Page 111 - The Jet Engine

Fuel system

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
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 Fig. 10-6 Acceleration control by dashpot
spill valve, so causing a reduction in pump stroke throttle.

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