Patent GB-849515-A (GB)
Title: Improvements relating to the control of gas turbine engines
Inventor: HAHN KENT LEMOYNE
IPC: F02C9/00
Filed: 1958-12-03
Abstract: 849,515. Gas turbine plant. GENERAL MOTORS CORPORATION. Dec. 3, 1958, No. 39069/58. Class 110 (3). [Also in Group XXIX] In a gas turbine engine comprising low and high pressure compressors and separate low and high pressure turbines coupled thereto, an air bleed valve fitted between the compressors to prevent surge is controlled by an actuator moved in valve opening direction by low pressure compressor outlet pressure and in valve closing direction by a pressure which is a function of high pressure compressor outlet pressure. As shown, the gas turbine engine comprises low and high pressure compressors C 1 , C 2  coupled respectively by concentric shafts 17, 18 to low and high pressure turbines T 1 , T 2  and includes a bleed valve 26 in a duct 24 communicating with a space 11 intermediate the compressors. The valve 26 is connected to a double-acting piston 28 under the control of an actuator 40 comprising bias and actuating pistons 53, 54 and an integral spool valve 51 having connections 58, 59 to opposite sides of the piston 28, exhaust connections 61 and a pressure fluid supply connection 57 which, e.g. may be connected to the outlet of the high pressure compressor. The actuator is under the control of air inlet pressure (P1), low pressure compressor outlet pressure (P2) and high pressure compressor outlet pressure (P3) sensed by probes 32, 33, 34 respectively, air inlet temperature (T1) sensed by a bulb 36 and low pressure compressor speed (N1). The speed and temperature signals are fed respectively to a speed governor 86 and a bellows unit 78, 81 which jointly through a rack and pinion 89, 91 and a three-dimensional cam 93 position a spring-loaded needle valve 101 to control a restriction 102 bleeding compressed air from lines 69, 67 connecting the upper face of the piston 54 with the P3 probe, the bled air passing through a line 71 to act on the upper face of the piston 53. The upper face of the piston 53 is also connected by a line 63 with the P1 probe and by a line 107 and a fixed but adjustable restriction 104 with the upper face of the piston 54 and the line 67. The lower faces of the pistons 53, 54 are connected respectively to the P3 and P2 probes. A fixed restriction 103, the flow through which equals the flow through the restrictions 102, 104 is fitted in the connection between the lines 69, 67. The P2 pressure acting on the lower face of the piston 54 tends to open the bleed valve whilst the pressure in the line 67 designated control pressure (PC) which is a function of the P3 pressure tends to close it. If ram pressure ratio in the engine is assumed constant, measurement of speed N1 and temperature T1 with the consequent adjustment of the needle valve 101 makes it possible to compute a minimum ratio of P3/P2 which must be maintained to avoid surge when the bleed valve 26 is closed. The purpose of the piston 53 is to cause the spool valve 51 to follow more closely the surge characteristic of the low pressure compressor and to lessen the variation required in the restriction 102. The P3 pressure acting on the lower face of this piston tends to open the bleed valve whilst the P1 pressure acting on its upper face tends to close it, so that the greater the pressure rise through the compressors the greater will be the tendency for the bleed valve to open. The result of this is that the PC pressure can be higher as P3 increases and lower as P3 decreases and provides compensation in general parallel to the N1 and T1 compensation effected by the cam 93. It may, in consequence (depending upon engine characteristic) be possible to dispense with the N1 and T1 compensation and fix the size of the restriction 102, when PC pressure is higher than P2 and high enough to maintain critical flow through the restrictions 102 and 104. In the line 69 between the probe 34 and the restriction 103 is fitted an acceleration control valve 44 comprising a plunger 126 actuated by a bellows 123 and a valve 46 under the control of a pilot's lever 21 which through a link 23 is coupled to fuel con-. trol mechanism 20. The valve 46 is provided to open the bleed valve at low speed and power, the lever 21 being moved so that a cam 116 depresses the valve plunger 111 to cut communication between the probe 34 and the upper face of the piston 54 to move the speed valve 51 upwardly. The acceleration valve 44 is provided to cause the bleed valve 26 to respond in a desired manner to engine acceleration. Assuming with the engine running steady the fuel supply is increased to increse power output. This will cause the high pressure compressor and turbine to accelerate, the speed of the low pressure compressor and turbine being held steady by a propeller governor, resulting in a sudden increase in turbine inlet temperature and small increases in the pressure ratio of each P2 of the compressors. The increase in the -- ratio P1 could be sufficient to cause surge, but since an P3 increase in the ratio -- causes movement of the P2  bleed valve towards closure it is possible for the valve 26 to remain closed. To overcome this if the P3 pressure is increasing faster than a predetermined rate, air at this pressure enters the bellows 123 to move the valve plunger 126 towards closure and again cut communication between the probe 34 and the upper face of the piston 54 to allow the bleed valve to open. A small orifice 127 in the bellows 123 allows leakage to the inside of the casing 121 to re-open the valve 44 when acceleration ceases. A further fixed but adjustable restriction 133 is fitted so that when the valve spool 51 rises to open the bleed valve 26 the PC pressure is further reduced and allows the bleed valve to remain open until the ratio P3/P2 has become slightly greater than that necessary to open the bleed valve and prevents hunting of the bleed valve. Adjustment of the needle 106 associated with the fixed but adjustable restriction 104 adjusts the actuating point of the bleed valve 26.