Lecture-8
Prepared under
QIP-CD Cell Project
Internal Combustion Engines
Ujjwal K Saha, Ph.D.
Department of Mechanical Engineering
Indian Institute of Technology Guwahati
1
Background
Air
standard
cycles
had
simplified
approximations, and therefore, performance
estimate of the engine is greater than the actual
performance.
‰
With a compression ratio of 7:1, the actual
indicated thermal efficiency of an SI engine is of
the order of 30 %, while the ideal (or air-standard)
efficiency is about 55 %.
‰
This divergence is due to partly due to noninstantaneous burning, incomplete combustion,
valve operation etc. However, the main reason lies
with the over-simplification of using values of
properties of the working fluid.
‰
2
Background-Contd.
Ideal Case:
‰
‰
‰
Working fluid is air
Air is a perfect gas
Has constant specific heats
Actual Case:
Working fluid is air + fuel + residual gas
Specific heats increases with increase in
temperature
‰ Combustion products are subjected to
dissociation at high temperature
‰
‰
3
Fuel-Air Cycle Considerations
– Actual composition of the cylinder gas (fuel
+ air + water vapor in air + residual gas)
– F/A ratio change during operation, and
hence changes in amount of CO2, water
vapor etc.
– Specific heat changes with temperature
(except for mono-atomic gas), and hence,
ratio of specific heats (k) also changes.
– Changes in no. of molecules in cylinder with
the change in pressure and temperature.
4
Fuel-Air Cycles - Assumptions
There is no chemical change in either fuel or air
prior to combustion.
‰
There is no heat transfer between the gases and
cylinder walls in any process (adiabatic).
‰
Compression and expansion processes are
frictionless.
‰
‰
The velocities are negligibly small.
5
Remark
# The air-standard analysis allows how the efficiency
is improved by raising the compression ratio of air.
# It does not give any idea on the effect of F/A
ratio on thermal efficiency.
Fuel – Air Cycles
‰ Allows study of F/A ratio on thermal efficiency.
‰ Allows study of pmax and Tmax as F/A ratio is
varied. This helps in structural design of the engine.
‰ Gives a good estimate of the power expected
from an actual engine.
6
Variable Specific Heats:
# Except mono-atomic gases, all other gases show
an increase in specific heats at high temperature.
This increase does not obey any law.
7
# Over the temperature range in general use for
gases in heat engines (300 K – 1500 K), the specific
heat curve is nearly a straight line, and can be
expressed as
Cp = a1 + k1T
Cv = b1 + k1T
R = Cp - Cv = a1-b1
# Above 1500 K, specific heats increase more rapidly,
and may be expressed in the form
Cp = a1 + k1T + k2T2
Cv = b1 + k1T + k2T2
8
Physical Explanation
Cp = 1.005 kJ/kgK at 300 K
Cp = 1.343 kJ/kgK at 2000 K
Cv= 0.718 kJ/kgK at 300 K
Cv = 1.055 kJ/kgK at 2000 K
‰ Increase
of specific heat is that as
temperature is raised, larger and larger fractions
of heat input go to produce the motion of atoms
within the molecules.
9
Explanation
‰ As temperature is an indication of motion of
molecules as a whole, therefore, the energy
that goes into the motion of atoms does not
contribute to temperature rise.
‰ This is the reason, why more heat is required
to raise the temperature of unit mass by one
degree (This heat, by definition, is the specific
heat). As Cp-Cv =Constant, and k (=Cp/Cv)
decreases with increase of temperature.
‰ Therefore, variation of specific heats leads
the FINAL temperature and pressure to lower
values (as compared to constant specific heats).
10
1-2-3-4
: with constant specific heats
1-2´-3´-4´´ : with constant specific heat from point 3´
1-2´-3´-4´ : with variable specific heats
2´ is lower than 2 : due to variable specific heats
3´ is lower than 3 : temperature rise due to a given heat
release ↓ as Cp ↑, and also as 2´ is lower than 2.
3´ to 4´´
: resulting adiabatic expansion.
3´ to 4´
: correct expansion (Specific heat ↓
as Temperature ↓ during expansion).
11
Dissociation Loss
Dissociation : disintegration of combustion products
at high temperature. During dissociation, heat is
absorbed, whereas during combustion heat is
liberated.
At 10000C, CO2 U CO + O2 + heat
At 13000C, H2O U H2 + O2 + heat
Presence of CO and O2 in the gases tends to
prevent the dissociation of CO2 in rich mixture,
which, by producing more CO suppresses the
dissociation of CO2. That means, there there is no
dissociation in the burnt gases of a lean mixture,
because the temperature produced is too low for
the phenomenon to occur.
