Patent GB-885486-A (GB)
Title: Improvements in or relating to control systems for gas turbine engines
Inventor: SLATTER BRIAN HUDSON
IPC: F02C9/28
Filed: 1958-12-20
Abstract: 885,486. Gas turbine plants. BRISTOL SIDDELEY ENGINES Ltd. March 11, 1960 [Dec. 20, 1958], No. 41207/58. Class 110 (3). [Also in Group XXIX] A control system, for a gas turbine engine, comprises an air/fuel ratio control unit, calibrated to provide a fuel delivery according to a predetermined relation between fuel delivery and compressor delivery pressure, and a trimming device operable on the air/fuel ratio control unit at a predetermined compressor outlet temperature to reduce the calibration of the air/fuel ratio control unit to provide a lower relation between fuel delivery and compressor delivery pressure, the device including means providing a time lag, such that the air/fuel ratio control unit will retain the reduced calibration during rough handling of the throttle control, i.e. during deceleration by the throttle control after the predetermined outlet temperature has been reached followed immediately by a sudden opening of the throttle control. The trimming device comprises a housing 1, mounted on the engine in the vicinity of the delivery duct 2 of a compressor and a perforated steel tube 4, shrouded by a tube 7, extends into the duct and contains a rod 5 of nickel-iron alloy having a low coefficient of expansion, the rod 5 being secured to the tube 4 at the end 6. The free end of the rod is arranged to engage a rocker arm 8 of a valve member 11 which is spring loaded towards its seat 12 and controls flow from the housing to a pipe 16, a by-pass orifice 23 being provided leading to the pipe 16. The air/fuel ratio control unit 3 comprises a pressure-sensitive capsule 59 controlling an arm 42 of a fuel control valve of an air/fuel ratio control unit as described in Specification 703,700. Pressure is transmitted to a chamber 58 under the control of a switch 140 responsive to compressor pressure ratio. The switch 140 comprises an evacuated bellows 122 secured to a bellows 120, the dividing wall 121 carrying a valve member 123 which co-operates with the end of a tube 139 connected to the pipe 16. The tube 139 is provided with orifices 133, 134 between which communication is provided between the tube and chamber 58 by a passageway 128. The interior of bellows 120 is connected to the compressor inlet or atmosphere by an outlet 130. A second pipe 26 leads from the housing 1 to an orifice 131 and then by passages 137, 135 to the exterior of the bellows, the orifice also leading to the outlet 130 via a passage-way 136. The valve member 123 closes when a predetermined compressor pressure ratio is reached. In operation with the valve 123 open and at engine speeds below a predetermined value, the air flowing through duct 2 passes through the open end of the tube 7, through the perforations in tube 4 to the housing 1 and the tube 4 is contracted, whereby the rod 5 holds the valve 11 open. The air passes through both the valve 11 and orifice 23 to the pipe 16 and hence to the tube 139, where part of the air flows to the outlet 130 and the remainder is fed to the chamber 58 by passageway 128. This results in the air/fuel ratio controller operating at its normal calibration setting. If the engine is then accelerated to above a predetermined speed, the rod 4 expands and allows the valve 11 to close. Air then flows to the bellows 120 through the by-pass orifice 23 and results in the controller running at a reduced calibration. If the engine is then decelerated to a speed at which the temperature would open the valve 11, there is a time lag on the opening due to the shrouding of the tube 4 and if the throttle is rough handled, i.e. opened immediately, the time lag prevents opening of the valve 11 and the controller still runs at the reduced calibration, preventing stalling of the engine. In a modification, the orifices 23, 133 may be omitted and a stop provided to prevent the valve 11 closing completely. If the engine is not to operate at widely varying altitudes, the pressure ratio switch 140 may be omitted and a restricted bleed provided in the chamber 58. When the valve 123 is closed, the trimming device has no effect until the valve opens again.