Diesel fuel, like gasoline, is made from petroleum. However, at the refinery, the petroleum is separated into three major components: gasoline, middle distillates, and all remaining substances. Diesel fuel comes from the middle distillate group, which has properties and characteristics different from gasoline.

The shape of the fuel spray, turbulence in the combustion chamber, beginning and duration of injection, and the chemical properties of the diesel fuel all affect the power output of the diesel engine. The significant chemical properties of diesel fuel are described briefly in the following paragraphs.

Viscosity

Viscosity is a measure of a fluid's resistance to flow, and like most fluids, diesel fuel viscosity varies with temperature.

The viscosity of diesel fuel directly affects the spray pattern of the fuel into the combustion chamber. Fuel with  a high viscosity produces large droplets that are hard to burn. Fuel with a low viscosity sprays in a fine, easily burned mist. If the viscosity is too low, however, it does not adequately lubricate and cool the injection pump and nozzles.

Wax Appearance Point and Pour Point

Temperature affects diesel fuel more than it affects gasoline. This is because diesel fuels contain paraffin, a wax substance common among middle distillate fuels. As temperatures drop past a certain point, wax crystals begin to form in the fuel. The point where the wax crystals appear is the wax appearance point (WAP) or cloud point. The better the quality of the fuel, the lower the WAP. As temperature drop, the wax crystals grow larger and restrict the flow of fuel through the filters and lines. Eventually, the fuel, which may still be liquid, stops following because the wax crystals plug a filter or line. As the temperature continues to drop, the fuel reaches a point where it solidifiers and no longer flows. This is called the pour point. In cold climates it is recommended that a low-temperature pour point fuel be used.

Volatility

Gasoline is extremely volatile compared to diesel fuel. The amount of carbon residue left by diesel fuel depends on the quality and the volatility of that fuel. Fuel that has a low volatility is much more prone to leaving carbon residue. The small, high-speed diesels found in automobiles require a high-quality and highly volatile fuel because they cannot tolerate excessive carbon deposits. Large, low-speed industrial diesels are relatively unaffected by carbon deposits and can run on low-quality fuel.

Cetane Number or Rating

Diesel fuel's ignition quality is measured by the cetane rating. Much like the octane number, the cetane number is measured in a single-cylinder test engine with a variable compression ratio. The diesel fuel to be tested is compared to cetane, a colorless, liquid hydrocarbon that has excellent ignition qualities. Cetane is rated at 100. The higher the cetane number, the shorter the ignition lag time (delay time) from the point the fuel enters the combustion chamber until it ignites.

In fuels that are readily available, the cetane number ranges from 40 to 55 with values of 40 to 50 being most common. These cetane values are satisfactory for medium-speed engines that have rated speeds from 500 to 1200 rpm and for high-speed engines rated over 1200 rpm. Low-speed engines rated below 500 rpm can use fuels in the above 30 cetane number range. The cetane number improves with the addition of certain compounds, such as ethyl nitrate, acetone peroxide, and amyl nitrate. Amyl nitrate is commercially available for this purpose.

Diesel Fuel Grades

Minimum quality standards for diesel fuel grades have been set by the American Society for Testing Materials. Two grades of diesel fuels, number 1 and number 2, are used to fuel cars and trucks.

Number 2 diesel fuel is the most popular and widely distributed. Number 1 diesel fuel is less dense than number 2, with a lower heat content. Number 1 diesel fuel is blended with number 2 to improve starting in cold weather. In the winter, passenger car diesel is likely to be a mixture of number 1 and 2 fuels. In moderately cold climates, the blend may be 90 percent number 2 to 10 percent number 1. In severe climates, the ratio may be as high as 50-50.

Diesel fuel economy can be expected to drop off during the winter months due to the use of number 1 diesel in the fuel blend.