KNOCK - FLAME ACCELERATION OR SPONTANEOUS IGNITION

There is no general agreement on the precise mechanism by which knock occurs; contrasting theories attribute the phenomenon to extremely rapid flame propagation or to the spontaneous ignition of the end gas. Experiments designed to resolve this question have been made with a rapid compression machine. The development of pressure, the emission spectrum and the nature of the moderately stable products formed during the spontaneous ignition of stoichiometric mixtures of 2-methylpentane with air have been compared with those during the spark-induced ignition of similar mixtures. The spectrum and products of the stable flame of such mixtures have also been investigated. The results show that spontaneous ignition and the smooth spark-induced propagation of a flame are different processes but that there are strong physical and chemical similarities between spontaneous ignition and the last, knocking, stage which can intervene in spark-induced reactions. This discontinuity in the spark-induced reaction is consistent with the opinion that knock represents an end-gas explosion. © 1968.