Actual cycles for internal combustion engines differ from air-standard cycles in many respects. Time loss factor. Heat loss factor. Exhaust blow down factor.

1. SOUMYTH

2. 2
Introduction
Air standard Cycle Actual Engine cycle
Compression ratio 7:1 7:1
Thermal efficiency 55 % 28 %
• Air standard cycle analysis gives an estimate of engine performance which is much
greater than the actual performance.
• Actual efficiency is much lower than the air-standard efficiency due to
various losses that occur in actual engine.
• The major losses are
1. Variation of specific heat with temperature.
2. Dissociation of the combustion products
3. Progressive combustion
4. Incomplete combustion of fuel
5. Heat transfer into the walls of the combustion chamber
6. Blowdown at the end of exhaust process
7. Gas exchange process
SOUMYTH

3. 3SOUMYTH
AIR CYCLE
Corrected for characteristics
of the Fuel-Air composition of
Cylinder gases, Variable
specific heat, Dissociation
FUEL-AIR
CYCLE
Modified to account
for combustion loss,
time loss, heat loss,
blowdown loss etc..
ACTUAL
CYCLE
Actual work losses
PLUS the friction losses
gives
Useful Work
1 2
3
4

4. Comparison of Air-Standard and Actual cycles
Actual cycles for internal combustion engines differ from air-standard
cycles in many respects. These differences are mainly due to:
1. The working substance mixing with the product of combustion left from the
previous cycle.
2. The change in chemical composition of the working substance.
3. The variation of specific heats with temperature.
4. The change in the composition, temperature and actual amount of fresh charge
because of the residual gases.
1. The progressive combustion rather than the instantaneous combustion.
2. The heat transfer to and from the working medium.
3. The substantial exhaust blowdown loss, i.e., loss of work on the expansion
stroke due to early opening of the exhaust valve.
4. Gas leakage, fluid friction etc., in actual engines.
SOUMYTH 4
FUELAIRCYCLESACTUALCYCLES

5. Major Losses
• Most of the factors listed above tend to decrease the thermal efficiency
and power output of the actual engines.
• Calculating thermal efficiencies while considering these factors are not
that different from those of the actual cycles.
Out of all the above factors, major influence is by
1. Time loss factor
Loss due to time required for mixing of fuel and air and also for combustion,
2. Heat loss factor
Loss of heat from gases to cylinder walls.
3. Exhaust blowdown factor
Loss of work on the expansion stroke due to early opening of the exhaust valve.
SOUMYTH 5

6. Time loss factor
• In air standard cycles the heat addition is an instantaneous process
whereas in an actual cycle it is over a definite period of time.
• The crankshaft will usually turn about 30 to 400 between the initiation of
the spark and the end of combustion (time loss due to progressive
combustion).
SOUMYTH
• Due to finite time of combustion, peak
pressure will not occur when the volume is
minimum (TDC) but will occur some time
after TDC.
• The pressure therefore rises in the first part
of the working stroke from b to c, as shown.
• This loss of work reduces the efficiency and
is called time loss due to progressive
combustion.
Peak pressure
6
Total Work

7. SOUMYTH
The time taken for combustion depends on
• The flame velocity which in turn depends on the
type of fuel and the air-fuel ratio.
• The shape and size of the combustion chamber.
• The distance from the point of ignition to the
opposite side of the combustion space.
In order that the peak pressure is not reached
too late in the expansion stroke, the time at
which the combustion starts is varied by varying
the spark timing or spark advance.
the peak pressure is low due
• Figure shows the effect of spark timing on P-V
diagram from a typical trial.
• With spark at TDC (00 spark advance), the peak
pressure is low due to the expansion of gases. Spark at TDC, advance 00
7
peak pressure is low

8. • If the spark is advanced to achieve complete
combustion close to TDC, additional work is
required to compress the burning gases.
• With or without spark advance the work area
could be less and the power output and
efficiency are lowered. 8
Optimum advance of
15◦ – 30°
Spark advance of 350
Therefore, a moderate or optimum spark
advance of (15 - 35°) is the best compromise
resulting in minimum losses on both the
compression ratio and expansion ratio.

