occurs in the cylinder of a spark-ignited internal
combustion engine, premature explosion of a portion of the air-fuel mixture,
independent 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 gasoline resist knocking. Also
called detonation.
Also see anti-knock,
octane number,
pre-ignition.