Patent GB-1491212-A (GB)
Title: Fuel control system for gas turbine engine
IPC: F02C9/26
Filed: 1975-02-13
Abstract: 1491212 Gas turbine engine fuel systems LUCAS INDUSTRIES Ltd 13 Feb 1975 [20 March 1974] 12323/74 Heading F1G A gas turbine engine fuel system comprises a metering valve 13, Fig. 1, including a ported control member 28 movable relative to a ported sleeve 27, the control member being axially movable in response to a pressure derived from the engine compressor and rotatable in response to a difference between a desired and an actual engine speed. The axial movement of the member 28 is effected by a servo pressure intermediate the pressures upstream of the metering valve 13 and upstream of the fuel pump 12 and existing at the junction between a restrictor 29 and a leak-off valve 30, the leak-off valve being controlled by a diaphragm 32 and an evacuated bellows 33 in accordance with the absolute valve of a pressure derived by an air potentiometer from the outlet pressure P 3  and inlet pressure P o  of the engine compressor. Negative feedback is provided by a spring 34. The rotary movement of the member 28 is effected by a piston 37 subjected to a servo pressure intermediate the pressures upstream of the metering valve 13 and upstream of the fuel pump 12 and existing at the junction between a restrictor 38 and a leak-off valve 39. The leak-off valve is controlled in accordance with the desired engine speed by a manual control 42 and in accordance with the actual engine speed by a diaphragm 41 subjected to a pressure intermediate the pressures upstream of the metering valve 13 and upstream of the fuel pump 12 and existing at the junction between a restrictor 25 and a leak-off valve 26, the latter being controlled by a governor driven at the speed N H  of the engine high-speed shaft. Negative feedback is provided by a spring 44. The total pressure drop across the metering valve 13 and a shut-off valve 19 is maintained constant by a by-pass valve 15 returning fuel to the upstream side of the fuel pump 12 and controlled by a valve 18 subjected via lines 20, 21 to the total pressure. The shut-off valve 19 comprises a valve element 46, Fig. 4, subjected to a servo pressure intermediate the pressures upstream of the metering valve 13 and upstream of the fuel pump 12 and existing at the junction between a restrictor 48 and a servovalve 51. The servovalve is actuated by a torque motor 54 in accordance with the speed NL of the engine low-speed shaft and the exhaust gas temperature T G  so as to reduce the opening of the valve 46 if N L  or T G  rise above acceptable limits. A manual control lever 59 operating a valve 57 has a shut-off position C in which the valve 57 vents the space 50 and the line 21 to the upstream side of the fuel pump 12, whereby the valve 46 is closed to shut off the fuel supply to the engine and the valve 18 is opened to vent the control line 16 of the by-pass valve 15 to the upstream side of the fuel pump, whereby the by-pass valve is opened to return substantially all the fuel delivered by the fuel pump. A cold-start position B of the control lever 59 provides an additional flow via a restrictor 64 by-passing the metering and shut-off valves.