# Autoignition temperature

The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical will ignite decreases as the pressure increases or oxygen concentration increases. It is usually applied to a combustible fuel mixture.

Autoignition temperatures of liquid chemicals are typically measured using a 500 mL flask placed in a temperature controlled oven in accordance with the procedure described in ASTM E659 <ref>E659 – 78 (Reapproved 2000), "Standard Test Method for Autoignition Temperature of Liquid Chemicals", ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959</ref>.

## Autoignition equation

The time $t_{ig}\,$ it takes for a material to reach its autoignition temperature $T_{ig}\,$ when exposed to a heat flux $q\,$ is given by the following equation

$t_{ig} = \left ( \frac{\pi}{4} \right ) \left (k \rho c \right )\left [ \frac{T_{ig}-T_\infty}{q} \right]$ <ref>Principles of Fire Behavior. ISBN 0-8273-7732-0. 1998. </ref>

where k = thermal conductivity (W/(m·K)), ρ = density (kg/m³), and c = specific heat capacity (J/(kg·K)) of the material of interest. $T_\infty$ is the temperature, in kelvins, the material starts at (or the temperature of the bulk material), and q″ is the heat flux (W/m²) incident to the material.