Preignition is defined as ignition of the fuel prior to normal ignition, or ignition before the electrical arcing occurs at the spark plugs.
Preignition may be caused by excessively hot exhaust valves, carbon particles or spark plug electrodes heated to an incandescent or glowing state. In most cases these local "hot spots" are caused by the high temperatures encountered during detonation.
This form of abnormal combustion has the same effect on the engine as an early or advanced timing of the ignition system, and is so harmful in its effects that an engine will continue to operate normally only for a short period of time. This holds especially true if detonation and preignition are in progress simultaneously. During preignition conditions, cylinder pressures are in excess of the normal limits of the cylinder and the engine structure.
One significant difference between preignition and detonation lies in the fact that if the conditions for detonation exists in one cylinder, they may exist in all cylinders; but, preignition may exist in only one or two cylinders. This can make preignition rather difficult to detect, because of the possibility of preignition occurring in a cylinder which is not the location of the thermocouple which measures cylinder head temperature. Probably the most reliable indication is a loss of power, but this also may be difficult to determine unless the engine has a torquemeter. Another indication of preignition may be the observation of glowing carbon particles being discharged from the exhaust system.
Corrective actions for preignition include any type of engine operation which would promote cooling, such as enriching the fuel/air mixtures, reducing cylinder/manifold pressures, and properly controlling engine cowl flaps when available.