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the rate of fuel flow increase.
38. The rapid initial increase of fuel flow causes a
rise in the pressure difference across the fuel
metering plunger and this is sensed by a diaphragm
in the pressure drop control section. At a fixed value
of over fuelling, the pressure drop control diaphragm
opens its servo spill valve to override the A.S.U, and
maintains a constant pressure difference across the
metering plunger.
39. The increased fuel supply causes the engine to
accelerate and the fuel metering plunger gives the
maximum permissible fuel flow to match the
increasing compressor delivery pressure. This it
achieves through the A.C.U. servo system, which is
under the control of a spill valve operated by
compressor delivery air pressure acting on a
capsule.
40. As the compressor delivery pressure continues
to rise, the capsule is compressed to open the spill
valve and to bleed pressure from above the metering
plunger. Pump delivery pressure acting underneath
the plunger causes it to lift, this increases the area of
the main fuel flow passage.
Fuel system
104
Fig. 10-8 Servo pressure control by kinetic
valve.
41. The pressure drop control spill valve closes to
increase the fuel pump delivery and maintains the
controlling pressure difference across the plunger.
The fuel flow, therefore, progressively rises as airflow
through the compressor increases. The degree of
overfuelling can be automatically changed by the air
switch, which increases the pressure signal on to the
capsule. The full value of compressor delivery
pressure is now passed on to the A.C.U. capsule
assembly, thus increasing the opening rate of the
metering plunger.
42. As the controlled overfuelling continues, the
pressure difference across the throttle valve
increases. When it reaches the controlling value, the
A.S.U. takes over due to the increasing proportional
flow and again gives a steady fuel flow to the spray
nozzles.
43. The engine speed governor can be of the
pressure control type described in para. 15, or a
hydro-mechanical governor as described in para. 23.
44. The control of servo pressure by the hydromechanical
governor is very similar to that of the
pressure control governor, except that the governor
pressure is obtained from pump delivery fuel passing
through a restrictor and the restricted pressure is
controlled by a rotating spill valve; this type of
governor is unaffected by changes in fuel specific
gravity.
45. At low engine speeds, the rotating spill valve is
held open; however, as engine speed increases,
centrifugal loading moves the valve towards the
closed position against the diaphragm loads. This
restricts the bleed of H.P. fuel to the L.P. side of the
drum until, at governed speed, the governor
pressure deflects the diaphragm and opens the fuel
pump servo pressure spill valve to control the
maximum fuel flow and engine speed.
46. If the engine gas temperature exceeds its
maximum limitation, the solenoid on the proportioning
valve unit is progressively energized. This causes
a movement of the rocker arm to increase the
effective flow area of one restrictor, thus increasing
the proportional flow and opening the A.S.U. spill
valve to reduce servo pressure. The fuel flow is thus
reduced and any further increase of gas temperature
is prevented.
47. To prevent the L.P. compressor from overspeeding,
some twin-spool engines have an L.P.
shaft r.p.m. governor. A signal of L.P. shaft speed is
fed to an amplifier and solenoid valve, which limits
the fuel output in the same way as the gas
temperature control.
48. An idling speed governor is often fitted to
ensure that the idling r.p.m. does not vary with
changing engine loads. A variation of idling r.p.m.
causes the rocker arm to move and alter the proportional
flow, and the A.S.U. adjusts the pump delivery
until the correct idling r.p.m. is restored.
49. On some engines, a power limiter is used to
prevent overstressing of the engine. To achieve this,
compressor delivery pressure acts on the power
limiter capsule. Excess pressure opens the power
limiter atmospheric bleed to limit the pressure on the
A.C.U. capsule and this controls the fuel flow through
the metering plunger.
50. To enable the engine to be relit and to prevent
flame-out at altitude, the engine idling r.p.m. is made
to increase with altitude. To achieve this, some
engines incorporate a minimum flow valve that adds
a constant minimum fuel flow to that passing through
the throttle valve.
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