Figure 7-9.—Mounting of the unit injector in the cylinder head.
On the compression stroke, the exhaust valves
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are closed, the intake ports are covered, and the
air is trapped in the cylinder. The rising piston
compresses the air and raises its temperature. By
the time the piston reaches the top of the stroke,
the volume of the combustion chamber has been
greatly reduced. The relation between the volume
of the cylinder with the piston at the bottom
of its stroke and the cylinder volume with the
piston at the top of its stroke is called the
COMPRESSION RATIO.
As the compression ratio is increased, the
temperature of the air in the cylinder increases.
Current gasoline engines operate at compression
ratios between 6:1 and 11:1, but compression
ratios of diesel engines range between 12:1 and
19:1. Remember, that on the compression stroke
of a diesel engine the air is compressed to a range
of 400 to 600 psi, which results in a temperature
ranging from 700°F to 1200°F. However, when
the fuel is injected into the cylinder and begins
to burn, the pressure may increase to more than
1500 psi and the temperature may rise as high as
1800°F.
compression ignition systems in Engineman 3,
NAVEDTRA 10539.
FUEL SYSTEM
The fuel system of a diesel engine draws fuel
from the service tank and injects it into the engine
cylinders. Figure 7-8 shows the units found in a
typical unit-injector fuel system. The fuel pump
draws the fuel from the tank through a primary
strainer and delivers it under low pressure to the
injector by way of the secondary filter. The
injector is operated by a rocker arm. It meters,
pressurizes, and atomizes the fuel as it is injected
into the combustion chamber. The outlet line
carries the excess fuel from the injector back to
the fuel tank. In some units, a transfer pump is
installed between the tank and the strainer. In
other units, the injection pump and injection
nozzles are separate units instead of a combined
unit, as shown in figure 7-9.
A diesel engine will not operate efficiently
unless clean fuel is delivered to the injector or
7-9
