Equations · equation
Otto Cycle Efficiency
Thermodynamics · 1876
- Equation
- η = 1 − r^(1−γ)
- Field
- Thermodynamics
- Named After
- Nikolaus Otto
- Symbols
- r compression ratio V_max/V_min · γ ratio of specific heats, about 1.4 for air
- Year
- 1876
Significance
The ideal efficiency of a spark-ignition engine depends on the compression ratio alone: r = 9 gives 58 % in theory. Real engines get about half that after friction, heat loss and incomplete combustion. Raise r and the fuel knocks — which is why octane rating exists, and why diesel, immune to knock, can go higher.
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