TROUBLESHOOTING THE HMMWV - OD1616 - LESSON 1/TASK 1
The air is compressed to about one-twentieth of its original volume
in a diesel engine.
In contrast, the fuel and air mixture in a
gasoline engine is compressed to about one-eighth of its original
volume. The diesel engine must compress the air tightly to generate
enough heat to ignite the fuel when it is injected into the cylinder.
The gasoline engine mixes the fuel and air before it reaches the
combustion chamber.
A diesel engine takes in air only through the
intake port. Fuel is then injected into the combustion chamber just
before completion of the compression stroke. As the fuel is injected
in, it mixes with the air and ignites because of the heat.
The engine speed and power output of the diesel engine are controlled
by the quantity of the fuel admitted to the combustion chamber. The
amount of air remains constant.
This contrasts with the gasoline
engine where the speed and power output are regulated by limiting the
air entering the engine.
(2) Operation.
In the paragraphs that follow, the operation of
the engine will be followed through each of the four strokes that
comprise one cycle of operation.
(a) Intake.
When the intake stroke begins, the piston is at
top dead center (TDC).
As the piston moves downward, the intake
valve opens and the downward movement of the piston creates a vacuum
that sucks the air into the cylinder. When the piston reaches bottom
dead center (BDC), the intake valve closes and the intake stroke is
complete.
(b) Compression. The compression stroke begins at BDC. As the
piston travels upward toward TDC, the air that was drawn into the
cylinder is compressed to a ratio of nearly 20 to 1, almost three
times the ratio of a typical gasoline engine. The compression stroke
ends when the piston reaches TDC.
When the piston reaches TDC, the fuel injector sprays a precisely
measured amount of fuel into the precombustion chamber.
The
superheated air in the combustion chamber ignites the fuel as it is
injected.
This ultra-high compression causes the air to superheat,
making it hot enough to ignite the diesel fuel.
This is where the
term compression ignition comes from. This very high
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