Post on 07-Feb-2018
ACTUAL CYCLE
Actual engine cycle
1
Introduction
Ideal Gas Cycle (Air Standard Cycle)
Idealized processes
Idealize working Fluid
Fuel-Air Cycle
Idealized Processes
Accurate Working Fluid Model
Actual Engine Cycle
Accurate Models of Processes
Accurate Working Fluid Model
2
Introduction
Air-Standard Cycle Analysis gives an estimate of engine
performance which is much greater than the actual
performance, For Example for SI
Air-Standard Cycle
Actual Engine Cycle
Compression ratio
7:1 7:1
Thermal Efficiency
55 % 28%
3
Introduction
The actual cycles for IC engines differ from the fuel-air cycles and air- standardcycles in many respects.
The actual cycle efficiency is much lower than the air-standard efficiency due tovarious losses occurring in the actual engine operation.
The major losses are due to:
Variation of specific heats with temperature
Dissociation of the combustion products
Progressive combustion
Incomplete combustion of fuel
Heat transfer into the walls of the combustion chamber
Blowdown at the end of the exhaust process
Gas exchange process
4
Introduction
Air CycleCorrected for the
Characteristics of the Fuel-AirComposition of Cy. Gases
Variable sp.heat, Dissociation etc..
Fuel-Air Cycle
modified to account for Combustion loss,Time loss, Heat lossBlowdown loss, etc…Actual Cycle
Actual work losesLess the friction losses
gives
Useful work
Theoretical Cycle
I II
III
IV
5
Comparison Of Air-standard And Actual Cycles
The actual cycles for internal combustion engines differ from
air- standard cycles in many respects
i. The working substance being a mixture of air and fuel vapor or
finely atomized liquid fuel in air combined with the products of
combustion left from the previous cycle.
ii. The change in chemical composition of the working substance.
iii. The variation of specific heats with temperature.
iv. The change in the pressure, temperature and actual amount of
fresh charge because of the residual gases
6
Comparison Of Fuel-Air Cycle And Actual Cycles
v. The progressive combustion rather than the instantaneous
combustion.
vi. The heat transfer to and from the working medium
vii. The substantial exhaust blowdown loss, i.e., loss of work on the
expansion stroke due to early opening of the exhaust valve.
viii. Gas leakage, fluid fiction etc., in actual engines.
Points (i) to (iv), are similar to fuel-air cycles
Points (v) to (viii) are the difference between fuel-air cycles
and actual cycles.
7
The Major Loss of Actual Cycle
Time loss factor
Loss due to time required for mixing of fuel and air and alsofor combustion.
Heat loss factor
Loss of heat from gases to cylinder walls.
Exhaust blowdown factor
Loss of work on the expansion stroke due to early openingof the exhaust valve.
8
Time Loss Factor
In air-standard cycles the heat addition is an instantaneousprocess whereas in an actual cycle it is over a definite periodof time.
The crankshaft will usually turn about 30 to 400 b/n theinitiation of the spark and the end of combustion (time lossdue to progressive combustion)
9
Time Loss Factor
Due to the finite time of combustion,peak pressure will not occur when thevolume is minimum (TDC) but will occursome time after TDC
The pressure, therefore, rises in thefirst part of the working stroke from bto c as shown in Fig.
This loss of work reduces theefficiency and is called time loss dueto progressive combustion.
10
Time Loss Factor
The time taken for combustion depends upon The flame velocity which in turn depend up on the type of
fuel and the fuel-air ratio
The shape and size of the combustion chamber.
The distance from the point of ignition to the opposite side ofthe combustion space
In order that the peak pressure is not reached too late in theexpansion stroke, the time at which the combustion starts is varied byvarying the spark timing or spark advance.
11
Time Loss Factor
Figure below shows the effect of spark timing on p-v diagram from a typical trial.
With spark at TDC (0o spark advance), the peak pressure is low due to theexpansion of gases.
12
Time Loss Factor
If the spark is advanced to achieve complete combustion close toTDC additional work is required to compress the burning gasses
35o Spark advance
13
Time Loss Factor
With or without spark advancethe work area could be less andthe power output and efficiencyare lowered.
Therefore a moderate oroptimum spark advance (15o-30o) is the best compromiseresulting in minimum losses onboth the compression andexpansion strokes
14
Time Loss Factor
Table shows the engine performance for various ignition timings(rc =6).
The effect of spark advance on the power output by means ofthe p-V diagram
15
Time Loss Factor
The effect of spark advance on imep and power loss16
Time Loss Factor
Some times a deliberate spark retarded from optimum may be necessary in order to
• avoid knocking
• reduce exhaust
• reduce emission of hydrocarbons and carbon monoxide
17
Time Loss Factor
At full throttle with the fuel-air ratio corresponding to maximumpower and with the optimum ignition advance, the time lossesmay account for a drop in efficiency of about
5 percent for actual Engine
2 percent fuel-air cycle efficiency
These losses are higher when the
mixture is richer or leaner
Ignition advance is not optimum and
at part throttle operations the losses are higher.
18
Time Loss Factor
It is impossible to obtain a perfect homogeneous mixture withfuel-vapor and air, since, residual gases from the previous arepresent in the clearance volume of the cylinder. further, verylimited time is available between the mixture preparation andignition
Under these circumstances, it is possible that a pocket excessoxygen is present in one part of the cylinder and a pocket ofexcess fuel in another part.
