PRINCIPLES GASOLINE/DIESEL FUEL SYSTEMS - OD1620 - LESSON 2/TASK 1
virtually no spontaneous ignition qualities, and pure cetane, which has what
are considered to be perfect spontaneous ignition qualities. The percentage
of cetane is increased gradually in the reference fuel until the fuel
matches the spontaneous ignition qualities of the fuel being tested. The
cetane number then is established for the fuel being tested based on the
percentage of cetane present in the reference mixture.
d. Diesel engines have a tendency to produce a knock that is
particularly noticeable during times when the engine is under a light load.
This knocking occurs due to a condition known as ignition delay or ignition
lag. When the power stroke begins, the first molecules of fuel injected
into the combustion chamber must first vaporize and superheat before
ignition occurs. During this period, a quantity of unburned fuel builds up
in the combustion chamber. When ignition occurs, the pressure increase
causes the builtup fuel to ignite instantly. This causes a
disproportionate increase in pressure, creating a distinct and audible
knock. Increasing the compression ratio of a diesel engine will decrease
ignition lag and the tendency to knock. This contrasts with a gasoline
engine, whose tendency to knock will increase with an increase in
compression ratio. Knocking in diesel engines is affected by factors other
than compression ratio, such as the type of combustion chamber, airflow
within the chamber, injector nozzle type, air and fuel temperature, and the
cetane number of the fuel.
e. Multifuel Engine Authorized Fuels. Multifuel engines are four
stroke cycle diesel engines that will operate satisfactorily on a wide
variety of fuels. The fuels are grouped accordingly:
(1) Primary and Alternate I Fuels. These fuels will operate the
multifuel engine with no additives.
(2) Alternate II Fuels. These fuels generally require the addition
of diesel fuel to operate the multifuel engine.
(3) Emergency Fuels. These fuels will operate the multifuel engine
with the addition of diesel fuel; however, extended use of fuels from this
group will cause eventual fouling of fuel injection
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