Patent GB-825531-A (GB)
Title: Improvements in fuel supply systems for gas turbine engines
IPC: F02C9/16
Filed: 1955-09-23
Abstract: 825,531. Gas-turbine plant. HOBSON Ltd., H. M., GREENLAND, L. S., and McCOURTY, W. D. Aug. 10, 1956 [Sept. 23, 1955], No. 27197/55. Class 110 (3). [Also in Group XXIX] In a gas-turbine fuel system controlled by a pilot's speed selecting lever and by an engine-driven governor, the pilot's speed selecting lever is arranged to select, in the lower speed band, a speed higher than that which a fuel flow control unit can provide, and, in the upper speed band, a speed lower than that providable by the flow control unit. Fuel is supplied to the engine by an engine driven centrifugal pump 1 through an acceleration control unit 122, such as described in Specification 825,530, and a flow control unit 123 to an engine driven pump 15 which circulates the fuel in a closed loop to a distributer 18 such as described in Specification 825,529. A gear pump 2, driven through a centrifugally-actuated clutch 19 supplies fuel for engine starting as described in Specification 764,072. The acceleration control unit 122 comprises a needle valve 4 controlling a metering orifice 31, the position of the needle 4 being controlled by an evacuated capsule 33 subjected to the compressor delivery pressure P2. The metering pressure across the orifice 31 is controlled by a balanced pressure-regulating valve 3 which is controlled by a spring 46 in conjunction with the pressure difference across a diaphragm 32, the pressure difference being determined by a fuel potentiometer controlled by a speed compensating valve 5 across the inlet and outlet of the pump 1. Increase in compressor delivery pressure P2 results in the needle 4 moving to the right to increase the fuel flow and increase in engine speed results in the regulating valve 3 moving to increase the metering pressure across the orifice 31. The flow control unit 123 comprises a metering orifice 49 controlled by a needle 6, the metering pressure across the orifice being controlled by a balanced pressure-regulating valve 8. The needle 6 is controlled by a cam 7 rotatable by a lever 17 coupled to the pilot's speed selecting lever 14 which is movable from the STOP position, in which the coupled shutoff cocks 16 are closed, through the ground idle, GI, position to the full power, FP, position. The cam 7 is also movable axially by a servopiston 54 and spring 53 in relation to the ram pressure applied to a capsule 50. The controls being such that increase in selected speed and/or increase in ram pressure results in movement of the needle 6 to increase the area of the orifice 49. The pressure-regulating valve 8 is controlled in accordance with the pressure across a diaphragm 59 loaded by a spring 60. During steady running the valve 8 exerts a throttling action on the flow to maintain a predetermined pressure drop across the metering orifice 49. This pressure drop is regulated by the relative areas of orifices 10 and 64 in a conduit 65 connecting the upstream and downstream sides of the orifice 49. The orifice 64 is a fixed restriction, but the orifice 10 can be varied by a valve 61A actuated by a centrifugal governor 9 as described in Specification 764,071. Displacement of the speed lever 14 varies the compression of a spring 62 loading the flyweights 63 such that below 80% maximum r.p.m. of the engine the speed selected is higher than that provided by the fuel metered by the control unit 123. At speeds above 80% maximum r.p.m. the speed selected is less than that provided by the metered flow and the governor operates to close the orifice 10 and cause the regulating valve 8 to reduce the metering pressure to bring the speed down to the selected valve. The governor includes an accelerationsensitive element 11 as described in Specification 764,071 which acts on the lever 61 in combination with the force produced by the flyweights. The regulating valve 8 of the control unit 123 may also be made responsive to changes in compressor intake temperature in which case the orifice 64 is replaced by a valve actuated by a temperature-responsive element, which causes the valve 8 to tend to close on increase in air intake temperature. In a modification the system is arranged to provide an increased rate of response of the control units 122 and 123 to their controlling factors. This is obtained for the control unit 122 by replacing the fuel potentiometer by an air potentiometer modifying the ram pressure P2 to the capsule 33. In the case of the flow control unit 123 the return flow from the orifice 61A is returned through a constant flow valve to the inlet of pump 1 instead of to the fuel chamber of the valve 8. The control unit 123, in this instance, being placed upstream of the acceleration control unit 122. To prevent the governor transiently operating at idling speeds on moving the speed lever 14, a needle valve operated by a cam on the spindle of the cam 7 closes the orifice 64 to prevent the governor exerting control over the flow control unit 123.