Which type of ignition is not initiated by an external source?

Auto ignition is a process where a material spontaneously ignites without the need for an external ignition source, such as a flame or spark. This phenomenon occurs when the temperature of the material reaches its ignition point due to internal heat generation, such as from chemical reactions, compression, or heat accumulation. In the context of fire behavior, understanding auto ignition is critical because it demonstrates how certain conditions can lead to combustion without obvious external triggers.

Piloted ignition requires an external heat source to ignite a fuel, while flashback ignition is also dependent on an external flame or spark traveling back along the path of a fuel-air mixture. Convection ignition relates to heat transfer between air and heated surfaces or materials, often requiring an external heat source as well. The unique characteristic of auto ignition distinguishes it as a self-initiating process, highlighting the importance of temperature and chemical properties in the ignition of fuels.