of the gas and the length of the absorption path. [IES-1999]Reason (R): Different gases are characterized by distinctive absorption bands within the infrared range.(a) Both A and R are individually true and R is the correct explanation of A(b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false(d) A is false but R is true

Previous Years IAS QuestionsQ1. Which of the following symptoms show that the combustion in air is

necessarily complete?(a) Absence of Oxygen in exhaust(b) Absence of Nitrogen in exhaust

[IAS-2002](c) Absence of free carbon in exhaust(d) Absence of carbon monoxide in exhaust


ANSWER WITH EXPLANATIONPrevious Years GATE Answers

1. Ans. (b) carbon di-oxide acts as thermal shield to atmosphere.2. Ans. (a)

Previous Years IES Answers1. Ans.(a)2. Ans. (d)

3-way catalytic converterPalladium CORhodium NOXPlatinum HC3-way catalytic converter consist platinum (Pt), palladium (Pd) and Rhodium (Rb). Rhodium first convert NOX into N2 and O2. Platinum convert hydrocarbon (HC) into H2O and CO2. Palladium convert CO (carbon mono-oxide) into CO2. It has a honey comb structure.

3. Ans. (d)4. Ans. (c)5. Ans. (d) Maximum temperature is produced at slightly rich air mixture and NOx emission is proportional to temperature.6. Ans. (c) Evaporative emissions account for 15 to 25 per cent of total hydrocarbon emission from a gasoline engine. The two main sources of evaporative emissions are the fuel tank and the carburettor.7. Ans. (c) In bright sunlight (i) NOx, (ii) HC, (iii) O2 intact chemically to produce powerful oxidants like ozone (O3) and peroxyacetylnitrate (PAN). It is photochemical smog.8. Ans. (c)9. Ans. (d) Nitrogen in flue gas means air is used for combustion10. Ans. (d)11. Ans. (b)12. Ans. (a) NOx is formed due to incomplete combustion and at high temperature.13. Ans. (d)

14. Ans. (b)15. Ans. (b)16. Ans. (c) Catalytic converters requires a non-leaded fuel because the lead compound, along with its scavengers, affects the performance of the catalysts.17. Ans. (a)

Previous Years IAS Answers

Emission & ControlS K Mondal’s Chapter 91. Ans. (d)

ic engines 2012 edition theory & questions

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