automatically and accurately regulate the pump
the range of operation. This characteristic requires
pressure and delivery to the engine.
the use of a pressure relief valve for disposing of
excess fuel. A constant-displacement gear-type
Where variable-displacement pumps are
pump is illustrated in figure 4-26.
installed, two similar pumps are provided,
connected in parallel. Either pump can carry the
load if the other fails during normal parallel
operations. At times, one pump is not enough to
meet power requirements. Pump duplication
The variable-displacement pump system dif-
increases safety in operation, especially in takeoffs
fers from the constant-displacement pump system.
Pump displacement is changed to meet varying
fuel flow requirements; that is, the amount of fuel
The positive-displacement, variable-stroke
that is discharged from the pump can be made
type of pump incorporates a rotor, a piston, a
to vary at any one speed. With a pump of variable
maximum speed governor, and a relief valve
flow, the applicable fuel control unit can
Figure 4-26.-Cutaway view of a dual-element gear-type pump.