Page 114 - The Jet Engine

Fuel system

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 propor-
tioning 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
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
Fig. 10-8 Servo pressure control by kinetic of over fuelling, the pressure drop control diaphragm
valve. opens its servo spill valve to override the A.S.U, and
maintains a constant pressure difference across the
34. The proportioning valve diaphragm is held open metering plunger.
in a balanced condition allowing fuel to pass to the
A.S.U. This means that the restrictor outlet pressure 39. The increased fuel supply causes the engine to
is equal to the throttle outlet pressure and, as their accelerate and the fuel metering plunger gives the
maximum permissible fuel flow to match the
inlet pressures are equal, it follows that the pressure increasing compressor delivery pressure. This it
difference across the restrictors and the throttle are achieves through the A.C.U. servo system, which is
equal; therefore, a constant fuel flow is obtained.
under the control of a spill valve operated by
compressor delivery air pressure acting on a
35. When the throttle is slowly opened, the
pressure difference across the throttle valve and the capsule.
proportioning flow restrictors decreases and the pro- 40. As the compressor delivery pressure continues
portioning valve diaphragm adjusts its position. This to rise, the capsule is compressed to open the spill
reduces the proportional flow, which closes the valve and to bleed pressure from above the metering
A.S.U. spill valve and increases the servo pressure. plunger. Pump delivery pressure acting underneath
The fuel pump increases its delivery and this restores the plunger causes it to lift, this increases the area of
the pressure difference across the throttle valve and the main fuel flow passage.

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