Diesel Fuel Characteristics

The main factors that affect the performance of diesel fuel are the cetane rating of the fuel, the fuel’s viscosity, its cloud point, and the extent to which the fuel is contaminated. As mentioned earlier, the cetane number, or rating, expresses ignition quality of diesel fuel and 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 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 number of 40 for direct injection diesel engines and 35 for indirect injection diesel engines Figure 54-5. 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 start-ups.

A cetane number, or rating chart.

Diesel fuel’s viscosity value measures its resistance to flow. Diesel fuel with viscosity that is either too high or too low can cause serious damage to the engine’s injection system. As discussed earlier, the clearances between the fuel injection components are very close. It is so close that improper fuel viscosity will cause a lack of lubrication. As a result, the injection components can be ruined.

The cloud point is the temperature at which fuel turns cloudy. When the fuel temperature drops to the fuel’s cloud point, paraffin waxes that occur naturally in diesel fuel crystallize 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 walls 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.

Engine Knock

The annoying rattle of older diesel engine designs has largely been subdued by a combination of engineering and fuel improvements. This has been accomplished through advanced injector design with better spray patterns, and injection timing control Figure 54-6. Also, improved compression temperatures and increased fuel cetane values have significantly lowered the noise levels of modern diesel engines. Additionally, fast computer processors allow precise electronic control of injection so that the combustion process can be smoothed to allow the up-and-down transitions of the pistons to occur without the traditional diesel clatter.

Fuel injector spray patterns.
An injector pattern from a 6.0L Ford Power-stroke.

In a modern quiet diesel engine, fuel injection timing and delivery are electronically controlled through pulse-width modulation, which is the time that the fuel injectors are turned off and on to inject a precise amount of fuel Figure 54-7. Duty cycle is another term that can be used for pulse-width modulation. Here is how it works: The injectors are first pulsed with a small lean pilot injection to start the ignition process with less noise, quickly followed by a main injection of fuel to develop engine torque and complete the power stroke, finishing with an additional pulse to assist with after-burning pollutants in the converter. The result of all the combined refinements is dramatically smoothed idle revolutions per minute (rpm) and roughness, far less combustion noise, no smoke, less odor and emissions, and improved fuel consumption.