unit 4+part+one

8/6/2019 UNIT 4+Part+One

1/14

UNIT-IV

I.C.ENGINES

IntroductionAn engine is machine that converts heat energy into mechanical force or motion or action.

There are different types of engines are used in practice such as heat engine, steam engine etc.

A heat engine which converts thermal energy into mechanical work is called heat engine. In heat

engine, chemical energy of the fuel (petrol, diesel etc) is first converted into the thermal energy by

means of combustion of air with fuel. Then thermal energy is converted into useful work by engine

mechanism.

Types of H ea t Engin e 1. Internal Combustion Engines (I.C. Engines)

2. External Combustion Engines (E.C. Engines)

1. Internal Combustion Engines (I.C. Engines): In this combustion of fuel-air mixture takes place inside

the working cylinder. Example: Buses, Cars, Mopeds etc.

2. External Combustion Engines (E.C. Engines): In this combustion of fuel-air mixture takes place outside

the working cylinder. Example: Steam engines, steam turbines, etc.

Class ific a tion of I.C. Engin es I.C. Engines are classified based upon

1. Number of Strokes: a) 4- stroke and b) 2-stroke

2. Fuel used: a) Petrol (or) Gasoline Engine b) Diesel Engine

c) Gas Engine d) Dual Fuel Engine

3. Working Cycle: a) Otto Cycle (Petrol Engine) b) Diesel Cycle

c) Dual Combustion Cycle

4. Fuel Supply: a) Carbureted type and b) Injection type

5. Method of Ignition: a) Spark Ignition Engine (S.I. Engine)

b) Compression Ignition Engine (C.I. Engine)

6. Method of Cooling: a) Air Cooled and b) Water Cooled

7. Number of Cylinders: a) Single Cylinder and b) Multi Cylinder


2/14

8 . Speed: a) Low Speed Engine b) Medium Speed Engine

c) High Speed Engine

9. Valve Arrangement: a) Over Head Valve Arrangement

b) Side Valve Arrangement

10. Arrangement of the Cylinder:

a) Vertical Engines

b) Horizontal Engines

c) Radial Engines

d) V-type Multi cylinder Engines

e) In- line Engines

f) Opposite Cylinder Engines

g) Opposite piston Engines11. Application:

a) Stationary Engines

b) Marine Engines

c) Locomotive Engines

d) Automotive Engines

e) Air Craft Engines

12. Lubrication:

a) Wet Sump Lubrication

b) Dry Sump

I. C. Engin e Te rminolog y 1. Bore: The inside diameter of the engine is known as bore.

2. Stroke: It is the linear distance travelled by the piston in the cylinder between the extreme upper andlower portions of the piston (TDC & BDC).

3. Top Dead Center (TDC) : It is the extreme portion of the cylinder on the top.

4. Bottom Dead Center (BDC) : It is the extreme portion of the cylinder on the bottom.

Note that in the vertical cylinder TDC & BDC are used. In the horizontal cylinder, the inner dead center(IDC) and outer dead center (ODC) are used.


3/14

5. Compression Ratio: It is the ratio of the volume when the piston is at BDC to the volume, when the

piston is at TDC.

Or Cr = Maximum Cylinder Volume / Minimum Cylinder Volume

6. Cylinder Volume (V): It is the sum of swept volume & the clearance volume.

V= Vs + Vc

7. Swept Volume (Vs): It is the volume of space generated by the movement of piston from one dead

center to another dead center.

8 . Clearance Volume (Vc): It is the space in the cylinder, when the piston is at TDC.

I. C. Engin e Pa rt s 1. Engine Cylinder: Cylinder is round sleeve in which piston reciprocates forward and backward.Combustion of air and fuel mixture takes place. The cylinder is closed by cylinder head.

2. Piston: The piston is connected to a mechanism which slides within the cylinder. It has three groovesto accommodate piston rings. A piston is a component of reciprocating engines. It is located in a cylinder and is made gas-tight by piston rings . In an engine, its purpose is to transfer force from expanding gas inthe cylinder to the crankshaft via a piston rod and/or connecting rod .


4/14

3. Piston rings: The piston rings are fitted into the grooves provided around the piston. These ringsprovide gas tight seal between piston and the cylinder wall to prevent leakage of fuel or gases of combustion.

4. Piston pin: Or Gudgeon pin connects the piston to the upper end of the connecting rod.

5. Valves: There are two types: (a) Inlet valve- which admits the fresh charge into the cylinder. (b)Outlet valve (or Exhaust valve): to send exhaust gases outside the cylinder.