9. SOUMYTH 9
Table shows the engine performance for various ignition timings
Cycle
Ignition
Advance
Max cycle
pressure
(bar)
mep
(bar)
efficiency
ηActual
ηFuel cycle
Fuel Air 0° 44 10.2 32.2 1.0
Actual 0° 23 7.5 24.1 0.75
Actual 17° 34 8.35 26.3 0.81
Actual 35° 41 7.6 23.9 0.74
P-V diagram showing power loss due to
ignition advance
Sometimes a deliberate spark retarded from
optimum may be necessary in order to
• Avoid knocking
• Reduce exhaust
• Reduce emission of hydrocarbons and carbon
monoxide.

10. • At full throttle with the fuel-air ratio corresponding to maximum power and
with the optimum ignition advance, the time losses may account for a drop in
efficiency of about
• 5 % for Actual engine
• 2 % for fuel-air cycle efficiency.
• These losses are higher when the
• Mixture is richer or leaner
• Ignition advance is not optimum
• At part throttle operations the losses are higher.
• It is impossible to obtain a perfect homogenous mixture with fuel-vapor and air,
since residual gases from the previous are present in the clearance volume of
the cylinder. Further, very limited time is available between the mixture
preparation and ignition.
• Under these circumstances, it is possible that a pocket excess oxygen is
present in one part of the cylinder and a pocket of excessive fuel in another
part.
• Therefore, some fuel does not burn or burns partially to CO and the unused O2
appears in the exhaust.
SOUMYTH 10

11. 11
1. Only about 95% of the energy is released with the stoichiometric fuel sir ratios.
2. Energy released in actual engine is about 90% of fuel energy input.
1. It should be noted that that it is necessary to use a lean mixture to eliminate
wastage of fuel, while a rich mixture is required to utilize all of the oxygen.
2. Slightly leaner mixture would give maximum efficiency but too lean a mixture
will burn slowly increasing the time losses or will not burn at all causing total
wastage of fuel.
3. In a rich mixture a part of the fuel will not get the necessary oxygen and will be
completely lost.
1. The flame speed in the mixtures more than 10% richer is low, thereby
increasing the time losses and lowering the efficiency.
2. Imperfect mixing of fuel and air may give different fuel-air ratios during suction
stroke or certain cylinders in a multi cylinder engine may get continuously
leaner mixtures than others.

12. Heat loss factor
• During combustion the heat flows from the
cylinder gases through
1. Cooling water
2. Lubricating oil
3. Conduction and convection and radiation
• Heat loss during combustion ill have the
maximum effect on the cycle efficiency.
• The effect of heat loss during the
combustion reduce the maximum
temperature and therefore the specific
heats are lower.
• Out of various losses heat losses contribute
over 12%.
SOUMYTH 12
Time loss, heat loss and exhaust loss in
petrol engines

13. Exhaust Gas Blowdown
The actual exhaust process consists of two phases:
1. Blowdown
2. Displacement
Blowdown : -
At the end of power stroke when the exhaust valve opens the cylinder pressure is
much higher than the exhaust manifold pressure which is much higher than the
exhaust manifold pressure which is typically at 1 atm (P4 > Pe), so the cylinder gas
flows out through the exhaust valve and the pressure drops to Pe.
Displacement : -
Remaining gas is pushed out of the cylinder by the piston from the BDC moving to
TDC.
SOUMYTH 13

14. SOUMYTH
When to open Exhaust valve?
• The cylinder pressure at the end of
expansion stroke is as high as 7 bar
depending on the compression ratio
employed.
• If the exhaust valve is opened at BDC,
the piston has to do work against high
cylinder pressure during the early part
of the exhaust stroke.
• If the exhaust valve is opened too
early, a part of the expansion stroke is
lost.
• The best compromise is to open the
exhaust valve is 40° - 70° before BDC
thereby reducing the cylinder
pressure to halfway (say 3.5 bar)
before the exhaust stroke begins.
Effect of exhaust valve opening time on blowdown
14

15. SOUMYTH
P5 = Pe = P6 ; T5 = Te = T6 ⟹ T5 = T4 ⋅
P5
P4
ൗγ−1
γ
= T4 ⋅
Pe
P4
ൗγ−1
γ
f =
m6
m1
=
m6
m4
=
ΤV6 v6
ΤV4 v4
=
1
rc
⋅
T4
T6
⋅
P6
P4
=
1
rc
⋅
T4
T6
⋅
P6
P4
The residual gas temperature T6 is equal to T5,
since,
T5
T4
=
P5
P4
ൗγ−1
γ
=
Pe
P4
ൗγ−1
γ
→ f =
1
rc
⋅
P5
P4
ൗ1
γ
=
1
rc
⋅
Pe
P4
ൗ1
γ
15
The exhaust stroke (4 to 5 to 6) illustrating residual mass.

