COMBUSTION-chamber deposits are the primary cause of preignition. This preignition may be silent or it may be manifested by a variety of engine noises ranging from light pings to violent explosions.
Its occurrence, according to these authors, is favored by changes in engine operating conditions that result in the development of higher pressures and temperatures during compression, or that cause an increase in the concentration of oxygen available to support the combustion of carbonaceous material.
Thus, increased compression ratio, supercharging, operation with retarded spark timing, and the combustion of lean mixtures are likely to increase the tendency for the occurrence of preignition.
Some hydrocarbons have a greater tendency to be ignited by hot deposit particles than others. Preflame reactions of the fuel tend to sensitize it to ignition. Under certain conditions, the effect of these preflame reactions can be minimized by tetraethyl lead.