Patent GB-1195987-A (GB)
Title: Improvements in Fuel Control System for Gas Turbine Engines.
IPC: F02C9/16
Filed: 1967-11-17
Abstract: 1,195,987. Gas turbine jet-propulsion plant. GENERAL ELECTRIC CO. Nov. 17, 1967 [Feb. 24, 1967], No.52466/67. Heading F1J. In a gas turbine engine fuel system, the pump supplying fuel to the main combustion equipment also supplies fuel to the reheat burner when the combined flow requirement of the main combustion equipment and reheat burner is low, a second pump supplying the reheat burner when the combined flow requirement is high, Fig. 1 shows a fuel system in which an engine-driven pump 22 supplies fuel to nozzles 14 of combustion equipment 12 via a main fuel control 24, a constant pressure drop across the main fuel control (or at least the metering valve thereof) being maintained by a bypass valve 26. The main control 24 is connected mechanically to the pilot's power lever 31 and the engine rotor to maintain a desired rotor speed as a function of the position of the power lever. When the power lever 31 is advanced beyond the setting for maximum engine speed and the engine rotor actually attains that speed, the main control 24 transmits a permissive fluid-pressure signal via a conduit 71 to a reheat fuel control 39 to enable fuel to be supplied to reheat burner nozzles 19, 20 under the control of a metering valve 46. A metering valve actuator 58 moves the metering valve 46 longitudinally in accordance with the setting of the power lever 31 and angularly in accordance with the discharge pressure (sensed at 29) of the engine compressor 11 whereby the reheat fuel flow raises with both of these factors. When the power lever 31 is set for reheat, it activates an initiation system 67 which then directs a constant pressure fluid, conveniently obtained from the main control 24 via a conduit 50, to a pump switching valve 72 via a conduit 73. At low rates of reheat fuel-flow, the valve 72 directs the fluid from conduit 73 to a conduit 45 to open a valve 44 whereby the main fuel pump 22 supplies fuel to the reheat metering valve 46. At rates of reheat fuel-flow to high to be handled by the main fuel pump, the valve 72 re-directs the fluid to a conduit 35 to open a valve 34 whereby the reheat fuel is supplied by a reheat fuel pump 32 instead of the main fuel pump 22. The initiation system 67 also provides fluid-pressure signals via conduits 68, 69 to open normally-closed throttle valves 61, 62. Initially, throttle valve 61 is opened to allow fuel to flow to inner nozzles 19 only. Beyond a predetermined position of the power lever 31, throttle valve 62 is opened so that fuel then also flows to outer nozzles 20. The throttle valves 61, 62 are actuated by the pressure drops across the corresponding portions of the metering valve 46, as sensed through the respective passageways 47, 65 and 48, 66, whereby the pressure drops are maintained constant. The components 46, 58, 67 and 72 of the reheat fuel control 39 are shown in more detail in Figs.3, 5, 6 and 7. At settings of the power lever 31 below the reheat range, the initiation system 67 is in the condition shown in Fig. 7. When the power lever 31 is moved into the reheat range, it rotates a valve 220 from a position in which it connects a line 234 to low pressure to a position in which it connects the line to the conduit 71, so that the permissive fluid-pressure signal indicating that the engine has attained the maximum speed is transmitted from conduit 71 via line 234 to a chamber 243 to move a valve piston 242 to the right. This allows pressure fluid to pass from conduit 50 to conduits 68 and 73, whereby the throttle valve 61 is opened to permit flow of fuel to the nozzles 19 and the pump switching valve 72 is activated to enable it to select the appropriate pump 22 or 32. When the power lever 31 is moved to the position in which a cam 224 allows a lever 282 to be pivoted by a spring into a position in which a plate 281 restricts flow of fluid from a bleed port 279, the resultant increase in pressure in a chamber 276 moves a valve piston 277 to the right to allow pressure fluid to pass from conduit 50 to conduit 69. The throttle valve 62 is thereby opened to permit flow of fuel to the nozzles 20. As the power lever 31 is advanced within the reheat range, it turns a cam 116, Fig. 3, to allow a pivoted lever 98 to be moved by a spring to cause a flapper valve 107 to restrict the flow from a bleed port 106, whereby the pressure in a chamber 96 tends to rise towards that in a chamber 95, with which it communicates via an orifice 104 in a piston 100. Consequently, the piston 100 is moved downwardly to bring the metering pistons 53, 54 of the metering valve 46 to positions of increased opening of their ports 56. 57. The downward movement of the piston 100 displaces the bleed port 106 from the flapper valve 107 to reduce the pressure in chamber 96 and consequently stabilize the piston in a new position. Increase in compressor discharge pressure, sensed at 29, Figs. 1 and 5, expands a bellows 161 to rotate a nozzle 144 whereby fluid discharged from the nozzle impinges to a greater extent on a passageway 151 so that the pressure in a chamber 135 increases relative to that in a chamber 136 communicating with a passageway 152. Consequently a piston 127 is moved to the right so that rack teeth 131, Fig.3, on the piston rotate a sector gear 108 to turn the metering pistons 53, 54 to increase the opening of their ports 56, 57. The movement of the piston 127 increases the feedback force exerted by a spring 136 on a lever 164 whereby the piston reaches a stabilized position. To prevent the fuel-flow from being reduced below a minimum level necessary to ensure proper distribution of fuel between the fuel nozzles and proper combustion, a movement of the piston 127 corresponding to a substantial reduction of the compressor discharge pressure causes a plate cam 125 carried by the piston and acting on a follower 124 to allow a pivoted arm 123 to be moved by a torsion spring 123' to displace the flapper valve 107 to restrict the flow from the bleed port 106. This causes an axial displacement of the metering pistons 53, 54 to maintain the flow at the minimum level. When the variable area jet nozzle 21, Fig. 1, of the engine is in the minimum area position, as during reheat initiation, it is necessary to limit the reheat fuel flow to a minimum level. For this purpose, a flapper valve 307, Fig.7, is displaced from a bleed port 306 when the jet nozzle area is a minimum to reduce the fluid pressure exerted in a chamber 310 upon a piston 309. The piston is then moved by a spring 311 to the position shown in which it pivots the lever 98 away from the cam 116. This causes the metering pistons 53, 54 to move axially to the minimum-flow position. Fig. 6 shows the pump switching valve 72. When the metering pistons 53, 54 have moved to increase the reheat fuel flow to the value at which the changeover from pump 22 to pump 32 is required, the resultant rotational and/or linear displacement of a cam 212 (see also Fig.3) pivots a lever 208 to the position shown, in which a plate 207 moves away from a bleed port 206 to allow pressure fluid to escape from a chamber 197 whereby a piston 193 is displaced leftward to the position shown to permit flow of pressure fluid from a bleed port 187. This causes a valve piston 170 to move rightward to the position shown, in which conduit 73 is disconnected from conduit 45 and connected instead to conduit 35 to effect the changeover from pump 22 to pump 32. The movement of piston 193 causes the lever 208 to pivot about its abutment with the cam 212 to further displace the plate 207 from the bleed port 206. Consequently, when the reheat fuel-flow decreases again, the change back to pump 22 takes place at a lower value of flow. This prevents hunting. At high rates of reheat fuel-flow, a flow of cooling air to the jet nozzle 21 is required. For this purpose, a cam 223, Fig.7, moves a plate 298 away from a bleed port 297 at high rates of reheat fuel-flow to reduce the pressure in a conduit 295 and thereby open a valve 293 (see also Fig. 1) to permit the flow of cooling air.