Patent GB-1269211-A (GB)
Title: Gas turbine engine control system
Inventor: LEWIS GEOFFREY ARTHUR
Filed: 1968-04-01
Abstract: 1,269,211. Gas turbine engine fuel systems; turbines. JOSEPH LUCAS (INDUSTRIES) Ltd. June 30, 1969 [April 1, 1968; May 27, 1968], Nos.15505/68 and 25190/68. Headings F1G and F1T. A gas turbine engine control system comprises an input servo-device producing first and second output signals in response to an input signal, the first output signal lagging behind the input signal but having a predetermined value for each steady state value of the input signal and the second output signal being changed substantially immediately when the input signal is changed to demand an increased engine speed, the change in the second output signal being cancelled when the first output signal attains its predetermined value corresponding to the changed value of the input signal; a fuel control servo-device for controlling the fuel-flow in response to the first output signal; and a vane adjusting servo-device for adjusting stator vanes of the work turbine and arranged so that the vanes can be situated in two alternative positions for each value of the first output signal and occupy the one of these positions in which an increased flow to the work turbine is permitted, whereby the combustion chamber is lowered to allow increased fuel-flow thereby creating an acceleration margin for the duration of the charge in the second output signal. The input and output signals may be mechanical, electrical, hydraulic or pneumatic. Fig. 1 shows a control system for a road vehicle gas turbine engine in which the accelerator pedal 10 operates a member 12 which coacts with the hollow piston rod of a servo-device 11 to form a valve controlling the flow of liquid from the left-hand side of the piston through passages 18, 19 to a drain line 20. Pressure liquid is supplied from a line 14 to the opposite sides of the piston via restrictors 15, 16. The effect of this arrangement is that the piston follows the movement of the pedal 10 but with a time lag imposed by the restrictor 15. The movement of the piston is transmitted to a lever 54 to vary the loading of a spring 53 tending to close a servovalve 49, 51 in opposition to governor 52 driven by the compressor turbine which tends to open it. The servovalve 49, 51 controls the by-pass valve 40 of a fuel pump 41 to determine the fuel-flow to the engine burners 39. The valve 49, 51 acts in conjunction with a restrictor 48 to determine the pressure applied from the delivery of the engine compressor to a chamber 47 on one side of a diaphragm 45. The chamber 46 on the other side is vented to the compressor intake pressure. Overspeeding of the work turbine causes a governor valve 55 to spill air downstream of the restrictor 48 to reduce the pressure in the chamber 47. If the fuel pump is of centrifugal type, a throttle valve may be used. The movement of the piston of the servo-device 11 is also transmitted to a cam 25 which acts through a follower 27 and a lever 24 to provide a corresponding adjustment of the stator vanes of the work turbine. During steady state condition, the follower 27 bears against a profile 25a of the cam 25. When the pedal 10 is depressed to accelerate the engine to a higher speed, the pressure at outlet 17 of the servo-device 11 immediately drops, allowing a spool 29 to be moved leftward by a spring 30 to the position shown, whereby the space above the piston of a servo-device 22 is disconnected from the pressure line 14 and vented to the drain line 20. The piston is then moved upwardly by a spring 26 into the position shown, in which the follower 27 bears against a second cam profile 25b and the opening of the vanes is thereby increased. Meanwhile, the piston of the servo-device 11 moves leftward at the speed allowed by the restrictor 15 to increase the fuel supply. When the piston has reached its new equilibrium position, the pressure at the outlet 17 rises again and the spool 29 resumes its normal position, causing the follower 27 to return to the cam profile 25a and to reduce the opening of the vanes correspondingly. The space above the piston of a servo-device 31 is connected to the pressure line 14 by a valve 32 which is partially closed during normal running. The space below the piston is connected to the space above the piston by a restrictor 37 and to the drain line 20 by a valve 38 controlled by the speed of the work turbine. Above a predetermined speed, the valve 38 begins to open to provide a flow through the valve 32 and the restrictor 37. When the resultant pressure drop across the restrictor 37 has risen sufficiently to overcome the loading of a spring 36, it moves the piston downwardly from the position shown, so that the pivotal axis of the lever 24 is moved to a position in which the vanes are turned to their fully open positions or beyond to reversed positions in which they effect engine braking. When the pedal 10 is released, the cam 25 engages a stem 35 to open the valve 32. The resultant increase of flow through the restrictor 37 lowers the speed at which the piston is moved downwardly to afford engine braking. The position of the pivot point of the lever 54 may be adjusted by a device sensitive to the compressor intake temperature or the combustion temperature. Alternatively, the cam profiles 25a, 25b may be external three-dimensional profiles on a cylindrical body movable axially like the cam 25 but also rotatable in accordance with the compressor intake temperature. In the embodiment shown in Fig.3, a piston 113 follows the movement of the accelerator pedal 110 as in the case of the corresponding piston and pedal of Fig. 1, and similarly controls the fuel supply, in this case via a cam acting on a lever 119. A three-dimensional cam 137 rotatable by the piston 113 and movable axially by a device 138 sensing the compressor intake temperature coacts with a servovalve 136 at the end of a piston rod 135 to control the escape of pressure fluid from a chamber 127 downstream of a restrictor 131, whereby a servopiston 126 adjusts the work turbine stator vanes through the linkage 123. During acceleration, the drop in pressure above a valve 134 corresponding to the spool 29 of Fig. 1 causes the valve to move to the position shown in which it vents the underside of the piston 126 to drain via a restrictor 133, whereby the piston moves to a mid-position determined by springs 128, 129 and opens the vanes. When the piston 113 has reached its new equilibrium position, the pressure above the valve 134 rises again, and the valve resumes its normal position. The piston 126 assumes the position dictated by the new position of the cam 137 and the opening of the vanes is reduced correspondingly. The vanes are turned to a reversed position for engine braking by means of a solenoid valve 139 which connects the upper end of the cylinder 127 directly to the fluid supply and connects the lower end to drain. The solenoid is energized for this purpose by a switch 140 when the accelerator pedal 110 is released, by a switch 143 on the clutch pedal during gear changing, by a switch 141 on the work turbine governor 142 when the work turbine is at rest or by a switch 144 on the governor when the work turbine overspeeds. In a modification, Fig.4 (not shown), a variable length link (149) sensitive to the compressor intake temperature may be provided in the coupling to the cam operating the lever (119), and the cam (155) controlling the piston (126) has its rotational axis displaceable by a actuator sensitive to the combustion chamber temperature. The pedal (110) does not coact directly with the servopiston operating the cams but instead controls the forces of a spring (143) opposing the tendency of the governor (142) to open a servovalve (145) controlling the servopiston. In both Figs.3 and 4, the valve 120 jointly controlled by the lever 119 and a speed governor 121 driven by the compressor turbine may control a fuel pump by-pass valve similar to the by-pass valve 40 of Fig. 1. Alternatively, it may control the spill from a spill burner or the secondary flow of a duplex burner, or it may control a throttle valve between the fuel pump and the burners if the fuel pump is of centrifugal type.