Cetane number

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Cetane number or CN a measure of the combustion quality of diesel fuel under compression, one measure of fuel quality. Cetane number is actually a measure of a fuel's ignition delay; the time period between the start of injection and start of combustion (ignition) of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. Cetane numbers are only used for the relatively light distillate diesel oils. For heavy (residual) fuel oil two other scales are used CCAI and CII.

Diesel engines run well with a CN between 45 to 50. There is no performance or emission advantage when the CN is raised past 50; after this point, the fuel's performance hits a plateau. Diesel at the pump can be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. Premium diesel has additives to improve CN and lubricity, detergents to clean the fuel injectors and minimize carbon deposits, water dispersants, and other additives depending on geographical and seasonal needs.

Alkyl nitrates and di-tert-butyl peroxide are used as additives to raise the cetane number.

1 Comparison to Octane Rating
2 Chemical Relavance
3 Measuring Cetane Number
4 Further Reading
5 See Also
6 External Links

[edit] Comparison to Octane Rating

Cetane rating of diesel is not equivalent to gasoline's octane rating. This is because gasoline's desirable property is to resist autoignition to prevent engine knocking whereas diesel's desirable property is to autoignite.[1]

[edit] Chemical Relavance

Cetane is an alkane molecule that ignites very easily under compression, so it was assigned a cetane number of 100. All other hydrocarbons in diesel fuel are indexed to cetane as to how well they ignite under compression. The cetane number therefore measures how quickly the fuel starts to burn (auto-ignites) under diesel engine conditions. Since there are hundreds of components in diesel fuel, with each having a different cetane quality, the overall cetane number of the diesel is the average cetane quality of all the components. There is very little actual cetane in diesel fuel.

[edit] Measuring Cetane Number

To measure cetane number properly is rather difficult, as it requires burning the fuel in a special, hard-to-find, diesel engine called a Cooperative Fuel Research (CFR) engine, under standard test conditions. For most practical purposes, fuel-users control quality using the Cetane Index, which is a calculated number based on the density and distillation range of the fuel. There are various versions of this, depending on whether you use metric or Imperial units, and how many distillation point are used. These days most oil companies use the '4-point method'.

[edit] Further Reading

Internal Combustion Engine Fundamentals, John B. Heywood, McGraw Hill, 1988 ISBN 0-07-100499-8
Automotive Fuels Reference Book, Keith Owen, Trevor Coley SAE, 1995, ISBN 1-56091-589-7