When the fuel/air mixture is subjected to a combination of excessively high temperature and high pressure within the cylinder, the spontaneous combustion point of the gaseous mixture is reached. When this critical detonation point is reached, normal progressive combustion is replaced by a sudden explosion, or instantaneous combustion (Fig. 2-14). Due to the piston's position in the cylinder at the time the detonation wave starts, extremely high pressures are reached, often in excess of the structural limits of the cylinder and engine parts. Tests have proven that pressures in excess of 4,000 PSI are reached during detonation. Since these pressures are virtually instantaneous, the effect on the piston is equivalent to a sharp blow with a sledge hammer. This shattering force is what is sometimes heard in an automobile as it is accelerated rapidly. In an automobile engine this is not so serious, because it is heard and can be readily remedied by reducing the engine power. In aircraft engines it is much more serious because it is difficult to detect above other aircraft noises and corrective action may then be too late to be effective. This form of combustion causes a definite loss of power, engine overheating, preignition and, if allowed to continue, physical damage to the engine.

Although detection of detonation may be extremely difficult, the indications are an otherwise unexplained rise in cylinder head temperature, an unexplained loss of power, especially at the higher power settings, and a whitish orange exhaust flame accompanied by puffs of black smoke.

Corrective action for detonation may be accomplished by adjusting any of the engine controls which will reduce both temperature and pressure of the fuel/air charge. Good engine operating procedures by the pilot will prevent detonation and from the pilot's standpoint, prevention is the best cure.