Detonation of Motor Fuels

(knock), a phenomenon that occurs in piston internal-combustion engines with spark ignition; it develops as a result of the formation and accumulation in the fuel charge of organic peroxides, which are the primary products of the oxidation of hydrocarbon fuel. If a certain critical concentration of peroxides is reached in the mixture, detonation occurs, which is characterized by an un-usually high velocity of flame propagation and the formation of shock waves. During normal engine operation the flame propagates at a velocity of 10-20 m/sec, but during detonation the velocity increases to 1,500-2,500 m/sec. Detonation of motor fuels is manifested by metallic “pinging,” smoky exhaust, vibration, and overheating of the engine, and it results in sticking of the rings, burning-out of the pistons and valves, destruction of the bearings, and loss of engine power.

The development and intensity of detonation depends on the operating conditions and design features of the engine and the chemical composition of the fuel. Fuels containing a large amount of nonbranched paraffin hydrocarbons knock more readily than those containing branched paraffin and aromatic hydrocarbons, which are detonation-resistant. The antiknock rating (octane number) of the individual fuel components depends on the composition of the fuel-air mixture (lean or rich mixtures).

The antiknock rating for lean gasoline mixtures is characterized by the octane number, and the antiknock rating for rich mixtures is based on the gasoline grade. The knock resistance of gasolines is increased by the use of antiknock compounds, such as tetraethyl lead.