The cetane number is used to measure the ignition quality of diesel fuel.
A diesel engine compresses air until it becomes hot, then injects fuel into that compressed air. Cetane number indicates how readily the fuel ignites under those conditions. A higher cetane number generally corresponds to a shorter ignition delay and smoother combustion, provided the engine and fuel system are suitable.
The cetane scale was historically associated with cetane, also called hexadecane, which was assigned a value of 100. A reference compound with poor ignition quality established the lower end of the original scale. Modern testing uses standardized engine methods and reference fuels rather than relying only on the pure compounds.
Cetane number is often confused with octane rating. Octane describes gasoline’s resistance to premature knocking in spark-ignition engines. Cetane describes diesel fuel’s readiness to ignite in compression-ignition engines, so the two measurements address opposite combustion qualities.