Patent GB-814078-A (GB)
Title: Improvements relating to fuel control systems for gas turbine engines
IPC: F02C9/16
Filed: 1955-11-30
Abstract: 814,078. Gas turbine plant. CURTISSWRIGHT CORPORATION. Nov. 30, 1955 [Dec. 6, 1954], No. 34275/55. Class 110 (3). [Also in Group XXIX] In a fuel system for a gas turbine engine including a turbine driving a compressor having a throttle lever connected to cam means which is in turn connected by linkage to a fuel valve, the cam means is profiled to pass fuel as a function of throttle lever position in proportion to a scheduled fuel to compressor discharge pressure ratio for the engine under standard operating conditions, the linkage is subject to forces which adjust the opening of the valve in accordance with the actual operating conditions and the pressure drop across the valve is regulated in accordance with the compressor discharge pressure. The invention is described with reference to a compound gas turbine jet propulsion engine having high and low pressure compressors driven by high and low pressure turbines respectively, a variable area propulsion nozzle and reheat fuel burnt in the jet pipe. The variable area propulsion nozzle is controlled in accordance with the speed of the low pressure rotor. The fuel supply to the reheat burners is controlled from the throttle lever so that the reheat fuel is supplied only when the throttle lever is advanced beyond the position corresponding to the maximum power setting of the engine proper. The reheat fuel is ignited by the injection of additional fuel into the main combustion chamber. The main fuel supply is controlled by a valve 56, Fig. 2, which is actuated by a throttle lever 38 operating through a cam member 72, a follower 102 and linkwork 104, 108, 112. The cam member 72 has an upper cam surface 73, shaped to conform to the ratio W#/P 4 , where Wf is the desired ratÞ of fuel flow and P 4  the absolute compressor discharge pressure under standard conditions, and a lower cam surface 74 shaped to conform to the desired rotor speed N 0 . The follower 76 is urged into engagement with the cam 74 by a spring 79 and has pivoted to it a bar 78 which is pivoted at its right-hand end 82 to a bar 80. The right-hand end of bar 80 is positioned according to the actual rotor speed N 1  by a tachometer unit 84 consisting of a governor 85 and a servomotor 86. The servomotor 86, Fig. 4, consists of a cylinder 190 having a cover 192 including a valve bore 193 and several fluid ports. The cylinder 190 contains a piston 194 having an output rod 196. The governor rod 198 has two lands 205, 206 which control the flow of fluid to and from a port 208 leading to the bottom of the cylinder 190. The resulting variations of pressure cause the piston 194 to move and vary the loading on the spring 202. This spring acting on the abutment 200 causes the flow of fluid through the port 208 to be cut off. With this arrangement, the piston 194 and rod 196 will assume an axial position in accordance with the rotational speed of the governor flyweights which will stabilize in the same position regardless of the rotational speed. A device 88 sensitive to the compressor inlet temperature T 2 energizes a servomotor 90, Fig. 3, to position an output element 91. A rod 174, connected to bellows 88 responsive to this temperature, has a valve end 178 which controls the flow of hydraulic fluid through a passage 184 and thereby controls the pressure drop across the piston 180. This pressure drop acting against the action of the spring 182 positions the piston 180 and the output rod 91 until the loads are balanced. The rod 91 is connected through a bell crank 92 and link 93 to a point 94 so that the position of point 94 may be located according to V# 2 , where # 2  = T 2 /T 0  and T 0  is the absolute compressor inlet temperature under standard conditions The point 82 is therefore located according to N/## 2 . The point 96 on the bar 78 will therefore assume a position dependent on the values of N 0  and N/## 2 . This is referred to as the " speed error." The position of this point 96 is transmitted to a bar 104 forming part of the linkwork connecting the cam follower 102 to the fuel control valve 56. The right-hand end of the bar 104 is thereby located vertically at a position corresponding to W#/P 4  modified by the existing speed errors. Bar 104 is coupled to another bar 108 which is pivoted on a link 110 positioned by the T 2  servomotor 90. The bell crank 92<SP>1</SP> and link 110 correct the position of bar 108 so that value of W#/P 4  is in accordance with the actual rather than the standard atmospheric conditions. Bar 108 is connected to the main fuel control valve 56. A by-pass regulator 64 regulates the pressure drop across the valve 56 by means of a by-pass valve 118. The valve 118 is urged to the closed position by the compressor discharge pressure acting on a diaphragm 120 which forms a wall of an evacuated cavity 122. This closing force is balanced by the actual pressure drop across the valve 56 acting on a diaphragm 123. To prevent the engine surging a cam 134 is provided which may be engaged by a follower 136 secured to one of the bars 104 or 108 at the pivot point 106. Upon drastic increase in fuel due to underspeed, the fuel increase is limited by the follower 136 engaging the cam 134. The position of the cam 134 which is pivoted at 138 is positioned according to the value of N/## 2  by the link 142 connected to the pivot point 82. In a similar manner, the maximum limit on the fuel feed during acceleration is imposed by a cam 144 which is engageable at times by a follower 148 secured to the speed error links 98, 100. The cam 144 is profiled to limit the fuel flow during acceleration. If excessive overspeed occurs this is controlled by the governor controlled propelling nozzle. A limit cam 158 is provided to limit the turbine inlet temperature. The cam 158 is shifted in position according to the temperature T 2  by a link 110 and pivot 159. In addition the cam 158 is rotated or adjusted by a connection 160 to the link 142 which moves according to N/## 2 . The cam 158 may be engaged at times by a follower 164 carried by an arm 162 secured to one of the bars 104, 108 at the pivot 106. A governor 166 responsive to the speed of the high-pressure rotor is connected through a servomotor 167, a rod 169 and levers 170, 171 to the bar 112 to limit the fuel supply if the speed is excessive. A number of modifications are described. For example, the propelling nozzle and tachometer 84 may be controlled by the speed of the high pressure rotor and the servomotor 167 by the speed of the low pressure rotor or the nozzle and servomotor 167 may be controlled by the speed of the low pressure rotor and the tachometer 84 by the speed of the high pressure rotor or the nozzle and servomotor 167 may be controlled by the speed of the high pressure rotor and the tachometer 84 by the speed of the low pressure rotor. The engine may also have only one rotor. An adjustable or fixed stop 130 is provided to limit the reduction of the fuel flow so to prevent the combustion chambers dying out. Specification 814,077 is referred to.