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In the context of diesel engines, the delay period is the time between when atomised fuel is injected into the combustion chamber and the time when this fuel begins to burn. During this time, the fuel absorbs heat from the compressed air in the cylinder. No burning, and therefore no temperature or significant rise in pressure occurs during this period. The shorter this period of time, the less diesel knock there is in the engine.

This period is sometimes referred to as the lag stage.

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