Page 121 - The Jet Engine

Fuel system

increases and the capsule moves the metering altitude is increased. This maintains the thrust
sleeve to increase the V.M.O. area. The effect of requirement with the throttle at a fixed position.
opening the V.M.O. is to reduce the pressure
difference and this is sensed by the pressure drop 73. To prevent the maximum L.P. compressor r.p.m.
governor, which opens the pressure drop control and engine gas temperature from being exceeded, a
orifice. The reduced system pressure difference is valve, known as the auxiliary throttling valve, is fitted
immediately sensed by the pressure drop spill valve, in the outlet from the fuel pump, Under steady
which moves towards the closed position and conse- running conditions, the valve is held open by spring
quently increases the fuel output. The increased fuel force, When limiting conditions are reached, the fuel
flow accelerates the engine with a subsequent flow is reduced in response to speed and
increase in pressure ratio (P4/P1). When the temperature signals from the engine. The signals are
required pressure ratio is reached, the pressure ratio amplified and passed to a rotary actuator that
control valve opens and the F.F.R. capsule chamber reduces the area of a variable restrictor. The effect of
pressure reduces. The capsule assembly expands, this is to increase the fuel pressure, which partially
moving the V.M.O. sleeve to reduce the orifice area. closes the throttling valve. H.P. fuel pressure acting
The resultant increased pressure difference is on the face of the pressure drop spill valve is
sensed by the pressure drop control governor, which increased and the spill valve opens to reduce the fuel
adjusts the pressure drop control orifice to a point at flow to the spray nozzles.
which the pressure drop spill valve gives a fuel 74. H.P. shaft speed is also governed by the
output consistent with steady running requirements.
auxiliary throttling valve. Should other controlling
70. During a rapid acceleration, the degree of devices fail and pump speed increases, the fuel
overselling is mechanically controlled by the acceler- pressure closes the throttling valve and opens the
ation stop, which limits the movement of the pressure pressure drop spill valve to reduce the fuel flow.
ratio control valve. A similar stop prevents the fuel 75. With the throttle closed, idling condition is
supply from being completely cut off during a rapid determined by controlling the amount of air being
deceleration. vented through the idling adjuster and the ground
71. When accelerating to a higher P4/P1 ratio, the idling solenoid valve, With both bleeds in operation,
throttle control orifice is increased. The reduced satisfactory flight idling for the air off-takes is
pressure allows the pressure ratio control capsule to ensured. By closing the solenoid valve a lower power
contract so that the valve contacts the acceleration condition for ground idling is obtained.
stop. F.F.R. capsule chamber pressure increases 76. This fuel system, like the combined acceleration
and the capsule moves to increase the V.M.O. area. and speed control system, has no pressurizing valve
This action continues until the required P4/P1 ratio is to divide the flow from the fuel pump into main and
reached. The increased P4 pressure allows the primary flows.
pressure ratio control capsule to re-expand and the
valve to return to the steady running position. ELECTRONIC ENGINE CONTROL

72. A change in altitude of the aircraft requires a 77. As stated in para. 8, some engines utilize a
variation in fuel flow to match the engine thrust and system of electronic control to monitor engine
aircraft climb requirement. The normal effect of an performance and make necessary control inputs to
altitude increase is to decrease the P1 and P4 maintain certain engine parameters within predeter-
pressures, thus opening the pressure ratio control mined limits. The main areas of control are engine
valve and allowing the F.F.R. capsule to expand to shaft speeds and exhaust gas temperature (E.G.T.)
reduce the V.M.O. area and, in consequence, the which are continuously monitored during engine
fuel flow. However, to match the engine thrust and operation. Some types of electronic control function
aircraft climb requirement it is necessary to increase as a limiter only, that is, should engine shaft speed or
the P4/P1 ratio with increasing altitude. This is done E.G.T. approach the limits of safe operation, then an
by a trimmer valve and a capsule that is subjected to input is made to the fuel flow regulator (F.F.R.) to
P1 pressure. As P1 pressure decreases, the trimmer reduce the fuel flow thus maintaining shaft speed or
valve moves across the P1 controlled orifice to E.G.T. at a safe level. Supervisory control systems
reduce the control pressure. This is sensed by the may contain a limiter function but, basically, by using
control capsule, which, by acting on the pressure aircraft generated data, the system enables a more
ratio control valve, slows the closure of the V.M.O. as appropriate thrust setting to be selected quickly and

111