Sylvie Willis
Octane ratings are measures of fuel stability, specifically the pressure at which a fuel will spontaneously combust in a testing engine. The octane number is the average of two different octane rating methods: motor octane rating (MOR) and research octane rating (RON). While gasoline is commonly rated by its octane number, diesel fuel is not. Instead, diesel fuel is rated by its cetane number, which is a measure of the relative ease with which diesel fuel ignites.
| Characteristics | Values |
|---|---|
| Octane rating of diesel fuel | 8-15 (some sources state that it could be as high as 25) |
| Cetane rating of diesel fuel | 40-55 |
| Cetane rating additive | Power Service Diesel Kleen and Power Service Diesel Fuel Supplement |
| Diesel fuel viscosity | Diesel #1 flows more easily than Diesel #2 |
| Diesel fuel and temperature | At lower temperatures, diesel fuel gets thicker and cloudier |
| Diesel fuel and sulfur | Diesel fuel sold in the USA after December 1, 2010, must be "Ultra-Low Sulfur," with a maximum of 15 ppm of sulfur |
| Diesel fuel and biofuel | B5, a blend of 5% biodiesel and 95% regular diesel, can be used in vehicles in the US and Canada without voiding manufacturer warranties |
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What You'll Learn
Diesel fuel is rated by cetane number, not octane
Octane ratings are measures of fuel stability. These ratings are based on the pressure at which a fuel will spontaneously combust (auto-ignite) in a testing engine. The octane number is the simple average of two different octane rating methods—motor octane rating (MOR) and research octane rating (RON)—that differ primarily in the specifics of the operating conditions. The higher the octane number, the more stable the fuel.
Diesel fuel is different. If your vehicle runs on diesel, you will notice that no octane figures are advertised. Instead, diesel fuel is rated with a "cetane number," which typically ranges from 40 to 55 worldwide. Cetane numbers are important because diesel engines (also called compression-ignition engines) do not compress the fuel, but rather compress only air, and then inject fuel into the air that was heated by compression.
Gasoline engines, on the other hand, rely on the ignition of a compressed air and fuel mixture, which is ignited only near the end of the compression stroke by electric spark plugs. Therefore, being able to compress the air/fuel mixture without causing detonation is important mainly for gasoline engines. Using gasoline with lower octane than an engine is built for may cause engine knocking and/or pre-ignition.
Diesel fuel also is measured by its viscosity. Like any oil, diesel fuel gets thicker and cloudier at lower temperatures. Under extreme conditions, it can become a gel and refuse to flow at all. Diesel #1 flows more easily than Diesel #2, so it’s more efficient at lower temperatures. The two types of oil can be blended, and most service stations offer diesel fuel blended for local weather conditions.
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Cetane number typically ranges from 40-55
Diesel fuel is not rated by octane number. Instead, diesel fuel is rated by cetane number (also known as cetane rating), which typically ranges from 40-55. Cetane number is a measure of a fuel's ignition delay, or 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 delays, providing more time for the fuel combustion process to be completed. Hence, higher-speed diesel engines operate more effectively with higher cetane number fuels.
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 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, of cetane's 100 and isocetane's 15.
The CN is an important factor in determining the quality of diesel fuel, but it is not the only one. Other measurements of diesel fuel quality include energy content, density, lubricity, cold-flow properties, and sulfur content. For example, in the USA, all diesel fuel sold after December 1, 2010, must be "Ultra-Low Sulfur," which holds a maximum of 15 ppm of sulfur.
Another method that fuel users employ to control quality is by using the cetane index (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 metric or imperial units are used, and the number of distillation points that are used. Most oil companies use the '4-point method', ASTM D4737, based on density and 10%, 50%, and 90% recovery temperatures.
The octane rating of gasoline, on the other hand, is a measure of fuel stability and is based on the pressure at which a fuel will spontaneously combust (auto-ignite) in a testing engine. The most common type of octane rating worldwide is the Research Octane Number (RON), which is determined by running the fuel in a test engine at 600 rpm with a variable compression ratio under controlled conditions. Another type of octane rating is the Motor Octane Number (MON), which is determined at 900 rpm engine speed.
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Diesel engines do not compress fuel
Diesel engines, also known as compression-ignition engines, do not compress fuel. Instead, they compress only air, and then inject fuel into the heated air. This is in contrast to gasoline engines, which rely on the ignition of a compressed air and fuel mixture. This mixture is ignited near the end of the compression stroke by electric spark plugs.
