Download - Basics of engine operation
-
7/29/2019 Basics of engine operation
1/47
Basic ofEngine Operating Characteristics
2103471 Internal Combustion Engine
-
7/29/2019 Basics of engine operation
2/47
Background on IC Engines
An internal combustion is defined as a
heat engine in which the chemical energyof the fuel is released inside the engineand converted directly into mechanicalwork on a rotating output shaft, asopposed to an external combustion engine
in which a separate combustor is used toburn the fuel.
-
7/29/2019 Basics of engine operation
3/47
Internal combustion engines are so calledbecause the heat required to drive them isreleased by oxidizing a fuel inside the engine
itself.
This approach has advantages anddisadvantages, but is still the most popular for
transport 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 efficiency
and output.
Background on IC Engines
-
7/29/2019 Basics of engine operation
4/47
Internal combustion engines include systemswhich function like "closed" systems (e.g. petrolengines) or as "open" systems (e.g. gas turbines).
All the engines we will examine contain thesame basic activities:
invest some work to compress a working fluid, inject heat into the fluid, recover a greater amount of work,
return to initial conditions by removal of someheat.
Background on IC Engines
-
7/29/2019 Basics of engine operation
5/47
Typical Processes
for an Internal Combustion Engine
-
7/29/2019 Basics of engine operation
6/47
Background on the Otto Cycle The Otto Cycle has four basic
steps 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 whichraises the temperature of themixture. A spark ignites the
mixture 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.
-
7/29/2019 Basics of engine operation
7/47
Crank shaft
90o
TC
0o
180o
BC
270o
Otto (SI Engine) Operating Cycle
Spark plug for SI engineFuel injector for CI engine
TopCenter(TC)
BottomCenter(BC)
Valves
Clearancevolume
Cylinderwall
Piston
Stroke
-
7/29/2019 Basics of engine operation
8/47
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 valveopens
Intake valvecloses
Exhaustvalvecloses
-
7/29/2019 Basics of engine operation
9/47
BC
L
TC
l
VC
s
a
B
as 2=
An average piston speed is:
LNUp 2=
Compression ratio:
For an engine with bore B; crank offset a, strokelength L, turning at an engine speed of N:
Average piston speed for all engines willnormally be in the range of 5 to 15 m/sec withlarge diesel engines on the low end and high-
performance automobile engines on the highend.
Engine Geometric Parameters
-
7/29/2019 Basics of engine operation
10/47
BC
L
TC
l
VC
s
a
B
( ) 2/1222 sincos alas +=
The cylinder volume at any crank angle is:
)(4
2
sal
B
VV c ++=
Cylinder volume when piston at TC (s=l+a)defined as the clearance volume Vc
Maximum displacement, or swept, volume:
LB
Vd
4
2=
Engine Geometric Parameters
The distance sbetween crank axis and wrist pinaxis is given by:
Compression ratio:
c
dc
TC
BCc
V
VV
V
Vr
+==
-
7/29/2019 Basics of engine operation
11/47
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 thesurface area of a flat-topped piston are given by:
4
2B
Ap
=
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:
( )
++= 22
sincos112
11
a
l
a
lr
V
Vc
c
+
++= 2
2
sincos12 a
l
a
lBSAAA pch
-
7/29/2019 Basics of engine operation
12/47
BC
L
TC
l
VC
s
a
B
( )( )
+=
2/122 sin/
cos1sin
2
alU
U
p
p
Average and instantaneous piston velocity are:
dt
dsU
LNU
p
p
=
= 2
Where N is the rotational speed of the crank shaftin units revolutions per second
( ) 2/1222 sincos alas +=
Average piston speed for standard high performanceauto engine is about 15 m/s. Ultimately limited bymaterial strength.
Therefore engines with large strokes run at lowerspeeds those with small strokes run at higher speeds.
Geometric Properties
-
7/29/2019 Basics of engine operation
13/47
-
7/29/2019 Basics of engine operation
14/47
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:
JNmbFT == :units
Engine Torque and Power Output
-
7/29/2019 Basics of engine operation
15/47
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&
WattJ
s
rev
rev
radTNTW =
== )(:units)2( &
JbFT :units=
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
-
7/29/2019 Basics of engine operation
16/47
Torque is a measure of an engines ability to do workand power isthe rate at which work is done
The term brake power, , is used to specify that the power ismeasured at the output shaft, this is the usable power delivered by theengine to the load.
