Cetane Number and Cetane Index of Diesel Fuel
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 the 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 minimum delay between fuel injection and autoignition. The cetane number plays a similar role for diesel as the octane rating does for gasoline fuel.
Cetane number (CN) is a relative measure of the delay time between the injection of fuel into the combustion chamber and the start of combustion. Fuels for compression ignition engines must auto-ignite readily. If ignition does not take place quickly when the fuel is injected into the cylinder then premixed fuel and air ignite in such a way that the rate of burning is too rapid. The prompt burning of fuel produces a high-pressure rise that results in engine knock that decreases the engine efficiency and in extreme cases can damage the engine.
Typically, diesel engines perform well with fuels having cetane numbers between 40 ~ 55. Higher cetane numbers mean a shorter time delay between fuel injection and autoignition. But the diesel engine hits a performance peak for cetane numbers higher than nearly 55. Hence fuels with higher cetane numbers have little added advantage. The cetane number for Euro-5 diesel fuel is 51.
Measuring the Cetane Number
The cetane number scale is based upon two hydrocarbons, Cetane (n-hexadecane, C16H34) and isocetane (2,3,4,5,6,7,8-heptamethylnonane, C11H10). The cetane number of cetane is 100 which has a short delay period during ignition. Whereas the cetane number of heptamethylnonane is 15 and has a long delay period.
To determine the cetane number of any 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. Under standard test conditions, the cetane number is calculated by determining the mixture of cetane and heptamethylnonane that results in the same ignition delay as the test fuel under standard conditions. The matching blend percentages to the first decimal are inserted in the following equation to obtain the cetane number
Cetane number = % n-cetane + 0.15(% heptamethylnonane)
Cetane = 40+0.15 (60) = 53.5
Other Methods of measuring the Cetane Number
1. Ignition Quality Tester (IQT)
The Ignition Quality Tester called IQT can also determine diesel fuel’s derived cetane number. This measurement technique is more basic but better than the Cooperative Fuel Research (CFR) engine. Fuel is injected into a constant volume combustion chamber at 575 °C and 21 bar conditions. Ignition delay is the interval between injection and pressure of the combustion chamber returning to 310 psi (21 bar). This measured ignition delay is then used to calculate the DCN of the fuel. Establishing an inverse empirical relationship between the derived cetane number and ignition delay determines the fuel’s DCN. The IQT has shown itself to be the most reliable method for checking the quality of DCN fuel because of its speed, accuracy, and low cost of materials.
2. 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. As the fuel burns, the chamber’s pressure increases and indicated the start of combustion. By measuring the time between fuel injection and the start of combustion, the fuel’s ignition delay may be determined. The fuel’s derived cetane number can then be calculated using an empirical inverse relationship to ignition delay.
The Calculated Cetane Index is the tool for estimating the cetane number where a test engine is not available. In cases where the cetane number of a fuel has been initially established, the cetane index is useful as a cetane number check on subsequent samples of that fuel. The cetane index formula represents a method for estimating the cetane number of distillate fuels from API gravity and mid-boiling point. The index value as computed from the formula is designated as a calculated Cetane Index (ASTM D976).
Advantages of High Cetane Number Diesel Fuel
- Improved engine performance: High cetane number diesel fuel ignites more easily and burns more completely, which can result in improved engine performance. This can include faster acceleration, better fuel economy, less noise, and lower emissions.
- Reduced emissions: Diesel fuel with a high cetane number burns more cleanly and generates fewer emissions than fuel with a low cetane number. This can be beneficial for reducing air pollution and meeting emissions regulations.
- Increased fuel efficiency: High cetane number diesel fuel can be more efficient, as it burns more completely and releases more energy. This can result in improved fuel economy and reduced fuel consumption
- Improved cold weather performance: High cetane number diesel fuel is more resistant to cold weather gelling, which can make it easier to start the engine in cold conditions
- Fewer engine problems: High cetane number of diesel fuel can reduce the risk of engine problems, such as knocking or pinging, which can be caused by poor ignition quality.
How to improve the Cetane number?
- Add cetane improvers: Cetane improvers are chemicals that are added to diesel fuel to increase its cetane number. These additives boost the fuel’s cetane rating by facilitating faster, more complete combustion. Cetane improvers are typically added to diesel fuel at a concentration of 1-2% by volume.
- Use a higher-cetane fuel: Alternate fuels with a higher cetane number may be used to boost the cetane rating of diesel. Some diesel fuels are formulated with higher-cetane components, such as certain types of distillates or additives, which can result in a higher cetane number.
- Refine the fuel: The refining process can also affect the cetane number of diesel fuel. By removing impurities and improving the quality of the fuel, the cetane number can be increased.
- Optimize engine design: The design of the engine can also impact the cetane number of the fuel. For example, improving the design of the fuel injectors or the combustion chamber can result in more efficient and complete combustion, which can increase the cetane number.
- W. Dabelstein, A. Reglitzky, A. Schütze, and K. Reders, “Automotive Fuels,” Ullmann’s Encyclopedia of Industrial Chemistry, Apr. 2007,
- J. G. Speight, Handbook of Petroleum Product Analysis: Speight/Handbook of Petroleum Product Analysis, vol. 182. Hoboken, NJ: John Wiley & Sons, Inc, 2014.
- J. S. Heyne, A. L. Boehman, and S. Kirby, “Autoignition studies of trans- and cis-decalin in an ignition quality tester (IQT) and the development of a high thermal stability unified/single battlefield fuel,” Energy and Fuels, vol. 23
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