TM 5-3895-346-14
ENGINE OVERHAUL
FUEL INJECTOR
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 in the plunger, into the fuel metering
recess, and into the supply chamber through the upper port until that 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.
When sufficient pressure is built up, it opens the flat, nonreturn check valve. The fuel in the check valve cage,
spring cage, tip passages, and tip fuel cavity is compressed until the pressure force acting upward 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 of its seat, the fuel is forced through the small orifices in the spray tip and atomized into
the combustion chamber.
When the lower land of the plunger uncovers the lower port in the bushing, the fuel pressure below the plunger
is relieved and the valve spring closes the needle valve, ending injection.
A pressure relief passage has been provided in the spring cage to permit bleed-off of fuel leaking past the
needle pilot in the tip assembly.
A check valve, directly below the bushing, prevents leakage from the combustion chamber into the fuel injector
in case the 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 4 shows the various phases of injector operation by the
vertical travel of the injector plunger.
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 metering of the fuel.
The fuel injector outlet opening, through which the excess fuel oil returns to the fuel return manifold and then
back to the fuel tank, is directly adjacent to the inlet opening.
Changing the position of the helices, by rotating the plunger, retards or advances the closing of the ports and
the beginning and ending of the injection period. At the same time, it increases or decreases the amount of
fuel injected into the cylinder. Figure 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 of the fuel is forced back into the supply
chamber and no injection of fuel takes place. With the control rack pushed all the way 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.
General Instructions for Injector Care and Overhaul
The fuel injector is one of the most important and precisely-built parts of the engine. The injection of the
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