The brake power is less than the power generated by the gas in thecylinders due to mechanical friction and parasitic loads (oil pump, airconditioner compressor, etc
The power produced in the cylinder is termed the indicated power, .
bW&
iW&
Engine Torque and Power Output
-
7/29/2019 Basics of engine operation
17/47
Indicated Work per Cycle
Given the cylinder pressure data over the operating cycle of the engine onecan 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
CompressionW0
IntakeW>0
ExhaustW 0
WB < 0
-
7/29/2019 Basics of engine operation
18/47
Gross indicated work per cycle net work delivered to the piston overthe compression and expansion strokes only:
Wi,g=area A + area C (>0)
Pump work net work delivered to the gas over the intake and exhauststrokes:
Wp=area B + area C (
-
7/29/2019 Basics of engine operation
19/47
Indicated power:
where N crankshaft speed in rev/snR number of crank revolutions per cycle
= 2 for 4-stroke= 1 for 2-stroke
Power can be increased by increasing: the engine size, V
d compression ratio, rc engine speed, N
cyclerev
srevcyclekJ
n
NWW
R
ii
))((=&
Indicated Power
-
7/29/2019 Basics of engine operation
20/47
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 significantcompared to gross indicated work (area A+C)
Pint
-
7/29/2019 Basics of engine operation
21/47
Indicated Work with Supercharging
Engines with superchargers or turbochargers can have intake pressuresgreater 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
-
7/29/2019 Basics of engine operation
22/47
Mechanical Efficiency
Some of the power generated in the cylinder is used to overcome enginefriction 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
,,
1&
&
&
&
==
fW&
WW
Friction power can be measured by motoring the engine.
The mechanical efficiency is defined as:
bgifW &&& = ,
-
7/29/2019 Basics of engine operation
23/47
Mechanical efficiency depends on throttle position, engine designand 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 soandrecall &&
Mechanical Efficiency (2)
-
7/29/2019 Basics of engine operation
24/47
There is a maximum in the brake powerversus engine speed called the ratedbrake power (RBP).
At higher speeds brake power decreases as
friction power becomes significant comparedto 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&&& = ,
Max brake torque
1 kW = 1.341 hp
Rated brake power
Power and Torque versus Engine Speed
-
7/29/2019 Basics of engine operation
25/47
Indicated Mean Effective Pressure (IMEP)
imepis a fictitious constantpressure that would produce the samework per cycle if it acted on the piston during the power stroke.
R
pp
R
d
id
Ri
d
i
n
UAimep
n
NVimep
WNV
nW
V
W
imep
=
=
== 2&
&
TWT cycle imepsorecall
imepdoes not depend on engine speed, just like torque
imepis a better parameter than torque to compare engines for design andoutput 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
Vbmep
TV
nT
V
W
bmep
=
==
2
2
-
7/29/2019 Basics of engine operation
26/47
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 kPaTurbocharged 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 enginedisplacement required to provide a given torque or power at a specified
speed.
-
7/29/2019 Basics of engine operation
27/47
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
2d
R
d
b
VnT
VWbmep ==
-
7/29/2019 Basics of engine operation
28/47
Vehicle Enginetype
Displ.(L)
Max Power(HP@rpm)
Max Torque(lb-ft@rpm)
BMEP atMax BT
(bar)
BMEP atRated 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
Mazda
Millenia S
L4
Turbo
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
Ferrari456 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
-
7/29/2019 Basics of engine operation
29/47
Road-Load Power
A part-load power level useful for testing car engines is the power requiredto 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( 2
Where CR = coefficient of rolling resistance (0.012 - 0.015)Mv = mass of vehicleg = gravitational acceleration
a = ambient air densityCD = drag coefficient (for cars: 0.3 - 0.5)Av = frontal area of the vehicleSv = vehicle speed
-
7/29/2019 Basics of engine operation
30/47
Specific Fuel Consumption
For transportation vehicles fuel economy is generally given as mpg, orL/100 km.
In engine testing the fuel consumption is measured in terms of the fuelmass flow rate .
The specific fuel consumption, sfc, is a measure of how efficiently the
fuel supplied to the engine is used to produce power,
fm&
b
f
W
mbsfc
&
&=
hrkW
g
W
misfc
i
f
= :units
&
&
Clearly a low value for sfc is desirable since for a given power levelless fuel is consumed
-
7/29/2019 Basics of engine operation
31/47
Brake Specific Fuel Consumption vs Engine Size
Bsfc decreases with engine size due to reduced heat losses from gas tocylinder wall.
rLr
rL
volumecylinder
areasurfacecylinder 12
2 =
Note cylinder surface to volume ratio increases with bore diameter.
