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 TM 55-1905-217-34
(3) Figure 3-4 illustrates the phase of injector operation by the vertical travel of the injector plunger.
(4) Both the injector plunger and plunger bushing are marked with identical part numbers to properly
identify them as mating parts. Therefore, if either plunger or bushing requires replacement, both must be replaced as an
assembly.
(5) The fuel injector has a circular disk pressed into a recess at the front side of the injector body for
identification purposes. The identification tag indicates the nominal output of the injector in millimeters. Each injector
control rack (fig. 3-2) is actuated by a lever on the injector control tube which, in turn, is connected to the governor by
means of a fuel rod. These levers can be adjusted independently on the control tube, thus permitting a uniform setting of
all injector racks.
b. Operation.
(1) Fuel under pressure, enters the injector at the inlet side through a filter cap (fig. 3-2) and a filter
element. From the filter element the fuel passes through a drilled passage into the supply chamber, that area between the
plunger bushing and the spill deflector, in addition to that area under 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.
(2) The motion of the injector rocker arm is transmitted to the plunger (fig. 3-2) by the follower which
bears against the follower spring. In addition to the reciprocating motion, the plunger can be rotated, during operation,
around its axis by the gear which meshes with the control rack. For metering the fuel, an upper helix and a lower helix are
machined in the lower part of the plunger. The relation of the helicies to the two ports, changes with the rotation of the
plunger.
(3) As the plunger moves downward, under pressure of the injector rocker arm, a portion of that fuel
trapped under the plunger is displaced into the supply chamber through the lower port until the port is closed off by the
lower end of the plunger. A portion of the fuel trapped below the plunger is then forced up through a central passage into
the plunger fuel metering recess and into the supply chamber through the upper port until the port is closed off by the
upper helix of the plunger. With the upper and lower ports both closed off, the remaining fuel under the plunger is
subjected to increased pressure by the continued downward movement of the plunger.
(4) When sufficient pressure is built up, the injector valve is lifted off its seat and the fuel is forced through
small orifices in the spray tip (fig. 3-2) and atomized into the combustion chamber.
(5) A check valve (fig. 3-2), mounted in the spray tip, prevents air leakage from the combustion chamber
into the fuel injector in case the injector valve is accidentally held open by a small particle of dirt. The injector plunger is
then returned to its original position by the injector follower spring. Figure 3-4 shows the various phases of injector
operation by the vertical travel of the injector plunger.
(6) On the return upward movement of the plunger, the high pressure cylinder within the bushing is again
filled with fuel oil through the ports, the constant circulation of fresh cool fuel through the injector renews the fuel supply in
the chamber, helps cool the injector and also, effectively removes all traces of air which might otherwise accumulate in the
system and interfere with accurate fuel metering.
(7) The fuel injector outlet opening through which the excess fuel oil returns to the fuel return passage and
then back to the fuel tank, is directly adjacent to the inlet opening and contains a filter element exactly the same as that in
the inlet side.
(8) Changing the position of the helices, by rotating the plunger, retards of advances the closing of the
ports and the beginning or ending of the injection period. At the same time, it in- creases or decreases the amount of fuel
injected into the cylinder. Figure 3-3 shows the various plunger positions from NO LOAD to FULL LOAD. With the control
rack pulled out all the way (no injection), the upper port is not closed by the helix until after the lower port is uncovered.
Consequently, with the rack in this position, all the fuel is forced back into the supply chamber so no injection of fuel takes
place. With the control rack pushed in (full injection), the upper port is closed shortly after the lower port has been covered
thus producing a maximum effective stroke and maximum injection. From this no injection position to full injection position
(full rack movement), the contour of the upper helix advances the closing of the ports and the beginning of injection.
c. Removal. Refer to TM 55-1905-217-12 (para 4-28) for removal of the fuel injector.
d. Fuel Injector Testing.
(1) Injector rack and plunger movement. Check to see if plunger (fig. 3-2) works freely in its bushing by
placing the injector against a bench (fig. 3-5) and depressing the follower to the bottom of its stroke while moving the rack
back and forth. Failure to produce a free rack indicates the internal parts of the injector are dirty or damaged.
3-5
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