6. Water Jacket: It is used to cool the engine.

7. Fly wheel: It is a big wheel attached on the crank shaft. It maintains the speed of the engine.

8 . Connecting rod: It is attached to the piston pin. It converts the up & down motion (reciprocatingmotion) of the piston to a rotary motion of the crank shaft.

9. Crank shaft: It is the device used for getting power from the motion of the piston and connecting rod

and this power is applied to the flywheel.

10. Cam shaft: It operates opening and closing of the engine valves. It has number of cams which aredriven by crank shaft through timing gears. The function of the cam is to convert the rotary motion intoreciprocatory motion.

11. Crank case: It is the bottom portion of IC engine which holds the cylinder. It also serves as pump forthe lubricating oil.

F our Strok e Cycle Engin es In this section, four stroke petrol and diesel engines working principle is discussed. In this there

will be 4 strokes namely Suction, Compression, Expansion/Working/Power and Exhaust Strokes. Thistype of engines consists of one power stroke in every four stroke or during two revolutions of the crank.

a) F our Strok e Cycl e Pe trol Engin e: Petrol engine is also called as Spark Ignition Engine (S.I) . It is also known as Otto Cycle . It requiresfour strokes of the piston to complete one cycle of operation.

P rinciple:

Petrol engine works on the principle of Otto Cycle (constant volume cycle) which was developed by aGerman Scientist by Nikolaus Otto in 1 8 76. Petrol from the fuel tank goes to the fuel filter where it isfiltered and then it goes to the carburetion process in the carburetor.

Carburetor is device which atomises and vaporizes the fuel and mixes with the air in the varyingproportions to suit the changing operating conditions of the engine. The process of breaking and mixingup of fuel with the air is called carburetion.


5/14

In the carburetor, the petrol evaporates and mixes with the air. Piston inside the cylinder slides up anddown to create the vacuum. Since the pressure is more inside the cylinder compared to the atmosphere,sucks the petrol air mixture and compresses it. An electric spark is produced from the spark plug ignitesthe mixture. The hot combustion products push the piston down. The crank shaft connected with thepiston converts the up and down motion into rotary motion. The burnt gases are sent out through

exhaust valves. This is known as Otto cycle.

D etailed D iscussions on different strokes:

1. S uction S troke: During the suction stroke, the inlet valve (I) opens, air and fuel mixture is suckedinto the cylinder. In this fuel is petrol and air- fuel mixture is called is C harge . The piston movesdownwards from TDC to BDC. During this stroke exhaust valve (E) is closed.

2. Compression S troke: During this stroke, both the inlet and exhaust valves are closed. The chargeis compressed as the piston moves upwards from BDC to TDC. As a result of compression,pressure and temperature of the charge are increased. Now the charge is ignited using the sparkplug. Thus increases the pressure and temperature of the products of the combustion, butvolume remains constant. These two strokes complete one revolution of the crank shaft.

3. Expansion (or) Working S troke: During this stroke, both the valves are closed. Due to the rise inpressure, the piston is pushed down with a great force. The hot burnt gases expand pushing thepiston from TDC to BDC. It is also called as Working Stroke as work is done by the expansion of hot gases.

4. Exhaust S troke: During this stroke, the exhaust valves opens as piston moves from BDC to TDC.This movement of the piston pushes out the hot gases from the cylinder. The exhaust gases areexhausted through the exhaust valves into the atmosphere. This completes the cycle. Again theinlet valve opens and the same operations are repeated.


6/14

b ) F our Strok e Cycl e Diese l Engin e: Diesel engine is also known as Compression Ignition engine since the ignition takes place due

to the high temperature produced during the compression of the air in the engine cylinder. Fuel is thediesel which cannot be vapourized, is injected into the cylinder in the form of fine spray with the help of fuel pump and the injector. The mixture ignites spontaneously and here no spark plug is required for


7/14

igniting the air fuel mixture. The temperature and pressure increases and this makes the piston tomove. The up and down motion of piston is converted into rotary motion by the crankshaft.

The working is similar to that of a petrol engine and the only difference is a fuel injector is presentinstead of a spark plug. In a four stroke cycle diesel engine the four strokes are completed in two

revolutions for one cycle. This is known as Diesel Cycle .

D etailed D iscussions on different strokes:

1. S uction S troke: During suction stroke, the inlet valve opens and the exhaust valve remainsclosed. The piston travels downwards from TDC. Air is drawn in, from outside to enter in thecylinder through the inlet valves till the piston reaches BDC. The air is taken inside at theatmospheric pressure.

