The basic fuel flow is as follows:
The fuel pump draws fuel from the tank past a
one-way no-return check valve into the primary
filter. Here the fuel passes through a 30-micron-
filtering-capacity, cotton-wound, sock-type
element. From the primary filter it passes up to
the suction side of the fuel pump. Here the fuel is
forced out at 65 to 75 psi to the secondary filter
that is a pleated paper element of lo-micron
Fuel then passes up to the inlet fuel manifold of
the cylinder head where it is distributed through
the fuel jumper lines into each injector.
All surplus fuel (not injected) returns from the
injectors through fuel jumper lines to the return
fuel manifold, through the restricted fitting,
which maintains adequate fuel pressure in the
cylinder head at all times, then back to the tank.
The fuel injector, or what is often referred to as a
unit injector (fig. 5-25), is used by Detroit diesel in all
series of engine that they build. Certainly, there are
some variations in basic design and in the actual testing
procedures used; however, the function and operation is
the same for all.
Unit injectors were designed with simplicity in
mind both from a control and adjustment outlook. They
are used on direct-injection, open-type, two-cycle
combustion chamber engines manufactured by General
Motors. No high-pressure fuel lines or air-fuel mixing
or vaporizing devices are required with these injectors.
The fuel from the fuel pump is delivered to the inlet fuel
manifold (cast internally within the cylinder head) at a
pressure of 65 to 75 psi. The fuel then flows to the
injectors through fuel pipes called jumper lines. Once
the fuel from the pump reaches the injector, it performs
the following functions:
1. Times injection:
liming of the injector is
accomplished by movement of the injector
control rack, which causes rotation of the
plunger within the injector bushing. Since the
plunger is manufactured with a helical chamber
area, this rotation will either advance or retard
closing of the ports in the injector bushing, and
therefore the start and end of the actual injection
period. Pushrod adjustment establishes the
height of the injector follower above the body.
In turn, this factor establishes the point or time
Figure 5-25.Unit injector.
that the descending plunger closes the bushing
ports, allowing injection to begin.
Meter the fuel: The rotation of the plunger by
movement of the injector control rack will
advance or retard the start and end of injection.
If the length of time that the fuel can be injected
is varied, the amount of fuel will be varied.
Pressurizes the fuel: Fuel that is trapped
underneath the plunger on its downward stroke
will develop enough pressure to force its way
past the check valve or needle valve, therefore
entering the combustion chamber.
Atomizes the fuel: Fuel under pressure that
forces its way past the check or needle valve
must than pass through small holes or orifices in
the injector spray tip. This passage breaks the
fuel down into a finely atomized spray, as it
enters the combustion chamber.
The two-stroke Detroit diesel engine unit fuel
injector is located in the cylinder head. The injector sits
in a copper tube in the head that is surrounded by water
for cooling purposes.
The injector is placed in the
cylinder head by a dowel pin on the underside of its
body. The injector is held in place by a single bolt and