Patent GB-1513738-A (GB)
Title: Fuel control system for a gas turbine engine
IPC: F02C9/32
Filed: 1974-09-26
Abstract: 1513738 Gas turbine engine fuel systems LUCAS INDUSTRIES Ltd 11 Aug 1975 [26 Sept 1974] 41823/74 Heading F1G The invention relates to a fuel control system for a gas turbine engine, the system comprising a first stage centrifugal pump 11 and a second stage pump 12 which is in the form of the gear-pump, the pump 12 discharging through line 13 to a control valve 14 which has a first outlet 15 which communicates with the inlet of the second stage pump, and a second outlet 16 through which fuel may flow to a variable metering device 18 and thence to the engine 10. The control valve 14 comprises a control member 19 which moves upwardly to increase fuel flow to the outlet 16 and downwardly to increase fuel flow to the outlet 15, the control member being acted on at its upper surface by fuel pressure between two restrictors 20, 21 and at its lower surface by a fuel pressure which is dependent on the pressure drop across the fuel metering device 18. The fuel metering device 18 comprises a sleeve 23 axially movable within an outer casing to control fuel flow from inlet 17 to outlet 41, the sleeve being movable by an actuator 25 which receives signals from unit 28 responsive to engine parameters O, the position of control lever 29, P 1  and T 1  compressor inlet pressure and temperature, P 2  compressor outlet pressure, the ratio T 3 /T 4  of turbine inlet and outlet temperatures, and N, engine speed. The system comprises also a combined throttle valve and shut-off cock 40 having an inlet 41 connected to the outlet of the metering device 18 and an outlet 42 connected to the engine 10, flow from inlet to outlet being controlled by spool 46. The spool 46 is acted on at its lower face by servo pressure in chamber 50 derived from outlet of pump 12 through line 47 and resistor 48; the spool is acted on in the opposite direction by spring 49. The servo pressure in chamber 50 is controlled by lever 52 associated with nozzle 51, the lever being controlled by a torque motor 54 in response to a signal from the unit 28 in dependence on engine speed N and temperature T 4  of the turbine exhaust gases such that should these parameters exceed a predetermined value, the lever 52 opens the nozzle 51, the resulting decrease in servo pressure in chamber 50 causing the spool 46 to move downwardly so reducing fuel flow through outlet 42. Fuel discharging from the nozzle 51 passes through non- return valve 56 back to the pump inlet. The assembly 40 comprises also a spool element 57 which is movable manually by means of lever 59 between an open position A as shown, a coldstart position B, and a shut position C. The pilot valve 30 is responsive to the total pressure drop across the metering device 18 and the valve assembly 40 so as to vary the servo pressure in line 22 applied to control valve 14; the pilot valve comprises a spool 31 which controls the interconnection between grooves 33, 34, the spool being acted on at its left-hand face by servo pressure derived between fixed orifice 36 and variable orifice 26, and at its right-hand face by the pressure at outlet side 42 of the valve assembly 40, this pressure being communicated through line 66. With lever 59 in position A, the duct 60 from chamber 50 is closed off so that the control member 19 of valve 14 is positioned in dependence on the total pressure drop across the metering device 18 and valve arrangement 40. In the position B of lever 59, the outlet of pump 12 is connected through line 47, restrictor 64 and passage 62 to the outlet 42 so as to provide additional fuel flow at starting. In the position C of the lever 59, the passage 60 from the chamber 50 is connected through axial bore 58 of valve 57  and non-return valve 56 to the suction side of pump 11; thus valve 40 shuts off fuel flow to the engine.