The Cool Flames of Hydrocarbons

Abstract

AbstractThis review describes the properties of the cool‐flame and two‐stage ignition processes that characterize the gaseous oxidation of hydrocarbons and discusses the chemical reactions that are responsible for these phenomena. Cool flames result from a chainthermal acceleration of reaction rate. It is probable that the free‐radical chain involved is propagated by the reaction of an alkyl radical with oxygen to give an alkyperoxy radical which isomerizes to a hydroperoxyalkyl radical. The decomposition of this radical produces a hydroxyl radical, which attacks the hydrocarbon rapidly and unselectively to regenerate an alkyl radical. Branching probably results from the pyrolysis of mono‐ and dihydroperoxides, from the oxidation of aldehydes, and from radical‐molecule reactions. This reaction scheme explains the existence of a low‐temperature ignition peninsula and the relation of the extent and shape of this peninsula to the molecular structure of the hydrocarbon. The chemical relevance of cool flames to abnormal combustion phenomena, such as knock, in gasoline engines is discussed.