What is defined as auto-ignition?

Auto-ignition refers to the phenomenon where a substance ignites without the need for an external ignition source, such as a spark or a flame. This self-ignition occurs when the temperature of the substance reaches a critical point due to heat produced by chemical reactions, compression, or other means.

In the context of combustion engines, for instance, auto-ignition is a critical factor for diesel engines, where the air and fuel mixture is compressed, raising its temperature sufficiently to ignite without the need for a spark plug. The ability to ignite spontaneously demonstrates the importance of temperature and pressure in determining when and how combustion will occur in various chemical systems.

The other choices describe different processes that do not accurately capture the essence of auto-ignition. For example, burning fuel without oxygen doesn’t constitute combustion; spontaneous formation of a compound doesn’t imply ignition, and while the reaction of fuel with oxygen is related to combustion, it typically relies on an ignition source rather than occurring spontaneously. The definition provided aligns perfectly with the phenomenon of auto-ignition as it emphasizes the timing and lack of external initiation.