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.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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