Distillation. The distillation range of diesel fuel has a significant influence on
engine performance. This is especially so in medium- and high-speed engines. If the
fuel is too volatile, the engine loses power and efficiency because of vapor lock in
the fuel system or poor droplet penetration into the cylinder. On the other hand, if
the volatility of the fuel is too low, the engine will lose power and efficiency as a
result of poor atomization of the fuel. Both the front end and the back end of the
distillation are important. If the 10 % vol point is too high, the engine will have
difficulty starting. A low 50 % point reduces particulate emissions and odor.
Because heavier molecules are more difficult to burn, both soot and the soluble
organic fraction (SOF) of the particulate emissions are increased if the 90 % point is
too high; the emission of unburned hydrocarbons will also increase.
Viscosity. Fuel viscosity has an important effect on the fuel pump and injector
system. The shape of the fuel spray is affected by viscosity. If this is too high, the
fuel will not be properly atomized into the cylinder, which will result in poor
combustion, loss of power, and efficiency, with an increase in CO and hydrocarbon
emission. Another effect of poor atomizing of the fuel will be to allow the fuel to
impinge on the cylinder walls and remove the lubricating film. This results in
excessive wear and increase of hydrocarbon emissions.
If the viscosity of the fuel is too low, the injection spray is too soft and will not
penetrate far enough into the cylinder. A loss of power and efficiency will occur due
to this. Where the fuel system is also lubricated by the fuel, as is the case in some
engine designs, increased wear to the system will result through low fuel viscosity.
Cold properties (cloud and pour point). Cold weather performance of diesel fuel
is a key consideration for users. Actual specifications for cold flow properties are
based on expected temperature extremes, and different test methods are used in
different parts of the world. In the USA, cloud point is used as an indicator of the
cold flow properties of the fuel. Cloud point is the temperature at which wax begins
to precipitate out of the fuel. The longer paraffin molecules in the fuel precipitate as
a wax when the temperature falls below the cloud point. This wax clogs unheated
fuel lines and filters. The more paraffinic a fuel, the higher will be its cloud point. In
some parts of the world, pour point is used as an indication of the lowest temperature at which a fuel can be pumped. Pour points are generally 4–5
C lower than
the cloud point. Increasingly, the Cold Filter Plugging Point (CFPP) test is
replacing or supplementing the other cold property specifications, since it directly
measures cold weather problems with fuels.
Flash point. The flash point of a fuel is the temperature that the fuel must be
heated to produce an ignitable mixture of vapor and air above the surface of the
liquid. This property is only important for safe handling and storage of the fuel. It
has no effect on the performance of the fuel or its emission properties. If the flash
point is too low, fire or explosion could occur when it is handled.
Meeting the Diesel Fuel Parameters
There are various solutions for meeting the parameters of diesel fuel in the design of
the fuel. These solutions range from fractionation, adding improvers, and more
complex hydroprocessing. Using modern developments in hydroprocessing, it is
Petroleum Products and a Refinery Configuration
79
engine performance. This is especially so in medium- and high-speed engines. If the
fuel is too volatile, the engine loses power and efficiency because of vapor lock in
the fuel system or poor droplet penetration into the cylinder. On the other hand, if
the volatility of the fuel is too low, the engine will lose power and efficiency as a
result of poor atomization of the fuel. Both the front end and the back end of the
distillation are important. If the 10 % vol point is too high, the engine will have
difficulty starting. A low 50 % point reduces particulate emissions and odor.
Because heavier molecules are more difficult to burn, both soot and the soluble
organic fraction (SOF) of the particulate emissions are increased if the 90 % point is
too high; the emission of unburned hydrocarbons will also increase.
Viscosity. Fuel viscosity has an important effect on the fuel pump and injector
system. The shape of the fuel spray is affected by viscosity. If this is too high, the
fuel will not be properly atomized into the cylinder, which will result in poor
combustion, loss of power, and efficiency, with an increase in CO and hydrocarbon
emission. Another effect of poor atomizing of the fuel will be to allow the fuel to
impinge on the cylinder walls and remove the lubricating film. This results in
excessive wear and increase of hydrocarbon emissions.
If the viscosity of the fuel is too low, the injection spray is too soft and will not
penetrate far enough into the cylinder. A loss of power and efficiency will occur due
to this. Where the fuel system is also lubricated by the fuel, as is the case in some
engine designs, increased wear to the system will result through low fuel viscosity.
Cold properties (cloud and pour point). Cold weather performance of diesel fuel
is a key consideration for users. Actual specifications for cold flow properties are
based on expected temperature extremes, and different test methods are used in
different parts of the world. In the USA, cloud point is used as an indicator of the
cold flow properties of the fuel. Cloud point is the temperature at which wax begins
to precipitate out of the fuel. The longer paraffin molecules in the fuel precipitate as
a wax when the temperature falls below the cloud point. This wax clogs unheated
fuel lines and filters. The more paraffinic a fuel, the higher will be its cloud point. In
some parts of the world, pour point is used as an indication of the lowest temperature at which a fuel can be pumped. Pour points are generally 4–5
C lower than
the cloud point. Increasingly, the Cold Filter Plugging Point (CFPP) test is
replacing or supplementing the other cold property specifications, since it directly
measures cold weather problems with fuels.
Flash point. The flash point of a fuel is the temperature that the fuel must be
heated to produce an ignitable mixture of vapor and air above the surface of the
liquid. This property is only important for safe handling and storage of the fuel. It
has no effect on the performance of the fuel or its emission properties. If the flash
point is too low, fire or explosion could occur when it is handled.
Meeting the Diesel Fuel Parameters
There are various solutions for meeting the parameters of diesel fuel in the design of
the fuel. These solutions range from fractionation, adding improvers, and more
complex hydroprocessing. Using modern developments in hydroprocessing, it is
Petroleum Products and a Refinery Configuration
79