Diesel Fuel and Cetane Rating

Like gasoline, diesel is a compound of hydrogen and carbon extracted from crude oil. There are different grades of diesel fuel for diesel engines. What is commonly sold in a service station is highly refined and is suitable for use in high-speed diesel engines, including those in light automotive use. Additionally, diesel fuel has a rating called the cetane number.

The cetane number is a rating of a diesel fuel that expresses ignition quality and defines how easily the fuel will ignite when it is injected into the cylinder. The lower a fuel’s cetane number, the longer it takes to reach ignition point. Using a fuel with a cetane number that is too low will increase the amount of diesel knock in an engine. A higher cetane number means the fuel is more prone to self-ignition. Better self-igniting properties of the fuel result in a shorter ignition delay period, which produces better control of the fuel combustion process. A shorter delay period results in easier starting, quieter engine operation, and lower emissions due to complete combustion of the fuel mixture in the cylinder.

It is interesting that when comparing the diesel fuel cetane number to the gasoline octane number, the self-igniting properties are opposite. That is, the higher the octane rating, the less self-igniting the fuel. Where the diesel fuel ignites from the heat of compression after fuel injection, ignition in a gasoline engine occurs when a spark is introduced to the air/fuel mixture; therefore, it is harmful for a gasoline fuel mixture to self-ignite. In fact, the term we use for self-ignition of a gasoline mixture is uncontrolled combustion, or pre-ignition.

Once fuel is ignited, a sharp pressure rise occurs in the combustion chamber due to the sudden rate of combustion of the fuel. This phase is called flame spread. The greater the rate of pressure increase, the greater the combustion “knock.” Combustion knock is the result of sudden and intense pressure changes within the cylinder.

As the rest of the fuel is injected into the combustion chamber, there is a more gradual pressure change in the cylinder. This is called the direct burning phase of combustion. It occurs when the engine is operated at full load. As a final note, as the piston continues to move down during the power stroke, fuel particles left in the cylinder are burned and assist the downward movement of the piston.

When diesel fuel is injected into the cylinder, it does not ignite instantly. It takes time for the heat of the compressed air in the cylinder to heat the fuel sufficiently for it to ignite. This period of time from the start of injection to the start of combustion is called the ignition delay period. During this delay period, fuel continues to be injected into the cylinder.

When the fuel is heated sufficiently, it erupts into flame and burns. Combustion occurs. The sudden pressure rise sends a shock wave through the combustion chamber that can be heard outside the engine. This sound is called diesel knock.

Diesel knock can also be caused by poor atomization of the fuel, which can take too long to reach combustion temperature. However, the knock referred to here is the result of uncontrolled combustion. This type of combustion means the fuel ignites when it wants to, not at the proper time for complete controlled combustion and heat energy transfer, as designed by the engine manufacturer. Uncontrolled combustion is damaging to the engine Figure 54-4.

FIGURE 54-4
Uncontrolled combustion is damaging to the engine.

Another important factor relating to diesel fuel is low-temperature operation. During low temperatures, the fuel becomes thicker. If the temperature is too low, paraffins in the fuel begin to solidify and form waxes. These waxes can block filters, causing fuel starvation and low power output. To help prevent this, filters are fitted close to the engine, and sometimes heaters are used. The term used for the beginning of fuel gelling or “change of state” of the fuel is “cloud point” or “waxing.”

Diesel fuel also acts as a lubricant for the fuel system components. Clearances between the components of a fuel injection system are extremely close. In fact, they can be in the hundred-thousandth of an inch range (0.00001")—so close, that if you were to hold the plunger of a pumping element in your hand, you would not be able to reinstall it until it cooled because of the expansion from the heat of your hand. Therefore, filtration is critical for a diesel fuel system. Usually, a primary and a secondary fuel filter are used to keep the fuel free of water and abrasive particles.

Applied Science
AS-22: Internal/External Combustion: The technician can demonstrate an understanding of how fuel characteristics affect combustion in an automotive engine.

A diesel fuel’s cetane number is a measure of the fuel’s ignition delay, or the length of time between the start of injection and the first identifiable pressure increase in the cylinder due to combustion. Light vehicle diesel engines, especially modern designs, require high cetane ratings to operate at the required high speeds, as well as meet current emission standards and noise-level targets.

Fuels with low cetane ratings have longer ignition delay periods. Longer ignition delay means more fuel is delivered into the cylinder before ignition, leading to a longer period of uncontrolled combustion and violent pressure and temperature increases in the cylinder. This rapid pressure increase results in shock loadings, commonly known as “diesel knock.” Longer periods of uncontrolled combustion also contribute to an increase in harmful exhaust emissions from diesel engines.