Patent GB-833493-A (GB)
Title: Improvements in reheat control systems for gas turbine engines
Inventor: GREENLAND LEONARD SIDNEY
IPC: F02K1/15
Filed: 1956-05-23
Abstract: 833,493. Gas-turbine jet-propulsion plant. HOBSON Ltd., H. M. Aug. 22, 1957 [May 23, 1956], No. 15936/56. Class 110 (3). [Also in Group XXIX] An aircraft jet-propulsion gas-turbine engine with reheat in the exhaust pipe and a variable area propelling nozzle has the area of the nozzle controlled by means responsive to the turbine pressure ratio so as to maintain this ratio substantially constant and means responsive to turbine inlet pressure to control the fuel flow to the reheat burners so that the fuel flow increases progressively with increase in turbine inlet pressure. When the pilot's lever 2 is moved into the reheat zone, a link 100 rotates an initially closed valve 102 so that air can be admitted to the turbine 7 when the valve 6 is opened. The link 100 actuates a cam 101 which allows the valve 32 to close and cause piston 160 to rise and open the valve 6. The turbine 7 drives the reheat fuel pump 8. If when the pilot's lever 2 is moved out of the reheat zone, the valve 6 fails to close, a solenoid 5 may be energized by a switch 103 to open the valve 32 and the piston 160 to close the valve 6. The valve 6 may be closed by the governor 44 acting on the bleed valve 45. As soon as the pump 8 operates the valve 105 opens and allows fuel to be supplied to the primary reheat burners 10 through a control unit 9A and to the secondary reheat burners 10A, 10B through a control unit 9B. The control unit 9A comprises a capsule 172 responsive to the turbine inlet pressure (P2) which controls the position of servo-piston 76A through a lever 74A varying the effective area of an orifice 75A. The piston 76A controls the area of a metering orifice 77A by a needle 34A. A valve 78A maintains a constant pressure difference across the metering orifice 77A. The control unit 9B comprises a metering orifice 77B the effective area of which is controlled by a needle 34B positioned by a servo-piston 76B in accordance with variations in effective area of an orifice 75B controlled by a beam 74B actuated by a capsule 272. The pressure drop across the metering orifice 77B is controlled by a pressure regulating valve 78B. The capsule 272 is initially exposed internally to the turbine inlet pressure and externally to a fraction of this pressure, the fraction being determined by the position of a valve 170. When reheat is switched on the micro-jet control 13 commences to move the nozzle flap 20 from the position A to a more open position due to an increase in the turbine exhaust pressure (PJ). This causes cam 52A to open up the valve 159 to permit a flow of air from the interior of the capsule 272 through a restrictor 158 and conduit 157. A link 106 connected to the lever 2 serves, as soon as the lever 2 is moved into the reheat zone to open a valve 107 to admit fuel to the burners 10B. As the lever 2 moves into the reheat zone, the link 106 progressively closes a valve 170 and progressively opens a valve 270. Closing of the valve 170 increases the external pressure on the capsule 272. This effect in conjunction with the reduction of internal pressure in the capsule, causes the needle 34B to increase the area of the metering orifice 77B. Opening of the valve 270 results in progressive movement of the diaphragm 83B to vary the pressure drop across the metering orifice 77B. By suitable profiling of the needles 170, 270 it is possible to cater for changes in combustion efficiency with change in altitude. An altitude control unit 53 permits the supply of fuel to additional reheat burners 10C at low altitude where the pump discharge pressure is sufficient to move piston 58 against the action of its spring 58A. If the turbine pressure ratio drops more than 5% below the setting of the control 13, a fuel override 27 reduces the fuel flow through the control unit 9B by closing the normally open orifice 82B. The reheat fuel is ignited by injecting fuel into the gas stream before the turbine through nozzles 41. When the booster pump is switched on, fuel is supplied to fill the cylinder 22. When the pilot selects reheat, the fuel pressure in the supply line to the burners 10 increases and causes the diaphragm 108 to move to the left and open a valve 110. This causes piston 111 to move to the left and open valve 112 and simultaneously close valve 112A so that the delivery pressure of the main fuel pump is applied to the underside of the piston 23 which then forces the fuel in the cylinder 22 through the jet 41. A turbine pressure ratio control 13 is constituted by a micro-jet comprising a diaphragm 14 mounted in a chamber 15 and subject at one side to the turbine exhaust pressure (PJ) and on the other to a pressure which is proportional to the turbine inlet pres sure (P2). When reheat is selected a cam 113 opens a valve 114 so that high pressure oil is supplied to the left-hand side of piston 115. This oil escapes to exhaust through a small gap between a port 116 in the piston 115 and the head 118 of a valve 119 coupled by a link 120 to the diaphragm 14. Movement of the diaphragm actuates the valve 119 to vary the flow through the restrictor 122 and thus vary the pressure on the left-hand side of the piston 115 which moves to cause the actuator 64 to open or close the nozzle 20. When the nozzle reaches the position B of maximum area, the cam 52A opens a valve 165 which reduces the pressure on the right-hand side of the diaphragm 14 and so prevents the nozzle opening further. The turbine pressure ratio can be modified as a function of compressor pressure ratio by means of a device 283 which comprises a pair of diaphragms 84 subject to the pressure difference across the compressor which control a needle 86 which varies the proportion of the compressor delivery pressure applied to the lefthand side of the diaphragm 14. The nozzle 20 is moved to the divergent position C by a device 163 responsive to Mach number but inoperative until the pilot has moved his lever into a position selecting about 80% of maximum reheat fuel. Before the lever 2 reaches this position, the valve 123, Fig. 3b occupies the position shown in which ram pressure (P1) alone is applied to the device 163. When the lever 2 reaches the 80% reheat position the cam 124 allows the valve 123 to be raised so that ambient pressure (PO) is applied to the space above the uppermost of the two diaphragms 130. Thereafter, when a predetermined Mach number is obtained, the diaphragms 130 move suddenly upwards and close valve 126 thereby removing the leak from the side of the diaphragm 14 through the valve 165. The resulting movement of the valve 117 moves the nozzle 20 to the divergent position C. In a modification, the valve 6 is controlled by a diaphragm the air pressure on one side of which is controlled by a solenoid operated valve and the overspeed control valve 45. The fuel supply to all of the reheat burners is controlled by a single control valve regulated by a capsule responsive to a pressure proportional to the turbine inlet pressure (P2), the proportion being determined by the position of the pilot's lever 2 controlling a leak-off valve from the capsule chamber. The supply to the secondary reheat burners is controlled by a valve also operated by the pilot's lever. The supply to these secondary reheat burners is under the control of a shut-off valve operated by the opening of the nozzle 20. In this case, the fuel override 27 actuates the valve controlling the pressure drop across the metering orifice. The arrangement and control of the ignition accumulator 21 also differs in detail. In this arrangement, the nozzle 20 is moved to the position C by a control responsive to the pressure ratio between the turbine inlet pressure (P2) and the ambient pressure (PO). In this arrangement the nozzle flaps are operated by a screw jack operated by an air motor under the control of the turbine pressure ratio control 13. A member connected to the moving member of the jack shuts off the air supply to the motor at the limits of travel. Specifications 757,004, 833,496, 833,497 and 833,498 are referred to.