autoignition: one construct, many systems

In engine combustion, autoignition is the spontaneous onset of burning driven purely by the temperature and pressure of a fuel–air mixture, with no spark or flame to start it: the same chemistry is harmful in a spark-ignition engine, where premature end-gas autoignition causes knock, and essential in a diesel or HCCI engine, where it is the intended ignition mechanism. Westbrook's 2000 Chemical kinetics of hydrocarbon ignition in practical combustion systems traces how low-temperature chain reactions build up to this self-ignition and set its timing. The referent is the kinetically controlled self-ignition of a mixture, measured by ignition delay, and it is the identical phenomenon that fire-safety engineers record as a material's autoignition temperature.[1]

Unit

The ignition delay before a mixture self-ignites at given temperature and pressure

Recovery means

Not applicable

Measured by

Characterised by ignition-delay times in shock tubes and rapid-compression machines and by the pressure–temperature conditions that trigger self-ignition

In fire and process safety the same phenomenon is captured as a material property: the autoignition temperature is the lowest temperature at which a substance will ignite on its own in air without any external flame or spark, a number used to judge whether a hot surface, process vessel, or stored chemical is safe. Babrauskas's Ignition Handbook compiles these values and explains how strongly they depend on test method, vessel size, and fuel concentration, so a single tabulated figure can mislead. The referent is a measured self-ignition threshold used for hazard assessment, the identical spontaneous-ignition construct as engine autoignition, read here as a conservative design limit rather than a combustion-timing event. It is distinct from flash point, which is about forming an ignitable vapour for an external spark, not igniting unaided.[2]

Unit

The lowest air temperature at which a substance ignites without an external source

Recovery means

Not applicable

Measured by

Determined in standardised hot-vessel or hot-surface tests, reported as the autoignition temperature with its strong dependence on apparatus and conditions noted