The octane rating of fuel is a measure of its stability, or resistance to detonation, and is based on the pressure at which it will spontaneously combust in a testing engine. The higher the octane number, the more stable the fuel. However, octane ratings are not indicators of the energy content of fuels.
Since diesel engines do not compress fuel, they are not limited by the need to prevent pre-ignition, which would cause engine damage. This allows diesel engines to have a higher compression ratio than gasoline engines, which is typically around 15:1, compared to 8:1 or 12:1 for gasoline engines. The higher compression ratio in diesel engines increases their efficiency, with a theoretical maximum efficiency of 75%. In practice, diesel engines have achieved efficiencies of up to 55% for large two-stroke marine engines.
While gasoline engines use spark plugs to ignite the fuel, diesel engines use compression ignition, where the fuel is ignited by the elevated temperature of the air in the cylinder due to mechanical compression. This is possible because diesel fuel can be compressed more than gasoline before it auto-ignites. As a result, diesel engines deliver their power in a rough and dirty manner, while gasoline engines are smoother, quieter, and less smoky.
Diesel fuel is rated with a cetane number, which ranges from 40 to 55 worldwide, rather than an octane number.
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Diesel fuel is thicker and cloudier at lower temperatures
Diesel fuel is not rated by octane number; instead, it is rated by cetane number, which ranges from 40 to 55 worldwide. Diesel engines, also known as compression-ignition engines, do not compress the fuel. Instead, they compress only air and then inject fuel into the air that has been heated by compression.
Diesel fuel contains paraffin wax, which solidifies and causes gelling when temperatures drop. This gelling phenomenon is known as "Cloud Point". The wax crystals formed during this process can clog fuel filters, making it difficult for the engine to run properly. The cloud point temperature for diesel fuel typically ranges from -18°F to +20°F, but it can be as high as +40°F depending on the refining process.
The process of gelling in diesel fuel occurs when the temperature remains below -10°F for extended periods, usually 48 to 72 hours. The wax crystals start to stick together, turning the fuel into a cloudy colour and affecting its flow characteristics. This can lead to issues with engine starting and fuel combustion, resulting in rough starting, misfires, and uneven power delivery.
To address fuel problems caused by cold weather, it is recommended to use a winter performance diesel fuel additive when temperatures approach 40°F. This can help prevent cold starts, fuel clouding, and gelling issues. Alternatively, keeping the vehicle in a warm place for 1-2 hours can also help the gelled diesel return to its original state.
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Diesel fuel is more stable at lower temperatures
Diesel fuel is not rated by octane number, but by cetane number, which typically ranges from 40 to 55. Octane ratings are a measure of fuel stability, and higher octane numbers indicate more stable fuel. However, diesel engines do not rely on the ignition of compressed air and fuel mixtures; instead, they compress only air and then inject fuel into the heated air.
As the temperature continues to drop, the fuel reaches the pour point, which is usually 6 to 10 degrees Fahrenheit lower than the cloud point. At this stage, the fuel thickens and can no longer flow properly. The final stage is the gel point, where the fuel has completely solidified and no longer flows, typically around 10 to 15 degrees Fahrenheit.
To prevent diesel fuel gelling and ensure smooth engine operation in cold weather, diesel fuel additives or anti-gels can be used. These products lower the cloud point and gelling point, preventing wax formation and fuel system blockages. It is important to choose an additive that improves water dispersion, lubricity, and cetane number for optimal performance.
In summary, while diesel fuel is not rated by octane number, it is more stable at lower temperatures than gasoline. The use of additives can further enhance the stability and performance of diesel fuel in cold climates, preventing issues with fuel gelling and engine operation.
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Frequently asked questions
Diesel fuel has a low octane rating of 8-15.
Cetane fuels are used in diesel engines, while octane fuels are used in gasoline engines. Octane ratings measure a fuel's resistance to spontaneous ignition, while cetane ratings measure the ease with which diesel fuel ignites.
Diesel engines do not compress the fuel, but rather compress only air and then inject fuel into the heated air. Therefore, diesel fuels have low octane but high cetane numbers.
The most common type of octane rating is the Research Octane Number (RON), which is determined by testing the fuel in a controlled engine at 600 rpm with a variable compression ratio.
Higher octane fuels are more resistant to ignition, which is important for gasoline engines to prevent engine knocking or pre-ignition.
