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The compression ratio is the ratio of the volume of the cylinder and the combustion chamber when the piston is at the bottom, and the volume of the combustion chamber when the piston is at the top. Automotive engineers can improve fuel efficiency and fuel economy by designing engines with high compression ratios.
That is done these days primarily to boost the fuelâs octane rating. A higher octane rating allows an engine to use a compression ratio of, say, 12-to-one instead of a more usual ten-to-one. The greater the compression, the higher the temperature within the combustion chamber. And the higher the temperature, the greater the thermal efficiency and power produced.