Cetane number measures diesel fuel’s combustible qualities on a scale of 0-100, with higher numbers indicating better quality. Cetane is one of many hydrocarbons in diesel fuel, but it ignites more easily, making it the standard for flammability. Low cetane numbers can cause engine problems, while high numbers improve efficiency and horsepower. Most diesel engines operate best on fuel with a cetane number of around 55, and many countries have set minimum standards. Cetane numbers are determined through testing with a variable compression diesel engine, but a cetane rating is often used instead.
A cetane number is a number from 0 to 100 that indicates the relative combustible qualities of diesel fuel. It is one of several measurable factors that denote the general 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 ignited most easily, it would be assigned a number of 100. Generally, the higher the cetane number of a given fuel, the higher the quality of the fuel.
Diesel fuel contains hundreds of different hydrocarbons, of which cetane is just one. Fuel must ignite under compression, and because cetane ignites more easily under conditions within the combustion chambers of diesel engines, it has been chosen as the standard to describe the relative flammability of a diesel fuel. The cetane number of any diesel fuel is actually an average of all the cetane numbers of the various hydrocarbons in that fuel.
Combustibility of diesel fuel affects the operation of a diesel engine. A fuel with a low cetane number can cause a diesel engine to run slowly and have higher emissions, because the fuel isn’t burned as efficiently as it could be. It could also make starting the engine more difficult. Diesel fuel with a high cetane number will ignite more easily, burn more completely, and deliver more horsepower than fuels with a lower cetane number.
The design of diesel engines and diesel fuel components dictates that most diesel engines achieve maximum fuel efficiency on fuel that has a cetane number of approximately 55. Higher numbers generally do not provide more horsepower, fuel efficiency or reduced emissions, although in some very high performance diesel engines, this level can be achieved at a cetane number closer to 60. Worldwide, 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. Because these engines are quite expensive and can be difficult to access for testing, another number, called the cetane rating, 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 considered interchangeable.
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