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TM 9-8000
General.  Fuels used in modern high-speed
Ignition Quality. The ignition quality of a fuel is
diesel engines are derived from the middle distillate
its ability to ignite spontaneously under the conditions
fraction of crude oil.  The middle distillates span the
existing in the engine cylinder. The spontaneous-ignition
boiling range between gasoline and heavy residual oil,
point of a diesel fuel is a function of the pressure,
and  typically include  kerosene,  jet  fuel  (aviation
temperature, and time.
Because it is difficult to
kerosene), diesel fuel, and burner fuel (home heating
reproduce the operating conditions of the fuel artificially
oil). Although large, slow-speed diesel engines used in
outside the engine cylinder, a diesel engine operating
stationary and marine applications will burn almost any
under controlled conditions is used to determine the
grade of heavy oil, the smaller, high-speed diesel
ignition quality of diesel fuel. The yardstick that is used
engines used in most military equipment require middle
to measure the ignition quality of a diesel fuel is the
distillate diesel fuels. These fuels must meet exacting
cetane-number scale. The cetane number of a fuel is
specification requirements to ensure proper engine
obtained by comparing it to the operation of a reference
fuel. The reference fuel is a mixture of alpha-methyl-
naphthalene, which has virtually no spontaneous-ignition
qualities, and pure cetane, which has what are
Cleanliness and Stability.  Cleanliness is an
considered to be perfect spontaneous-ignition qualities.
important characteristic of diesel fuel because the
The percentage of cetane is increased gradually in the
extremely close fit of the injector parts can be damaged
by particles.  Dirt or sand particles in the fuel cause
spontaneous-ignition qualities of the fuel being tested.
scoring of the injector parts, leading to poor performance
The cetane number then is established for the fuel being
or seizure.  Moisture in the fuel can also damage or
tested based on the percentage of cetane present in the
cause seizure of injector parts when corrosion occurs.
reference mixture.
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 long periods
Knocking. Diesel engines have a tendency to
without formation of gum or sludge.  Good thermal
produce a knock that is noticeable particularly during
stability prevents the formation of carbon in hot parts
times when the engine is under a light load.  This
such as fuel injectors or turbine nozzles.  Carbon
knocking occurs due to a condition known as Ignition
deposits disrupt the spray patterns and cause inefficient
delay or ignition lag. When the power stroke begins, the
first molecules of fuel injected into the combustion
chamber first must vaporize and superheat before
ignition occurs.
During this period, a quantity of
The viscosity of a fluid is an
unburned fuel builds up in the combustion chamber.
indication of its resistance to flow. What this means is
When ignition occurs, the pressure increase causes the
that a fluid with a high viscosity is heavier than a fluid
built-up fuel to Ignite instantly.
This causes a
with a low viscosity. The viscosity of diesel fuel must be
disproportionate increase in pressure, creating a distinct
low enough to flow freely at its lowest operational
and audible knock. Increasing the compression ratio of
temperature, yet high enough to provide lubrication to the
a diesel engine will decrease ignition lag and the
moving parts of the finely machined injectors. The fuel
tendency to knock.  This contrasts with a gasoline
must also be sufficiently viscous so that leakage at the
engine, whose tendency to knock will Increase with an
pump plungers and dribbling at the injectors will not
Increase in compression ratio.
Knocking in diesel
occur. Viscosity also will determine the size of the fuel
engines is affected by factors other than compression
droplets, which, in turn, govern the atomization and
ratio, such as the type of combustion chamber, airflow
penetration qualities of the fuel injector spray.
within the chamber, Injector nozzle type, air and fuel
temperature, and the cetane number of the fuel.
Change 1 5-1


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