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pump delivery pressure is quite low. The fuel pump
output is controlled to give a constant pressure
difference across the throttle valve at a constant air
intake condition. Various devices are also used to
adjust the fuel flow for air intake pressure variations,
idling and acceleration control, gas temperature and
compressor delivery pressure control.
30. A variation of the flow control system is the proportional
flow control system (fig 10-7), which is more
suitable for engines requiring large fuel flows and
which also enables the fuel trimming devices to
adjust the fuel flow more accurately. A small
controlling flow is created that has the same characteristics
as the main flow, and this controlling or proportional
flow is used to adjust the main flow.
31. A different type of spill valve, referred to as a
kinetic valve, is used in this system. This valve
consists of two opposing jets, one subjected to pump
delivery pressure and the other to pump servo
pressure, and an interrupter blade that can be moved
between the jets (fig. 10-8). When the blade is clear
of the jets, the kinetic force of the H.P. fuel jet causes
the servo pressure to rise (spill valve closed) and the
fuel pump moves to maximum stroke to increase the
fuel flow. When the blade is lowered between the
jets, the pressure jet is deflected and the servo
pressure falls, so reducing the pump stroke and the
fuel flow, When the engine is steadily running, the
blade is in an intermediate position allowing a slow
bleed from servo and thus balancing the fuel pump
output.
32. All the controlling devices, except for the engine
speed governor, are contained in one combined fuel
control unit. The main parts of the control unit are the
altitude sensing unit (A.S.U.), the acceleration
control unit (A.C.U.), the throttle and pressurizing
valve unit, and the proportioning valve unit.
33. At any steady running condition below governed
speed, the fuel pump delivery is controlled to a fixed
value by the A.S.U. The spill valve in this unit is held
in the controlling position by a balance of forces,
spring force and the piston force. The piston is
sensitive to the pressure difference across the
sensing valve, the pressure difference being created
by fuel flowing from the proportioning valve back to
the fuel pump inlet.
Fuel system
103
34. The proportioning valve diaphragm is held open
in a balanced condition allowing fuel to pass to the
A.S.U. This means that the restrictor outlet pressure
is equal to the throttle outlet pressure and, as their
inlet pressures are equal, it follows that the pressure
difference across the restrictors and the throttle are
equal; therefore, a constant fuel flow is obtained.
35. When the throttle is slowly opened, the
pressure difference across the throttle valve and the
proportioning flow restrictors decreases and the proportioning
valve diaphragm adjusts its position. This
reduces the proportional flow, which closes the
A.S.U. spill valve and increases the servo pressure.
The fuel pump increases its delivery and this restores
the pressure difference across the throttle valve and
equalizes the pressure difference across the
restrictors. The proportional flow is restored to its
original value and the balance of forces in the A.S.U.
returns the spill valve to the controlling position.
36. A variation of air intake pressure, due to a
change of aircraft forward speed or altitude, is
sensed by the capsule in the A.S.U. A pressure
reduction causes the A.S.U. capsule to expand, thus
increasing the bleed from the spill valve. This
reduces fuel pump delivery until the fuel flow
matches the airflow and results in a lower pressure
difference across the throttle valve and the proportioning
valve restrictors. The reduced proportional
flow restores the balance in the A.S.U. which returns
the spill valve to its controlling position. Conversely,
an increase in aircraft forward speed increases the
air Intake pressure, which reduces the bleed from the
spill valve and increases the fuel flow.
37. During a rapid acceleration, the sudden
decrease in throttle pressure difference is sensed by
the A.S.U., causing the spill valve to close, Such a
rapid increase in fuel supply would, however, create
an excessive gas temperature and also cause the
compressor to surge (Part 3). This occurs because
the inertia of the rotating assembly results in an
appreciable time lag in the rate of airflow increase. It
is essential therefore, to have an acceleration control
to override the A.S.U. to give a corresponding lag in
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