Patent GB-1474170-A (GB)
Title: Fuel control system for a gas rutbine engine
IPC: F02C9/26
Filed: 1974-08-02
Abstract: 1474170 Gas turbine engine fuel systems LUCAS INDUSTRIES Ltd 2 Aug 1974 [19 Sept 1973] 43879/73 Heading F1G [Also in Division G3] A gas turbine engine fuel system includes a variable metering device 52 having a member 54 controlling fuel flow through ported sleeve 53 in response to servopressure in line 55 controlled by electrically operated pilot valve 58, the latter and metering member 54 being interengageable so that, in a position of member 54 in which fuel flow to the engine is shut-off, pilot valve 58 is urged to a position in which the servopressure maintains member 54 in the shut-off position. Pilot valve 58 comprises a flapper 59 movable by torque motor 60 to simultaneously vary the flow through nozzles 61, 62 in response to an electronic control device 11 supplied with signals representing compressor output pressure, speeds of the two engine shafts, and setting of throttle lever 12. Nozzle 61 is supplied via restriction 65 with fuel from first stage pump 13 whereas nozzle 62 is supplied via restriction 66 and a shut-down valve 67, a pair of fluid potentiometers thus being defined. The higher pressure in the latter is communicated via ball type shuttle valve 68 and line 55 to act as the controlling servopressure on metering member 54. During normal running the engine speed is prevented from falling below a given value by a port 80 which at a minimum flow position of member 54 vents line 55 to low-pressure via nozzle 62 and line 64. Should the engine speed exceed a predetermined value, a device 81 operates via linkage 82 to cause shutdown valve 67 to simultaneously dump fuel downstream of member 54 and to isolate restriction 66 from nozzle 62. The servopressure in line 55 thus rises causing member 54 to move leftwards to shutoff the fuel flow and simultaneously engage a spring finger 63 on flapper 59 so as to hold the latter closed against nozzle 62. In addition, a spring-loaded closure 78 carried by member 54 shuts-off the adjacent end of a line 75 causing the servopressure below a further dump piston 73 to collapse so that fuel manifolds 69, 70 are vented to dump line 74 and igniter connection 32 is isolated from its supply line 39. Upstream of metering device 52 is a fuel pressure control valve 19 having a member 20 positioned by the servopressure in line 21 derived from a fluid potentiometer formed by restriction 22 and pilot valve 23. The latter includes a flapper 24 positioned by torque motor 25 in response to signals from controller 11, a stand by motor 26 being provided on the same shaft as motor 25. Servoline 21 includes a locking valve 40 comprising a piston 41 with a restricted through passage 42. Should the pressure in line 21 fall below a permissible value the pressure drop across passage 42 decreases and spring 43 urges piston 41 leftwards to block the line. Valvemember 20 is thus locked in the position it occupied when valve 40 closes, until the servopressure again increases. The head 45 of valvemember 20 slidably contains a piston 46 having a through passage 47 which can be shut by engagement of opposing end portions of member 20 and piston 46. A port 51 in head 45 allows fuel from the downstream side of valve 19 to flow via passages 47, 50 to line 27 so that piston 46 follows movement of valve-member 20. An extension 49 of piston 46 is linked to a vane valve 17 controlling the flow of compressor bleed air to the driving turbine 15 of a second stage fuel pump 14. The engine is started by running an electrically driven pump 30 which primes the system via non-return valve 33 and delivers fuel via restriction 31 and line 39 to igniter connection 32 when the servopressure in line 75 has increased sufficiently to move dump piston 73 to its upwards position. Fuel flow to manifolds 69, 70 is distributed by a pair of plates 71, 72 of different weights which unseat at lower and higher fuel pressures respectively.