-
7/29/2019 Basics of engine operation
32/47
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 heatlosses 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
-
7/29/2019 Basics of engine operation
33/47
Performance Maps
Performance map is used to display the bsfc over the engines full loadand speed range. Using a dynamometer to measure the torque and fuelmass flow rate you can calculate:
d
R
V
nTbmep
=
2
b
f
W
m
bsfc&
&
=)2( TNWb = &
bmep@WOT
Constant bsfc contours from atwo-liter four cylinder SI engine
Engine Thermodynamic Efficiencies
-
7/29/2019 Basics of engine operation
34/47
Engine Thermodynamic Efficiencies
While bsfc is commonly used because it is a fairly directmeasurement, it is also possible to work out theengine's thermodynamic efficiency if you know theheating value of the fuel.
Typical hydrocarbon fuel heating values are:
Fuel Heating Value
(lower heating value, fuel is liquidif that is its normal state at STP)
Methane 50 MJ/kgLPG 46 MJ/kgGasoline 44.5 MJ/kgDiesel 43 MJ/kgMethanol 20 MJ/kg
Engine Efficiencies
-
7/29/2019 Basics of engine operation
35/47
Engine Efficiencies
The time for combustion in the cylinder is very short so not all the fuelmay 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
in
c Qm
Q
Qm
Q
utl heat inptheoretica
t inputactual hea
=
==
&
&
Where Qin= heat added by combustion per cyclemf= mass of fuel added to cylinder per cycle
QHV= heating value of the fuel (chemical energy per unit mass)
Engine Efficiencies (2)
-
7/29/2019 Basics of engine operation
36/47
Engine Efficiencies (2)
The thermal efficiency is defined as:
HVfcinth
Qm
W
Q
W
===
cycleperinputheat
cycleperwork
HVfcin
thQm
W
Q
W
===
&
&
&
&
inputheatofrate
outpower
or in terms of rates
Thermal efficiencies can be given in terms of brake or indicated values
Indicated thermal efficiencies are typically 50% to 60% and brake thermalefficiencies are usually about 30%
Engine Efficiencies (3)
-
7/29/2019 Basics of engine operation
37/47
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 actualfuel combusted.
Recall:
Therefore, the fuel conversion efficiency can also be obtained from:
Wmsfc f&
&
=
HVf
Qsfc =)(
1
Volumetric Efficiency
-
7/29/2019 Basics of engine operation
38/47
o u et c c e cy
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
=
==
&airtheor.
inductedairactual
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 conditionsa= 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.
Volumetric Efficiency
-
7/29/2019 Basics of engine operation
39/47
y
Volumetric efficiency is used in two ways. Some engineers want to measure the tuning effectiveness of the
intake manifold and valve system . They use volumetric efficiency as
their indicator. For this purpose, the "i" conditions would refer to thedensity 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.
Air-Fuel Ratio
-
7/29/2019 Basics of engine operation
40/47
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
m
AF&
&
==
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 theoperating conditions.
For a CI engine, where the mixture is highly non-homogeneous, theAF is in the range of 18 to 70.
-
7/29/2019 Basics of engine operation
41/47
Engines Comparison
-
7/29/2019 Basics of engine operation
42/47
Engines Comparison
mep= work done per unit displacement volume Or average pressure that results in the same amount
of 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 thegas during compression and expansion
bmep: brake mean effective pressure
the external shaft work per unit volume done by theengine
BMEP
-
7/29/2019 Basics of engine operation
43/47
BMEP
Based on torque:
dVbmep
=4
(4 stroke)
(2 stroke)
dVbmep
=2
Engines Comparison
-
7/29/2019 Basics of engine operation
44/47
Brake specific fuel consumption (bsfc)
Measure of engine efficiency
They are in fact inversely related, so a lower bsfcmeans a better engine
Often used over thermal efficiency because anaccepted universal definition of thermal efficiency
does not exist
N
fm
bW
fmbsfc
==
2
&
&
&
g p
bsfc
-
7/29/2019 Basics of engine operation
45/47
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
==
2
&
&
&
qcbsfcqcfm
bW
=
=1
&
&
Engines Comparison
-
7/29/2019 Basics of engine operation
46/47
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
)(2 && +=
0=fm&
g p
Others Engines Comparison
-
7/29/2019 Basics of engine operation
47/47
First law analysis- energy conservation
For a system open to the transfer of enthalpy, mass,
work, and heat, the net energy crossing the controlsurface is stored into or depleted from the controlvolume
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)
g p