PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 2/TASK 1
Characteristics of Diesel Fuels
Probably the most necessary characteristic of
diesel fuels is cleanliness.
Any foreign material present in diesel fuel
will certainly cause damage to the finely machined injector parts. Damage
occurs in two ways:
(1) Particles of dirt cause scoring of the injector components.
Any damage to the fuel injectors will cause poor operation or render the
engine inoperative. Controlling dirt and moisture content in diesel fuel is
more difficult because it is heavier than gasoline.
This causes foreign
material to remain in suspension longer, so that sediment bowls do not work
The viscosity of a fluid is an indication of its
resistance to flow. What this means is that a fluid with a high viscosity
is heavier than a fluid with a low viscosity. The viscosity of diesel fuel
must be low enough to flow freely at its lowest operational temperature, yet
high enough to provide lubrication to the moving parts of the finely
The fuel must also be sufficiently viscous so that
leakage at the pump plungers and dribbling at the injectors will not occur.
Viscosity also will determine the size of the fuel droplets which, in turn,
govern the atomization and penetration qualities of the fuel injector spray.
c. Ignition Quality. The ignition quality of a fuel is its ability to
ignite spontaneously under the conditions existing in the engine cylinder.
The spontaneous-ignition point of a diesel fuel is a function of pressure,
temperature, and time. Because it is difficult to reproduce the operating
conditions of the fuel artificially outside the engine cylinder, a diesel
engine operating under controlled conditions is used to determine the
ignition quality of diesel fuel. The yardstick that is used to measure the
ignition quality of a diesel fuel is the cetane number scale. The cetane
number of a fuel is obtained by comparing it to the operation of a reference
The reference fuel is a mixture of alpha-methyl-naphthalene, which