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Basic of
Engine Operating Characteristics
2103471 Internal Combustion Engine
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Background on IC Engines
An internal combustion is defined as a
heat engine in which the chemical energy
of the fuel is released inside the engine
and converted directly into mechanical
work on a rotating output shaft, as
opposed to an external combustion engine
in which a separate combustor is used toburn the fuel.
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Internal combustion engines are so called
because the heat required to drive them isreleased by oxidizing a fuel inside the engineitself!
This approach has advantages anddisadvantages" but is still the most popular fortransport and small power generation plant!
We will be looking at some common types of
engine" examining some ways of analysing theirperformance parameters" and some of the
problems encountered in improving efficiencyand output!
Background on IC Engines
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Typical Processes
for an Internal Combustion Engine
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Background on the Otto Cycle
The Otto Cycle has four basicsteps or strokes: F-A : An intake stroke that
draws a combustible mixtureof fuel and air into the cylinder
A-B : A compression strokewith the valves closed which
raises the temperature of themixture. A spark ignites themixture towards the end of thisstroke.
C-D : An expansion or powerstroke. Resulting fromcombustion.
E-F : An Exhaust stroke thepushes the burned contentsout of the cylinder.
Figure idealized representation of the
Otto cycle on a PV diagram.
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Crank shaft
90o
TC
0o
180o
BC
270o
Otto (SI Engine) Operating Cycle
Spark plug for SI engineFuel injector for CI engine
Top
Center
(TC)
Bottom
Center
(BC)
Valves
Clearance
volume
Cylinderwall
Piston
Stroke
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Pressure-Volume digram of a 4-stroke SI engine
One power stroke for every two crank shaft revolutions
1 atm
Spark
TC
Cylinder volume
BC
Pressure
Exhaust valve
opens
Intake valve
closes
Exhaust
valve
closes
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BC
L
TC
l
VC
s
a
B
as !
An average piston speed is:
LNUp !
Compression ratio:
For an engine with bore B; crank offset a, stroke
length L, turning at an engine speed of N:
Average piston speed for all engines will
normally be in the range of 5 to 15 m/sec with
large diesel engines on the low end and high-
performance automobile engines on the high
end.
Engine Geometric Parameters
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BC
L
TC
l
VC
s
a
B
!"#!!! $%&'($ alas
The cylinder volume at any crank angle is:
)*+
!
salB
VV c
Cylinder volume when piston at TC (s=l+a)
defined as the clearance volume Vc
Maximum displacement, or swept, volume:
LB
Vd
+
!
Engine Geometric Parameters
The distance s between crank axis and wrist pin
axis is given by:
Compression ratio:
c
dc
TC
BCc
V
VV
V
Vr
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BC
L
TC
l
VC
s
a
B
The combustion chamber surface area at anycrank angle is:
)* salBAAA pch
The combustion chamber surface area is:
The cross-sectional area of a cylinder and the
surface area of a flat-topped piston are given by:
+
!BA
p
For most engines B ~ L (square engine)
Engine Geometric Parameters
Cylinder volume at any crank angle can also be
written in a non-dimensional form as:
!!
$%&'($##!##
al
alr
VV
c
c
!
!
$%&'($#! a
l
a
lBSAAA pch
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BC
L
TC
l
VC
s
a
B
!"#!! $%&"
'($#$%&
!
alU
U
p
p
Average and instantaneous piston velocity are:
dt
dsU
LNU
p
p
!
Where N is the rotational speed of the crank shaft
in units revolutions per second
!"#!!! $%&'($ alas
Average piston speed for standard high performance
auto engine is about 15 m/s. Ultimately limited by
material strength.
Therefore engines with large strokes run at lower
speeds those with small strokes run at higher speeds.
Geometric Properties
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Torque is measured off the output shaft using a dynamometer.
Load cell
Force FStator
Rotor
b
N
The torque exerted by the engine is T:
NmbFT ,-&%.$
Engine Torque and Power Output
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Torque is measured off the output shaft using a dynamometer.
