Patent GB-1459416-A (GB)
Title: Fuel system for a gas turbine engine
IPC: F02C9/26
Filed: 1973-05-16
Abstract: 1459416 Gas Turbine engine fuel systems LUCAS INDUSTRIES Ltd 25 April 1974 [16 May 1973] 23265/73 Heading F1G A gas turbine engine fuel system including, in series, a fuel pump 10, main throttle valve 17 and servo throttle valve 23, has a sensing valve 28 which derives a pressure signal dependent upon the pressure drop across main valve 17, the signal acting in opposition to a further pressure signal to position the spool 24 of servovalve 23 to thus maintain the pressure drop constant. As shown pump 10 is a first stage pump driven by the highpressure spool of a twin spool engine so that its output pressure is related to the HP spool speed NH. A second stage pump 13 is driven by an air turbine 14 supplied with air from the engine compressor under the control of valve 15. The latter is positioned by piston 47 operated by a servopressure derived from control valve 50 responsive to the total pressure drop through main valve 17, servovalve 23 and a further valve 48. Sensing valve 28 includes a spool 34 urged leftwards by spring 38 to tend to cut-off communication between ports 35, 36. Spring 38 is assisted by the pressure downstream of main valve 17. The opposite end of spool 34 is acted upon by a pressure derived from a point upstream of main valve 17. To accomplish this a non-return valve 39 connects the outlet of pump 13 via restrictor 40 to auxiliary port 22 of main valve, the pressure between restrictor 40 and port 22 being communicated to the left end of spool 34. Thus communication between ports 35, 36 becomes progressively less restricted as the pressure drop across main valve 17 increases. The pressure between restrictor 29 and port 35 is communicated to the lower end of servovalve spool 24, restrictor 29 being connected at its upstream side to a servo supply line 32 branched-off via an electrically driven starting pump 43 and tapping 30 from a point between pumps 10, 13. Line 32 is also connected via restrictors 27, 26, return line 51 and non-return valve 33 to the inlet of pump 10, the pressure between restrictors 26, 27 being applied to the upper end of servo-spool 24. The latter thus acts to maintain the pressure drop across main valve 17 constant irrespective of variations in the setting of the main valve or of the pump output pressure. The outlet of servovalve 23 is connected to the engine burners via a further valve 48 controlled in response to pressure in a line 52. This pressure is varied by speed governors 45, 46 respectively controlled by electrical and mechanical inputs from the L.P. and H.P. turbines. Servosupply line 32 is pressurized at starting by pump 43, the latter also supplying fuel via non-return valve 44 to restrictors 40, 42, 49. Restrictor 42 is variable and is disposed in a by-pass line 41 around main valve 17 to permit control during engine starting, whereas variable restrictor 49 is in series with port 21 of main valve 17 to permit idling control when the main ports 20 of the latter are closed. Line 32 is also connected via a restrictor 53 and line 54 to the engine igniters. Restrictor 53 can be by-passed by a relief valve 31.