Studies of knock and antiknock by kinetic spectroscopy

Abstract

The method of flash photolysis has been used to ignite mixtures of acetylene, oxygen and amyl nitrite, the latter serving as fuel and as sensitizer. The course of the explosive combustion was followed by flash spectroscopy and by photoelectric technique. Two phases have been conveniently distinguished in the explosion : (1) An induction period following the immediate disappearance of the spectrum of amyl nitrite during which no radical spectra were visible. (2) The fast build-up of diatomic radical spectra—predominantly OH, with a little NH and CN— at the point of ignition accompanied by strong light emission upon which were superimposed high-temperature emission peaks of only a few microseconds’ duration indicating detonation waves travelling through the reacting gases. On the addition of small amounts of lead tetraethyl the induction period was greatly lengthened, and during that time the waxing and waning of the lead oxide band system was observed. 0∙2 mm (Hg) of lead tetraethyl increased phase 2 up to 200%, and the strong emission caused by heavy detonation was often quenched completely. The entirely different behaviour of aromatics is also shortly discussed. It was established that the antiknock effect of lead tetraethyl is a completely homogeneous gas-phase reaction, and it was concluded that its effect is twofold in nature: first, by reacting with oxygenated intermediates produced in phase 1 of the combustion with the formation of lead oxide, it removes the sources of auto-ignition which lead to knock in phase 2; secondly, the lead monoxide so formed reduces the propagating and branching mechanisms leading to the final explosion by preferential reaction with the free radicals involved, and thus contributes to conditions which do not lead to detonation and knock. This reaction and also the conditions obtaining at the end of the explosion promote the final reduction of the lead monoxide to atomic lead.