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Knocking (also called pinking or pinging)—technically detonation—in internal combustion engines occurs when fuel/air mixture in the cylinder has been ignited by the spark plug and the smooth burning is interrupted by the unburned mixture in the combustion chamber exploding before the flame front can reach it. Combusting stops suddenly, because of the explosion, before the optimum moment of the four-stroke cycle. The resulting shockwave reverberates in the combustion chamber and pressures increase catastrophically, creating a characteristic metallic "pinging" sound.
Detonation
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 33.5 m/second (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 detonation 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 damage or destroy 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 (lower gear), decreasing the manifold pressure (throttle opening) or reducing the load on the engine, or any combination.
- Reduction of charge (in-cylinder) temperatures (such as through cooling, water injection or compression ratio reduction).
- Retardation of spark plug ignition.
- Improved combustion chamber design that 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 retard (delay) the ignition (spark plug firing) to prevent it, allowing engines to safely use petrol of below-design octane rating, with the consequence of reduced maximum power output and efficiency.
A knock sensor consists of a small piezoelectric microphone, on the engine block, connected to the engine's ECU. Spectral analysis is used to detect the trademark frequency produced by detonation at various RPM. When detonation is detected the ignition timing is retarded, reducing the knocking and protecting the engine. See also Automatic Performance Control (APC).
Pre-ignition
Pre-ignition is a different phenomenon from detonation, explained above, and occurs when the air/fuel mixture in the cylinder (or even just entering the cylinder) ignites before the spark plug fires. Pre-ignition is caused by an ignition source other than the spark. Heat or hot spots can buildup in engine intake or cylinder components due to improper design, for example, spark plugs with heat range too hot for the conditions, or due to carbon deposits in the combustion chamber, or also due to overheating of the air/fuel mixture during compression.
Pre-ignition and "dieseling" or "run on" are the same phenomenon, except in the latter case the engine continues to run after the ignition is shut off with a hot spot as an ignition source. Pre-ignition might cause rough running due to the advanced and erratic effective igniton timing and may cause noise if it leads to detonation. It may also cause "rumble" which is fast and premature but not detonating combustion.
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 and fuel economy is reduced and engine damage may result. Pre-ignition may lead to detonation and detonation may lead to pre-ignition or either may exist separately.
References
- 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
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"Engine knocking"