Regan Brown
Cetane number (cetane rating) is a measure of the combustion quality of diesel fuel. It is an indicator of the speed of combustion and the compression needed for ignition. The higher the cetane number, the quicker the ignition, resulting in better engine performance, improved fuel efficiency, and reduced emissions. While the cetane number of diesel fuel is not published at the pump, it is an important factor in determining fuel quality, with diesel engines generally operating well with a cetane number between 45 and 55. Various factors influence the cetane number, including the presence of additives, and it is measured through a standardized ASTM test or calculated based on fuel properties such as density.
| Characteristics | Values |
|---|---|
| What is Cetane Number? | It is an indicator of the combustion speed of diesel fuel and compression needed for ignition. |
| What does a higher Cetane Number mean? | A higher cetane number means the fuel ignites more quickly and efficiently in the engine's combustion chamber. |
| What is the difference between Cetane and Octane? | While a good octane rating means a fuel doesn't ignite too soon, a good cetane number means a fuel ignites as soon as possible. |
| What is the range of Cetane Number? | The scale for measuring cetane ranges from 0 to 100, with 100 being the cetane number for cetane. |
| What is a good Cetane Number? | Generally, diesel engines operate well with a CN from 48 to 50. Modern highway diesel engines require fuel with a cetane number ranging from 45 to 55. |
| What is the minimum Cetane Number for diesel fuel? | In North America, the minimum cetane number is set at 40, with typical values in the 42-45 range. In Europe, the minimum diesel cetane number was set at 38 in 1994, 40 in 2000, and 46 from 2001 onwards. |
| What is the Cetane Number for premium diesel fuel? | Premium diesel fuel can have a cetane number as high as 60. |
| How to improve Cetane Number? | Cetane number can be improved by adding chemicals or cetane improvers to the fuel that raise its cetane rating. |
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What You'll Learn
Cetane number vs. cetane index
The cetane number or cetane rating is a measure of the ignition quality or knocking tendency of diesel fuel in a diesel engine. It is used to predict how easily diesel fuel will ignite and burn when injected into the combustion chamber of a diesel engine. A high cetane number indicates a fuel that burns smoothly in the engine cylinder with a minimum delay between fuel injection and auto-ignition. The cetane number plays a similar role for diesel as the octane rating does for gasoline fuel. Cetane number (CN) is an important factor in determining the quality of diesel fuel, but not the only one. Other measurements of diesel fuel quality include energy content, density, lubricity, cold-flow properties, and sulfur content.
The cetane number scale is based on the ignition characteristics of two hydrocarbons: n-hexadecane (cetane) and 2,3,4,5,6,7,8-heptamethylnonane. The cetane number of cetane is 100, while that of heptamethylnonane is 15. The cetane number of a fuel is defined by finding a blend of cetane and isocetane with the same ignition delay. Once the blend is known, the cetane number is calculated as a volume-weighted average, rounded to the nearest whole number.
The cetane index is a calculated number based on the density and distillation range of the fuel. It is a substitute for the cetane number of diesel fuel and is used to estimate the cetane number of distillate fuels from API gravity and mid-boiling point. The cetane index is roughly approximate to the cetane number and is useful as a cetane number check on subsequent samples of a fuel whose cetane number has already been established. Cetane index calculations cannot 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 merely an estimation of the base (unadditized) cetane number.
The Fuel Ignition Tester (FIT) is a reliable method for measuring the derived cetane number of diesel fuel. 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.
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How to improve cetane in diesel fuel
Cetane number (CN) 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 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 quality include energy content, density, lubricity, cold-flow properties, and sulfur content.
The cetane number 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. 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 isocetane's 15. A higher cetane number signifies that the fuel ignites more readily when sprayed into hot compressed air during the fuel injection cycle. This results in better starting performance, smoother engine operation, less engine noise, and fewer emissions.
If you suspect your diesel fuel’s cetane rating isn’t high enough, there are options to boost it. The industry standard for cetane improver is 2-ethyl hexyl nitrate. Not only is it effective at raising the diesel cetane rating, but it also doesn’t cause problems in the engine or harm engine materials. The one thing to watch out for with cetane improvers is a similar problem that people find with octane-improving additives. Lots of people sell products marketed as cetane improvers and octane improvers because consumers think of these as quick fixes. Unless the cetane improver you’re looking at contains 2-ethyl hexyl nitrate, you probably want to stay away from it.
Pay attention to treat rates. Raising fuel cetane rating is not difficult, but you have to have enough active ingredients in the fuel. If the additive claims to raise fuel cetane rating by a significant amount but you only have to use a small amount of it, it probably won't work. To raise fuel cetane rating by any meaningful amount, you probably need a minimum of 1 ounce per 10 gallons. There are no shortcuts, but a cetane improver additive will do what you want if you use enough of it.
