Cetane number
Encyclopedia
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.
and CII
.
advantage when the CN is raised past approximately 55; after this point, the fuel's performance hits a plateau.
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
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 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 diesels may or may not have higher cetane, depending on the supplier. Premium diesel often use additives to improve CN and lubricity
, detergent
s to clean the fuel injectors and minimize carbon
deposits, water dispersants, and other additives depending on geographical and seasonal needs.
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 http://www.dorfketal.com/additivesphlox.htm) and di-tert-butyl peroxide are used as additives to raise the cetane number.
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.
molecule
that ignites very easily under compression, so it was assigned a cetane number of 100, while alpha-methyl naphthalene was assigned a cetane number of 0. All other hydrocarbon
s 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.
) and isocetane
(2,2,4,4,6,8,8-heptamethylnonane) will result in the same ignition delay.
(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% 50% and 90% recovery temperatures. The '2-point method' is defined in ASTM D976, and uses just density and the 50% recovery temperature. This 2-point method tends to over-estimate cetane index and is not recommended. Cetane index
calculations can not account for cetane improver additives and therefore do not measure 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.
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...
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.
Definition
Cetane number or CN is actually a measure of a fuel's ignition delay; the time period between the start of injection 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 CCAICalculated Carbon Aromaticity Index
The calculated carbon aromaticity index is an index of the ignition quality of residual fuel oil.The running of all internal combustion engines is dependent on the ignition quality of the fuel. For spark-ignition engines the fuel has an octane rating. For diesel engines it depends on the type of...
and CII
Calculated Ignition Index
The Calculated Ignition Index is an index of the ignition quality of residual fuel oil.The running of all internal combustion engines is dependent on the ignition quality of the fuel. For spark-ignition engines the fuel has an octane rating. For diesel engines it depends on the type of fuel, for...
.
Typical values
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 emissionEmission standard
Emission standards are requirements that set specific limits to the amount of pollutants that can be released into the environment. Many emissions standards focus on regulating pollutants released by automobiles and other powered vehicles but they can also regulate emissions from industry, power...
advantage when the CN is raised past approximately 55; after this point, the fuel's performance hits a plateau.
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
European Union
The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...
, Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
, Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
and Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
is set in EN 590
EN 590
EN590 describes the physical properties that all automotive diesel fuel must meet if it is to be sold in the European Union, Croatia, Iceland, Norway and Switzerland....
, with a minimum cetane index
Cetane index
Cetane index is used as a substitute for the cetane number of diesel fuel. The cetane index is calculated based on the fuel's density and distillation range . There are two methods used, ASTM D976 and D4737. The older D976, or "two-variable equation" is outdated and should no longer be used for...
of 46 and a minimum cetane number of 51. Premium diesel fuel can have a cetane number as high as 60.
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 diesels may or may not have higher cetane, depending on the supplier. Premium diesel often use additives to improve CN and lubricity
Lubricity
Lubricity is the measure of the reduction in friction of a lubricant. The study of lubrication and mechanism wear is called tribology.-Measurement of lubricity:...
, detergent
Detergent
A detergent is a surfactant or a mixture of surfactants with "cleaning properties in dilute solutions." In common usage, "detergent" refers to alkylbenzenesulfonates, a family of compounds that are similar to soap but are less affected by hard water...
s to clean the fuel injectors and minimize carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
deposits, water dispersants, and other additives depending on geographical and seasonal needs.
Dimethyl ether
Dimethyl ether
Dimethyl ether , also known as methoxymethane, is the organic compound with the formula . The simplest ether, it is a colourless gas that is a useful precursor to other organic compounds and an aerosol propellant. When combusted, DME produces minimal soot and CO, though HC and NOx formation is...
may prove advantageous as a future diesel fuel as it has a high cetane rating (55) and can be produced as a biofuel
Biofuel
Biofuel is a type of fuel whose energy is derived from biological carbon fixation. Biofuels include fuels derived from biomass conversion, as well as solid biomass, liquid fuels and various biogases...
.
Alkyl nitrates (principally 2-ethyl hexyl nitrate http://www.dorfketal.com/additivesphlox.htm) and di-tert-butyl peroxide are used as additives to raise the cetane number.
Biodiesel
Biodiesel
Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl esters. Biodiesel is typically made by chemically reacting lipids with an alcohol....
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.
Chemical relevance
Cetane is an un-branched open chain alkaneAlkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
that ignites very easily under compression, so it was assigned a cetane number of 100, while alpha-methyl naphthalene was assigned a cetane number of 0. All other hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s 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
Diesel engine
A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...
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.
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(CFRTM) engine, under standard test conditions. The operator of the (CFRTM)engine uses a hand-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 2.407ms. The resulting cetane number is then calculated by determining which mixture of cetane (hexadecaneHexadecane
Hexadecane is an alkane hydrocarbon with the chemical formula C16H34. Hexadecane consists of a chain of 16 carbon atoms, with three hydrogen atoms bonded to the two end carbon atoms, and two hydrogens bonded to each of the 14 other carbon atoms.Cetane is often used as a short-hand for cetane...
) and isocetane
Isocetane
Isocetane is a highly branched alkane used as a reference in determining the cetane number of diesel. It is given a cetane number of 15. Isocetane replaced 1-methylnaphthalene as the lower reference for cetane number owing to the expense of 1-methylnaphthalene, and difficulty in safe handling....
(2,2,4,4,6,8,8-heptamethylnonane) will result in the same ignition delay.
Ignition quality tester
Another reliable method of measuring the derived 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.Fuel Ignition Tester
Another reliable method of measuring the derived cetane number of diesel fuel is the Fuel Ignition Tester (FITTM). 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 different 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 indexCetane index
Cetane index is used as a substitute for the cetane number of diesel fuel. The cetane index is calculated based on the fuel's density and distillation range . There are two methods used, ASTM D976 and D4737. The older D976, or "two-variable equation" is outdated and should no longer be used for...
(CI), which is a calculated number based on the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
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
Petroleum industry
The petroleum industry includes the global processes of exploration, extraction, refining, transporting , and marketing petroleum products. The largest volume products of the industry are fuel oil and gasoline...
use the '4-point method', ASTM D4737, based on density, 10% 50% and 90% recovery temperatures. The '2-point method' is defined in ASTM D976, and uses just density and the 50% recovery temperature. This 2-point method tends to over-estimate cetane index and is not recommended. Cetane index
Cetane index
Cetane index is used as a substitute for the cetane number of diesel fuel. The cetane index is calculated based on the fuel's density and distillation range . There are two methods used, ASTM D976 and D4737. The older D976, or "two-variable equation" is outdated and should no longer be used for...
calculations can not account for cetane improver additives and therefore do not measure 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 D-613 (ISO 5165) for the CFR engine, D-6890 for the IQT, and D-7170 for the FIT.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