What kind of ignition does not need a spark or flame to start?

Autoignition refers to the process in which a substance spontaneously ignites without the presence of an external flame or spark. This phenomenon occurs when the temperature of a combustible material reaches its ignition temperature due to heat from its environment or due to its own chemical reactions.

This form of ignition demonstrates the principles of thermodynamics and combustion science, indicating that under certain conditions, materials can ignite purely through the buildup of heat. Common examples include combustible gases that can ignite when exposed to high temperatures, such as in the case of diesel engines or certain materials in a fire scenario that may reach the necessary temperatures for autoignition.

In contrast, the other types of ignition discussed do rely on specific external factors to initiate combustion. Chemical ignition typically involves a reaction with an oxidizing agent, piloted ignition requires an external source like a spark or flame, and manual ignition indicates direct intervention to cause combustion, thus making autoignition a unique and critical concept in understanding fire behavior and prevention.