Knocking (also called pinking or pinging)—technically detonation—in internal combustion engines occurs when fuel in the cylinder is ignited by the firing of the spark plug and smooth burning proceeds but some of the unburned mixture in the combustion chamber explodes before the flame front can reach it, combusting suddenly before the optimum moment of the four-stroke cycle. The resulting shockwave reverberates in the combustion chamber creating a characteristic metallic "pinging" sound.
The fuel/air mixture is normally ignited slightly before the point of maximum compression to allow a small time for the flame-front of the burning fuel to expand throughout the mixture, so that maximum pressure occurs at the optimum point. The flame-front moves at roughly 110 feet/second during normal combustion. It is only when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time that the knocking effect occurs. It is caused by an instantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinder pressure rises dramatically beyond design limits. If allowed to persist detonation will cause vibration and damage to engine parts.
Detonation can be prevented by:
- The use of a fuel with higher octane rating
- The addition of octane-increasing "lead," isooctane, or other fuel additives.
- Reduction of cylinder pressure by increasing the engine revolutions or reducing the load on the engine, or both.
- Reduction of in-cylinder temperatures (such as through cooling or compression ratio reduction).
- Retardation of spark plug ignition.
- Adding water in the form of vapor in the air intake.
- Proper combustion chamber design which concentrates mixture near the spark plug and generates high turbulence to promote fast even burning.
- Use of a spark plug of colder heat range in cases where the spark plug insulator has become a source of pre-ignition leading to detonation.
Correct ignition timing is essential for optimum engine performance and fuel efficiency. Modern automotive and small-boat engines have sensors that can detect knock and delay the spark plug firing to prevent it, allowing engines to safely use petrol of lower octane ratings, with the consequence of reduced power and efficiency.
A knock sensor consists of a small piezoelectric microphone on the engine block, connected to the engine's electronic control unit. Spectral analysis is used to detect the trademark frequency produced by detonation at various RPM. When detonation is detected, ignition timing is retarded, reducing the knocking and protecting the engine. See also Automatic Performance Control (APC).
Detonation, explained above, is a different phenomenon from pre-ignition, which occurs when the air/fuel mixture in the cylinder (or even just entering the cylinder) ignites before the spark plug fires. Pre-ignition is accompanied by a loud hacking noise and caused by an ignition source other than the spark. Heat can buildup in engine intake or cylinder components due to improper design e.g. spark plugs with too low a heat rating or due to maintenance, e.g. carbon deposits in the combustion chamber, or also due to overheating of the air/fuel mixture during compression. This heat buildup can only be prevented by eliminating the overheating (through redesign or cleaning) or the compression effects (by reducing the load on the engine or temperature of intake air). As such, if pre-ignition is allowed to continue for any length of time, power output reduces drastically and engine damage can result.
Pre-ignition may lead to detonation and detonation may lead to pre-ignition or either may exist separately.
- Charles Fayette Taylor, Internal Combustion Engine in Theory and Practice: Vol. 2, Revised Edition, MIT Press, 1985, Chapter 2 on "Detonation and Preignition", pp 34-85. ISBN 0-262-20052-X