Patent GB-1465480-A (GB)
Title: Fuel control system for gas turbine engine
IPC: F02C9/38
Filed: 1973-07-18
Abstract: 1465480 Gas turbine engine fuel systems LUCAS INDUSTRIES Ltd 2 July 1974 [18 July 1973 31 Oct 1973] 34136/73 and 50565/73 Heading F1G In a gas turbine engine fuel system, the fuel flows successively through a pump, a variable metering device and a control valve, the latter having first and second outlets respectively communicating with burners and with the upstream side of the pump and a control element movable in response to increase in a servo pressure to open the first outlet and shut the second outlet, and a pilot valve responsive to increase in engine speed to reduce the servo pressure. Fuel is supplied by a centrifugal pump 11, Fig. 2, and a positive-displacement pump 12 to a metering valve 14 comprising a sleeve 15a slidable relative to a fixed sleeve 15b in a closing direction by means of flyweights 20 driven by the high-speed shaft of a three-spool engine 10, Fig. 3, and slidable in the opening direction by means of a spring 36, Fig. 1, acting on the sleeve 15a via a roller 35 and a lever 21 pivoted at 22. The spring 36 reacts via a roller 34 against a face 26 of a cam 25, the other face 27 of which is supported by rollers 28, 29. The movement about the pivot 22 exerted by the spring 36 is variable by a power demand lever 38 acting via a cam 37 on a pivoted carrier 31. The position of the cam 25 and hence also the force of spring 36 is adjustable by means of a cam 43 rotatable by a stepping motor 44 controlled by a digital circuit 45 in accordance with the speed NH of the high-speed shaft, the engine inlet temperature T 1 , the Mach number MN and altitude of the aircraft, and the combustion chamber temperature T 7 . Required operating modes, e.g., take-off, maximum climb, and maximum cruise are selected by a switch 46. If circuit 45 fails, lever 40 can be lifted away from cam 43 by means of a cam 47 by putting a selector lever 48 into a position A. The metering valve 14 comprises a further sleeve 17 movable in the opening direction in response to decrease in an intermediate pressure P 3  of the engine compressor acting on the interior of a bellows 52 or increase in a pressure P4p acting on the exteriors of the bellows 52 and an evacuated bellows 51, the pressure P 4 p being derived via a fluid potentiometer 53, 54 from the pressure P3 and a pressure P 4  derived from the compressor output. Movement of sleeve 17 in the closing direction is limited by a stop 57, the position of which is variable via a cam 58 by the selector lever 48, according to whether the latter is in the normal running (B), fuel-rich starting (c), or fuel-lean starting (D) positions. A by-pass valve connected across the pump 12 comprises a sleeve 61 movable in the closing direction by flyweights 71 driven at the speed NH and acting via a spring 72 and a stem 60. A throttle valve downstream of the metering valve 14 and leading via a duct 68 and a further valve 69 to the main burners comprises a sleeve 65 slidable on stem 60 and urged downwardly in the opening direction by the pressure downstream of metering valve 14 and upwardly by a spring 70 reacting against the sleeve 61. The force exerted by the flyweights 71 is opposed by the pressure drop across the metering valve 14 acting on the combination of the sleeves 61, 65 but aided by a force exerted by a spring 75 and variable via a cam 76 by the selector lever 48 so that when the latter is in position C, fuel-rich starting, the spring force is increased and the spill flow through the sleeve 61 is reduced. During starting, the spring 70 overcomes the pressure downstream of the metering valve 14 to close the throttle valve. Substantially the whole of the fuel is then delivered via a duct 73 to the pilot burners. When the engine speed has risen sufficiently to increase the pressure downstream of the metering valve 14 and acting on the sleeve 65 to the extent that the force of spring 70 is overcome and the sleeve 65 engages the sleeve 61, the sleeves 61, 65 thereafter move as a unit to maintain the pressure across the metering valve 14 constant at any given value of the speed N H . The valve 69 comprises a spool 79 controlling, in opposite senses, the flow from an outlet 78 to the main burners and from an outlet 77 to a by-pass duct 64 leading back to a point upstream of the metering valve 14. The spool 79 has lands of different diameters so as to be biased by the pressure in duct 68 in the direction to increase the spill flow through outlet 77 and decrease the flow through outlet 77. It is also biased in the same direction by a spring 80 and a high-pressure signal applied via a line 81 from the outlet of pump 12. It is biased in the opposite direction by a pressure within a chamber 82, derived from the pressures in line 81 and duct 68 by a potentiometer comprising a restrictor 83 and a restriction afforded by a valve 84 in series with a pilot valve 86, the latter being controlled via a torque motor 87 by an electronic circuit responsive to the temperature T 7  and the speeds N L , N I  of the low-speed and intermediate-speed engine shafts, whereby increase beyond predetermined levels of these variables opens the pilot valve 86 to reduce the flow to the main burners. If the lever 50 has moved more than a predetermined amount in response to changes in P 3  and P 4P , it acts via a lever 89 on the pilot valve 86 to reduce the fuel flow. When the selector lever 48 is in the shut-down position E, it opens a cock 92 to vent the chamber 82 to the duct 68 to shut outlet 78 completely and open spill outlet 77 fully. In a modification, Fig. 4 (not shown), the pivoted element (112) of the pilot valve is acted on by a spring (114) and by an opposing spring (111), the latter being interposed between the element (112) and the valve spool (101), whereby the force exerted by the spring (111) on the element (112) is dependent on the position of the spool (101).