Diesel compresses harder because it doesn’t pre-mix

Diesel engines run compression ratios of 16–22:1; petrol engines top out around 8–12:1.

Why it works

Petrol engines compress a fuel/air mixture that would pre-ignite ("knock") at high ratios; diesel compresses air alone and injects fuel only at the top of the stroke, sidestepping that limit entirely.

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Why do diesel engines have higher compression ratios?

Diesel engines run compression ratios of 16–22:1; petrol engines top out around 8–12:1. Petrol engines compress a fuel/air mixture that would pre-ignite ("knock") at high ratios; diesel compresses air alone and injects fuel only at the top of the stroke, sidestepping that limit entirely.

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