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.
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:
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 that the
descending plunger closes the bushing ports, allowing injection to begin.
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.
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.
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.
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 clamp arrangement. The clamp sits low on
the injector body, which allows clearance for the valve bridge operating mechanism. The
injector is also known as an offset body because the fuel inlet and outlet are offset to
one another. This arrangement allows sufficient clearance between the valves.
has a circular disc pressed into a recess at the front side of the injector for
identification purposes. The identification tag indicates the nominal output of the
injector in cubic millimeters. Both the plunger and bushing are marked with corresponding
numbers to identify them as mating parts. Therefore, if either the plunger or bushing
requires replacement, both must be replaced as an assembly.
control rack for each injector is actuated by a lever on the injector control tube that,
in turn, is connected to the governor by mean of a fuel rod. hese levers can be adjusted,
thus permitting a uniform setting of all injector racks. Basic operation of the unit
injector is as follows:
under pressure, enters the injector at the inlet side through a filter cap and filter
element. rom the filter element, the fuel passes through a drilled passage into the supply
chamberthat area between the plunger bushing and the spill deflector and the area
underneath the injector plunger within the bushing. The plunger operates up and down in
the bushing, the bore of which is open to the fuel supply in the annular chamber by two
funnel-shaped ports in the plunger bushing.
plunger descends, under pressure of the injector rocker arm, first closing of the lower
port and then the upper. Before the upper port is shut off, fuel being displaced by the
descending plunger flows up through the "T" drilled hole in the plunger and
escapes through the upper port and into the supply chamber.
the upper and lower ports closed off, the remaining fuel is subjected to increased
pressure by the continued downward movement of the plunger. When sufficient pressure is
built up, it opens the flat, non-return, check valve. The fuel is compressed until the
pressure force acting on the needle valve is sufficient to open the valve against the
downward force of the valve spring. As soon as the needle valve lifts off its seat, the
fuel is forced through the small orifices in the spray tip and atomized into the
the plunger continues to descend, it uncovers the lower port, so fuel pressure is
relieved, and the valve spring closes the needle valve, ending injection. Then the plunger
returns to its original position and waits for the next injection cycle.