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Internal Combustion Engine

The document summarizes two common gas power cycles used in internal combustion engines: the Otto cycle and the Diesel cycle. The Otto cycle consists of four processes: 1) isentropic compression, 2) constant volume heat addition, 3) isentropic expansion, 4) constant volume heat rejection. Thermal efficiency increases with higher compression ratios. The Diesel cycle differs from the Otto cycle in that it uses constant pressure heat addition instead of constant volume. While it has lower efficiency than the Otto cycle at the same compression ratio, Diesel engines can operate at much higher compression ratios, giving them higher overall efficiency than spark-ignition engines.

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158 views5 pages

Internal Combustion Engine

The document summarizes two common gas power cycles used in internal combustion engines: the Otto cycle and the Diesel cycle. The Otto cycle consists of four processes: 1) isentropic compression, 2) constant volume heat addition, 3) isentropic expansion, 4) constant volume heat rejection. Thermal efficiency increases with higher compression ratios. The Diesel cycle differs from the Otto cycle in that it uses constant pressure heat addition instead of constant volume. While it has lower efficiency than the Otto cycle at the same compression ratio, Diesel engines can operate at much higher compression ratios, giving them higher overall efficiency than spark-ignition engines.

Uploaded by

nidhidarklord
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PPT, PDF, TXT or read online on Scribd
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Gas Power Cycle - Internal Combustion Engine

Otto Cycle
Otto Cycle
P
v
1
2
3
4
T
s
1
2
3
4
( )
cycle 3 4 1 2
34 12 4 1
in 23 3 2 3 2
4 1 4 1 1 4 1
v
3 2 3 2 2 3 2
4 1
Thermal efficiency of the system:
W [ ( )]
( )
= 1
Q ( ) ( )
( ) ( ) / 1
For an ideal gas, u=C , =1 1 1
( ) ( ) / 1
Since /
v
v
m u u u u
W W u u
Q m u u u u
C T T u u T T T
T
u u C T T T T T
T T
q
q
+
+
= = =

| |
= =
|

\ .
3 2
1
2
1
1 2 1
1
2 1 2
/ (why?)
1 . From isentropic compression relation for an ideal gas
1
, where r= is the volume compression ratio
k
k
T T
T
T
T V V
T V r V
q

=
=
| | | |
= =
| |
\ . \ .
1-2 isentropic compression
2-3 constant volume heat
transfer
3-4 isentropic expansion
4-1 constant volume heat
rejection
Otto Cycle-2
0 3 6 9 12 15
0
20
40
60
80
100
compression ratio
t
h
e
r
m
a
l

e
f
f
i
c
i
e
n
c
y
q( ) r
r
Thermal efficiency of an Otto cycle,
q =

1
1
1
r
k
Typical value of r for a real engine:
between 7 and 10
The higher the compression ratio, the higher the thermal
efficiency.
Higher r will led to engine knock (spontaneous ignition)
problem.
Improvement of Performance
Increase the compression ratio

Increase the engine displacement: more power

Compress more air into the cylinder during intake: using
supercharger and turbocharger.

Cool the air before allowing it to enter the cylinder: cooler
air can expand more, thus, increase the work output.

Reduce resistance during intake and exhaust stages: multiple
valve configuration: 4 cylinders/16 valves engine

Fuel injection: do away with the carburetor and provide
precise metering of fuel into the cylinders.
Diesel Cycle
P
v
1
2
3
4
T
s
1
2
3
4
2-3: a constant pressure
process (instead of a
constant volume process)
and is the only difference
between an idealized
Diesel cycle and an
idealized Otto cycle.
Fuel injection for an extended period during the power stroke and therefore
maintaining a relatively constant pressure.
Diesel cycle has a lower thermal efficiency as compared to an Otto cycle
under the same compression ratio.
In general, Diesel engine has a higher thermal efficiency than spark-ignition
engine because the Diesel engine has a much higher compression ratio.
Compression-ignition: very high compression ratio 10 to 20 or even higher.
Diesel Cycle Internal Combustion Engine

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