diesel fuel with that of a reference fuel of known cetane
The properties to consider in selecting a fuel for a
number in a test engine. This reference fuel is a mixture
diesel engine are volatility, cleanliness, viscosity, and
of methylnaphthalene, which is difficult to ignite alone,
ignition quality.
and cetane, which will ignite readily at temperatures
Volatility
and pressures comparable to those in the cylinders of a
diesel engine. The cetane rating indicates the
The volatility of a diesel fuel is measured by the
percentage of cetane in a reference fuel that will just
90-percent distillation temperature. This is the
match the ignition quality of the fuel being tested. The
temperature at which 90 percent of a sample of the fuel
higher cetane numbers indicate more efficient fuels.
has been distilled off. The lower this temperature, the
The large, slow diesels can use 30 cetane fuel, but
higher the volatility of the fuel. In small diesel engines,
the high-speed diesels must use at least a 40 cetane fuel,
a fuel of high volatility is more necessary than in large
while some require as high as a 60 cetane fuel.
engines if there is to be low fuel consumption, low
exhaust temperature, and little exhaust smoke.
Also, the ignition quality of a diesel fuel depends
on its flash point and its fire point. The flash point is the
Cleanliness
temperature to which the fuel vapors must be heated to
flash or ignite. The minimum flash point for diesel fuel
Cleanliness of diesel fuel is very important. Fuel
is 150F. A fuel with too low a flash point is dangerous
should not contain more than a trace of foreign
both to handle and to store.
substances; otherwise, fuel pump and injector
The fire point is that temperature at which the fuel
difficulties will develop. Because it is heavier and more
vapors will continue to burn after being ignited. It is
viscous, diesel fuel will hold dirt particles in suspension
usually 50 to 120 degrees higher than the flash point.
for longer periods than will gasoline. In the refining
process, not all foreign matter can be removed, and
You will sometimes hear knocks in diesel engines.
harmful matter like dirt and water can get into the fuel
They are believed to be caused by the rapid burning of
while it is being handled. Water especially will rust an
the fuel that accumulates during the delay period
engine. Water will also cause hard starting and
between injection and ignition. This delay is known as
misfiring. Dirt will clog injectors and spray nozzles and
ignition lag or ignition delay. When the fuel is injected
may cause an engine to misfire or stop altogether.
into the cylinders, it must vaporize and be heated to the
flash point to start combustion. The lag between
Viscosity
vaporization and flash point depends upon the ignition
quality of the fuel and the speed of the engine and its
The viscosity of fuel is the measure of its resistance
compression ratio. In high-speed engines, the delay
to flow. Viscosity is expressed by the number of
varies from 0.0012 to 0.0018 of a second. Ignition lag
seconds required for a certain volume of fuel to flow
decreases with the increase in engine speed because of
through a hole of a certain diameter at a given
a swifter air movement in the cylinders that makes the
temperature. The viscosity of diesel fuel must be low
injected fuel heat better. Diesel combustion chambers
enough to flow freely at low temperatures, yet high
are designed to provide swift movement of air in the
enough to lubricate the pump and injector plungers
cylinder. The air assists in providing a uniform mixture
properly and lessen the possibility of leakage at the
when the fuel is injected into the cylinder.
pump plungers and dribbling at the injectors. Viscosity
is measured by an instrument called the Saybolt
COMBUSTION CHAMBERS
viscosimeter and is expressed in Saybolt seconds,
universal (SSU).
Several types of combustion chambers are used in
modern diesel engines. They are designed to create
Ignition Quality
turbulence in the cylinder in order to mix the air and
fuel effectively. All modern combustion chambers may
The ignition quality of a diesel fuel is its ability to
be classified under one of the following three designs:
ignite when it is injected into the compressed air within
open type, precombustion type, and turbulence
the engine cylinders. Ignition quality is measured by
chambers. In support equipment, the most common
the cetane rating of the fuel. A cetane number is
types are the open combustion chamber and the
obtained by comparing the ignition quality of a given
precombustion chamber.
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