ICE Design & Performance Parametersteacher.buet.ac.bd/zahurul/ME401/ME401_performance.pdf · Engine...

ICE Design & Performance Parameters

Dr. M. Zahurul Haq

ProfessorDepartment of Mechanical Engineering

Bangladesh University of Engineering & Technology (BUET)Dhaka-1000, Bangladesh

[email protected]://teacher.buet.ac.bd/zahurul/

ME 401: Internal Combustion Engines

c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 1 / 21

Engine Performance Parameters

Indicator Diagrams of SIE

1 2 3 4 5 6 7 8 9 10 12

1

10

100

300

Actual cycle, rc=12, actual k , w/ HT

Actual cycle, rc= 12, actual k , w/o HT

Ideal Otto cycle, rc= 7, actual k

End of Combustion

P/P

o

V/Vclearance

Start of Combustion

Air-standard Otto cycle, k = 1.4, rc= 12

e1021

c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 2 / 21Engine Performance Parameters

Air-Standard Efficiency, as& Real Gas Efficiency, o

as is not as much like the actual efficiency of the real processes:

working fluid is not air:

intake & compression stroke: air + fuel

Power & exhaust stroke: mix of N2, O2, CO2, CO, H2O, N0 ...

k = 1.3 0.05, rather than 1.4 for air.

heat loss

combustion is not instantaneous

Real gas efficiency, o: takes into account of real gases, but ignores

the heat loss and combustion delay effects. Mixture composition during

power and exhaust stroke is a function of temperature and is varying.o/as v primarily a function of , as it changes k

v very weak function of compression ratio

v essentially independent of inlet & exhaust conditions

v 0.69, for CFR engine, = 1.0



Indicated Efficiency, i

i =Indicated Power

Heat Input Rate=

Pi_mf QLHV 100%

actual, real, but measured at the cylinder, not at flywheel

does not contain mechanical efficiency, menergy expended in pumping the gases into & out of cylinder

driving some accessories

turning the cam-shaft & crank-shaft.

rubbing the piston rings up & down against cylinder walls

i/o : primarily a function of , weak function of combustion

chamber, surface/volume ratio influencing heat loss.

i/o = 0.86: for CFR engine, = 1.13

i/as = 0.69 0.86 = 0.59



180 210 240 270 300 330 360 390 420 450 480 510 5400

2000

4000

6000

8000

10000

Actual cycle(Rakopoulos)

Actual cycle (simulated)

Pres

sure

(kP

a)

Crank Angle (deg)

Ideal Otto cycle

Iso-octane= 1.0

rc = 7.0

N = 2500

s = 330 deg CA

d = 90 deg CA

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180 210 240 270 300 330 360 390 420 450 480 510 5400

1000

2000

3000

4000

5000

Actual cycle HT

Actual cycle w/o HT

End of Combustion

Tem

pera

ture

(K)

Crank Angle (deg)

Start of Combustion

Air standard Otto cycle

rc = 12.0

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

k

Cv (

kJ/k

gK),

k =

Cp/C

v

Cv

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Effect of Compression Ratio

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Air Standard Efficiency as,Otto = 1 (

1rc

)k1



A More Realistic Cycle

a = spark ignitionb = end of combustionc = EVO

Time Loss = hatched area to the left of a-b

yz = isentropic curve through b

= Actual Cycle

Blow down loss

Heat Loss

and below curve y-b

e938

Effects of real intake & exhaust strokes are not considered; these are

normally included in mechanical efficiency.



Time Loss: Combustion is not instantaneous. It is necessary to

start the combustion before TDC and combustion will go

substantially past TDC. Usually, greatest efficiency is obtained

when the point of ignition and the point at which combustion is

completed are roughly symmetric.

Time Loss 30% of (i o)Heat Loss: heat loss during compression is negligible as

temperature is low.

Heat Loss 60% of (i o)Blow-down Loss: the difference between the ideal and actual

cycle at EVO represents unavailable work.

Blow by Loss 10% of (i o)c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 9 / 21


Pumping Losses

negligible at WOT (wide open throttle) condition.

become a substantial fraction of the total at partial throttle when

the manifold vacuum is high.

at idle, the engine produce no net work, the work produced by the

cylinder must balance the mechanical losses (pumping, accessories,

friction etc.)

