Patent GB-1418494-A (GB)
Title: Fuel system for an engine having an air bleed valve control  arrangement
IPC: F02C3/04
Filed: 1972-07-28
Abstract: 1418494 Gas turbine engine fuel systems PLESSEY CO Ltd 10 May 1973 [28 July 1972] 35306/72 Heading F1G The degree of air bleed valve opening necessary during acceleration is determined by comparing the maximum acceleration fuel flow which could be tolerated by the engine without surging, overheating or running adversely with the or each bleed valve fully open, with the actual fuel flow. Fuel is supplied via the chamber 12 of the gas generator turbine govenor 50 to a gear-pump 20, also driven by the gas generator turbine. The govenor 50 centrifuges the fuel to boost its pressure before it enters the pump 20. Overspeeding of the gas generator turbine is prevented by spilling fuel back from the outlet of the pump 20 via valves 46 controlled by the governor weights 44. Excessive fuel pressure is prevented by a relief valve 66. Fuel delivered by the pump 20 passes via the metering ports 58 of an acceleration control valve 52 to a throttle and sequencing valve 77 controlled by a lever 80 where it passes through a profiled port 78 to a duct 82 leading to a valve 114 which is movable in a closing direction by the power turbine governor weights 112 is opposition to a spring force controlled by a control lever 132. The total pressure drop across the ports 58 and 78 in series is maintained constant by a valve 90 subjected to the pressure drop and controlling the return of fuel to the inlet of the pump 20. A similar valve 124 maintains the pressure drop across the valve 114 at a constant value. After passing through the valve 114, the fuel returns via a conduit 136 to further metering ports 214 of the acceleration control valve 52, and then returns via a conduit 216 to the throttling and sequencing valve 77, where it is directed by one or other of two ports 138 either via a conduit 140 to a torch igniter manifold or via a conduit 142 to a main fuel nozzle manifold, according to the position of the lever 80. The acceleration control valve 52 is controlled by a capsule 74 subjected to a control pressure and an evacuated capsule 72, whereby it is controlled in accordance with the absolute value of the control pressure. The control pressure may be the compressor delivery pressure or a proportion thereof which is varied in accordance with ambient temperature by means of a bi-metallic element controlling a valve which in turn controls a fluidic device which effects the variation. The acceleration control valve 52 is positioned to provide a fuel flow through the ports 58 which would provide the maximum acceleration that can be sustained without the engine surging, overheating or running adversely with the engine bleed valve fully opened. Since, for any given position of the valve 52, the restriction provided by the ports 214 is identical to that provided by the ports 58, the pressure drop across the ports 214 will be the same as that across the ports 58 if the fuel flow through the ports 214 (which is the actual fuel flow supplied to the engine fuel nozzles) is the same as the fuel flow through the ports (which is the fuel flow corresponding to the maximum permissible acceleration). The engine bleed valve will then be fully open. If, however, the fuel flow through the ports 214 is less than this value, the resultant difference between the pressure downstream of the ports 58 and the pressure downstream of the ports 214 will move a diaphragm 208 which is subjected to this pressure difference, thereby changing the position of an obturator 224 relative to a nozzle 225 to thereby vary a signal pressure in a conduit 228. This signal pressure reduces the opening of the engine bleed valve to the appropriate value.