Patent GB-1174991-A (GB)
Title: Gas Turbine Engine Fuel Control
Inventor: GARDNER WILLIAM GEORGE BESLER
IPC: F02C9/32
Filed: 1967-02-20
Abstract: 1,174,991. Gas turbine engine fuel control systems. UNITED AIRCRAFT CORP. Feb.20, 1967 [March 2, 1966], No. 8029/67. Heading F1G. [Also in Division G3] The invention relates to a fuel control system for a gas turbine engine, the system including a metering valve for controlling the fuel flow to the engine. A speed selector and a temperature selector are connected to the metering valve, the first selector selecting a desired engine rotor speed and the second selector selecting a desired turbine inlet temperature. A first linkage responsive to an actual rotor speed sensor is connected to the metering valve for modifying fuel flow in response to deviations of engine speed from the selected speed, calculating means are connected to a plurality of engine parameter sensing means for calculating the actual temperature at the turbine inlet, and a second linkage responsive to the calculating means is connected to the metering valve for modifying fuel flow in response to deviations of turbine inlet temperature from the selected turbine inlet temperature. In the embodiment shown fuel flows through inlet 10 and filter 12 to the line 14, through the metering valve 18 to the line 14 and to the engine combustion system. The pressure drop across the metering valve is maintained constant by a by-pass valve 20. The metering valve comprises a sleeve 126 formed with metering ports 134 and with a piston 128 which is subjected to either servo-pressure PS or discharge pressure Poin the chamber 124. Pressures Psor Poare communicated to the chamber 124 through line 122 under control of spool valve 118, the spool valve being acted upon at its left-hand end by a lever 100 pivotally mounted at its mid-point 102, the lower end of the lever being connected to the metering valve by spring 130. The lever 100 is acted upon by rollers 98 which also bear against a lever 104, the lever 104 being pivotally mounted at 106 and connected at its upper end to a capsule 112 which is subject at its outside to compressor discharge pressure and at its inside to atmospheric pressure. The rollers 98 are carried by a member 96 which is connected to a bell-crank lever 90; clockwise movement of the lever about its pivotal point 92 causes downward movement of the rollers 98 and so movement to the right of spool valve 118, thus the chamber 124 is connected to low-pressure P0 and the metering valve 126 moves to the right thus increasing fuel flow to the combustion section. Anti-clockwise movement of the bell-crank lever 90 on the other hand causes reduced fuel flow. An engine speed N0 selector cam 24 and a turbine inlet temperature TT4 selector cam 26 are provided being actuated by a pilot's lever. A lever 56 pivotally mounted at 58 co-operates with the cam 24, the lever 56 being connected to a member 60 which is in turn connected to a lever 76 which bears against a face on the lever 78, the right-hand end of the lever 78 engaging against a finger 82 of a cam member 84 which in turn bears against the bellcrank lever 90, clockwise movement of the cam causing the bell-crank lever 90 to rotate anticlockwise so causing reduction in fuel flow, and vice versa. The actual engine speed is sensed by a centrifugal governor 136, an increase in speed causing upward movement of spool valve 138 sothat servofluid at pressure Ps flows through line 140 to the chamber 144 above the cam 148, the cam thus moving downwardly and carrying with it the rod 80. The downward movement of the rod 80 causes anti-clockwise movement of the lever 78 about its pivotal point 76 of lever 70, the upward movement of the righthand end of the lever 78 causing the cam 84 to rotate clockwise and thus resulting in reduction in fuel flow. Downward movement of the rod 80 causes the lever 154 to rotate clockwise above pivot 156 thus returning the spool valve 138 to the null position. Upward movement of the spool valve resulted in chamber 146 below the cam 148 being connected through line 142 to discharge pressure P0. The cam 148 which is a threedimensional cam is rotated by means of a rack 208 formed on a servopiston 42 as shown in Fig. 2. A signal responsive to engine inlet temperature TT2 is communicated to capsule 32, Fig.3, an increase in temperature causing the rod 36 and so the lever 38 which is pivotally