Ameer Randolph
Diesel fuel is rated based on its combustion quality, which is measured by its cetane rating or number (CN). The CN indicates the speed of combustion and compression needed for ignition. The higher the cetane rating, the more efficiently the diesel fuel will ignite and burn. Premium diesel fuels generally have a higher cetane rating, improving engine performance and efficiency. The cetane rating is calculated by measuring the ignition delay, or the time between the start of ignition and the first pressure increase during combustion. Diesel engines are more efficient than gasoline engines because they contain 10% more energy per gallon and have lower risks of explosion or ignition. Diesel fuel standards have become stricter over time, addressing concerns about toxic emissions and environmental impact.
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Cetane number
The cetane number can be measured using a Fuel Ignition Tester (FIT). This instrument injects fuel 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.
In North America, most states have adopted 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 a higher cetane number depending on the supplier. Biodiesel from vegetable oil sources has been recorded as having a cetane number range of 46 to 52, and animal-fat-based biodiesels have cetane numbers ranging from 56 to 60.
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$82.08
Fuel Ignition Tester (FIT)
The Fuel Ignition Tester (FIT) is a device used to measure the ignition quality of diesel fuel. Ignition quality is a crucial attribute of fuel, reflecting its suitability for use in compression ignition engines.
The FIT is a valuable tool for characterizing the ignition behaviour of diesel fuel. It offers a simpler and more robust approach to measuring the derived cetane number (CN) of diesel fuel compared to other methods, such as the Cooperative Fuel Research (CFR) engine. The CN is an important factor in determining the quality of diesel fuel, playing a similar role as the octane rating does for gasoline.
- Fuel is injected into a constant volume combustion chamber.
- The chamber's ambient temperature is maintained at 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 is calculated by measuring the time elapsed between the start of fuel injection and the start of combustion.
- The fuel's CN can then be determined using an empirical inverse relationship to ignition delay.
The CN of a fuel is defined by blending cetane and isocetane to achieve the same ignition delay. Cetane has a CN of 100, while isocetane's CN is 15. Once the blend is known, the CN is calculated as a volume-weighted average, rounded to the nearest whole number.
The FIT helps ensure diesel fuel quality and engine performance. It is a valuable tool for fuel testing and research, providing insights into the ignition behaviour of diesel fuel and contributing to the development of cleaner and more efficient diesel engines.
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Energy content
Diesel fuel is denser than petrol and contains about 10% more energy by volume (roughly 36.9 MJ/litre compared to 33.7 MJ/litre). The calorific value of diesel fuel is roughly 45.5 megajoules per kilogram (MJ/kg), slightly lower than petrol, which is 45.8 MJ/kg. Diesel fuel offers a higher volumetric energy density: the density of EN 590 diesel fuel is defined as 0.820 to 0.845 kg/L at 15 °C, about 9.0-13.9% more than EN 228 gasoline (petrol) at 0.720–0.775 kg/L. When burned, diesel offers a net heating value of 43.1 MJ/kg as opposed to 43.2 MJ/kg for gasoline.
The higher energy density of diesel fuel makes it more efficient than gasoline. It also has safety benefits, as its vapours do not explode or ignite as easily as gasoline vapours.
The energy content of diesel fuel is one of the factors that determine its quality. Other measurements of diesel fuel quality include cetane number, density, lubricity, cold-flow properties, and sulfur content.
In the United States, all diesel fuel sold after December 1, 2010, must be "Ultra-Low Sulfur," with a maximum sulfur content of 15 parts per million (ppm). Similar legislation has been implemented in Europe, reducing the maximum sulfur content to 10 ppm in 2009. These regulations have significantly reduced the sulfur content in diesel fuels, contributing to cleaner diesel exhaust.
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Sulphur content
In 2007, stricter diesel fuel standards were implemented, limiting the sulphur content in diesel fuels to a maximum of 15 parts per million (ppm). This was a significant improvement from the previous "dirty diesel" formulation, which averaged approximately 550 ppm of sulphur.
The reduction of sulphur content in diesel fuel is essential for mitigating the toxic effects of diesel emissions on humans and other living organisms. Conventional diesel exhaust has been linked to severe health issues, including cancer, necessitating the development of cleaner diesel engines and stricter regulations.
To ensure compliance with these regulations, diesel fuel sold in the United States after December 1, 2010, must be labelled as "Ultra-Low Sulfur," adhering to the maximum sulphur content of 15 ppm. Similarly, Europe has implemented comparable legislation, reducing the maximum sulphur content to 10 ppm in 2009.
It is important to note that the use of diesel fuel with a low sulphur content is crucial for engine performance. Diesel fuel with a poor cetane rating, indicating high sulphur content, can result in slower combustion, leading to reduced engine performance, increased emissions, and elevated engine noise. Therefore, it is recommended to use diesel fuel with the appropriate sulphur content and cetane rating for optimal engine functionality.
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Biodiesel
Diesel fuel is rated using the cetane number (CN) which indicates the combustion speed of the fuel and the compression needed for ignition. 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 energy content, density, lubricity, cold-flow properties, and sulfur content.
The cetane number of biodiesel is an important quality parameter that describes the auto-ignition characteristics of the fuel. The standard test methods for evaluating the cetane number of biodiesel are ASTM D613 and ISO 5165 procedures using a monocylindrical cetane engine. However, these conventional procedures are challenging and time-consuming, so alternative techniques such as color indicator titration, aniline point, and fatty acid composition-based methods have been proposed.
The cetane number range for biodiesel derived from vegetable oil sources is 46 to 52, while animal-fat-based biodiesels have cetane numbers ranging from 56 to 60. The minimum values of the cetane number for biodiesel dictated by ASTM D6751 and EN 14214 standards are 47 and 51, respectively. Biodiesel generally has a greater cetane number than pure conventional diesel fuels due to the presence of saturated molecules, longer fatty acid carbon chains, and higher oxygen content.
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Frequently asked questions
Cetane rating, or cetane number (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.
The cetane rating is calculated by finding a blend of cetane and isocetane with the same ignition delay. Cetane has a cetane number of 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.
In North America, the minimum cetane number is set at 40, with typical values ranging from 42 to 45. In Europe, the minimum diesel cetane number is currently 40, but it was previously set at 38 in 1994.
Other factors that determine the quality of diesel fuel include energy content, density, lubricity, cold-flow properties, and sulfur content.
It is important to refer to your vehicle's owner's manual for specific recommendations. In general, for driving in cold weather, it is advisable to choose diesel fuel rated at least 10 degrees lower than the expected coldest temperature.
