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Gas Power Cycle - Internal Combustion Engine

The Otto cycle describes the ideal thermodynamic cycle of a gasoline internal combustion engine. It consists of: 1) Isentropic compression of the air-fuel mixture. 2) Constant-volume heat addition from combustion. 3) Isentropic expansion of the combustion gases. 4) Constant-volume heat rejection. The thermal efficiency of the Otto cycle increases with higher compression ratios. Real engines typically operate between compression ratios of 7 to 10.

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Hashem Mohamed Hashem
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87 views5 pages

Gas Power Cycle - Internal Combustion Engine

The Otto cycle describes the ideal thermodynamic cycle of a gasoline internal combustion engine. It consists of: 1) Isentropic compression of the air-fuel mixture. 2) Constant-volume heat addition from combustion. 3) Isentropic expansion of the combustion gases. 4) Constant-volume heat rejection. The thermal efficiency of the Otto cycle increases with higher compression ratios. Real engines typically operate between compression ratios of 7 to 10.

Uploaded by

Hashem Mohamed Hashem
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Gas Power Cycle - Internal Combustion Engine

Otto Cycle
Otto Cycle
P 3 T 3 • 1-2 isentropic compression
• 2-3 constant volume heat
transfer
4 2
2 • 3-4 isentropic expansion
4
• 4-1 constant volume heat
1 1 rejection
v s
Thermal efficiency of the system:
Wcycle W34 − W12 m[(u3 − u4 ) − (u2 − u1 )] (u − u )
η= = = = 1− 4 1
Qin Q23 m(u3 − u2 ) (u3 − u2 )
(u4 − u1 ) C (T − T ) T  T / T −1 
For an ideal gas, u=C vT , η =1 − = 1− v 4 1 = 1− 1  4 1 
(u3 − u2 ) Cv (T3 − T2 ) T2  T3 / T2 − 1 
Since T4 / T1 = T3 / T2 (why?)
T1
η = 1− . From isentropic compression relation for an ideal gas
T2
k −1
T1  V2  1  V1 
=  = k −1
, where r=   is the volume compression ratio
T2  V1  r  V2 
Otto Cycle-2
100
Thermal efficiency of an Otto cycle,
thermal efficiency

80
1
60
η = 1− k −1
η( r ) r
40
Typical value of r for a real engine:
20 between 7 and 10
0
0 3 6 9 12 15
r
compression ratio

• 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
2-3: a constant pressure
P 2 3 T process (instead of a
3 constant volume process)
and is the only difference
2 between an idealized
4
4 Diesel cycle and an
idealized Otto cycle.
1 1
v s
• 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.

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