Autoignition of combustible materials, particularly hydrocarbon fuels, play an important role in the fire hazard in the storage and handling of these materials. The minimum autoignition (or spontaneous ignition) temperature of a given fuel is an important flammability property of the material, but its experimentally determined value is markedly dependent on the method and apparatus employed for its determination. Some of the complex phenomena associated with autoignition, such as cool flames, zones of nonignition, multiple ignition, ignition delay, and hot-surface ignition, are defined and discussed. The numerous experimental and other factors which may influence autoignition temperatures are discussed, and their relations to autoignition phenomena are described. Some of the factors are chemical structure and composition, fuel-air ratio, concentration of oxygen, surface-volume ratio, geometry of the containing vessel, and pressure.

This study develops an experimental method for the reliable determination of autoignition temperatures under a variety of conditions which involve short duration exposures of controlled fuel/air mixtures on three metal surfaces. Over 1100 autoignition temperature determinations have been made for the ignition of 15 hydrocarbons on heated nickel, stainless steel, and titanium surfaces for three different fuel/air mixtures. The measured autoignition temperatures generally decrease for the larger hydrocarbons and for richer mixtures, with the C[2 hydrocarbons having particularly low values. The highest autoignition temperatures are observed for nickel surfaces and the lowest for the stainless steel, with titanium being an intermediate case.

A new apparatus has been designed, built, and extensively tested for making short-duration autoignition temperature measurements of hydrocarbon fuels under conditions where the fuel/air stoichiometry, the nature of the hot metal surface, and the contact time are well controlled. This approach provides a much more reliable database to establish the importance of fuel structure effects than the current ASTM E659 procedure. Over 1100 individual autoignition temperature determinations have been made for the ignition of 15 hydrocarbon fuels on heated nickel, stainless steel, and titanium surfaces for three different fuel/air mixtures (stoichiometry theta = 0.7, l.0, and l.3). Excellent reproducibility has been achieved with the new apparatus. The measured autoignition temperatures generally decrease for the larger hydrocarbons and for richer mixtures, with the C2 hydrocarbons (ethane, ethylene, and acetylene) having particularly low values. The highest autoignition temperatures are observed for nickel surfaces and the lowest for stainless steel, with titanium being an intermediate case. A review of the autoignition literature suggests that the branched alkanes should be more resistant to autoignition than the linear isomers, and thus present a reduced hazard. Limited data obtained in this study are consistent with this prediction. Promising directions for substantiating these observations and additional areas for future research are outlined.