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altitude increases, the compressor delivery pressure
falls and the capsule assembly expands to reduce
the V.M.O. The increased system pressure drop is
sensed by the fuel pump servo piston, which adjusts
the pump output to match the reduced airflow and so
maintain a constant engine speed. Conversely, an
increase in aircraft forward speed causes the
capsule assembly to be compressed and increase
the V.M.O. The reduced system pressure drop
causes the fuel pump to increase its output to match
the increased airflow.
60. To prevent the maximum gas temperature from
being exceeded, fuel flow is reduced in response to
signals from thermocouples sensing the temperature
(Part 12). When the maximum temperature is
reached, the signals are amplified and passed to a
rotary actuator which adjusts the throttle mechanism.
This movement has the same effect on fuel flow as
manual operation of the throttle.
61. To ensure that the engine is not overstressed,
the H.P. compressor delivery pressure is controlled
to a predetermined value. At this value, a pressure
limiting device, known as a power limiter, reduces the
pressure in the capsule chamber, thus allowing the
capsule assembly to expand and reduce the V.M.O.
so preventing any further increase in fuel flow.
62. A governor prevents the L.P. compressor shaft
from exceeding its operating limitations and also acts
as a maximum speed governor in an event of a
failure of the F.F.R. The governor provides a variable
restrictor between the regulator and the main fuel
spray nozzle manifold. Should the L.P. compressor
reach its speed limitation, flyweights in the governor
move a sleeve valve to reduce the flow area, The
increased system pressure drop is sensed by the
fuel pump servo piston, which reduces the fuel flow
to the spray nozzles.
63. This fuel system has no pressurizing valve to
divide the flow from the fuel pump into main and
primary fuel flows. Primary fuel pressure is taken
from the fixed-area orifice of the pressure drop
control valve. This pressure is always higher than the
Fuel system
107
Fuel system
108
Fig. 10-10 Governor movement and fuel flow control.
main fuel pressure and it is not shut off by the
pressure drop control piston. It therefore gives a satisfactory
idling fuel flow at all altitudes.
64. On engines featuring water injection (Part 17), a
reset device (fig. 10-11), operated by a piston and
reset cam, increases the loading on the throttle
control spring and stirrup arm, thus selecting a higher
engine speed during water injection. To prevent the
power limiter (fig, 10-9) cancelling the effect of water
injection, a capsule in the limiter is subjected to water
pressure to raise the compressor delivery pressure
at which the power limiter operates.
Pressure ratio control
65. The pressure ratio control (fig. 10-12) is a
mechanical system similar to the combined acceleration
and speed control system, but uses the ratio of
H.P. compressor delivery pressure to air intake
pressure (P4/P1) as the main controlling parameter.
It needs no separate governor unit for controlling the
maximum r.p.m. The controlling mechanism is
contained in one unit, which is usually referred to as
a fuel flow regulator (F.F.R.). A gear-type pump is
used, as described in para. 88, and the pump output
to the F.F.R. is controlled by a pressure drop spill
valve.
66. The F.F.R. is driven by the engine through a
gear train and has two rotating valves. One valve,
known as a variable metering sleeve, has a
triangular orifice, known as the variable metering
orifice (V.M.O.), and this sleeve is given axial
movement by a capsule assembly. The other valve,
known as the pressure drop control valve, is
provided with axial movement by a centrifugal
governor, known as a pressure drop control
governor, Both valves form variable restrictors which
control the fuel flow to the spray nozzles.
67. Control of the V.M.O. area is a function of a
pressure ratio control unit housed in the F.F.R. A
pressure ratio control valve, subjected to P4 and P1,
pressures, regulates the movement of the F.F.R.
capsule and thus controls the V.M.O. area to produce
the pressure ratio dictated by the throttle or power
lever.
68. At any steady running condition, the output of
the fuel pump is greater than the engine requirement.
The pressure drop spill valve is open to allow surplus
fuel to return to the inlet side of the pump. This action
controls the fuel delivery to that demanded by the
F.F.R.
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