Page 117 - The Jet Engine

Fuel system

53. At any steady running condition, the engine governor sleeve. A similar stop also prevents the fuel
speed is governed by the regulator controlling the supply from being completely cut off by the governor
fuel flow. The fuel pump delivery is fixed at a constant sleeve during a rapid deceleration.
value by applying the system pressure difference to
the fuel pump servo piston. This is arranged to 59. Changes in altitude or forward speed of the
balance the servo piston spring forces. aircraft vary the fuel flow required to maintain a
constant engine speed. To provide this control, the
54. When the air intake pressure is at a constant capsule assembly senses changes in H.P.
value, the rotating V.M.O. sleeve is held in a fixed compressor inlet and delivery pressures and adjusts
axial position by the capsule loading. The fixed the V.M.O. accordingly. For instance, as the aircraft
throttle setting maintains a set load on the speed altitude increases, the compressor delivery pressure
control governor and, as the r.p.m. is constant, the falls and the capsule assembly expands to reduce
governor sleeve is held in a fixed position. the V.M.O. The increased system pressure drop is
sensed by the fuel pump servo piston, which adjusts
55. The fuel pump delivery is passed to the annulus the pump output to match the reduced airflow and so
surrounding the V.M.O.; the annulus area is maintain a constant engine speed. Conversely, an
controlled by the governor sleeve, and the exposed increase in aircraft forward speed causes the
area of the orifice is set by the axial position of the capsule assembly to be compressed and increase
V.M.O. sleeve. Consequently, fuel passes to the the V.M.O. The reduced system pressure drop
inside of the sleeve at a constant flow and therefore causes the fuel pump to increase its output to match
at a constant pressure difference. the increased airflow.
56. The pressure drop control valve, which also 60. To prevent the maximum gas temperature from
forms a piston, senses the pressure difference being exceeded, fuel flow is reduced in response to
across the V.M.O. and maintains the fuel flow at a signals from thermocouples sensing the temperature
fixed value in relation to a function of engine speed, (Part 12). When the maximum temperature is
by controlling the exposed area of the pressure drop reached, the signals are amplified and passed to a
control orifice. rotary actuator which adjusts the throttle mechanism.
This movement has the same effect on fuel flow as
57. When the throttle is slowly opened, the load on
the speed control governor is increased, so moving manual operation of the throttle.
the governor sleeve to increase the V.M.O. annulus 61. To ensure that the engine is not overstressed,
area. The effect of opening the V.M.O. is to reduce the H.P. compressor delivery pressure is controlled
the pressure difference and this is sensed by the to a predetermined value. At this value, a pressure
pressure drop control governor, which opens the limiting device, known as a power limiter, reduces the
pressure drop valve. The reduced system pressure pressure in the capsule chamber, thus allowing the
difference is immediately sensed by the fuel pump capsule assembly to expand and reduce the V.M.O.
servo piston, which increases the pump stroke and so preventing any further increase in fuel flow.
consequently the fuel output. The increased
compressor delivery pressure acts on the capsule 62. A governor prevents the L.P. compressor shaft
assembly, which gradually opens the V.M.O. so that from exceeding its operating limitations and also acts
the fuel flow and engine speed continue to increase. as a maximum speed governor in an event of a
At the speed selected, centrifugal forces acting on failure of the F.F.R. The governor provides a variable
the speed control governor move the governor restrictor between the regulator and the main fuel
sleeve to reduce the V.M.O. annulus area. The spray nozzle manifold. Should the L.P. compressor
resultant increased pressure difference is sensed by reach its speed limitation, flyweights in the governor
the pressure drop control governor, which adjusts the move a sleeve valve to reduce the flow area, The
pressure drop valve to a point at which the pump increased system pressure drop is sensed by the
servo system gives an output to match the engine fuel pump servo piston, which reduces the fuel flow
requirements. The function of the governors and the to the spray nozzles.
control of the fuel flow is shown diagrammatically in
fig. 10-10. 63. This fuel system has no pressurizing valve to
divide the flow from the fuel pump into main and
58. During a rapid acceleration, the initial degree of primary fuel flows. Primary fuel pressure is taken
overselling is mechanically controlled by a stop that from the fixed-area orifice of the pressure drop
limits the opening movement of the speed control control valve. This pressure is always higher than the

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