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
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
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.
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.