Patent GB-789999-A (GB)
Title: Improvements in and relating to engine acceleration control systems
Inventor: LAWRY CLINTON CHAMBERLAIN
IPC: F02K1/16
Filed: 1955-05-25
Abstract: 789,999. Gas turbine jet-propulsion plant. GENERAL ELECTRIC CO. May 25, 1955, No. 15126/55. Class 110 (3). [Also in Group XXXV] In an automatic control system for a gasturbine engine, optimum values of engine speed, reheat fuel flow and exhaust nozzle area are set in accordance with a desired engine thrust. A rapid acceleration to the optimum speed is effected by disconnecting the reheat fuel valve and exhaust nozzle regulators from their respective elements and by setting a high exhaust temperature limit on the main fuel demand, the possibility of compressor " surge " being prevented by setting a comparatively low maximum limit to the main fuel flow. When the optimum speed is attained the regulators are reconnected to impose the preset optimum values of reheat fuel flow and exhaust area corresponding to the desired thrust, the exhaust temperature limit is transferred to these regulators at a reduced level and the maximum main fuel limit is increased. A maximum speed limit is imposed. Acceleration to the optimum speed. Movement of a manual control 22 to select a desired thrust results in the setting of predetermined optimum values of speed, reheat fuel flow and exhaust area by the positioning of the appropriate regulators (as shown) and a speed reference potentiometer 25. The " underspeed " error signal resulting from the comparison of the D.C. reference set on potentiometer 25 and a rectified speed signal from an engine-driven tachometer 32 is applied over resistors 36, 38, 40 to an amplifier 42 whose output controls a motor 44 positioning a main fuel valve 20 whereby the increased fuel demand results in the engine accelerating to the optimum speed. The error signal is also applied over a resistor 60 to an amplifier 61, so that, if sufficiently positive, the amplifier output overcomes a biasing source 67 to render a valve 64 conductive to energize a relay 65, whereby contacts 99, 108 are opened to disconnect the reheat fuel and nozzle area regulators from their respective elements while the engine is accelerating. Contacts 90, 93 are closed by operation of relay 65 to allow an error signal, resulting from a comparison of the exhaust temperature sensed by thermocouples 82 with a relatively high reference set by combined potentiometers 84, 94, to be applied over an amplifier 87 to the cathode of a diode 92, whereby, if the reference is exceeded, the valve conducts to modify the speed signal applied to amplifier 42 and thus limit the fuel demand. Operation of the relay also causes a contact 58 to close, whereby a high maximum limit to the main fuel flow is set by the short-circuiting of a resistor 55 connected in series with a follow-up potentiometer 54 whose slider 57 is positioned according to the setting of main fuel valve 20. Potentiometer 54 is supplied from a source 56 in parallel with a potentiometer 50 whose slider 49 is positioned in accordance with the turbine discharge pressure as sensed by a bellows 52 provided with an ambient pressure compensation bellows 51. If a maximum setting of the fuel valve, as modified by the discharge pressure, is exceeded, the voltage applied to the cathode of a diode 47 from the slider 49 becomes negative, whereby the valve conducts to modify the input to amplifier 42 and thus limit the fuel demand. Control at the optimum speed.-When the optimum speed has been substantially attained relay 65 is de-energized, whereby contacts 99, 108 close to reconnect the regulators to their respective elements and the optimum values of reheat fuel flow and nozzle area set by lever 22 are enforced. The opening of contact 58 removes the short-circuit from resistor 55, whereby a lower maximum limit is set on the main fuel flow; the opening of contact 93 causes the application of a comparatively low temperature reference from potentiometer 84 only to the temperature limit circuit for the regulators; and the opening of contact 90 prevents the application of a temperature limit signal to the main fuel control circuit. Maximum speed limit.-An inductance-capacity network 69, 70, 71 responds to the output frequency of tachometer 32 at the desired maximum speed, whereby the A.C. signal applied to a diode 73 is of sufficient amplitude to overcome the D.C. bias of a source 74 during negative half-cycles and the valve conducts to reduce the speed error signal applied to amplifier 42 in spite of an increased speed demand set on potentiometer 25. In order that relay 65 may be de-energized somewhat below the maximum speed, a diode 78, connected through a capacitor 79 to the output of the filter network, is arranged to conduct before valve 73, thereby preventing the continued energization of the relay as a result of a setting on potentiometer 25 above the permitted maximum.