Load cell
Force FStator
Rotor
b
N
The torque exerted by the engine is T:
W
WattJs
rev
rev
radTNTW
)*,-&%.$)!*
JbFT ,-&%.$
The power delivered by the engine turning at a speed N andabsorbed by the dynamometer is:
Note: is the shaft angular velocity in units rad/s
Engine Torque and Power Output
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Indicated Work per Cycle
Given the cylinder pressure data over the operating cycle of the engine one
can calculate the work done by the gas on the piston.
This data is typically given as P vs V diagram.
The indicated work per cycle is given by PdVWi
Compression
W0
Intake
W>0
Exhaust
W 0
WB < 0
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Gross indicated work per cycle net work delivered to the piston over
the compression and expansion strokes only:
Wi,g=area A + area C (>0)
Pump work net work delivered to the gas over the intake and exhaust
strokes:
Wp =area B + area C (
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Indicated power:
where N crankshaft speed in rev/s
nR number of crank revolutions per cycle= 2 for 4-stroke
= 1 for 2-stroke
Power can be increased by increasing:
the engine size, Vd compression ratio, rc
engine speed, N
cyclerev
srevcyclekJ
n
NWW
R
ii
))**
Indicated Power
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Indicated Work at Part Throttle
At WOT the pressure at the intake valve is just below atmospheric pressure,
However at part throttle the pressure is much lower than atmospheric
Therefore at part throttle the pump work (area B+C) can be significant
compared to gross indicated work (area A+C)
Pint
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Indicated Work with Supercharging
Engines with superchargers or turbochargers can have intake pressures
greater than the exhaust pressure, giving a positive pump work
Wi,n = area A + area B
Supercharges increase the net indicated work but is a parasitic load
since they are driven by the crankshaft
Pint
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Mechanical Efficiency
Some of the power generated in the cylinder is used to overcome engine
friction and to pump gas into and out of the engine.
The term friction power, , is used to collectively describe these power
losses, such that:
gi
f
gi
bm
W
W
W
W
00
#
fW
WW
Friction power can be measured by motoring the engine.
The mechanical efficiency is defined as:
bgifW 0
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Mechanical efficiency depends on throttle position, engine design
and engine speed.
Typical values for car engines at WOT are:
90% @2000 RPM and 75% @ max speed.
Throttling increases pumping work and thus decreases the brake powerso the mechanical efficiency drops and approaches zero at idle.
Power varies with speed but torque is independent of engine speed
cyclecycle WTTNWWNW $(1&234'155
Mechanical Efficiency (2)
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There is a maximum in the brake power
versus engine speed called the rated
brake power(RBP).
At higher speeds brake power decreases asfriction power becomes significant compared
to the indicated power
There is a maximum in the torque versus
speed called maximum brake torque (MBT).Brake torque drops off:
at lower speeds do to heat losses
at higher speeds it becomes more difficult to
ingest a full charge of air.
cyclecycle WTWNW
fgib WWW 0
Max brake torque
1 kW = 1.341 hp
Rated brake power
Power and Torque versus Engine Speed
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Indicated Mean Effective Pressure (IMEP)
imep is a fictitious constantpressure that would produce the same
work per cycle if it acted on the piston during the power stroke.
R
pp
R
di
d
Ri
d
i
n
UAimep
n
NVimepW
NV
nW
V
Wimep
!
TWT cycle %647$(34'155imep does not depend on engine speed, just like torque
imep is a better parameter than torque to compare engines for design and
output because it is independent of engine speed, N, and engine size, Vd.
Brake mean effective pressure (bmep) is defined as:
R
d
d
R
d
b
n
VbmepT
V
nT
V
Wbmep
!
!
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The maximum bmep of good engine designs is well established:
Four stroke engines:
SI engines: 800-1000 kPa*
CI engines: 500 -900 kPa
Turbocharged SI engines: 1200 -1700 kPa
Turbocharged CI engines: 1000 - 1400 kPa
Two stroke engines:
Standard CI engines comparable bmep to four stroke
Large slow CI engines: 500 - 1600 kPa (with supercharging)
*Values are at maximum brake torque at WOT
Note, at the rated (maximum) brake power the bmep is 10 - 15% less
Can use above maximum bmep in design calculations to estimate engine
displacement required to provide a given torque or power at a specified
speed.
