Patent GB-783617-A (GB)
Title: Improvements in or relating to fuel control systems for gas turbine engines
IPC: F02C9/16
Filed: 1954-09-21
Abstract: 783,617. Gas turbine plant. BENDIX AVIATION CORPORATION. Sept. 21, 1954 [Oct. 28, 1953], No. 27304/54. Class 110 (3). [Also in Group XXIX] The fuel supply to a gas turbine is controlled in accordance with the pressure in a chamber 90 connected to the delivery and intake of a compressor driven by the engine through respective passages 140, 144 containing restricted orifices 142, 146, the area relation between the orifices being such that the modulated pressure PM in the chamber 90 closely follows the stall characteristic of the particular compressor so that the velocity of flow through the orifice 146 reaches sonic velocity at a predetermined compressor pressure ratio and thereafter the modulated pressure PM becomes a fixed percentage of the compressor delivery pressure for all higher compressor ratios. In this way optimum acceleration is achieved over the entire operating range of the engine. The pressure PM acts on a bellows 92 connected at its free end to a force balance lever 100 pivoted at 104 and loaded by an adjustable trimmer spring 40 and a further tension spring 38 connected to a metering valve 30 regulated by a servo-piston 128 under the control of a servo vent valve 42 carried by the lever 100. The valve 30 is therefore positioned in accordance with the modulated compressor delivery pressure PM. The pressure drop P1-P4 across the metering valve 30 is regulated in accordance with the instantaneous engine speed by a governor unit 60 and the pressure drop for a selected speed is determined by a further control unit 62. An upper limit to the pressure drop is imposed on the control by a unit 64 operative at a predetermined maximum compressor delivery pressure. The pressure drop P1-P4 between the inlet 22 from the fuel pump and the outlet 17 to the fuel burners is determined by a balanced bypass valve 52 operated by a spring-loaded diaphragm 190 under the difference of pressure P1-P1<SP>1</SP> between chambers 186, 178 connected by a restricted orifice 188. The chamber 186 communicates with the fuel inlet 22, the chamber 178 is connected to the pump inlet pressure PO through passages 176, 252 controlled respectively by servo-vent valves 54, 58. The vent valve 58 is normally closed and opens to cause the by-pass valve to open or open further only when the compressor delivery pressure acting in a chamber 236 is sufficient to depress a spring-loaded piston 230 connected to the vent valve 58 by a lever 234. The vent valve 54 is similarly operated by a lever 152 operatively connected to a piston 148 subject to the fuel pressure difference P1-P3 across an orifice 72. The piston 148 is acted on in the opposite direction, by a thrust element 156 moved by centrifugal weights 82 driven from the engine through a shaft 86. The pressure drop P1-P3 is therefore proportional to the square of the engine speed. The downstream side of the orifice 72 is connected to the fuel outlet 17 through a further restricted orifice 76. The passage 74 between the orifices 72, 76 has a lateral bleed orifice 208, controlled by a vent valve 56 connected to a piston 196 through a lever 202. The piston 196 is adjustably loaded by a spring 216 controlled by a pilot's lever 88 and is subject to the pressure P3 in the passage 74 and the pressure P1 in the fuel inlet 22. The lever 88 thus selects the value of the pressure drop P1-F3 for a given engine speed and hence determines the controlled engine speed. As the same flow passes through the orifice 72 and the orifices 76, 208, the pressure drop P1-P4 across the metering valve 30 is proportioned to P1-P3 which is itself proportional to the square of the engine speed. Hence the flow past the metering valve is directly proportional to engine speed.