Cetane number is a measure of the ease with which a liquid fuel self-ignites. Cetane number is the diesel fuel equivalent to a gasoline octane rating, and expresses the fuel's ignition quality and its suitability for spontaneous self-ignition in an engine. The fuel must ignite spontaneously and quickly after being injected into the hot, compressed air inside the combustion chamber. A higher cetane number means a shorter ignition delay and a smaller amount of fuel in the combustion chamber when the fuel ignites.