Knocking (also knock, detonation, spark knock, pinging or pinking) in spark-ignition internal combustion enginesoccurs when combustion of the air/fuel mixture in the cylinder does not start off correctly in response to ignition by thespark plug, but 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. Knock occurs when the peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic “pinging” sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive.
Knocking should not be confused with pre-ignition – they are two separate events. However, pre-ignition is usually followed by knocking.
Detonation can be prevented by any or all of the following techniques:
- the use of a fuel with high octane rating, which increases the combustion temperature of the fuel and reduces the proclivity to detonate
- enriching the air–fuel ratio which alters the chemical reactions during combustion, reduces the combustion temperature and increases the margin above detonation
- reducing peak cylinder pressure
- decreasing the manifold pressure by reducing the throttle opening or boost pressure
- reducing the load on the engine
- retarding (reduce) ignition timing. Because pressure and temperature are strongly linked, knock can also be attenuated by controlling peak combustion chamber temperatures by compression ratio reduction, exhaust gas recirculation, appropriate calibration of the engine’s ignition timing schedule, and careful design of the engine’s combustion chambers and cooling system as well as controlling the initial air intake temperature.