TM 55-2815-574-24
0003
FUEL SYSTEM - Continued
Combustion required for satisfactory engine operation is obtained by injecting, under pressure, a small quantity of
accurately timed, metered, and finely atomized fuel oil into the combustion chamber. Metering and timing during
fuel injection is accomplished by an upper and lower helix machined in the lower end of the injector plunger. The
continuous fuel flow through the injector serves, in addition to preventing air pockets in the fuel system, as a
coolant for those injector parts subjected to high combustion temperatures.
To vary the power output of the engine, injectors having different fuel output capacities are used. The fuel output
of the various injectors is governed by the effective stroke of the plunger and the flow rate of the spray tip. Since
the helix angle and the plunger design determines the operating characteristics of a particular injector it is
imperative that the specified injectors are used for each engine. If injectors of different types are mixed in an
engine, erratic operation will result and may cause serious damage to the engine or to the equipment which
it powers.
Figure 5. Injector.
Each fuel injector has a circular disc pressed into a recess at the front side of the injector body for identification
purposes. Each injector control rack 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 or fine tuning of all injector racks. The fuel injector combines in a single unit all of the
parts necessary to provide complete and independent fuel injection at each cylinder.
Fuel, under low pressure, enters the injector at the inlet side through a filter cap and filter positioned over the
racks. From the filter, 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, and is supplied fuel through the two tunnel-shaped ports in the
bushing wall. 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 around its axis by
the gear which meshes with the control rack. To accomplish fuel metering, an upper helix and a lower helix are
machined in the lower part of the plunger. The helix relationship to the ports changes with the rotation of
the plunger.