Diesel Fuel Characteristics
A proper understanding of diesel fuel characteristics is important for anyone operating a vehicle equipped with a diesel engine. Knowledge of a few basic characteristics can help you ensure that the fuel you are putting into your engine is not going to cause long-term harm. Each is easily identifiable, and requires no professional expertise in the identification or handling of fuels.
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Cetane Number
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In layman's terms, the cetane number (or rating) is a measure of how fast the fuel ignites after the compression stroke in a diesel engine. Diesel fuel is engineered to quickly and completely burn after the compression stroke (after it reaches top dead center). Diesel fuels with lower cetane numbers will tend to burn slower, which is not optimal in most automotive applications. According to CDX eTextbook, using diesel with a cetane number higher than 35 to 40--depending on the engine's ignition system--is important for the power output of the engine. Using diesel fuel with a cetane number that is too low will cause starting difficulties as well as a reduction in fuel economy. Chevron sets a minimum cetane rating of 40 for all diesel fuels it produces.
Viscosity
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According to Chevron, viscosity is an important factor in the atomization and combustion of the fuel through the injectors. Chevron warns that diesel fuels that are not compliant with the ASTM D 975 standard might be made to incorrect viscosity specifications. That can lead, much like an incorrect cetane number, to loss of power and poor fuel economy. CDX eTextbook notes that improper fuel viscosity can damage the engine's injection system.
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Cloud Point
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The cloud point of diesel fuel is important for low-temperature diesel fuel operation. Crude oil naturally contains paraffin wax, which in diesel fuel remains in significant quantities even after refinement. According to Chevron, this wax will melt and become inert at 100 to 180 degrees Fahrenheit. This is normal. However, when the fuel is cooled to lower temperatures, the wax will begin to "precipitate" out of the fuel, giving the fuel a cloudy appearance. The point at which the paraffin wax becomes noticeable is considered the cloud point. According to the Independent Towers of Washington, typical diesel fuels have a cloud point of about 40 degrees Fahrenheit. Most engines will continue to run at this stage, but be aware that this is a precursor to the pour point.
Pour Point
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Cooling diesel fuel further below the cloud point will bring the fuel to the pour point. The pour point is the point in which the paraffin becomes so thick that the fuel takes on a gel-like consistency and will not pour. Chevron approximates that this occurs at 6 to 10 degrees Fahrenheit below the cloud point for fuels that do not have Pour Point depressants added. The Independent Towers of Washington states that the engine will fail to run at and below the pour point.
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