2. Compression: At the end of the suction stroke, both the inlet and the exhaust valves remainclosed. The piston moves upwards from BDC to TDC. The air is sucked in during suction strokeand is compressed to a high pressure and temperature with a decrease in volume.


8/14

3. Expansion (or) P ower stroke: Just before beginning of this stroke, fuel (diesel) is injected in theform of fine spray into the cylinder through the nozzle known as fuel injection valve . At thismoment, the fuel is ignited by the hot compressed air as compression ratio of diesel engine ishigh (16 to 20) and it starts burning at constant pressure. The fuel is continuously injected for20% of the expansion stroke. The ignited mixture of air and fuel (diesel) expands and forces the

piston downwards from TDC to BDC. During the expansion stroke both the valves remain closed. 4. Exhaust stroke: During the exhaust stroke, the inlet valve is closed and the exhaust valve is

opened. The piston is on its upstroke from BDC to TDC forcing the burnt gases out of thecylinder through the exhaust valve. This completes the cycle and the engine cylinder is ready tosuck the fresh air once again.

Tw o Strok e Cycle Engin es In a two stroke cycle engine, one cycle is completed in two strokes of the piston in one

revolution of the crank shaft. It has only ports at the cylinder walls and has no valves.

a) Tw o Strok e Cycle Pe trol Engin e: S cavenging: In the two stroke petrol engine the exhaust gases are removed from the cylinder with

the help of fresh compressed charge. This process of removing exhaust gases is called scavenging. Aspecific shape is given to the piston as shown, called deflector, which helps to prevent the loss of incoming charge and helps for exhausting the hot gases effectively.

In the case of single cylinder engines used in scooters and motor cycles, three ports are providednamely, inlet, transfer and exhaust ports. Through the inlet port fresh charge from the carburetor istaken into the cylinder crank case. Through the transfer port, the fresh charge from the bottom of thepiston is supplied to the cylinder. Through the exhaust port, the hot gases are pushed out. The twostrokes consist of one upstroke (BDC to TDC) and one downward stroke (TDC to BDC) for every powerstroke.

The two strokes are:

1. First stroke (or) Upstroke (Compression, Suction & Ignition)2. Second stroke (or) Down stroke ( Expansion & Exhaust)

In this engine there is no separate stroke for the suction and exhaust strokes, so an alternate method isused to scavenge the cylinder. The downward motion of the piston is used to pressurize fresh charge inthe crank case through the transfer port in the cylinder through the transfer port in the cylinder wall.

Two stroke cycle petrol engines are small and light for their power output and mechanically very simple.The efficiency of this engine is very low and it is more polluting than 4 stroke engine. As regards topower cubic centimeter, a two stroke engine produces more power than an equivalent four strokeengine due to the advantage of having one power stroke for every 360 degree of crank shaft rotationwhere as 4 strokes engine requires 720 degree of crank shaft rotation to produce one power stroke.

FIRS T S TROKE:


9/14

Initially piston is at its BDC. At this stage air fuel mixture has already filled in the cylinder. When thepiston moves from BDC to TDC, the transfer port and exhaust port are closed. Due to the movement of the piston towards TDC, the mixture or charge in the cylinder is compressed. At the same time theupward movement of the piston creates a partial vacuum in the crank case and a fresh charge is drawninto the crank case through the inlet port which is in open condition.

S ECOND S TROKE:

An electric spark is produced by the spark plug just before the completion of compression stroke overthe compressed charge. Thus the charge is ignited in the combustion chamber. Due to the combustionof air petrol mixture the pressure and temperature of hot gas produced increases and this increasedpressure moves the piston downwards and the work is done by the hot gas. During the downward

motion of the piston the inlet port is covered by the piston and the charge already passed to the crankcase is now compressed. During this process, the exhaust port is opened first and then the transfer portwill be opened. When the exhaust port is opened, the expanded gas escapes through the port. Thecompressed air- petrol mixture passes through the transfer port and fills the upper portion of thecylinder due to the deflector on the top of the piston. Simultaneously, the burnt exhaust gas is forcedout through the exhaust port that remains open.


10/14

Note: Deflector deflects the fresh charge to the top of the cylinder expelling the burnt gases through theexhaust port. Then the piston is pushed down wards and the cycle is repeated.

b ) Tw o Strok e Cycle Diese l Engin e: The two stroke cycle diesel engine also uses one upstroke and one down stroke for every one powerstroke. This also does not contain suction and exhaust stroke. So an alternate method is used toscavenge the cylinder. The downward motion of the piston is used to pressurize fresh air in the crankcase, which is then blown to the cylinder through transfer port in the cylinder wall.

