Fuel Systems: Diesel Fuel Systems: Diesel fuel systems
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Topic IntroductionHelp

Diesel fuel characteristics

Summary
The primary factors that affect the performance of diesel fuel are: the cetane rating, viscosity, its cloud point, the the extent to which the fuel is contaminated.
Diesel fuel

The main factors that affect the performance of diesel fuel are:

The Cetane rating is a comparative measure of the speed with which diesel fuel ignites under compression. The higher the number, the faster the fuel ignites. If the fuel does not burn rapidly after the piston has reached TDC (Top Dead Center) then it may not burn completely during the power stroke, in which case it will not deliver the maximum possible power, and unburned fuel may then escape into the atmosphere as black exhaust smoke.

Diesel fuel should have a minimum Cetane rating of 40 for direct injection diesel engines, and 35 for indirect injection diesel engines. Fuels with lower Cetane ratings contribute to harder starting, ignition delay, power loss and decreased fuel economy. Cetane improver additives can improve ignition and reduce white smoke during cold weather startups.

Diesel fuel’s viscosity value measures its resistance to flow. Diesel fuel with viscosity which is either too high or too low can cause serious damage to the engine’s injection system.

The ‘Cloud Point’ is the temperature at which fuel turns cloudy to look at. When the fuel temperature drops to the fuel's cloud point, paraffin waxes that occur naturally in diesel fuel crystalize and cling together, making the fuel appear cloudy. This is known as “waxing” and, if not prevented, can clog filters and stop fuel flow to the engine. Clouding can be combated by using fuels with a lower cloud point, providing heat to the tanks, or including a cloud point improver to the fuel. This improver separates the clinging wax particles so they can pass through the fuel filters. Some oil companies produce special winter grade fuels for cold weather operation.

Diesel fuel is vulnerable to contamination, particularly from water in the tank, and from various types of sediment. As the fuel is fed into the system from the bottom of the tank, it is easy for contaminants to enter the system with the fuel. As the fuel in the tank is used, air from the atmosphere enters the tank. Water condenses on the walks of the tank and runs down into the fuel. The water is heavier than the fuel and usually ends up in the bottom of the tank along with sediment such as rust, or scale, or weld slag. Water in the fuel can cause injector seizures and engine failure. It also accelerates component wear. Water traps or separators are very important in diesel fuel systems. Dirt and other debris can clog fuel filters and form deposits, resulting in reduced power and excessive fuel system wear.