Cetane number (cetane rating) is an indicator of the combustion speed of diesel fuel and compression needed for ignition. It plays a similar role for diesel as

octane rating An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating. The higher the octane number, the more compression the fuel can withstand before detonating ...

does for gasoline. The CN is an important factor in determining the quality of diesel fuel, but not the only one; other measurements of diesel fuel's quality include (but are not limited to) energy content, density, lubricity, cold-flow properties and sulphur content.Werner Dabelstein, Arno Reglitzky, Andrea Schütze and Klaus Reders "Automotive Fuels" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2007, Wiley-VCH, Weinheim.

Definition

The cetane number (or CN) of a fuel is defined by finding a blend of cetane and isocetane with the same ignition delay. Cetane has a cetane number defined to be 100, while isocetane's measured cetane number is 15, replacing the former reference fuel alpha-methylnaphthalene, which was assigned a cetane number of 0. Once the blend is known, the cetane number is calculated as a volume-weighted average, rounded to the nearest whole number, of cetane's 100 and heptamethylnonane's 15. :cetane number = % ''n''-cetane + 0.15(% heptamethylnonane) Cetane number is an inverse function of a fuel's ignition delay, the time period between the start of ignition and the first identifiable pressure increase during combustion 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.

Typical values

Generally, diesel engines operate well with a CN from 48 to 50. Fuels with lower cetane number have longer ignition delays, requiring more time for the fuel combustion process to be completed. Hence, higher speed diesel engines operate more effectively with higher cetane number fuels. In Europe, diesel cetane numbers were set at a minimum of 38 in 1994 and 40 in 2000. The current standard for diesel sold in European Union, Iceland, Norway and

Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...

is set in EN 590, with a minimum cetane index of 46 and a minimum cetane number of 51. Premium diesel fuel can have a cetane number as high as 60. In Finland, premium diesel fuels sold by filling station chains

St1 St1 Oy is a Finnish energy company owning the St1 service stations' chain in Finland, Sweden, Norway and Poland. The company was established in 1995 under the name Greenergy Baltic Oy. In 1997, the St1 chain was established in Finland . In 2 ...

(Diesel Plus), Shell (containing GTL) and ABC (Smart Diesel) have a minimum cetane number of 60 with the typical value being at 63. Neste MY

Renewable Diesel Vegetable oil can be used as an alternative fuel in diesel engines and in heating oil burners. When vegetable oil is used directly as a fuel, in either modified or unmodified equipment, it is referred to as straight vegetable oil (SVO) or pure ...

sold in Finland has a minimum cetane number of 70. In North America, most states adopt ASTM D975 as their diesel fuel standard and the minimum cetane number is set at 40, with typical values in the 42-45 range. Premium diesel fuel may or may not have higher cetane number depending on the supplier. Premium diesel often use additives to improve CN and lubricity,

detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are more ...

s to clean the fuel injectors and minimize carbon deposits, water dispersants, and other additives depending on geographical and seasonal needs.. California diesel fuel has a minimum cetane of 53. Under the Texas Low Emission Diesel (TxLED) program there are 110 counties where diesel fuel must have a cetane number of 48 or greater, or else must use an approved alternative formulation or comply with the designated alternative limits. Neste MY Renewable Diesel sold in North America has a cetane number of 75-85.

Additives

Alkyl nitrates (principally 2-ethylhexyl nitrate) and di-''tert''-butyl peroxide are used as additives to raise the cetane number.

Alternative fuels

Biodiesel from vegetable oil sources have been recorded as having a cetane number range of 46 to 52, and animal-fat based biodiesels cetane numbers range from 56 to 60. Dimethyl ether is a potential diesel fuel as it has a high cetane rating (55-60) and can be produced as a

biofuel Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. According to the United States Energy Information Administration (E ...

. Most simple ethers, including liquid ones, such as diethyl ether can be used as diesel fuels, although the lubricity can be of concern.

Chemical relevance

Cetane is the hydrocarbon with chemical formulas C₁₆H₃₄ and CH₃(CH₂)₁₄CH₃. Named n-hexadecane, it is an unbranched saturated

alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...

. Cetane ignites with a short delay under compression, and is assigned a cetane number of 100. Alpha-methylnaphthalene, which has a long delay period, was assigned a cetane number of 0, but has been replaced as a reference fuel by 2,3,4,5,6,7,8-heptamethylnonane, whose cetane number is 15. All other hydrocarbons in diesel fuel are indexed to cetane as to how rapidly they ignite under compression, i.e., diesel engine conditions. Since hundreds of components in comprise diesel fuel, the overall cetane number of that fuel is the average cetane quality of all the components. High-cetane components have a disproportionate influence, hence the use of high-cetane additives.

Measuring cetane number

Cetane numbers are rather difficult to measure accurately, as it requires a special diesel engine called a Cooperative Fuel Research (CFR) engine. Under standard test conditions, the operator of the CFR engine uses a manual-wheel to increase the compression ratio (and therefore the peak pressure within the cylinder) of the engine until the time between fuel injection and ignition is 2.407 ms. 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.

Ignition Quality Tester (IQT)

Another reliable method of measuring the derived cetane number (DCN) of diesel fuel is the Ignition Quality Tester (IQT). This instrument applies a simpler, more robust approach to CN measurement than the CFR. Fuel is injected into a constant volume combustion chamber at approximately 575 °C and . The time between the start of injection and the recovery of the combustion chamber pressure to is defined as the ignition delay. This measured ignition delay is then used to calculate the DCN of the fuel. The fuel's DCN is then calculated using an empirical inverse relationship to ignition delay. Because of the reproducibility, material cost, and speed of the IQT, this has been the definitive source for DCN measurements of fuels since the late 2000s.

Fuel ignition tester

Another reliable method of measuring the derived cetane number of diesel fuel is the Fuel Ignition Tester (FIT). 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 elapsed 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.

Cetane index

Another method that fuel-users control quality is by using the cetane index (CI), which is a calculated number based on the density and a distillation range of the fuel. There are various versions of this, depending on whether metric or Imperial units are used, and the number of distillation points are used. These days most oil companies use the '4-point method', ASTM D4737, based on density, 10% 50% and 90% recovery temperatures. The '2-point method' is defined in

ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...

D976 and uses just density and the 50% recovery temperature. This 2-point method tends to overestimate the cetane index and is not recommended. Cetane index calculations can not account for cetane improver additives and therefore do not measure the total cetane number for additized diesel fuels. Diesel engine operation is primarily related to the actual cetane number and the cetane index is simply an estimation of the base (unadditized) cetane number.

Industry standards

The industry standards for measuring cetane number are ASTM D613 (ISO 5165) for the CFR engine, D6890 for the IQT, and D7170 for the FIT.

References

External links

Article by Bruce Hamilton

Explanation of Cetane from BP
Diesel fuel Fuel technology Petroleum production Scales