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Internal Combustion Engines

Lecture-5

Ujjwal K Saha, Ph.D.Department of Mechanical Engineering

Indian Institute of Technology Guwahati

Prepared underQIP-CD Cell Project

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Chem.Energy Thermal Energy Mech. Work

Losses

Energy in Fuel Heat Not wholly convertible to drive the piston

Loss to coolant, radiation and exhaust

Remainder is converted to Power (to drive the piston), and this is the indicated power.

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Transmission Loss

The brake power is always less than the indicated power because of frictional losses.

(from piston to crankshaft via the connecting rod)

Friction lossPumping loss fp

ip fp bp∴ − =

Indicated power (ip), is the power actually developed in the cylinder.

Brake power (bp), is the output power measured at the crankshaft.

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where, ip = indicated power (kW)imep = indicated mean effective pressure (kN/m2)L = length of stroke (m)

A = cross-sectional area of piston (m2)n = number of power strokes n=N/2 for four strokes, and n=N for two-strokesN = crankshaft speed (revolutions per minute)

and K = number of cylinders

Indicated power (ip) can be expressed as

( )60×1000

imep LAnKip =

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Brake power (bp) can be expressed as

( )60×1000

bmep LAnKbp =

Brake power (bp) obtained at the output shaft can also be related as

260×1000

NTbp π=

where bp = brake power (kW)N = crankshaft speed (revolutions per minute)

and T = engine torque (N-m)

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Specific Fuel Consumption: It is defined as the fuel flow rate per unit power output, and can be expressed as

Depending upon whether it is brake power or indicated power, the terms brake specific fuel consumption (bsfc), or indicated specific fuel consumption (isfc) is used. Accordingly,

fmsfc

P=

&fmbsfc

bp= fm

isfcip

=

sfc is a measure of how efficiently the fuel supplied to the engine is used to produce power. Clearly, a low value of sfc is desirable since for a given power level less fuel is consumed.

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Brake Specific Fuel Consumption vs Engine Size

bsfc generally decreases with engine size due to reduced heat losses from gas to cylinder wall.

rLrrL

volumecylinderareasurfacecylinder 12

2 ∝=ππ

• Note cylinder surface to volume ratio increases with bore diameter.

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Brake Specific Fuel Consumption vs Engine Speed

• At high speeds the bsfc increases due to increased friction losses.

• There is a minimum in the bsfcversus engine speed curve

• As compression ratio is increased, fuel consumption decreases due to greater thermal efficiency

• At lower speeds, the bsfc increases due to increased time for heat losses from the gas to the cylinder and piston wall, and thus a smaller ip

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Engine Efficiency

Efficiency is the relation between the power delivered and the power that could be obtained if the engine operates without loss of power.

Engine efficiency can be calculated two ways viz.,

Thermal efficiency and Mechanical efficiency.

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Combustion EfficiencyAs time available for combustion is very short, a

small fraction of fuel does not react and exits with the exhaust flow.

A Combustion Efficiency is defined to account for the fraction of fuel burnt, and typically has values in the range of 95 % to 98 % when an engine is operating properly.

in f f cQ m Q η=where mf = mass of fuel

= calorific value of fuelfQcη = combustion efficiency

in f f cQ m Q η∴ =

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Thermal Efficiency: It is the ratio of power produced to the energy in the fuel burned to produce this power, and can be expressed as

Depending upon whether it is brake power or indicated power, the terms brake thermal efficiency or indicated thermal efficiency is used. Accordingly, following two expressions can be used.

where = fuel mass flow rate= calorific value of fuel

f

Pmth

fQη =

f

;mbth

f

bpQ

η =fmith

f

ipQ

η =

fQfm

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Mechanical efficiency:

Mechanical efficiency usually lies between 80 to 90 %. It can also be defined as the ratio of brake thermal efficiency to indicated thermal efficiency.

mbpip

η =

It also follows that

bthm

ith

isfcbsfc

ηηη

= =

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Air-Fuel Ratio:

SI engines may have A/F ratio in the range of 12 to 18 based on the operating conditions such as starting, accelerating, cruising etc.

a

f

mAF m

=

a

f

mAF m=

CI engines, on the other hand, may have A/F ratio in the range of 18 to 70.

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Chemically Correct or Stoicheometric F/A: The mixture that contains optimum proportion of fuel air ratio.

FA

FA

Equivalence RatioActual Ratio

Stoicheometric Ratio

φ =

=

1;1;1 ;

Chemically CorrectLean MixtureRich Mixture

φφφ

=⟨⟩

:0.056 0.083:0.014 0.056

FA

FA

SI enginesCI engines

≤ ≤

≤ ≤

:12 18:18 70

AF

AF

SI enginesCI engines

≤ ≤

≤ ≤

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Volumetric Efficiency: The power output of an engine depends directly on the amount of charge that can be inducted in the cylinder.

