Expansion ratio

Expansion ratio is a measure of the amount that the combustion gases expand in the cylinder after combustion. The greater the expansion ratio, the more work is done and the more thermally efficient the engine is.

Usually the expansion ratio is the same as the compression ratio, but by using sophisticated valve control the compression stroke can be shortened, reducing the compression ratio to avoid knock while still providing a high expansion ratio. This is the basis of the Miller cycle.

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