Engine knocking
In spark-ignition internal combustion engines, knocking (also called knock, detonation, spark knock, or pinging) occurs when combustion of some of the air/fuel mixture in the cylinder results from a source other than the flame front ignited by the spark plug, in other words, when one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel–air charge is meant to be ignited by the spark plug only, and at a precise point in the piston's stroke.