This is often referred to as the breathing capacity of the engine, and is expressed quantitatively as volumetric efficiency.

It can be defined as the ratio of the volume of air induced to the swept volume of the cylinder, and can be expressed as

av

a d

mv

ηρ

=

av

a d

nmv N

ηρ

=

where, ma = mass of air into the engine in one cycle (kg)= mass flow rate of air into the engine (kg/s)

ρa = air density at atmospheric conditions (kg/ m3)Vd = displaced volume (m3)N = engine speed (revolutions per minute)

and n = number of revolutions per cycle

am

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Volumetric Efficiency:

Actual mass is always less than theoretical mass because of pressure losses in the ducting system and due to heat transfer (process is not adiabatic).

argargv

Actual mass of ch e inductedTheoretical mass of ch e inducted

η =

The volumetric efficiency for a normally aspirated engine is about 80 %, and this value can be increased by supercharging or turbocharging methods.

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Improving Volumetric Efficiency:

Modifying the intake passages that make it easier for the mixture to flow through as shown in Figure. Other changes include reshaping ports to smooth bends, reshaping the back of the valve heads, or polishing the inside of the ports.

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Specific Weight

Specific Volume

Engine weight wEngine power bp

= =

Indicates the relative economy with which materials are used.

dVEngine volumeEngine power bp

= =

Indicates the relative effectiveness with which engine space is utilized.

Specific Power ( ) p

Engine power bpPiston face area all pistons A

= =

Measures the effectiveness with which piston area is used regardless of cylinder size.

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For same power generation, air input in a 2-stroke engine is greater than a 4-stroke engine.

As there is a loss in the scavenging period, the term volumetric efficiency (as applied to a 4-stroke engine) is replaced by the terms delivery ratio and charging efficiency.

Two-stroke Engines:

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Two-stroke Engines:

arg /

d

a d c

i

Cylinder Volume Swept Volume VCylinder Mass V mMass of Fresh Ch e Delivered Ingested m

ρ= =

= ==

( )( . Re )

arg Re /arg

arg

t

i t

tc

Short circuitingincluding Exh siduals

Mass of Fresh Ch e tained Trapped mMass of Ch e Lost m mMass of Ch e Trapped m

−=

= −=

Delivery Ratio: idr

c

mm

λ =

Charging Efficiency: tce

c

mm

λ =

dr ceλ λ∴ ⟩ Because some mixture is lost out of exhaust port before it is closed

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Two-stroke Engines:

arg /

d

a d c

i

Cylinder Volume Swept Volume VCylinder Mass V mMass of Fresh Ch e Delivered Ingested m

ρ= =

= ==

( )( . Re )

arg Re /arg

arg

t

i t

tc

Short circuitingincluding Exh siduals

Mass of Fresh Ch e tained Trapped mMass of Ch e Lost m mMass of Ch e Trapped m

−=

= −=

Trapping Efficiency: tte

i

mm

λ =

Scavenging Efficiency: tse

tc

mm

λ =

Relative Charge: tc cerc

c se

mm

λλλ

= =

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Charging Efficiency = Delivery Ratio x Trapping Efficiency

Charging Efficiency = Relative Charge x Scavenging Efficiency

0.65 0.95drλ⟨ ⟨0.50 0.75ceλ⟨ ⟨0.65 0.80teλ⟨ ⟨0.75 0.90seλ⟨ ⟨0.60 0.90rcλ⟨ ⟨

Typical values

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Road-Load Power

• A part-load power level useful for testing car engines is the power required to drive a vehicle on a level road at a steady speed.

• The road-load power (Pr) is the engine power needed to overcome rolling resistance and the aerodynamic drag of the vehicle.

vvvDavRr SSACgMCP ⋅+= )21( 2ρ

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vvvDavRr SSACgMCP ⋅+= )21( 2ρ

where CR = coefficient of rolling resistance (0.012 - 0.015)Mv = mass of vehicleg = gravitational accelerationra = ambient air densityCD = drag coefficient (for cars: 0.3 - 0.5)Av = frontal area of the vehicleSv = vehicle speed

Road-Load Power

Modern midsize aerodynamic cars only need 5-6 kW (7-8 HP) power to cruise at 90 km/hr, hence the attraction of hybrid cars!

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Specific volume, specific weight and specific power are the important parameters for engines used in transportation vehicles such as boats, automobiles, airplanes, where keeping weight to a minimum is necessary. For land-based stationary engines, weight is insignificant.

Modern automobile engines usually have brake power per displacement in the range of 40 to 80 kW/L.

Summary

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Summary

The Honda eight-valve/cylinder V4 motorcycle engine generates about 130 kW/L, an extra example of a high-performance racing car engine.

One main reason for continued development in two-stroke engines is that they produce 40 % greater power output per unit weight.

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References

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