1
ACTUAL CYCLE
Actual engine cycle
Introduction
2



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
Introduction

Air-Standard Cycle Analysis gives an estimate of engine
performance
which
is
much
greater
than
the
actual
performance, For Example for SI
Compression
ratio
Thermal
Efficiency
Air-Standard
Cycle
7:1
Actual Engine
Cycle
7:1
55 %
28%
3
Introduction
4



The actual cycles for IC engines differ from the fuel-air cycles and air- standard
cycles in many respects.
The actual cycle efficiency is much lower than the air-standard efficiency due to
various 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
Introduction
5
Theoretical Cycle
I
Corrected for the
Characteristics of the Fuel-Air
Air Cycle
Composition of Cy. Gases
Variable sp.heat, Dissociation etc..
II
Fuel-Air Cycle
IV
Useful work
Actual work loses
Less the friction losses
gives
III
Actual Cycle
modified to account
for Combustion loss,
Time loss, Heat loss
Blowdown loss, etc…
Comparison Of Air-standard And Actual Cycles
6

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
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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.
The Major Loss of Actual Cycle
8

Time loss factor


Heat loss factor


Loss due to time required for mixing of fuel and air and also
for combustion.
Loss of heat from gases to cylinder walls.
Exhaust blowdown factor

Loss of work on the expansion stroke due to early opening
of the exhaust valve.
Time Loss Factor
9


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 b/n the
initiation of the spark and the end of combustion (time loss
due to progressive combustion)
Time Loss Factor
10



Due to the 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 in Fig.
This loss of work reduces the
efficiency and is called time loss due
to progressive combustion.
Time Loss Factor
11

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 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.
Time Loss Factor
12


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 the
expansion of gases.
Time Loss Factor
13
If the spark is advanced to achieve complete combustion close to
TDC additional work is required to compress the burning gasses
35o Spark advance
Time Loss Factor
14


With or without spark advance
the work area could be less and
the power output and efficiency
are lowered.
Therefore a moderate or
optimum spark advance (15o30o) is the best compromise
resulting in minimum losses on
both the compression and
expansion strokes
Time Loss Factor
15


Table shows the engine performance for various ignition timings
(rc =6).
The effect of spark advance on the power output by means of
the p-V diagram
Time Loss Factor
16
The effect of spark advance on imep and power loss
Time Loss Factor
17

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
Time Loss Factor
18


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 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.
Time Loss Factor
19



It is impossible to obtain a perfect homogeneous 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
excess fuel in another part.
Therefore, some fuel does not or burns partially to CO and the
unused O2 appears in the exhaust
Time Loss Factor
20

...
Composition exhaust gases for
various fuel-air ratio
Time Loss Factor
21





Only about 95 % of the energy is released with stoichiometric fuelair 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 to
eliminate wastage of fuel, while a rich mixture is required to utilize
all the oxygen.
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
In a rich mixture a part of the fuel will not get the necessary oxygen
and will be completely lost.
Time Loss Factor
22


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-air
ratios during suction stroke or certain cylinders in a multi cylinder
engine may get continuously leaner mixtures than others.
Heat Loss factor
23


During combustion the heat flows
from the cylinder gases through
 Cooling water
 Lubricating oil
 Conduction and convection and
radiation
Heat loss during combustion will
have the maximum effect on the
cycle efficiency
Heat Loss factor
24



The effect of heat loss during combustion reduce the
maximum temperature and therefore the specific
heats are lower.
Out of various losses heat losses contribute around
12 %
For further details, read
 John B. Heywood, chapter 12 (page 668- 711)
Exhaust Gas Blowdown
The actual exhaust process consists of two phases:
P
i) Blowdown
PT
ii) Displacement
i
e
i
Products
State 6 (TC) State 5 (BC)
Blowdown
Displacement
Blowdown – At the end of the power stroke when the exhaust valve opens
the cylinder pressure 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.
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Displacement – Remaining gas is pushed out of the cylinder by the piston from
BDC moving to TDC.
Exhaust Gas Blowdown
26





When to open the exhaust valve?
The cylinder pressure at the end of expansion stroke is 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 400 to 700 before
BDC thereby reducing the cylinder pressure to halfway (say 3.5
bar) before the exhaust stroke begins
Exhaust Gas Blowdown
P5 = Pe = P6 , T5 = Te = T6
P 
T5 = T4  5 
 P4 
k −1
k
P 
= T4  e 
 P4 
k −1
k
m6 m6  V6 v6  1  v4 
 =  
=
= 
m1 m4  V4 v4  rc  v6 
1  T P  1  T  P 
=  4 6  =  4  6 
rc  T6 P4  rc  T5  P4 
f =
The residual gas temperature T6 is equal to T5
T5  P5 
=  
T4  P4 
since
f =
1
k
k −1
P 
=  6 
 P4 
k
1  P5 
1P 
  =  e 
rc  P4 
rc  P4 
1
k
k −1
k
Blowdown
Displacement
27
TC
BC
Exhaust Gas Blowdown
28

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, because
throttling reduce the suction pressure
 Pumping
 Pumping
loss also increase with speed
loss affect the Volumetric efficiency when Pi
less than Pe
Exhaust Gas Blowdown
Unthrottled (WOT): Pi = Pe = 1 atm
EV opens
imep =
Throttled: Pi < Pe
W5−6−1 = ( Pi − Pe )Vd
EV closes
IV opens
W3−4 − W1−2
IV closes (state1)
Vd
EV opens
EV closes
Pumping work
IV closes
6’
IV opens
Supercharged: Pi > Pe
IV opens
6’
1
29
EV closes
Exhaust Gas Blowdown
30

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
Volumetric Efficiency

The density of fresh charge

As the fresh charge arrives in the hot cylinder, heat is transferred to it
from

The hot chamber walls

The hot residual gases

Temperature rise reduces the density , which decrease the mass of
fresh charge admitted and a reduction in volumetric efficiency

The volumetric efficiency increased by

Low temperature

High pressure of fresh charge
31
Volumetric Efficiency
32

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.
Volumetric Efficiency
33

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
Volumetric Efficiency
34

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 for
smooth operation
Volumetric Efficiency
35

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
Volumetric efficiency
36

For high speed

Opening @10o before TDC

Closing @60o after TDC
Loss due to Running Friction
37

The losses are due to friction between
 the
 In
piston and the cylinder walls
various bearings
 Energy
spent in operating the auxiliary equipment
(cooling pump, ignition system, fan…)

The piston ring friction increases rapidly with engine
speed.
Loss @ part and Full load r=8
38