Diesel Cycle Thermal Efficiency Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 26, 2026Diesel Cycle Thermal Efficiency Calculator
The Diesel cycle models compression-ignition engines with combustion occurring at constant pressure rather than constant volume, requiring a more complex efficiency formula than the Otto cycle - but one that reflects why diesels run such high compression ratios.
The Formula
Efficiency = 1 - (1/CRγ-1) × [(rcγ - 1) / (γ × (rc-1))], using γ = 1.4
Worked Example
Compression ratio 18:1 with a cutoff ratio of 2: efficiency ≈ 63.2%.
How to Use This Calculator
- Enter the compression ratio.
- Enter the cutoff ratio (the ratio of cylinder volume at the end versus start of the constant-pressure combustion phase).
At an identical compression ratio, the Diesel cycle is theoretically less efficient than Otto - but real diesel engines run compression ratios of 15:1 to 22:1 or higher (versus roughly 9:1 to 12:1 for gasoline), more than making up the theoretical gap in practice.