autoignite

Autoignite, or autoignition, refers to the ignition of a fuel–air mixture without an external ignition source. It occurs when the mixture reaches the conditions needed for the chemical reaction between fuel and oxidizer to begin spontaneously, typically due to sufficiently high temperature and/or pressure. The temperature at which ignition occurs is called the autoignition temperature (AIT); this value varies widely among fuels and is affected by mixture composition, pressure, heat transfer, and contaminants.

In propulsion and power systems, autoignition is a central concept in compression-ignition engines. In diesel engines, air is compressed to high temperature and pressure, and fuel injected into the hot charge autoignites. In spark-ignition engines, autoignition is generally undesirable and can manifest as knocking or pre-ignition when hot spots or excessive compression raise the local temperature above the fuel’s AIT before the spark can ignite the mixture. Engine designers manage this through fuel choice, ignition timing, compression ratio, and charge cooling.

Safety and engineering practice treat autoignition as a hazard for flammable liquids and vapors. Reliable AIT data and standardized testing support hazard classification and risk assessment for storage, handling, and transport. In process design, understanding autoignition helps prevent unwanted ignition and informs safety margins. Autoignition concepts also relate to broader topics such as reaction runaway in chemical processes and the distinction between autoignition and other ignition phenomena like pre-ignition and detonation.