torquemeter gives an accurate indication of the torque being produced by the power turbine. The torque
pressure value is obtained by tapping the two outlets on the top of the reduction gearbox case. The
pressure differential between the two outlets is then read on an instrument which indicates the correct
torque pressure.
9.18.
FUEL SYSTEM
The T74 basic fuel system consists of a single, engine-driven, fuel pump; p fuel control unit,
temperature compensator, and starting control; and a dual fuel manifold with 14 simplex fuel nozzles.
Two drain valves are mounted on the gas generator case to insure drainage of residual fuel after engine
shutdown after a false or aborted start. The -702 fuel system is illustrated in figure 9.14.
a.
The fuel pump is a positive displacement gear-type pump and is driven off the
accessory gearbox. Fuel from a booster pump enters the fuel pump through a screen filter and then on
to the pump gear chamber, from where the fuel is pumped at high pressure to the fuel control unit.
The filter screen is spring loaded and should it become blocked, an increase in fuel pressure differential
overcomes the spring, lifts the screen from its seat, and allows unfiltered fuel to flow into the system.
The fuel control unit returns bypassed fuel from the fuel control unit to the pump inlet; see the internal
line in figure 9.14.
b.
The fuel control unit is mounted on the engine-driven fuel pump and is driven at a
speed proportional to compressor turbine speed (N1). The control determines the proper fuel schedule
for the engine to produce the power required as established by controlling the speed of the compressor
control governs the N thereby governing the power output of the engine. Control of N1 is accomplished
by regulating the amount of fuel supplied to the combustion chamber.
c.
The temperature compensator is mounted on the compressor case with the bimetallic
disks extending into the inlet air stream. The temperature compensator is illustrated in figure 9.15.
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