The self-ignition of a volume of explosive gas mixture is understood to be the transition from slow to very rapid chemical reaction with approximately adiabatic heat evolution. Two known processes bring this about. They are distinguished as thermal and branched-chain explosion. In the former, the chemical reaction becomes self-accelerating owing to the temperature rise. In the latter, the self-acceleration is caused by the formation of chemically active particles at a rate that exceeds the rate of destruction of these particles. A peculiar process of the latter type plays an important contributory part in engine knock. Local ignition, as by a spark, results in the formation of a combustion wave which propagates through the explosive mixture in a manner analogous to Huyghen’s principle for the propagation of a light wave. Small sparks do not produce ignition. The minimum ignition repuirement of electric capacitance sparks in mixtures of methane or other hydrocarbons with oxygen and nitrogen is found to be only the spark energy. This is theoretically explained, and the minimum spark-ignition energy is shown to be a function of the burning velocity, the width of the combustion wave, and other variables of the gas mixture in agreement with experimental data. A combustion wave in a stream of explosive gas mixture becomes stationary, i.e., the flame becomes stable, when the burning velocity is equal to the gas velocity somewhere in the wave and is nowhere larger than the gas velocity. The mechanism is described by which the condition of equality of burning velocity and gas velocity is realized at the rim of a burner tube. It is shown that the limits for flash back and blowoff correspond to critical values of the gas-velocity gradient at the stream boundary. This applies equally to the outer boundary and any inner boundary formed by solid objects in the stream.

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