Cetane number

Cetane number or CN is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of diesel fuel quality among a number of other measurements that determine overall diesel fuel quality. Cetane number of a fuel is defined as the percentage by volume of normal cetane in a mixture of normal cetane and alpha-methyl naphthalene which has the same ignition characteristics (ignition delay) as the test fuel when combustion is carried out in a standard engine under specified operating conditions..

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.

Generally, diesel engines run well with a CN from 40 to 55. Fuels with higher cetane number which have shorter ignition delays provide more time for the fuel combustion process to be completed. Hence, higher speed diesels operate more effectively with higher cetane number fuels. There is no performance or emission advantage when the CN is raised past approximately 55; after this point, the fuel's performance hits a plateau. In North America, diesel at the pump can be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. Premium diesel may have 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.

In Europe, diesel cetane numbers were set at a minimum of 49 in 1994 and 51 in 2000.

Dimethyl ether may prove advantageous as a future diesel fuel as it has a high cetane rating (55) and can be produced as a biofuel. Alkyl nitrates (principally 2-ethyl hexyl nitrate ) and di-tert-butyl peroxide are used as additives to raise the cetane number.

Chemical relevance
Cetane is an un-branched open chain alkane molecule that ignites very easily under compression, so it was assigned a cetane number of 100, while alpha-methyl napthalene was assigned a cetane number of 0. 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.

Fuels used in spark ignition engines are typically branched alkanes. These are particularly suited for these engines for which the desirable property is to resist autoignition and to burn quickly once ignited. The lack of premature ignition (engine knocking) is characterised by the Octane Number (ON), which is also measurable in an appropriate test engine. Similar to the cetane test, a two-fuel mixture (iso-octane and normal-heptane) shows an inverse relationship with the CN of the same fuel. (Good CN = bad ON and vice versa)

Measuring cetane number
To measure the 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. The operator of the CFR engine uses a hand-wheel to increase the pressure within the cylinder of the engine until the time between fuel injection and ignition is 2.407ms. The resulting cetane number is then calculated by determining which mixture of cetane (hexadecane) and isocetane (2,2,4,4,6,8,8-heptamethylnonane) will result in the same ignition delay.

Another reliable and more precise method of measuring the cetane number of diesel fuel is the Ignition Quality Tester (IQTTM). This instrument applies a simpler, more robust approach to CN measurement than the CFR. Fuel is injected into a constant volume combustion chamber in which the ambient temperature is approximately 575°C. The fuel combusts, and the high rate of pressure change within the chamber defines the start of combustion. The ignition delay of the fuel can then be calculated as the time difference between the start of fuel injection and the start of combustion. The fuel's derived cetane number can then be calculated using an empirical inverse relationship to ignition delay.

Another method that fuel-users control quality is by using the Cetane index (CI), 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 points are used. These days most oil companies use the '4-point method', ASTM D4737, based on density, 10% 51% and 89% evaporation temperatures.. The '2-point method'' is defined in ASTM D976, and uses just density and 50% evaporation. However, CI can not be used with cetane improver additives.

The industry standards for measuring cetane number are ASTM D-613 (ISO 5165) and D-6890.