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compression ratio[kəm′presh·ən ‚rā·shō]
the ratio of the volume of the working fluid at the beginning of compression in the cylinder of an internal-combustion engine to the volume at the end of compression. The working fluid in carburetor engines is an air-fuel mixture; in diesel engines it is air. An increase in the compression ratio causes a decrease in the volume of the working fluid at the end of the compression stroke; the pressure and temperature of the working fluid are increased correspondingly, combustion is accelerated, and heat losses are reduced. A higher compression ratio increases the power output of an engine and improves fuel economy. However, increases in the compression ratio are limited by the fuel’s ability to prevent detonation. Compression ratios for carburetor engines range from 6.5:1 to 9.5:1; those for diesel engines are between 16:1 and 21:1.
In a cylinder, the piston displacement plus clearance volume, divided by the clearance volume. This is the nominal compression ratio determined by cylinder geometry alone. In practice, the actual compression ratio is appreciably less than the nominal value because the volumetric efficiency of an unsupercharged engine is less than 100%, partly because of late intake valve closing. In spark ignition engines the allowable compression ratio is limited by incipient knock at wide-open throttle. See Combustion chamber, Internal combustion engine
ii. The ratio of the compressor discharge pressure to the compressor inlet pressure. Also called compressor pressure ratio.