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ICE Design & Performance Parameters
Dr. M. Zahurul Haq
ProfessorDepartment of Mechanical Engineering
Bangladesh University of Engineering & Technology (BUET)Dhaka-1000, Bangladesh
zahurul@me.buet.ac.bdhttp://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
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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
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 3 / 21
Engine Performance Parameters
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
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 4 / 21
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Engine Performance Parameters
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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c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 5 / 21
Engine Performance Parameters
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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c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 6 / 21Engine Performance Parameters
Effect of Compression Ratio
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Air Standard Efficiency as,Otto = 1 (
1rc
)k1
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 7 / 21
Engine Performance Parameters
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
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Effects of real intake & exhaust strokes are not considered; these are
normally included in mechanical efficiency.
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 8 / 21
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Engine Performance Parameters
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
Engine Performance Parameters
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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c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 10 / 21Engine Performance Parameters
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.
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 11 / 21
Engine Performance Parameters
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.
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 12 / 21
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Engine Performance Parameters
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.
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 13 / 21
Engine Performance Parameters
Friction Mean Effective Pressure
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tfmep(bar) = 0.97 + 0.15
(
N
1000
)
+ 0.05
(
N
1000
)2
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 14 / 21Engine Performance Parameters
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.
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 15 / 21
Engine Performance Parameters
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.
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 16 / 21
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Engine Performance Parameters
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.
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 17 / 21
Engine Performance Parameters
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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c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 19 / 21
Engine Performance Characteristics
Performance Parameters of an CI Engine
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c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 20 / 21
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Engine Performance Characteristics
Performance Maps
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SI Enginee1032
CI Engine
c Dr. M. Zahurul Haq (BUET) ICE Design & Performance Parameters ME 401 (2011) 21 / 21
Engine Performance ParametersEngine Performance Characteristics
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