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Maximum BMEP
The maximum bmep is obtained at WOT at a particular engine speed
Closing the throttle decreases the bmep
For a given displacement, a higher maximum bmep means more torque
For a given torque, a higher maximum bmep means smaller engine
Higher maximum bmep means higher stresses and temperatures in the
engine hence shorter engine life, or bulkier engine.
For the same bmep 2-strokes have almost twice the power of 4-stroke
!
d
R
d
b
V
nT
V
Wbmep
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Vehicle Engine
type
Displ.
(L)
Max Power
(HP@rpm)
Max Torque
(lb-ft@rpm)
BMEP at
Max BT
(bar)
BMEP at
Rated BP
(bar)
Mazda
Protg LX
L4 1.839 122@6000 117@4000 10.8 9.9
Honda
Accord EX
L4 2.254 150@5700 152@4900 11.4 10.4
MazdaMillenia S
L4Turbo
2.255 210@5300 210@3500 15.9 15.7
BMW
328i
L6 2.793 190@5300 206@3950 12.6 11.5
Ferrari
F355 GTS
V8 3.496 375@8250 268@6000 13.1 11.6
Ferrari
456 GT
V12 5.474 436@6250 398@4500 12.4 11.4
Lamborghini
Diablo VT
V12 5.707 492@7000 427@5200 12.7 11.0
Typical 1998 Passenger Car Engine Characteristics
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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 )!
#* !
Where CR = coefficient of rolling resistance (0.012 - 0.015)
Mv = mass of vehicle
g = gravitational acceleration
a = ambient air densityCD = drag coefficient (for cars: 0.3 - 0.5)
Av = frontal area of the vehicle
Sv = vehicle speed
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Specific Fuel Consumption
For transportation vehicles fuel economy is generally given as mpg, or
L/100 km.
In engine testing the fuel consumption is measured in terms of the fuel
mass flow rate .
The specific fuel consumption, sfc, is a measure of how efficiently thefuel supplied to the engine is used to produce power,
fm
b
f
W
mbsfc
hrkW
g
W
misfc
i
f
,-&%.$
Clearly a low value for sfc is desirable since for a given power level
less fuel is consumed
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Brake Specific Fuel Consumption vs Engine Speed
At high speeds the bsfc increases due to increased friction i.e. smaller
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
Bsfc increases with compression ratio due to higher thermal efficiency
bW
iW
There is a minimum in the bsfc versus engine speed curve
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Performance Maps
Performance map is used to display the bsfc over the engines full load
and speed range. Using a dynamometer to measure the torque and fuel
mass flow rate you can calculate:
d
R
V
nTbmep
!
b
f
W
mbsfc
)!* TNWb
bmep@WOT
Constant bsfc contours from a
two-liter four cylinder SI engine
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Engine Thermodynamic Efficiencies
While bsfc is commonly used because it is a fairly direct
measurement, it is also possible to work out theengine's thermodynamic efficiency if you know the
heating value of the fuel.
Typical hydrocarbon fuel heating values are:
Fuel Heating Value(lower heating value, fuel is liquid
if that is its normal state at STP)
Methane 50 MJ/kg
LPG 46 MJ/kg
Gasoline 44.5 MJ/kg
Diesel 43 MJ/kg
Methanol 20 MJ/kg
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Engine Efficiencies
The time for combustion in the cylinder is very short so not all the fuel
may be consumed or local temperatures may no favour combustion
A small fraction of the fuel may not react and exits with the exhaust gas
The combustion efficiency is defined as:
HVf
in
HVf
inc
Qm
Q
Qm
Q
utl heat inptheoretica
t inputactual hea
Where Qin = heat added by combustion per cycle
mf= mass of fuel added to cylinder per cycle
QHV= heating value of the fuel (chemical energy per unit mass)
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Engine Efficiencies (2)
The thermal efficiency is defined as:
HVfcinth
Qm
W
Q
W
'8'54743%&7-.941.
'8'54743:(3;
HVfcinth
QmW
QW
%&7-.941.(
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Engine Efficiencies (3)
Fuel conversion efficiency is defined as:
HVfHVff
Qm
W
Qm
W
Note: f is very similar to th, difference is th takes into account actual
fuel combusted.
