Otto Cycle Efficiency Calculator

About this calculator

This calculator finds the theoretical thermal efficiency of an Otto cycle engine — the idealized four-stroke cycle (intake, compression, combustion, expansion, exhaust) that models a gasoline spark-ignition engine — using η = 1 − 1/r^(γ−1), where r is the compression ratio and γ is the heat capacity ratio of the working gas (about 1.4 for air). The formula shows that efficiency rises with compression ratio but with diminishing returns, which is why real engines balance higher compression against knock resistance and fuel octane requirements rather than pushing r indefinitely.

Automotive and mechanical engineers use this to evaluate the theoretical efficiency ceiling when optimizing engine compression ratios during design; thermodynamics students use it as a standard worked cycle alongside the Diesel and Brayton cycles; and engine designers reference it when weighing compression ratio changes against fuel requirements and emissions. Enter the compression ratio and gas heat capacity ratio to get the theoretical Otto cycle efficiency.