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Otto cycle
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Idealised petrol engine cycle.
η = 1 − r^{1−γ}Idealised petrol engine: adiabatic compression, constant-volume heating (spark), adiabatic expansion and constant-volume cooling. η = 1 − r^{1−γ} with compression ratio r.
r = 9, γ = 1.4: η = 58%.
Real engines achieve about 30%.
In more detail
The Otto cycle is the idealised model of a petrol engine, with adiabatic compression and expansion and constant-volume heating and cooling. Its efficiency is 1 minus the inverse of the compression ratio raised to the power γ minus one.
Real-world uses
- Petrol car engines are modelled by the Otto cycle.
- Compression ratio improves Otto cycle efficiency.
- Engineers model a petrol engine using the Otto cycle to estimate how efficiency depends on the compression ratio.
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Frequently asked questions
Why does a higher compression ratio help?
It increases the temperature before combustion, improving the efficiency.
Why don't engines reach the ideal?
Because of knock, friction, heat losses and incomplete combustion.
Why does a higher compression ratio give more efficiency?
The gas gets hotter before combustion, so more of the heat is converted into work.
Related topics in Heat & Thermodynamics
Carnot engine · Carnot cycle · Refrigerators and heat pumps · Diesel cycle · Entropy · Entropy change