Recall:
Therefore, the fuel conversion efficiency can also be obtained from:
W
msfc
f
HVf
Qsfc )*
#
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Volumetric Efficiency
Due to the short cycle time and flow restrictions less than ideal amount of
air enters the cylinder.
The effectiveness of an engine to induct air into the cylinders is measuredby the volumetric efficiency:
NV
mn
V
m
da
aR
da
av
1%3.94(3=
%&2-'.421%31'.-15
where a is the density of air at atmospheric conditions Po, To and for anideal gas a =Po/ RaTo and Ra = 0.287 kJ/kg-K (at standard conditions
a= 1.181 kg/m3)
Typical values for WOT are in the range 75%-90%, and lower when the
throttle is closed.
If an engine is throttled, the volumetric efficiency will be much less than 1,
(eg 25-30%), and
If it is running at full torque, volumetric efficiency can be about 1.
Supercharged engines will have a volumetric efficiency greater than 1.
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Volumetric Efficiency
Volumetric efficiency is used in two ways.
Some engineers want to measure the tuning effectiveness of theintake manifold and valve system . They use volumetric efficiency as
their indicator. For this purpose, the "i" conditions would refer to the
density at intake manifold temperature and pressure. The idealvolumetric efficiency would be around 1 (ie 100%). Actual V would
be reduced by flow losses at the valve but could also be increased
by pulsation tuning.
The more common use of volumetric efficiency is to indicate how
much mixture is flowing through the engine, (without worryingwhether it ought to be 100%). For this purpose, the calculation is
usually done including fuel/air mixture and with the reference density
set at ambient atmospheric conditions.
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Air-Fuel Ratio
For combustion to take place the proper relative amounts of air and fuel
must be present in the cylinder.
The air-fuel ratio is defined as
f
a
f
a
m
m
m
mAF
The ideal AF is about 15:1, with combustion possible in the rangeof 6 to 19.
For a SI engine the AF is in the range of 12 to 18 depending on the
operating conditions.
For a CI engine, where the mixture is highly non-homogeneous, the
AF is in the range of 18 to 70.
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Engines Comparison
Engine performance can be compared by the following
parameters: Mean effective pressure
Brake specific fuel consumption
Engine efficiency
Volumetric efficiency
First law analysis energy conservation
Second law analysis entropy conservation
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Engines Comparison
mep= work done per unit displacement volume
Or average pressure that results in the same amountof indicated or brake work produced by the engine
Scales out effect of engine size
Two useful types: imep and bmep
imep: indicated mean effective pressure
the net work per unit displacement volume done by the
gas during compression and expansion
bmep: brake mean effective pressure
the external shaft work per unit volume done by theengine
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BMEP
Based on torque:
dV
bmep
+
!" $%&'()*
!+ $%&'()*
dVbmep
!
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Brake specific fuel consumption (bsfc)
Measure of engine efficiency
They are in fact inversely related, so a lower bsfc
means a better engine
Often used over thermal efficiency because an
accepted universal definition of thermal efficiency
does not exist
fm
bW
fmbsfc
!
Engines Comparison
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bsfc
bsfc is the fuel flow rate divided by the brake power
We can also derive the brake thermal efficiency if we givean energy to the fuel called heat of combustion or, qc
N
fm
bW
fmbsfc
!
qcbsfcqcfm
bW
#
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Volumetric Efficiency, ev The mass of fuel and air inducted into the cylinder
divided by the mass that would occupy the displacedvolume at the density i in the intake manifold
Note its a mass ratio and for a 4 stroke engine
For a direct injection engine
NV
mme
di
fav
)*!
>fm
Engines Comparison
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First law analysis- energy conservation For a system open to the transfer of enthalpy, mass,
work, and heat, the net energy crossing the control
surface is stored into or depleted from the control
volume
Second Law Analysis entropy conservation
This approach takes into account the irreversibility
that occurs in each process
Another outcome of this analysis is the developmentof the usefulness of each type of energy (exergy)
Others Engines Comparison