Therefore, some fuel does not or burns partially to CO and theunused O2 appears in the exhaust
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Time Loss Factor
Composition exhaust gases forvarious fuel-air ratio
...
20
Time Loss Factor
Only about 95 % of the energy is released with stoichiometric fuel-air ratios.
Energy released in actual engine is about 90% of fuel energy input.
It should be noted that it is necessary to use a lean mixture toeliminate wastage of fuel, while a rich mixture is required to utilizeall the oxygen.
Slightly leaner mixture would give maximum efficiency but too leana mixture will burn slowly increasing the time losses or will not burnat all causing total wastage of fuel
In a rich mixture a part of the fuel will not get the necessary oxygenand will be completely lost.
21
Time Loss Factor
The flame speed in mixtures more than 10% richer is low,thereby, increasing the time losses and lowering the efficiency.
Imperfect mixing of fuel and air may give different fuel-airratios during suction stroke or certain cylinders in a multi cylinder
engine may get continuously leaner mixtures than others.
22
Heat Loss factor
During combustion the heat flowsfrom the cylinder gases through Cooling water Lubricating oil Conduction and convection and
radiation Heat loss during combustion will
have the maximum effect on thecycle efficiency
23
Heat Loss factor
The effect of heat loss during combustion reduce themaximum temperature and therefore the specificheats are lower.
Out of various losses heat losses contribute around12 %
For further details, read John B. Heywood, chapter 12 (page 668- 711)
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Blowdown – At the end of the power stroke when the exhaust valve opensthe cylinder pressure is much higher than the exhaust manifold pressurewhich is typically at 1 atm (P4 > Pe), so the cylinder gas flows out through theexhaust valve and the pressure drops to Pe.
Displacement – Remaining gas is pushed out of the cylinder by the piston fromBDC moving to TDC.
Exhaust Gas Blowdown
State 6 (TC)
The actual exhaust process consists of two phases:
i) Blowdown
ii) Displacement
Blowdown Displacement
State 5 (BC)
Pi TiPe
Products
25
Exhaust Gas Blowdown
When to open the exhaust valve?
The cylinder pressure at the end of expansion stroke is high as 7bar depending on the compression ratio employed.
If the exhaust valve is opened at BDC, the piston has to do workagainst high cylinder pressure during the early part of the exhauststroke
If the exhaust valve is opened too early, a part of the expansionstroke is lost
The best compromise is to open the exhaust valve 400 to 700 beforeBDC thereby reducing the cylinder pressure to halfway (say 3.5
bar) before the exhaust stroke begins
26
Exhaust Gas Blowdown
kk
ek
ke
PPT
PPTT
PP1
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6e565 TT T , P −−
=
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kk
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PP
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TT
1
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6
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TC BC
DisplacementBlowdown
The residual gas temperature T6 is equal to T5
ke
c
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rPP
rf
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PP
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rPP
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r
vv
rvVvV
mm
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27
Exhaust Gas Blowdown
Loss due to Gas Exchange process (pumping loss) The work done for intake and exhaust stroke cancelled
each other
The pumping loss increased at part throttle, becausethrottling reduce the suction pressure
Pumping loss also increase with speed
Pumping loss affect the Volumetric efficiency when Pi
less than Pe
28
Exhaust Gas Blowdown
dei VPPW )(165 −=−−
dVWWimep 2143 −− −
=
Unthrottled (WOT): Pi = Pe = 1 atm
Throttled: Pi < Pe
Supercharged: Pi > Pe
1
EV closesIV opens
EV closes
IV opens
EV closes
IV opens6’
6’
EV opens
IV closes (state1)
EV opens
IV closesPumping work
29
Exhaust Gas Blowdown
Volumetric efficiency affected by
The density of fresh charge
The exhaust gas in the clearance volume
The design of intake and exhaust manifold
The timing of intake and exhaust valves
30
Volumetric Efficiency
The density of fresh charge
As the fresh charge arrives in the hot cylinder, heat is transferred to itfrom
The hot chamber walls
The hot residual gases
Temperature rise reduces the density , which decrease the mass offresh charge admitted and a reduction in volumetric efficiency
The volumetric efficiency increased by
Low temperature
High pressure of fresh charge
31
Volumetric Efficiency
Exhaust gas in the clearance volume
The residual gas occupy a portion of piston displacement
volume, thus reducing the space available to the incoming
charge.
These exhaust products tend to rise the temperature of the fresh
charge.
32
Volumetric Efficiency
The design of intake and exhaust manifold
The exhaust manifold should be designed to enables the
exhaust products to escape readily,
The intake manifold should be designed so as to bring in
maximum possible fresh charge flowing in to the cylinder
33
Volumetric Efficiency
The timing of intake and exhaust valves Valve timing is the regulation of the points in the cycle at
which the valves are set to open and close.
Valves requires a finite period of time to open or close forsmooth operation
34
Volumetric Efficiency
The effect of intake valve timing on the engine air capacity is
indicated by its effect on the air inducted per cylinder, per cycle.
The intake valve timing for both a low and high speed SI engine
For low speed
Opening @10o before TDC
Closing @10o after BDC
35
Volumetric efficiency
For high speed
Opening @10o before TDC
Closing @60o after TDC
36
Loss due to Running Friction
The losses are due to friction between
the piston and the cylinder walls
In various bearings
Energy spent in operating the auxiliary equipment(cooling pump, ignition system, fan…)
The piston ring friction increases rapidly with enginespeed.
37
Loss @ part and Full load r=838