A flammable mixture residing at sufficiently high temperature will ignite without further interaction after some temporal delay. This phenomenon is called ‘autoignition’ or ‘selfignition’. The underlying process is fuel reforming and buildup of intermediate species and radicals. In the initial stage of combustion this is accompanied by only a small fraction of the total heat release (‘low’ temperature combustion). When the radicals and intermediates have reached critical concentrations, the chemistry transitions to the ‘high’ temperature regime with significant heat release.

The time expiring until the transition to the high temperature regime occurs, the ignition delay time, is determined by detailed chemistry with dozens of species and hundreds of reactions involved. For CFD it is impractical to calculate this process in all details. Instead, the progress of fuel reforming and radical buildup is correlated with the elapsed fraction of the delay time, . A transport equation is solved in order to account for variations of the ignition delay time:

(7–104)

The production term is disabled when no fuel is available, such that there is no further growth behind the flame front. The elapsed fraction of ignition delay time, , may also be interpreted as a normalized radical concentration.

Autoignition is modeled to occur when the scalar exceeds a threshold, . That is, reaction occurs if, and only if, the delay time has expired. This leads to a distinction in two kinds of autoignition models: ‘Ignition Delay’ and ‘Knock’.

The physical information about ignition delay is provided by the ignition delay time, , which is the primary input parameter to the model.