Copied from Powerplants for Aerospace Vehicles, Northrup Institute of Technology,Third edition Copyright 1965
The preignition knock
A knock similar to the one just described may occur in an aircraft engine when the compressed charge is, in some way, ignited before the spark jumps the gap between the spark-plug electrodes, or points. This is called a preignition knock. Of fuel knocks, this one is probably the most destructive. If the condition which causes it is not immediately corrected, backfire pressure in the induction system may damage the supercharger and the untimed combustion, may melt pistons and crack cylinder heads through excessive overheating.
Preignition, also called surface ignition because it is caused by surface conditions within the combustion chamber, is the ignition of the compressed charge before the action of the spark plug. It may be caused by valves which have been ground, or lapped, to feather edges during an overhaul and which consequently be¬come red hot in operation owing to their thinness, by excessive carbon in cylinder heads, and by spark plugs thickly coated with carbon. The normal heat of the engine may cause the carbon formation in the combustion chambers, the thin valves, or the faulty spark plugs to glow incandescently, thus firing the charge prior to regular spark-plug action.
Aside from the fact that preignition and the use of unsuitable fuel can do great damage to the aircraft engine, the possibility of its causing engine failure while in fight should be of sufficient importance to guarantee every measure being taken to correct this faulty condition once its existence is known.
Detonation
The word detonation is generally used by automotive and aircraft engine specialists to describe severe fuel knock. Actually, in plain English, detonation means an explosion, but when the word is used by internal combustion-engine men, it is not employed in its normal, everyday sense. Fuel knock is merely the result of abnormal burning or uncontrolled combustion.
The availability of improved metal alloys has made it possible to construct engines of high power output, with higher compression ratios and higher manifold pressures than before. However, the improvement of engines has been limited by the fact that detonation will occur when any vaporized fuel in an engine cylinder is compressed to a critical pressure which is characteristic of that particular fuel.
Detonation means that, after the fuel is ignited, the fuel/air mixture bums at a normal rate until a substantial portion of the mixture is consumed and then the remainder burns with explosive rapidity. There is an unusual rise in pressure, accompanied by a distinct metallic knock. The gases of combustion, the engine valves, and various parts of the engine are heated to excessive temperatures. Overheating of the engine causes expansion of the fuel/air mixture; less fuel is burned; hence the engine power output is decreased. A large part of the power loss resulting from detonation is due to the fact that the piston cannot accelerate rapidly enough to convert the abnormally high transient pressures into useful power. Cylinder walls and other metal parts vibrate and chatter owing to the sudden pressure increase of "explosion." Detonation may cause the engine to stop; may cause cracked pistons, burned valves, and other mechanical damage in the engine; and may also lead to preignition.
Preignition always occurs before the normal period of ignition. Detonation, on the other hand, occurs after the fuel has been ignited. Since preignition occurs when metal parts within the combustion chamber are overheated from any cause, thus igniting the fuel/air mixture before the ignition spark jumps the gap, it is apparent that the excessive temperatures which exist in a condition of detonation can lead to preignition, as mentioned above.
Preignition advances the ignition period abnormally and further reduces the engine efficiency. If the ignition switch of an overheated engine is opened, the engine may continue to run for several minutes because of this unintentional ignition of the fuel.
Causes of detonation
Some of the various factors which contribute to cause detonation are the cylinder temperature, the amount of the induced charge, the temperature of the fuel/air mixture, the mixture ratio of gasoline and air, the intake manifold pressure, the positions of the spark plugs, and the shape of the combustion chamber. All these factors pertain to the design or adjustment of the carburetor, the supercharger, or the engine itself. In addition to these factors, one of the most important factors contributing to detonation is the antiknock or octane value of the fuel. Since this topic pertains directly to the subject of this chapter, it is discussed at this point.
