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A cetane number is a number from 0 to 100 that indicates the relative combustive qualities of diesel fuel. It is one of a number of measurable factors that denote the overall characteristics and quality of diesel fuel. It is also one of the most important of these factors. If a diesel fuel were composed of pure cetane, the component of diesel fuel that ignites the most easily, it would be assigned a number of 100. Generally, the higher the cetane number of a particular fuel, the higher the quality of the fuel.
Diesel fuel contains hundreds of different hydrocarbons, of which cetane is only one. The fuel must ignite under compression, and since cetane ignites the most readily under the conditions inside the combustion chambers of diesel engines, it was chosen as the standard to describe a diesel fuel's relative combustibility. The cetane number of any diesel fuel is actually an average of all the cetane numbers of the various hydrocarbons found in that fuel.
The combustibility of diesel fuel affects how a diesel engine runs. A fuel with a low cetane number can cause a diesel engine to run sluggishly and have higher emissions, as the fuel is not burned as efficiently as it might be. It may also cause the engine to be more difficult to start. A diesel fuel with a high cetane number will ignite more readily, burn more completely, and deliver more power than fuels with lower numbers.
The design of diesel engines and the components of diesel fuel dictate that most diesel engines will reach maximum fuel efficiency with a fuel that has a cetane number of around 55. Higher numbers generally do not provide any more power, fuel efficiency, or reduction in emissions, although in some very high performance diesel engines, this plateau may be reached at a cetane number closer to 60. Around the world, many countries have set standards for minimum cetane numbers for diesel fuel. Most of these ratings range from 40 to 51.
To determine a cetane number, a fuel is tested using a specially designed, variable-compression diesel engine. As these engines are quite expensive and can be difficult to gain access to for testing, another number, called a cetane index is often used to describe fuel quality. This number is obtained by a different method and is not the same as the cetane number, although the two are often mistakenly assumed to be interchangeable.