What is compression ratio (CR) ?

The compression ratio (CR) is defined as the ratio of the volume of the cylinder and its head space when the piston is at the bottom of its stroke to the volume of the head space when the piston is at the top of its travel (Top dead centre - TDC). Typically, petrol engines have a CR of 8–10, while diesel engines have a CR of 15–20. The CR of petrol engines is limited by the requirement that the fuel burns uniformly in the cylinder and does not ignite thermally prior to the spark -It’s also called Engine knocking.

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