Patent GB-1413882-A (GB)
Title: Speed-adjustable automatic fuel-control apparatus for internal- combustion turbine engines
IPC: F02C9/32
Filed: 1973-01-19
Abstract: 1413882 Gas turbine engine fuel systems PLESSEY CO Ltd 19 Jan 1973 2760/73 Heading F1G The rate of fuel flow to a gas turbine engine is varied by the application to a fuel flow control valve device of a flow-control pressure obtained from a fluidic circuit which is connected to two pressures whose ratio is determined by the speed of an engine-driven dynamic compressor which supplies combustion air to the engine, the fluidic circuit including an adjustable restrictor operable by the pilot's control lever and including a portion arranged to derive from the pressures a primary control pressure which varies with the engine speed in accordance with the steady-running fuel-supply graph of the engine, a further portion arranged to produce a speed-error signal by comparison of a speed-determined pressure or pressure ratio corresponding to the momentary engine speed with a speed-controlling pressure or pressure ratio selected by the adjustable restrictor, and a fluidic proportional amplifier having a power jet to which the primary control pressure is applied, this power jet being subjected to control-jet action produced by the error signal to provide an output from which the flow-control pressure is derived. The fuel valve is actuated by an actuator comprising a bellows contained in an ambient pressure chamber 3, Fig. 5, and having an evacuated portion 6 containing a spring 7 and a portion 4 connected to an output leg 24 of a proportional fluidic amplifier 20. The amplifier 20 is preceded by proportional amplifiers 32, 33 in cascade, the first amplifier 32 being fed with a control signal from the pilot's lever 37. The control signal is derived from a potentiometer comprising a variable restrictor 35, controlled by the lever 37, connected in series with a choked Venturi 34 between a line 9 at the engine compressor delivery pressure P4 and ambient pressure P1. A jet pump 39 fed with air at the pressure P3 of an intermediate stage of the compressor ensures that the Venturi 34 is choked even at low values of the ratio P4/P1. The control signal developed at the throat of the Venturi 34 is fed to one control port of the first amplifier 32. Since the signal is a variable proportion, selected by the pilot's lever 37, of the ratio P4/P1, this ratio being a measure of the engine speed, the signal represents the error between the actual speed and the desired speed. The other control port of the amplifier 32 is fed with a reference pressure which may be the ambient pressure P1 or instead may be a modified pressure P1' derived from P1 by means of a potentiometer comprising a variable restrictor 41 in series with a de Laval nozzle 43 which is maintained choked by a jet pump 44 connected to the junction between the variable restrictor 35 and the Venturi 34. The variable restrictor 41 is controlled by a spring- loaded evacuated bellows 42 subjected to the pressure P1. To reduce the risk of accumulation of moisture or ice from the incoming flow of air at pressure P1, this flow may be replaced by a flow of warm air derived from a source 47 at the pressure P3 via a jet collector 46 and a restrictor 45, this flow being slightly in excess of the required flow, the excess being blown off through the P1 inlet. A variable restrictor 38 shunting the output legs of the amplifier 32 is linked to the lever 37 so that the gain of the amplifier is increased when higher values of P4/P1 are selected to compensate for a reduction of sensitivity of the Venturi arrangement at the higher values. Matching of the control characteristics of the engine to those of other engines is effected by an adjustable by-pass restrictor 40. A vortex amplifier 31 is connected between the output line 5 and the vent line 27 of the amlifier 20 and associated with adjustable restrictors 28, 30, the effect of this arrangement being that the restrictors provide substantially independent adjustment of the deceleration rate and acceleration rate respectively. Fig. 7 shows a modification in which the control pressure applied to the amplifier 32 is derived from a potentiometer connected between the pressures P4 and P1 and comprising a range adjustment restrictor 49 and a choked variable restrictor 48 controlled by the pilot's lever 37. The reference pressure PJC applied to the amplifier 32 is the output pressure of a jet collector 50 supplied with operating fluid from the P4 line 9 via a variable restrictor 51 controlled by the lever 37 and venting to ambient pressure P1 via an orifice 41 actuated by an evacuated bellows 42 acted on by the pressure P1. In the further modification shown in Fig. 9, the reference pressure applied to the amplifier 32 is derived from a potentiometer comprising a fixed restrictor 63 and a choked de Laval nozzle 64 connected between the pressures P4 and P1, while the control pressure Pb is derived from the output of two cascaded proportional amplifiers 52, 56. The amplifier 52 is provided with a feedback network comprising a variable restrictor 53 controlled by an evacuated bellows 54 subjected to the pressure P1, and a choked Venturi 55. A feedback network for the amplifier 56 comprises a potentiometer connected between the pressures Pb and P1 and comprising a range-adjustment restrictor 59 and a variable restrictor 57 operated by the pilot's lever 37, the restrictor 57 being shunted by an adjustable restrictor 58.