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an arbitrary quantitative characteristic of the spontaneous combustion of diesel fuels in the cylinder of an engine. As is the case with the octane number of a gasoline, the cetane number of a diesel fuel is determined under strictly controlled conditions; cetane and α-methylnaphthalene, the cetane numbers of which are assumed to be 100 and 0, respectively, are used as the standard references. The ignition quality of a fuel being tested is characterized by determining the composition of a blend of cetane and α-methylnaphthalene that has an equivalent ignition quality. Quantitatively, the cetane number indicates the percentage by volume of cetane in the reference blend. The higher the cetane number of a diesel fuel, the quicker the ignition of the fuel in the engine, the smoother the pressure increase, and the quieter the operation of the engine. The cetane numbers of present-day diesel fuels range from 40 to 55. Additives, such as organic peroxides and nitroalkanes, are used to improve the cetane number.