knock - in the cylinder of a spark-ignited internal combustion engine, premature explosion of a portion of the air-fuel mixture, in dependent of spark plug ignition, as a result of excessive heat and pressure during compression. The high local pressures resulting from the explosion are the source of the objectionable clatter or ping associated with knock. Knock reduces efficiency and can be destructive to engine parts. Knock tendency increases with high ambient temperature, dry atmosphere, low altitude, high barometric pressure, or high power demands such as hill climbing and trailer towing. High-octane gasolines resist knocking. Also called detonation. See anti-knock, octane number, pre-ignition.