Patent GB-891194-A (GB)
Title: A fuel control system for gas turbine engines
IPC: F02C9/16
Filed: 1959-11-25
Abstract: 891,194. Gas turbine plant. BENDIX CORPORATION. Nov. 25, 1959 [Dec. 31, 1958], No. 40051/59. Class 110 (3). [Also in Group XXIX] A fuel control system for gas turbine engines having a compressor, comprises a movable fuel control member, a pressure-responsive member connected to said fuel control member and subjected to an air pressure generated by the compressor to control the fuel control member as a function of variations in said pressure, a second pressure-responsive member having a surface of predetermined area greater than the area of the first pressure-responsive member and connected thereto and subject on its opposite sides to air pressures transmitted from the compressor, and means responsive to an engine operating variable other than a compressor generated pressure operative to vary the pressure acting on one side of the second pressureresponsive member as a function of said variable, whereby the fuel control member is positioned in response to the combined pressure forces acting on the first and second pressure-responsive members. The invention is described as applied to an engine 10, Fig. 1, including a compressor 12 connected to a first turbine 14, combustion chamber 18 and a second or free turbine 26 connected to a load 30. The main fuel control 32 controls the position of the fuel supply valve 94, Fig. 2, in accordance with (a) a compressor generated pressure sensed by pickup device 44, Fig. 1, and modified by means sensitive to ambient air temperature; (b) the speed of the first turbine 14 as detected by a manually settable governor 162, Fig. 2 ; and (c) the speed of the free turbine 26 as detected by a manually settable governor 202. A by-pass valve 96 maintains a constant pressure difference across the metering valve 94. The controller 32 includes a housing 86 divided by a bellows-diaphragm assembly 104, 108, 116, 120 into chambers 100, 118, 122 and 124. A rod 110 supported by the assembly is connected by a lever 114 to the metering valve 94. Air pressure generated by the compressor 12 is transmitted to a conduit 46 and passes via a restrictor 132 adjusted by ambient air temperature responsive means 134 to passages 146, 148, 150 leading via resistrictors 152, 154, 156 respectively to chambers 124, 122, 118. The flyweights 162 of the first speed-responsive means control two pivoted levers 168, 170, the left-hand ends of which are urged apart by a spring 180 and the right-hand ends of which control leak-off valves 174, 182 controlling air flow from the chambers 118, 124 respectively. The manually settable lever 64 acts on the lever 168 through a cam 188, a pivoted lever 190 and a spring 192. The flyweights 202 of the second speed-responsive means control a pivoted lever 208 which controls a leak-off valve 212 controlling air flow from the chamber 124. The manually settable lever 68 acts on the lever 208 through a cam 216, a pivoted lever 218 and a spring 220. In the steady state condition the air pressure generated by the compressor and modified by the ambient air temperature-responsive means 134 is transmitted to the chambers 118, 122, 124 and bellows 104, 108 are subject to atmospheric pressure via orifice 158 so that the bellows-diaphragm assembly maintains the valve 94 in a desired position. The first speed-responsive means keeps the leak-off valve 174 wide open and the leak-off valve 182 in an intermediate position. The second speed-responsive means keeps the leak-off valve 212 closed. If the lever 64 is moved to call for an increase of speed, the spring 192 is compressed to close leak-off valve 182 whereupon the air pressure in chamber 124 rises and the rod 110 is raised to further open the valve 94. The engine accelerates so that the pressure generated by the compressor and reflected in the chamber 118, 122, 124 increases and the valve 94 opens further. The first speed-responsive means responds to the increase of speed and by opening the leak-off valve 182 reduces the pressure in the chamber 124 to return the valve 94 to a new balanced position. Any tendency of the speed of the turbine 26 to overshoot maximum safe value causes the flyweights 202 to open the leak-off valve 212 to reduce the pressure in the chamber 124. If the lever 64 is moved to call for a decrease in engine speed the flyweights 162 cause the leak-off valve 182 to be fully opened whereupon the air pressure in the chamber 124 rapidly falls and the rod 110 moves the valve 94 in a closing direction. The decleration of the engine causes a decrease in the air pressure supplied to the chambers 118, 122, 124 so that the valve 94 is moved further in a closing direction until the flyweights 162 reflect the decrease of speed and move the lever 168 and cause closing of the valve 182 whereupon the pressure in the chamber 124 builds up and the valve 94 assumes a new balanced position. If it is desired to closely control the speed of the turbine 26, the lever 64 is set to a maximum overspeed position whereby the leak-off valve 182 is closed. Governing is then obtained by the second speed-responsive device. If the engine is operating near the stall range an increased rate of acceleration just outside the stall range is achieved by the action of the first speed responsive in opening the leak-off valve 174 to exhaust air from the chamber 118 and cause the rod 110 to move upwardly and open the metering valve 94. This increase rate of acceleration may be varied by adjusting the restriction 156. The pressure delivered by the compressor 12 at start conditions may be close to atmospheric pressure but fuel flows less than that at start conditions are obtainable because the bellows 108 and diaphragms 116, 120 are of progressively increasing areas. The bellows-diaphragm assembly may be replaced by an arrangement of two bellows of different sizes.