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constant fuel flow.
19. When the throttle is opened, the control valve
closes the low pressure (L.P.) fuel port in the sleeve
and the throttle servo pressure increases. The
throttle valve moves towards the selected throttle
position until the L.P. port opens and the pressure
balance across the throttle valve is restored. The
decreasing fuel pressure difference across the
throttle valve is sensed by the pressure drop control
diaphragm, which closes the spill valve to increase
the pump servo pressure and therefore the pump
output. The spill valve moves into the sensitive
position, controlling the pump servo mechanism so
that the correct fuel flow is maintained for the
selected throttle position.
20. During initial acceleration, fuel control is as
described in para. 19; however, at a predetermined
throttle position the engine can accept more fuel and
at this point the throttle valve uncovers an annulus,
so introducing extra fuel at a higher pressure (pump
delivery through one restrictor). This extra fuel further
increases the throttle servo pressure, which
increases the speed of throttle valve travel and the
rate of fuel supply to the spray nozzle.
21. On deceleration, movement of the control valve
acts directly on the throttle valve through the servo
spring. Control valve movement opens the flow ports
through the control valve and throttle valve, to bleed
servo fuel through the L.P. port. Throttle control
pressure then moves the throttle valve towards the
closed position, thus reducing the fuel flow to the
spray nozzles.
22. Changes in air intake pressure, due to a change
in aircraft altitude or forward speed, are sensed by
the capsule assembly in the fuel control unit. With
increased altitude and a corresponding decrease in
air intake pressure, the evacuated capsule opens the
spill valve, so causing a reduction in pump stroke
Fuel system
101
Fig. 10-6 Acceleration control by dashpot
throttle.
Fuel system
102
Fig. 10-7 A proportional flow control system.
until the fuel flow matches the airflow. Conversely, an
increase in air intake pressure closes the spill valve
to increase the fuel flow.
23. H.P. compressor shaft r.p.m. is governed by a
hydro-mechanical governor which uses hydraulic
pressure proportional to engine speed as its
controlling parameter. A rotating spill valve senses
the engine speed and the controlling pressure is
used to limit the pump stroke and so prevent overspeeding
of the H.P. shaft rotating assembly. The
controlling pressure is unaffected by changes in fuel
specific gravity.
24. At low H.P. shaft speeds, the rotating spill valve
is held open, but as engine speed increases,
centrifugal loading moves the valve towards the
closed position against the diaphragm loads. This
restricts the bleed of fuel to the L.P. side of the valve
until, at governed speed, the governor pressure
deflects the servo control diaphragm and opens the
servo spill valve to control the fuel flow and thereby
the H.P. shaft speed.
25. If the engine gas temperature attempts to
exceed the maximum limitation, the current in the
L.P. speed limiter and temperature control solenoid is
reduced. This opens the spill valve to reduce the
pressure on the pressure drop control diaphragm.
The flow control spill valve then opens to reduce the
pump servo pressure and fuel pump output.
26. To prevent the L.P. compressor from overspeeding,
multi-spool engines usually have an L.P.
compressor shaft speed governor. A signal of L.P.
shaft speed and intake temperature is fed to an
amplifier and solenoid valve, the valve limiting the
fuel flow in the same way as the gas temperature
control (para. 25).
27. The system described uses main and starting
spray nozzles under the control of an H.P. shut-off
valve. Two starting nozzles are fitted in the
combustion chamber, each being forward of an
igniter plug. When the engine has started, the fuel
flow to these nozzles is cut off by the H.P. shut-off
valve.
28. To ensure that a satisfactory fuel pressure to the
spray nozzles is maintained at high altitudes, a back
pressure valve, located downstream of the throttle
valve, raises the pressure levels sufficiently to
ensure satisfactory operation of the fuel pump servo
system.
Flow control
29. A flow control fuel system is generally more
compact than a pressure control system and is not
sensitive to flow effect of variations downstream of
the throttle. The fuel pump delivery pressure is
related to engine speed; thus, at low engine speeds
　
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