Two stroke cycle diesel engines are bigger and heavier for their power output compared to petrolengines. They are mechanically simple compared to 4 stroke engines. The efficiency of this engine is verylow.

Working P rinciple:

FIRS T STROKE: When the piston moves from BDC to TDC the transfer port is closed and exhaust port isclosed. During the movement of piston from BDC to TDC, the pressure in the crank case decreases as thevacuum is created so that fresh air is drawn into the crank case through the inlet port.

The diesel fuel is forced under pressure in the form of fine spray to the engine cylinder through the fuelinjector nozzle into the hot compressed air just before the end of compression. At this moment, the


11/14

temperature of the compressed air is high enough to ignite the fuel. The temperature and pressure of the products of combustion are increased suddenly. The pressure of the hot gases produced moves thepiston downwards and the work is done by the hot gases. When the exhaust port is opened due todownward motion of the piston, the expanded gas starts to escape through the exhaust port.

S ECOND S TROKE: when the piston moves down from TDC to BDC during the power stroke, it initiallyopens the exhaust port. The cylinder pressure drops as the expanded gases of the combustion come outfrom the cylinder. Next to the opening of exhaust port, the piston opens the transfer port and airpartially compressed in the crank case enter the engine cylinder from the crank case. This air moves upto the top of the cylinder due to the deflector on the top of the piston. The entry of fresh air pushes outthe remaining exhaust gases through the exhaust port at the same time fuel is injected and this cycle isrepeated.


12/14

COMPA RS ION BETWEEN FOUR S TROKEA ND TWO S TROKE CYCLE ENGINE

S L.NO FOUR S TROKE TWO S TROKE

1

The cycle is completed in two

revolutions of the crankshaft,thus 4 strokes of the piston

The cycle is completed in one revolutionof the crank shaft

2One power stroke is obtainedfor every two revolutions of the crank shaft

One power stroke is obtained for everyone revolutions of the crank shaft

3

For a given size of the enginepower is produced is less sinceonly one power stroke isobtained for two revolutions of the crank shaft

For a given size of the engine power isproduced is more since only one powerstroke is obtained for one revolution of the crank shaft

4

Heavy flywheel is needed asthe turning moment is not souniform because of only oneworking stroke for tworevolutions of crank shaft

Light flywheel is needed as the turning

moment is more uniform because of only one working stroke for eachrevolutions of crank shaft

5It contains valves and valvemechanisms

It has ports instead of valves

6 Thermal efficiency is high Thermal efficiency is low

7 Less rate of wear and tear Higher rate of wear and tear

8 Engine is heavy and bulky forthe same power developed

Engine is light and compact for thesame power developed

9Less cooling and lubricationrequirement

Higher cooling and lubricationrequirement

10Four stroke engines are usedwhere efficiency is of greatimportance

Two stroke engines are used where lowcost and light weight are of primeconsideration.


13/14

COMPA RS ION BETWEEN P ETROL ENGINEA ND D IES EL ENGINE

S L.NO PA RTICULA RS P ETROL ENGINE D IES EL ENGINE

1 Fuel Ignition By spark plug (S.I. Engine)By compressed air (C.I.Engine)

2Charge duringsuction stroke

Air + Fuel mixtureAir alone is admitted andfuel is injected

3Compressionratio

Low (6 to 8 ) High (16 to 20)

4 Fuel Admission Through carburetor Through fuel injector

5Cycle of operation

Otto Cycle (ConstantVolume Cycle)

Diesel Cycle (Constantpressure cycle)

6 Engine SpeedHigh speed about 3000rpm

Low speed 1500 rpm

7Engine starting incold weather

Easy Difficult

8 Engine cost Less More

9Fuel

ConsumptionLess More

10 Fuel Cost More Less

11 MaintenanceRequires change of sparkplug after few thousandskilometer

Fuel injection does notrequire frequentmaintenance

12 Weight Less More

13 Uses Automobiles, airplanesBuses, Tractors, Trucksetc.

14 Vibration andNoise

Almost nil More

15 Vehicle Chassis

Due to less weight of engine and smoothworking vehicle chassis isnot made very strong

Due to heavy weight of engine and morevibration, vehicle chassisis made very strong


14/14

unit 4+part+one

Documents