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Cetane and octane compared
Cetane and octane are both indicators of fuel combustibility. However, they work in opposite ways. A higher cetane rating in diesel fuel means it will ignite faster in the cylinder, which is important for better performance in a diesel engine. On the other hand, a higher octane rating in gasoline means that the gas ignites more slowly, preventing pre-ignition, which is why gasoline requires a higher octane rating.
Cetane number (CN) is an indicator of the combustion speed of diesel fuel and the compression needed for ignition. It is an important factor in determining the quality of diesel fuel. The CN is calculated by finding a blend of cetane and isocetane with the same ignition delay and then taking the volume-weighted average, rounded to the nearest whole number. Cetane has a cetane number of 100, while isocetane has a number of 15. The CN is measured using a specialised engine, and additives can be used to improve it.
Octane rating, often seen as numbers like 87, 91, or 93 at the gas pump, is a measure of a fuel's ability to resist knocking when ignited in a cylinder. It is determined by comparing the knock intensity of the fuel with blends of iso-octane and heptane, with iso-octane having an octane number of 100 and heptane, 0. Higher compression engines require higher-octane fuel, which is more stable.
While cetane and octane serve similar purposes as indicators of combustibility, the key difference is that cetane measures the speed of combustion and ignition delay in diesel fuel, while octane measures the fuel's ability to resist knocking during combustion in gasoline engines.
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Cetane number calculation
The cetane number (CN) is an important factor in determining the quality of diesel fuel. It is a measure of the ignition quality or knocking tendency of diesel fuel in a diesel engine. It is used to predict how easily the diesel fuel will ignite and burn when injected into the combustion chamber of a diesel engine.
The CN is defined as an important index that evaluates the ignition behaviour of diesel and other fuels in an engine. It indicates the fuel's autoignition performance and ignition delay period, with higher numbers leading to better cold start performance. The cetane number is similar to the octane rating in that it is a rating assigned to a fuel to rate the quality of its combustion. However, octane ratings are for gasoline, whereas cetane ratings are for diesel. Pure cetane represents the highest purity of diesel fuel possible and has a cetane rating of 100.
The cetane number scale is based on the ignition characteristics of two hydrocarbons: n-hexadecane (cetane) and 2,2,4,4,6,8,8-heptamethylnonane (isocetane). Cetane has a cetane number of 100, while isocetane's measured cetane number is 15. To determine the cetane number of a test fuel, its ignition delay is compared in a special standard test engine called a Cooperative Fuel Research (CFR) engine with a blend of reference fuels. The resulting cetane number is then calculated by determining which mixture of cetane and isocetane will result in the same ignition delay as the test fuel under standard conditions. This calculated cetane number is then rounded to the nearest whole number.
Another method to determine the cetane number 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 310 psi (21 bar). The measured ignition delay is then used to calculate the derived cetane number (DCN) of the fuel using an empirical inverse relationship to ignition delay.
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Cetane number by region
The cetane number (CN) is an indicator of the combustion speed of diesel fuel and the compression needed for ignition. It is an important factor in determining the quality of diesel fuel. The cetane number of a fuel is defined by finding a blend of cetane and isocetane with the same ignition delay. The CN scale is defined by blends of two pure hydrocarbon reference fuels. Cetane (n-hexadecane), a compound with high ignition quality, represents the top of the scale with a cetane number of 100. A higher cetane number means a shorter ignition delay period.
In Europe, the minimum diesel cetane number was set at 38 in 1994 and 40 in 2000. The standard for diesel sold in the European Union, Iceland, Norway, and Switzerland is defined by standard EN 590, which demands 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 (Diesel Plus), Shell (containing GTL), and ABC (Smart Diesel) have a minimum cetane number of 60, with a typical value of 63. Neste MY Renewable Diesel, sold in Finland, has a minimum cetane number of 70.
In North America, most states have adopted ASTM D975 as their diesel fuel standard, with a minimum cetane number of 40 and typical values in the 42-45 range. California diesel fuel has a minimum cetane number 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. Neste MY Renewable Diesel, sold in North America, has a cetane number of 70+.
Modern highway diesel engines tend to require fuel with a cetane number ranging from 45 to 55. Regular No. 2 diesel typically has a cetane number of 40 to 42, while Cenex Premium Diesel has a cetane number of 47 to 52.
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Frequently asked questions
Cetane number (cetane rating) (CN) is an indicator of the combustion speed of diesel fuel and compression needed for ignition. It is an important factor in determining the quality of diesel fuel.
A good octane rating means a fuel doesn't ignite too soon, whereas a good cetane number means a fuel ignites as soon as possible. A higher cetane number means the fuel ignites more readily, resulting in better engine performance.
The cetane number of diesel fuel varies. Regular no. 2 diesel has a cetane number of 40 to 42, while Cenex Premium Diesel has a cetane number of 47 to 52. In North America, the minimum cetane number is 40, while in California, the minimum cetane number is 53.
