Any rapid chemical reaction accompanied by noise and often heat and light, e.g., explosions.
Internal Combustion Engine
The inlet charge in the combustion chamber ignites uncontrollably, usually before the spark ignites the mixture. Potentially very damaging to engine components.
This sometimes results in a noise called "pinging." Detonation is caused by autoignition of the "end gas" i.e., that part of the charge not yet consumed in the normal flame-front reaction. Detonation occurs because piston motion and compression of the end gas raise its temperature and pressure to the point where the end gas autoignites. The pinging or knocking noise is the result of intense pressure waves in the charge which cause the cylinder walls to vibrate.
Usually a piezoelectric accelerometer, mounted near the cylinders which can detect engine knocking or frequencies of detonation. This signal is processed by the engine control unit and the ignition timing is retarded to prevent damage to the engine.
See also: Explosion.