mounted in the servopiston 42 to move upwardly thus causing the spool valve 40 to move upwardly whereby servofluid at pressure Ps passes to chamber 44 at the right-hand side of the servopiston, the piston thus moving to the left thus causing the lever 52 which is connected thereto to move anti-clockwise about its pivotal point 54 thus moving the threedimensional cams, the turbine inlet temperature TT4 selector cam 26 and the engine speed Nc selector cam 24 which are engaged by lower end portions 52a, 52b of the lever 52 to move to the right. The lever 38 connected to the servopiston 42 is of cam formation so that as the piston moves to the left the spool valve is caused to return to its null position. The turbine inlet temperature TT4 selector cam is engaged by a lever 182 pivotally mounted at 184, descreased temperature setting causing the lever to pivot anti-clockwise thus causing floating lever 188 to move clockwise about the point X shown on Fig. 2, so causing the servopiston 190 to move downwardly whereupon servofluid at pressure Ps passes through line 192 to the chamber 196 at the left-hand end of integrating servopiston 198 which thus moves to the right causing lever 200 which is pivotally mounted at 202 to move anti-clockwise so causing rod 204 to move to the left. The rod 204 is connected to the lower end of the bell-crank lever 72, Fig.1, the connection not being shown, so that the leftward movement of the rod 204 causes the bell-crank lever 72 to move clockwise about its pivot point 74 so causing the lever 70 to move anticlockwise about pivotal point 68. The end 76 of the lever 70 thus moves upwardly moving the lever 78 anti-clockwise about its pivotal point on rod 80, the movement of the lever 78 causing the cam 84 to rotate clockwise which movement as previously described results in movement to the left of the fuel metering valve and thus reduction in fuel flow. The left-hand end of the floating lever 188 - point X on Figs. 2 and 1 is connected to a rod 180 which bears against a turbine inlet temperature TT4 calculation cam 160. The cam is mounted on rod 96 which carries at its upper end the rollers 98 and is connected to the bellcrank lever 90 so that rotational movement of the lever 90, which causes movement of the fuel metering valve 126 and so increase or reduction in fuel flow, simultaneously causes axial movement of the TT4 calculation cam 160. The cam 160 is caused to move angularly in dependence upon combustion chamber inlet temperature TT3, a signal responsive to this temperature being communicated to amplifier 162. Signals from the amplifier 162 are transmitted to a motor device 164 which causes movement of flapper valve 166 which controls escape of servofluid at pressure PM in chamber 170 at the left-hand side of servopiston 172, axial movement of the piston causing angular movement of the TT4 calculating cam 160 through a rack and pinion device. A feedback coil 178 is provided between the servopiston 172 and the amplifier 162. The arm 90a of the bellcrank lever 90 is connected by linkage 212, 214 to the cam follower rod 180. Thus downward movement of the cam 160 as a result of increased fuel flow causes the cam follower 180 to move to the right thus causing the link 214 to move anticlockwise about its pivotal point 210 on the arm 90a so that the follower 216 will be brought into contact with the cam 148. Thus regardless of the rotation of the cam 84 in a direction calling for increased fuel flow, movement of the bell-crank lever 90 will be regulated by the cam 148, the cam 84 rotating anti-clockwise and out of contact with the follower 94 on the bell-crank lever 90. Movement of the rod 96 to increase fuel flow is thus restricted in accordance with maximum turbine inlet temperature TT4 as determined by engine rotor speed Nc and engine inlet temperature TT2. Actual turbine inlet temperature TT4 is compared with the permissible maximum to determine when acceleration limiting must occur. The link 78 receives inputs of selected speed and actual speed so that the position of the bearing surface at the right hand end of the link 78 is a function of speed error. Similarly the output from the servo 194 is a function of turbine inlet temperature TT4 error so that the input to link 78 through bell-crank lever 72 from servo 194 may be regarded as trimming the speed or speed error setting as a function of turbine inlet temperature error.