16. SOUMYTH 16
Loss of Gas exchange process (pumping loss)
• The work done for intake and exhaust stroke cancelled each other.
• The pumping loss increased at part throttle, because throttling reduces the suction
the pressure.
• Pumping loss also increases with speed.
• Pumping loss affect the volumetric efficiency when P1 is less than Pe.
Unthrottled (WOT)
Pi = Pe = 1 atm
Throttled :
Pi < Pe
Supercharged :
Pi > Pe

17. Volumetric Efficiency
• Volumetric efficiency is an indication of the breathing ability of the
engine and is defined as the ratio of the volume of air actually inducted
at ambient condition to swept volume.
• It may also be defined on mass basis as the ratio of the actual mass of
air drawn into the engine during a given period of time to the theoretical
mass which should have been drawn in during that same period of time,
based upon the total piston displacement of the engine, and the
temperature and pressure of the surrounding atmosphere.
• Volumetric efficiency is affected by
1. The density of fresh charge.
2. The exhaust gas in the clearance volume.
3. The design of intake and exhaust manifold.
4. The timing of intake and exhaust valves.
SOUMYTH 17

18. • The density of fresh charge
• As the fresh charge arrives in the hot cylinder, heat is transferred to it from the
1. Hot chamber walls 2. The hot residual gases
• Temperature rise reduces the density, which decrease the mass fresh charge
admitted and a reduction in volumetric efficiency.
• The volumetric efficiency is increased by
1. Low temperature 2. High pressure of fresh charge
• 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.
• Timing of intake and exhaust valves
• Valve timing is the regulation of the points in the cycle in which the cycle at which
the valves are set to open and close.
• Valves require a finite period of time to open or close for smooth operation.
SOUMYTH 18

19. 19
Losses due to running friction
Losses are due to friction between the piston and the cylinder walls, in various bearings
and energy spent in operating the auxiliary equipment (cooling pump, ignition system,
fan etc..)
The piston ring friction increases rapidly with engine speed.
S. No Item
At load
Full Load (%) Half Load (%)
(a) Air standard cycle efficiency 56.5 56.5
1 Losses due to variation of specific heat and chemical equilibrium 13 13
2 Loss due to progressive combustion 4 4
3 Loss due to incomplete combustion 3 3
4 Direct heat loss 4 5
5 Exhaust blowdown loss 0.5 0.5
6 Pumping loss 0.5 1.5
7 Rubbing friction loss 3 6
(b) Fuel air cycle efficiency = Air std cycle efficiency – (1) 43.5 43.5
(c) Gross indicated thermal efficiency = Fuel air cycle efficiency – (2+3+4+5) 32 31
(d) Actual brake thermal efficiency = Fuel air cycle efficiency – (6+7) 28.5 23.5
Typical losses in a gasoline engine for r = 8

20. High speed & Low speed Engines Valve timing Diagram
The effect of intake valve
timing in the engine air
capacity is indicated by its
effect on the air inducted
per cylinder, per cycle.
For high speed
• Opening 10° before TDC.
• Closing 60° after BDC.
For low speed
• Opening 10° before TDC.
• Closing 10° after BDC.
20
The intake valve timing for a 4 stroke engine

21. Intake Valve timing
• Theoretically, the intake valve should open at TDC.
• In almost all SI engines the intake valve opens few degrees before TDC to ensure
the valve fully opens, flow fresh charge to cylinder as piston reaches TDC.
• The intake valve opens 10° before TDC for both Low speed and High speed engine.
As piston moves away from TDC, the engine draws fresh charge into cylinder.
• When the piston reaches BDC and ascent again during compression stroke, the
inertia of flowing air-fuel mixture tends to continue the flow of charge in to the
cylinder.
• The inertia tends to continue to keep open the intake valve for a short period. If the
intake valve keeps open much beyond BDC, the compression stroke force out
some fresh charge with consequent of reduction of volumetric efficiency. Hence
the intake valve should close relatively early after it reaches the BDC.
• For low speed engine the inertia of flowing charge is also low and intake valve
close 10° after BDC.
• For high speed engine, the intake charge has higher inertia, it causes a RAM effect
as piston moving up during the compression stroke. Ram effect tends to pack more
fresh charge into cylinder. So, to take advantage of this, intake valve closing is
delayed in high speed engine. For high speed engine, intake valve closes 60° after
BDC.
SOUMYTH 21

