Patent GB-1228308-A (GB)
IPC: F02C9/32
Filed: 1969-03-27
Abstract: 1,228,308. Gas turbine engine fuel systems. GENERAL ELECTRIC CO. March 27, 1969 [April 29, 1968], No. 16227/69. Heading F1G. A gas turbine engine fuel system comprises means for generating an error signal proportionate to the difference between the desired engine power and the actual engine power, a control loop comprising means in the forward postion of the loop for positioning the fuel metering valve so as to change the fuel-flow rate at a rate proportionate to a control signal input to the means, thereby providing an integrating action, cam means in the feedback portion of the loop for generating a signal reflecting the rate of change of fuel-flow rate, thereby providing a differentiating action, amplifier means for summing this rate signal and the error signal to provide the control signal input to the metering valve positioning means, and amplifier means for limiting the magnitude of one of the signals as a direct function of the discharge pressure of the compressor of the engine, whereby the rate of change of fuel-flow rate is proportionate to the magnitude of the error signal but does not exceed a limit value which proportionately increases with the compressor discharge pressure. The rotor speed N c  and the air inlet temperature T 2  of a gas turbine engine are applied to a fluidic speed error signal generator 30 to provide a corrected rotor speed signal which is compared with the corrected rotor speed demanded by the throttle lever 28, the resultant error signal appearing as a difference between the air-pressure in lines 32, 34. This pressure difference is applied as an input to a flueric limiter amplifier 40 which is followed by further flueric amplifiers 54, 60, 62 and 107. Integrators, formed by restrictors 64, 74 and volumes 66, 76, are provided between the amplifiers 54 and 60. The amplifier 60 is provided with positive feedback through restrictors 92, 94 connected between output receivers 84, 86 and the volumes 76, 66. The output receivers 108, 109 of the amplifier 107 are connected to opposite ends of the operating piston of the fuel metering valve 26, the pressure drop across the metering valve orifice being maintained constant. The position of the metering valve 26, represented by the angular position of a cam 110 coupled to the metering valve, is sensed by a pair of discharge nozzles 112, 114 directed towards the cam and connected to a fixed reference source 117 of pressurized air via restrictors 116, 118, the resultant pressures upstream of the nozzles being applied via further restrictors 120, 122 to the control ports 104, 106 of the amplifier 107 to provide negative feedback. The rate of change of pressure upstream of the nozzle 112, which is representative of the rate of change of fuel-flow provided by the movement of the metering valve 26, is sensed by a differentiator circuit in which the pressure upstream of the nozzle 112 is applied to one control port 134 of an amplifier 130 through a restrictor 132 and to the other control port 128 through a restrictor 124 and a volume 126. The output of the amplifier 130 is applied vÝa restrictors 140, 142 to the input of the amplifier to provide negative feedback. As a result, the rate of change of fuelflow is directly proportional to the signal input to the amplifier 54 from the amplifier 40. However, to prevent compressor surge or flameout due to a rate of change of fuel-flow which is excessive in relation to the compressor discharge pressure, the amplifer 40 is a limiter amplifier and has its power stream nozzle 144 connected to a source of pressurized air which is controlled to provide a pressure having a linear or non-linear relationship to the compressor discharge pressure sensed by a probe 152, so that the maximum output of the amplifier is limited to a value dependent on the compressor discharge pressure. Each amplifier may comprise several cascaded amplifying devices. Instead of fluid signals, electrical or mechanical signals may be used. The orifice of the metering valve 26 may be shaped to provide a non-linear relationship between the fuel-flow rate and the displacement of the metering valve.