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addition, certain sensing devices are incorporated to
provide automatic control of the fuel flow in response
to engine requirements. On the turbo-propeller
engine, the fuel and propeller systems are coordinated
to produce the appropriate fuel/r.p.m.
combination.
10. The usual method of varying the fuel flow to the
spray nozzles is by adjusting the output of the H.P.
fuel pump. This is effected through a servo system in
response to some or all of the following:
(1) Throttle movement.
(2) Air temperature and pressure.
(3) Rapid acceleration and deceleration.
(4) Signals of engine speed, engine gas
temperature and compressor delivery
pressure.
Pressure control (turbo-propeller engine)
11. The pressure control system (fig. 10-4) is a
typical system as fitted to a turbo-propeller engine
where the rate of engine acceleration is restricted by
a propeller speed controller. The fuel pump output is
automatically controlled by spill valves in the flow
control unit (F.C.U.) and the engine speed governor.
These valves, by varying the fuel pump servo
pressure, adjust the pump stroke to give the correct
fuel flow to the engine.
12. At steady running conditions, at a given air
intake pressure and below governed speed, the spill
valve in the F.C.U. is in a sensitive position, creating
a balance of forces across the fuel pump servo
piston and ensuring a steady pressure to the throttle
valve.
13. When the throttle is slowly opened, the
pressure to the throttle valve falls and allows the
F.C.U. spill valve to close, so increasing the servo
pressure and pump delivery. As the pressure to the
throttle is restored, the spill valve returns to its
sensitive or controlling position, and the fuel pump
stabilizes its output to give the engine speed for the
selected throttle position. The reverse sequence
occurs as the throttle is closed.
14. A reduction of air intake pressure, due to a
reduction of aircraft forward speed or increase in
altitude, causes the F.C.U. capsule to expand, thus
increasing the bleed from the F.C.U. spill valve. This
reduces fuel pump delivery until the fuel flow
matches the airflow and the reduced H.P. pump
delivery (throttle inlet pressure), allows the spill valve
to return to its sensitive position. Conversely, an
increase in air intake pressure reduces the bleed
from the spill valve and increases the fuel flow. The
compensation for changes in air intake pressure is
such that fuel flow cannot be increased beyond the
pre-determined maximum permissible for static
International Standard Atmosphere (I.S.A.) sea-level
conditions.
15. The engine speed governor prevents the engine
from exceeding its maximum speed limitation. With
increasing engine speed, the centrifugal pressure
from the fuel pump rotor radial drillings increases and
this is sensed by the engine speed governor
diaphragm. When the engine reaches its speed
limitation, the diaphragm is deflected to open the
governor spill valve, thus overriding the F.C.U. and
preventing any further increase in fuel flow. Some
pressure control systems employ a hydromechanical
governor (para. 23).
16. The governor spill valve also acts as a safety
relief valve. If the fuel pump delivery pressure
exceeds its maximum controlling value, the servo
pressure acting on the orifice area of the spill valve
forces the valve open regardless of the engine
speed, so preventing any further increase in fuel
delivery pressure.
Pressure control (turbo-jet engine)
17. In the pressure control system illustrated in fig.
10-5, the rate of engine acceleration is controlled by
a dashpot throttle unit. The unit forms part of the fuel
control unit and consists of a servo-operated throttle,
which moves in a ported sleeve, and a control valve.
Fuel system
99
Fuel system
100
Fig. 10-5 A pressure control system (turbo-jet engine).
The control valve slides freely within the bore of the
throttle valve and is linked to the pilot’s throttle by a
rack and pinion mechanism. Movement of the throttle
lever causes the throttle valve to progressively
uncover ports in the sleeve and thus increase the
fuel flow. Fig. 10-6 shows the throttle valve and
control valve in their various controlling positions.
18. At steady running conditions, the dashpot
throttle valve is held in equilibrium by throttle servo
pressure opposed by throttle control pressure plus
spring force. The pressures across the pressure drop
control diaphragm are in balance and the pump
servo pressure adjusts the fuel pump to give a
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