12
Remarks
‰ Lean Mixture : No dissociation takes place
due to low temperature.
‰ Maximum dissociation : Chemically correct
mixture when the temperature is high.
‰ Rich Mixture : Dissociation is prevented by
the available CO and O2.
Further, heat transfer to cooling medium
causes a reduction in maximum temperature
and pressure. As temperature falls (during the
expansion stroke) the separated constituents
recombine and heat absorbed (during
dissociation) gets released. But, it becomes
too late to recover.
13
The curve shows the reduction in exhaust gas temperature
due to dissociation with respect to air-fuel ratio.
14
Effect of dissociation on Power (SI Engine)
Power Output is
maximum
at
stoichiometric
ratio where there
is no dissociation.
Shaded
area
represents loss of
power due to
dissociation.
For Lean mixture : No dissociation.
For Stoichiometric : Maximum dissociation.
For Rich mixture : Effect declines due to incomplete combustion
and also due to increased quantity of CO.
15
Effect of Operating Variables
Compression Ratio: For a
given φ, efficiency (fuelair cycle) increases with
compression ratio (r) in
a similar manner as that
of air standard cycle.
pmax
increases
with
increasing
r
and
liberation of chemical
energy at high pressure
gives more scope for
expansion work. Thus,
there is higher efficiency
but to a certain value of
compression ratio (r).
16
At the same compression
ratio, efficiency (fuel-air)
decreases with increasing φ.
φ = Equivalence Ratio
=
Actual F
A
Ratio
Stoicheometric F
A
Ratio
φ < 1 implies a lean mixture.
Tmax becomes lower due to
excess air. This results in
lower specific heats and
higher values of k. Hence,
efficiency increases with
decreasing φ (gases expand
to a larger temperature before
exhaust).
When φ > 1, efficiency (fuelair)
decreases
with
increasing
φ,
because
insufficient air leads to
incomplete oxidation of fuel.
17
Effect of Equivalence Ratio on Temperature
At a given r, maximum
temperature is reached
when the mixture is slightly
rich (about 6 - 8 %). This is
because, at φ=1, there is
still some oxygen present
at point 3 because of
chemical
equilibrium
effects, and a rich mixture
will cause more fuel to
combine with oxygen at
that point thereby raising
the
temperature
T 3.
However, at rich mixtures
increased formation of CO
counteracts this effect.
18
Effect of Equivalence Ratio on Pressure
The pressure of a gas
in a given space
depends
upon
its
temperature and the
number of molecules.
The
curve
of
p3,therefore follows T3,
but because of the
increased
no.
of
molecules, p3 starts
decreasing when the
mixture is about 18 to
20 % rich.
19
References
Crouse WH, and Anglin DL,
DL (1985), Automotive Engines, Tata McGraw Hill.
2. Eastop TD, and McConkey A, (1993), Applied Thermodynamics for Engg.
Technologists, Addison Wisley.
3. Fergusan CR, and Kirkpatrick AT, (2001), Internal Combustion Engines, John
Wiley & Sons.
4. Ganesan V, (2003), Internal Combustion Engines, Tata McGraw Hill.
5. Gill PW, Smith JH, and Ziurys EJ, (1959), Fundamentals of I. C. Engines, Oxford
and IBH Pub Ltd.
6. Heisler H, (1999), Vehicle and Engine Technology, Arnold Publishers.
7. Heywood JB, (1989), Internal Combustion Engine Fundamentals, McGraw Hill.
8. Heywood JB, and Sher E, (1999), The Two-Stroke Cycle Engine, Taylor & Francis.
9. Joel R, (1996), Basic Engineering Thermodynamics, Addison-Wesley.
10. Mathur ML, and Sharma RP, (1994), A Course in Internal Combustion Engines,
Dhanpat Rai & Sons, New Delhi.
11. Pulkrabek WW, (1997), Engineering Fundamentals of the I. C. Engine, Prentice Hall.
12. Rogers GFC, and Mayhew YR,
YR (1992), Engineering Thermodynamics, Addison
1.
Wisley.
13. Srinivasan S, (2001), Automotive Engines, Tata McGraw Hill.
14. Stone R, (1992), Internal Combustion Engines, The Macmillan Press Limited, London.
15. Taylor CF, (1985), The Internal-Combustion Engine in Theory and Practice, Vol.1 & 2,
The MIT Press, Cambridge, Massachusetts.
20