Actual Cycle and their analysis
ME 432 ICE
2
Learning Objectives
• Introduction
• The Major Losses of Actual Cycle
• Time loss factor
• Heat loss factor
• Exhaust blowdown factor
3
4
Introduction
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
5
Introduction
v.
The progressive 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
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The Major Loss of Actual Cycle

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
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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
40° between the time the spark occurs and the
time the charge is completely burnt (time
loss due to progressive combustion)
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Time Loss Factor

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
progressive combustion
to
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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
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
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Time Loss Factor
Case : Too Early
• If the spark is advanced to
achieve complete combustion
close to TDC additional work
is required to compress the
burning gasses
• Again the work area is less
and the power output and
efficiency are lowered
35o Spark advance
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Time Loss Factor
Case : Too Late

With spark at TDC (0o spark
advance) or late, the combustion
process will be completed later
in the expansion stroke

The peak pressure is low due to
the expansion of gases

The work area is less and the
power output and efficiency are
lowered
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Time Loss Factor
• With or without spark advance the
work area could be less and the power
output and efficiency are lowered
• Moderate or optimum spark advance
(15°-30°) is the best compromise
resulting minimum losses on both the
compression and expansion strokes
• Timing
at
which
the
engine
performance is maximum --- MBT
(Maximum Brake Torque) spark timing
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Time Loss Factor

Table shows the engine performance for various
ignition timings (rc =6).
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Time Loss Factor

The effect of spark advance on the power output by
means of the p-V diagram
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Time Loss Factor


As seen from fig., when the ignition
advance is increased there is a drastic
reduction in the imep and the
consequent loss of power.
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
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Time Loss Factor
• 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
• 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
17
Heat Loss factor
During combustion 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
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Heat Loss factor
• Heat loss during combustion will naturally have
the maximum effect on the cycle efficiency while
heat loss just before the end of the expansion stroke
can
have
very
little
effect
because
of
its
contribution to the useful work is very little
• About 15 per cent of the total heat is lost during
combustion and expansion
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Exhaust Gas Blowdown
• The actual exhaust process consists of two phases:
▫
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 1atm
(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 BDC moving to TDC
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Exhaust Gas Blowdown




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
before the exhaust stroke begins
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Exhaust Gas Blowdown
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Loss Ratio
Ratio of loss area to the fuelair cycle area
i.
Time Loss Ratio
ii. Heat Loss Ratio
iii. Blowdown Loss Ratio
iv. Lost Work Ratio