Why diesel engines are more efficient in practice than petrol engines

Compare the cycles at the same compression ratio and then at the same peak pressure. What compression ratios do real engines actually use?

Model answer.

What the theory says. The Diesel cycle efficiency is
η_Diesel = 1 − (1/r^(γ−1)) × [(ρ^γ − 1) / (γ(ρ − 1))],
where ρ is the cut-off ratio. For ρ > 1, the bracketed term is greater than 1, so at the same compression ratio, η_Otto > η_Diesel. Constant-pressure heat addition happens partly while the piston is moving away, so some heat is added at a lower effective expansion ratio.

Why the comparison is misleading. The two engines do not run at the same compression ratio.

1. Higher compression ratios in diesels. A petrol (SI) engine compresses an air-fuel mixture, so it is limited by knock to roughly 9 to 13. A diesel (CI) engine compresses only air and injects fuel at the end of compression, so there is no knock limit of that kind. Diesels typically run at about 15 to 22. When compared at the same peak pressure and temperature, which is the realistic structural limit, the Diesel cycle is more efficient than the Otto cycle.

1. No throttling losses. A petrol engine controls load by throttling the intake air, which creates pumping losses, especially at part load. A diesel controls load by varying the fuel quantity with unthrottled air.

1. Lean overall mixtures. Diesels run with excess air. A lean charge burns at lower peak temperatures, so the working gas keeps a higher effective γ and dissociation losses are smaller, which helps efficiency.

Summary for the interviewer. At equal compression ratio, Otto wins. At equal maximum pressure, Diesel wins. Real diesels use much higher compression and avoid throttling, so they achieve higher brake thermal efficiency in practice.

Common mistake. Saying "the Diesel cycle is more efficient" without any qualification. The interviewer is usually testing whether you know the same-compression-ratio result.

Likely follow-up. "What is the dual (Sabathe) cycle?" A cycle with part of the heat added at constant volume and part at constant pressure. It models real high-speed diesel engines more closely.