22. Exhaust Valve timing
• The exhaust valve is usually open before piston reaches the BDC. This reduces the
work done, but decreases the work required to expel the burned gas during the
exhaust stroke, the result will be an overall gain in output.
• For low speed engine exhaust valve opens 25° before BDC.
• For high speed engine exhaust valve opens 55° before piston reaches BDC.
• The exhaust valve is set to close some time after the piston reaches the TDC, so that
the inertia of exhaust gas tends to give better scavenging by carrying out the burned
product left in the clearance volume.
• For low speed engine exhaust valve close 5° after TDC and for high speed engine
exhaust valve closes 20° after TDC.
• The opening and closing of intake and exhaust valve may get overlapped during the
operation.
• This overlap should not be excessive enough to fresh charge expel through the
exhaust valve or burned gas product sucked into the cylinder during the intake stroke.
SOUMYTH 22

23. SOUMYTH 23
Theoretical & Actual Engines Valve timing Diagram
Theoretical and actual valve timing diagram for 4-stroke petrol engine

24. In reality the opening and closing of valve is not instantaneous as like in the theoretical
assumption. Time taken for opening of these valves needs to be considered.
SOUMYTH 24
Suction
Theoretical:
In theoretical cycle the inlet valve will
open when the piston is at TDC and it
starts moving downwards. Thus the
air will be drawn into the cylinder.
Actual:
In actual cycle, the inlet valve will be
started opening just before the piston
reaching to the TDC from the previous
cycle.
Because in the actual engine the valve
cannot be opened instantaneously, so
it has to be started opening a bit early.
Compression
Theoretical:
In theoretical cycle, on completion of the
suction stroke, the compression stroke
starts when the piston reaches BDC. At
BDC, the inlet valve will close and piston
will start to move upward. The air in the
cylinder will be compressed
Actual:
In actual cycle, the inlet valve starts
closing right after the piston starts
moving upwards, because to close the
valve completely it will take some time.

25. Exhaust stroke
Theoretical:
In theoretical cycle, at the BDC the
exhaust valve will open, all the
combustion particles will be thrown
out of cylinder with piston upward
movement.
Once the piston reaches TDC, all the
combustion particles will be thrown
out of the cylinder completely and
the suction stroke will start again for
the second cycle.
Actual:
In actual cycle, the exhaust stroke
will start a bit early as before piston
reaches the BDC and the exhaust
valve closing should be maintained
properly or there is a chance of
exhaust blowdown.
SOUMYTH 25
Expansion stroke
Theoretical:
In theoretical cycle, when piston reaches TDC, now
the fuel is injected into the cylinder by high pressure
fuel injector at the end of compression stroke.
Due to the high compression of air in the cylinder,
pressure and temperature od air are increased,
which is sufficient to self-ignite the fuel
instantaneously which is injected at the end of
compression stroke in which the piston is at TDC
Actual:
In actual cycle, the fuel will be injected before the
piston reaches to TDC.
The ignition starts immediately right after the
injection of fuel into cylinder. But the reason behind
injecting the fuel right before the piston reaches to
the TDC is that the fuel complete combustion is not
that instantaneous as like in the theoretical
assumption.
So, it has to start burning before the piston reaches
to TDC, that is the way we can take full advantage of
power stroke.

26. Theoretical & Actual valve timing diagram for
2-stroke petrol engine
There are mainly two stokes in 2 stroke engine:
Expansion stroke:-
In this stroke, piston moves from TDC to BDC. Combustion forces the piston
downwards. Piston uncovers first exhaust port, combustion gases escape
through exhaust port and uncovers transfer port next to allow fresh charge flow
into combustion chamber through transfer port.
Swiping of exhaust gases by incoming charge is called Scavenging.
Compression stroke: -
In this stroke, piston moves from BDC to TDC. It covers the transfer port and when
it further moves up, covers the exhaust port completely, to stop scavenging.
It uncovers the inlet port when moving upwards, where it inlets the gas into crank
case.
SOUMYTH 26

27. SOUMYTH 27
Valve timing diagram for 2-stroke petrol and Diesel engine