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69. When the throttle is slowly opened, the throttlecontrolled
orifice is increased and the control
pressure falls, thus allowing the pressure ratio
control valve to move towards the closed position
(acceleration stop). F.F.R. capsule chamber pressure
Fuel system
109
Fig. 10-11 Effect of water reset on speed control governor.
Fuel system
110
Fig. 10-12 A pressure ratio control system.
increases and the capsule moves the metering
sleeve to increase the V.M.O. area. The effect of
opening the V.M.O. is to reduce the pressure
difference and this is sensed by the pressure drop
governor, which opens the pressure drop control
orifice. The reduced system pressure difference is
immediately sensed by the pressure drop spill valve,
which moves towards the closed position and consequently
increases the fuel output. The increased fuel
flow accelerates the engine with a subsequent
increase in pressure ratio (P4/P1). When the
required pressure ratio is reached, the pressure ratio
control valve opens and the F.F.R. capsule chamber
pressure reduces. The capsule assembly expands,
moving the V.M.O. sleeve to reduce the orifice area.
The resultant increased pressure difference is
sensed by the pressure drop control governor, which
adjusts the pressure drop control orifice to a point at
which the pressure drop spill valve gives a fuel
output consistent with steady running requirements.
70. During a rapid acceleration, the degree of
overselling is mechanically controlled by the acceleration
stop, which limits the movement of the pressure
ratio control valve. A similar stop prevents the fuel
supply from being completely cut off during a rapid
deceleration.
71. When accelerating to a higher P4/P1 ratio, the
throttle control orifice is increased. The reduced
pressure allows the pressure ratio control capsule to
contract so that the valve contacts the acceleration
stop. F.F.R. capsule chamber pressure increases
and the capsule moves to increase the V.M.O. area.
This action continues until the required P4/P1 ratio is
reached. The increased P4 pressure allows the
pressure ratio control capsule to re-expand and the
valve to return to the steady running position.
72. A change in altitude of the aircraft requires a
variation in fuel flow to match the engine thrust and
aircraft climb requirement. The normal effect of an
altitude increase is to decrease the P1 and P4
pressures, thus opening the pressure ratio control
valve and allowing the F.F.R. capsule to expand to
reduce the V.M.O. area and, in consequence, the
fuel flow. However, to match the engine thrust and
aircraft climb requirement it is necessary to increase
the P4/P1 ratio with increasing altitude. This is done
by a trimmer valve and a capsule that is subjected to
P1 pressure. As P1 pressure decreases, the trimmer
valve moves across the P1 controlled orifice to
reduce the control pressure. This is sensed by the
control capsule, which, by acting on the pressure
ratio control valve, slows the closure of the V.M.O. as
altitude is increased. This maintains the thrust
requirement with the throttle at a fixed position.
73. To prevent the maximum L.P. compressor r.p.m.
and engine gas temperature from being exceeded, a
valve, known as the auxiliary throttling valve, is fitted
in the outlet from the fuel pump, Under steady
running conditions, the valve is held open by spring
force, When limiting conditions are reached, the fuel
flow is reduced in response to speed and
temperature signals from the engine. The signals are
amplified and passed to a rotary actuator that
reduces the area of a variable restrictor. The effect of
this is to increase the fuel pressure, which partially
closes the throttling valve. H.P. fuel pressure acting
on the face of the pressure drop spill valve is
increased and the spill valve opens to reduce the fuel
flow to the spray nozzles.
74. H.P. shaft speed is also governed by the
auxiliary throttling valve. Should other controlling
devices fail and pump speed increases, the fuel
pressure closes the throttling valve and opens the
pressure drop spill valve to reduce the fuel flow.
75. With the throttle closed, idling condition is
determined by controlling the amount of air being
vented through the idling adjuster and the ground
idling solenoid valve, With both bleeds in operation,
satisfactory flight idling for the air off-takes is
ensured. By closing the solenoid valve a lower power
condition for ground idling is obtained.
76. This fuel system, like the combined acceleration
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