Cetane number is an important indicator of quality of a diesel fuel. Cetane number represents the ignition quality of a fuel. High cetane number indicates a fuel that burns smoothly in the engine cylinder with minimum delay between fuel injection and autoignition.
Cetane (n-hexadecane) ignites readily and burns smoothly in a diesel engine. This hydrocarbon is arbitrarily assigned the cetane number of 100. Other fuels are compared with cetane for autoignition quality and then assigned with a cetane number. α-methylnaphthalene behaves very poorly in a diesel engine and is assigned with 0 cetane number.So, cetane number of a fuel is equal to the percentage of cetane in a mixture of cetane and α-methylnaphthalene which has the same ignition performance in a diesel engine as of the fuel under question.
Typically, diesel engines perform well with fuels having cetane number between 40 to 55. Higher cetane numbers mean shorter time delay between fuel injection and autoignition and provide more time for fuel combustion process to be complete. But performance of a diesel engine hits a plateau for cetane numbers higher than approximately 55. Hence fuels with higher cetane numbers have little added advantage.
Cetane index which can be calculated based on the API gravity and distillation range of a fuel is also used as an indicator of the fuel quality. There are numerous ways to determine the cetane index for a fuel depending on the units used, distillation points used etc.
See also octane number, which is use to indicate the antiknocking quality of fuels to be used in an ignition engine.