with which a liquid changes to a vapor is known as the
volatility of the liquid The 90 percent distillation
temperature measures volatility of diesel fuel. This is
the temperature at which 90 percent of a sample of the
fuel has been distilled off. The lower the distillation
temperature, the higher the volatility of the fuel. In
small diesel engines higher fuel volatility is needed than
in larger engines in order to obtain low fuel
consumption, low exhaust temperature, and minimum
The viscosity is a measure of the resistance to flow
of the fuel, and it will decrease as the fuel oil
temperature increases. What this means is that a fluid
with a high viscosity is heavier than a fluid with low
viscosity. A high viscosity fuel may cause extreme
pressures in the injection systems and will cause
reduced atomization and vaporization of the fuel spray.
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 machined injectors. 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.
Recommended fuel oil viscosity for high-speed
diesel engines is generally in the region of 39 SSU
(Seconds Saybolt Universal) which is derived from
using a Saybolt Viscosimeter to measure the time it
takes for a quantity of fuel to flow through a restricted
hole in a tube. A viscosity rating of 39 SSU provides
good penetration into the combustion chamber,
atomization of fuel, and suitable lubrication.
Sulfur has a definite effect on the wear of the
internal components of the engine, such as piston ring,
pistons, valves, and cylinder liners. In addition a high
sulfur content fuel requires that the engine oil and filter
be changed more often. This is because the corrosive
effects of hydrogen sulfide in the fuel and the sulfur
dioxide or sulfur triioxide that is formed during the
combustion process combines with water vapor to form
acids. High additive lubricating oils are desired when
high sulfur fuels are used. Refer to the engine
manufacturers specifications for the correct lube oil
when using high sulfur fuel.
Sulfur content can only be established by chemical
analysis of the fuel. Fuel sulfur content above 0.4% is
considered as medium or high and anything below 0.4%
is low. No. 2D contains between 0.2 and 0.5% sulfur,
whereas No. 1D contains less than 0.1%.
Sulfur content has a direct bearing on the life
expectancy of the engine and its components. Active
sulfur in diesel fuel will attack and corrode injection
system components in addition to contributing to
combustion chamber and injection system deposits.
Cloud and Pour Point
Cloud point is the temperature at which wax
crystals in the fuel (paraffin base) begin to settle out
with the result that the fuel filter becomes clogged. This
condition exists when cold temperatures are
encountered and is the reason that a thermostatically
controlled fuel heater is required on vehicles operating
in cold weather environments. Failure to use a fuel
heater will prevent fuel from flowing through the filter
and the engine will not run. Cloud point generally
occurs 9-14°F above the pour point.
Pour point of a fuel determines the lowest
temperature at which the fuel can be pumped through
the fuel system. The pour point is 5°F above the level at
which oil becomes a solid or refuses to flow.
Cleanliness and Stability
Cleanliness is an important characteristic of diesel
fuel. Fuel should not contain more than a trace of
foreign substances; otherwise, fuel pump and injectors
difficulties will develop leading to poor performance or
seizure. Because it is heavier and more viscous, diesel
fuel will hold dirt particles in suspension for a longer
period than gasoline. Moisture in the fuel can also
damage or cause seizure of injector parts when
Fuel stability is its capacity to resist chemical
change caused by oxidation and heat. Good oxidation
stability means that the fuel can be stored for extended
periods of time without the formation of gum or sludge.
Good thermal stability prevents the formation or carbon
in hot parts, such as fuel injectors or turbine nozzles.
Carbon deposits disrupt the spray patterns and cause
COMBUSTION CHAMBER DESIGN
The fuel injected into the combustion chamber must
be mixed thoroughly with the compressed air and