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 TM 55-1905-221-14-2
5-18. FUEL INJECTOR (Continued).
i. Fuel, under pressure, enters the injector at the inlet side through a filter cap 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.
j. The motion of the injector rocker arm is transmitted to the plunger 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.
k. 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 upper 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.
l. 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 and atomized into the combustion chamber.
m. A check valve 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. The figure below shows the various phases of injector operation by
the vertical travel of the injector plunger.
5 -401
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