GB1514166A - Pressure signal generating arrangement for a gas turbine engine fuel control system - Google Patents

Abstract

1514166 Gas turbine engine fuel systems LUCAS INDUSTRIES Ltd 8 Aug 1975 [23 Oct 1974] 45919/74 Heading F1G A pressure signal generating arrangement for a gas turbine engine fuel system comprises means, responsive to the difference between a desired and an actual engine speed, for generating a first servo pressure signal to control a variable fuel metering device; means, responsive to the engine speed and to a pressure difference across the metering device for generating a second servopressure signal to control a throttle valve in series with the metering device; and pressure-responsive means for modifying the first servo pressure signal in accordance with the delivery pressure of a compressor of the engine. A positive-displacement pump 11 driven at a speed proportional to the engine high speed shaft speed NH delivers fuel to the engine 10 via a control valve 12 and a combined shut-off and flow limiting valve 83. Control valve 12 comprises concentric slidable valve elements 17, 21 which co-operate with one another to define a throttle valve 24 which is in series with a metering orifice 25 defined by the element 17 in relation to the outlet 15. The element 17 also controls the spill of fuel via a spill outlet 16 to the inlet side of the pump 11. A spool valve 65 operated by a governor 66 develops a pressure signal in a line 63 which is representative of the engine speed NH, the pressure signal being applied to a diaphragm 64 to position a leak-off valve 60, 61 which is urged in the opposite direction in accordance with desired engine speed by a speed selector lever 62. The pressure signal is also applied to a leak-off valve 68 to increase its opening as the speed increases. Another leak-off valve 47 is connected via a fixed restrictor 46 to the outlet side of the pump 11 and is acted on by the outlet pressures P 2 and P 3 of the engine L.P. and H.P. compressors via respective restrictors 48, 49, a diaphragm unit 50 and an evacuated bellows 51. A valve 53 is controlled in accordance with the inlet temperature T 1 of the L.P. compressor. The valves 60, 61 and 68 are connected in parallel with one another and in series with the valve 53 and a fixed restrictor 52, the upstream end of the fixed restrictor 52 being connected to the junction between the fixed restrictor 46 and the valve 47. The resultant variable control pressure (varied by the valves 47; 53; 60, 61 and 68) at the downstream side of the fixed restrictor 52 is applied via a line 54 to act on the control member 44 of a leak-off valve 42 which is connected to the outlet side of the pump 11 via a fixed restrictor 41, the resultant variable control pressure occurring at the junction of the restrictor 41 and the leak-off valve 42 being applied to a piston portion 30 of the valve element 17, which is resiliently coupled to the control member 44. A further piston 32 abutting the valve element 21 is acted on by a pressure controlled by a spool valve 80 operated in accordance with the engine speed NH by a governor 81 and also in accordance with the pressure drop across the metering orifice 25, whereby the throttle valve 24 controls flow through the metering orifice so that the pressure drop across the latter remains substantially constant for a given engine speed. If the control pressure in the line 54 falls below a predetermined value, a spring-loaded diaphragm 72 closes the orifice 60, while if it rises above a further predetermined value, a relief valve 74 opens. The pressure at the junction between a fixed restrictor 92 and a leak-off valve 93 which are connected in series between the outlet and inlet of the pump 11 is applied at 89 to the control element 88 of the shut-off and flow limiting valve 83 against the action of springs 90, 96, the latter also engaging the valve 93 in opposition to a spring 98. An increase in the speed NL of the engine low speed shaft or the temperature TG of the gases leaving the combustion chamber causes a torque motor 95 to open valve 93 and thereby move the control element 88 to reduce the fuel flow to the engine and to spill fuel to the inlet side of the pump 11. A manual valve 99 can vent the space 89 to shut the valve 83 fully, all the fuel then being spilled.