Otto Cycle Thermal Efficiency Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 26, 2026Otto Cycle Thermal Efficiency Calculator
The air-standard Otto cycle is the idealized thermodynamic model behind every spark-ignition engine, and its efficiency depends on exactly one variable: compression ratio.
The Formula
Efficiency = 1 - (1/CR)γ-1, using γ = 1.4 for air
Worked Example
At a compression ratio of 10:1 - a common figure for naturally aspirated gasoline engines - this gives exactly 60.2% theoretical efficiency, a classic textbook benchmark value.
How to Use This Calculator
- Enter the engine's compression ratio (e.g. enter 10 for a 10:1 ratio).
Important caveat: this is the theoretical maximum for an idealized cycle with no real-world losses - actual engines achieve considerably less than this ideal figure due to friction, heat transfer to cylinder walls, and incomplete combustion. Compare against the Diesel Cycle Thermal Efficiency Calculator for the compression-ignition equivalent.