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Losses Due to Unburned Fuel

because of poor mixing control, either within cylinder or from

cylinder to cylinder, some of the mixture is not burned.

some of the charge next to the cylinder walls and crevices around

the spark plug and the valves are chilled, and will not burn.

loss of energy and HC pollution.

for 5 1 : comb u 0.98, and for > 1 : comb = 1/: in fact,

combustion falls little faster than this. Example: = 1.2, only

93% of the fuel theoretically burnable is burned, then

comb = 0.93/1.2 = 0.777.



Leakage Losses

Piston rings and sometimes the valves, do not seal properly and

consequently the cylinder pressures do not rise as high as they

should, represents a loss.

For an engine in good condition: loss is negligible.

Leakage losses are less serious at higher engine speeds, as it takes

finite time for fluid at an elevated pressure to leak through an

orifice or obstruction.

Many racing engines use only one piston ring in order to reduce

the friction at higher piston speeds, the resulting leakage not

being a problem at operating speeds.



Mean Effective Pressures

Power = mep (

Ap s Ncyl)

N

X= mep Vdisp

N

X

Ap: piston area, Vdisp = Ap s Ncyl : total displacement volume,

X = 1: for 2s engine & X = 2: for 4s engine; N : engine speed.

bmep: when power is measured at flywheel.

imep: power developed within cylinder, no friction effects.

bmep = imep fmep : Pb = Pi Pf : m =Pb

Pi100%

typical WOT bmep = 0.9 1.1 MPa.



Friction Mean Effective Pressure

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e1027

tfmep(bar) = 0.97 + 0.15

(

N

1000

)

+ 0.05

(

N

1000

)2


Mean Piston Speed, V P & Specific Power

Piston Speed, V P = 2 N s : comparable among engines.

Maximum gas flow in engine is limited by the sonic velocity in the

valve aperture related to V P .

Specific Power, PbAPNcyl

= bmep V P2X .

Bore/Stroke Ratio, s/b

Pb = bmep AP Ncyl V P

X

Pb

Vdisp=

bmep

sV P

2X

for similar specific power and piston speed, bmep for 4s will be

twice the bmep for a 2s engine.

for same bmep and piston speed: s Pb/Vdisp .

Now-a-days: bmep = 1.1 MPa,V P = 15 m/s, s/b

0.92,Pb/(ApNcyl) = 0.38 kW/cm2,Pb/Vdisp 48 kW/L.



Power Equation

Pi = i comb _mfuel QLHVPb = _ma (F/A) QLHV i comb m_ma = i s Ap Ncyl N

Xv = i Vdisp

N

Xv

Pb = i comb m v i Vdisp N

X (F/A) QLHV

bmep = i comb m v {i (F/A) QLHV }

i (F/A) QLHV = bmep for i = comb = m = v = 1.0: a

value of bmep, far above anything practically attainable, as it

would correspond to efficiencies of unity.

For gasoline: i = 1.2 kg/m3,QLHV = 42.7 MJ/kg, (F/A) =

1/14.7 = i (F/A) QLHV = 3.44 MPa.



Effect of

At full power, engines operate rich, upto perhaps 1.2: i falls as

properties of the gases are not the same, comb falls as not all of

the fuel is burnt.

icomb

as= 0.921 0.327 : CFR engine

icomb/as = 0.594 at = 1.0, and = 0.528 at = 1.2: a decrease

in 11 %.

CFR engine, rc = 8 & = 1.0 as = 0.565 icomb = 0.336:

If v = 0.85 & m = 0.85 = bmep = 0.815 MPa.

For, = 1.2 bmep = 0.872 MPa: 7% more power.

If inlet air is warm and heating in the manifold: density 0.86 of

the ambient value bmep = 0.75 MPa.



Racing Engine Design

Exotic fuel: 5% nitrobenzene + 80% methanol + 15% acetone.

Two important factors:

1 High latent heat of evaporation, 1.1 MJ/kg chilling of charge.

2 Very high resistance to knock.

rc = 16 as = 0.67, = 1.2 icomb/as = 0.528 icomb =

0.354.

QLHV = 20.9 MJ/kg, (F/A) = 0.15 i (F/A) QLHV =

4.51 MPa 3.55 MPa

= bmep = 1.1 MPa. with tuning : bemp 1.4 MPa.

c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 18 / 21Engine Performance Characteristics

Performance Parameters of an SI Engine, WOT

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Engine Performance Characteristics

Performance Parameters of an CI Engine

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Engine Performance Characteristics

Performance Maps

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SI Enginee1032

CI Engine


Engine Performance ParametersEngine Performance Characteristics

ICE Design & Performance Parametersteacher.buet.ac.bd/zahurul/ME401/ME401_performance.pdf · Engine...

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