GB804702A - Improvements in a fuel flow control for engines - Google Patents

Abstract

804,702. Gas turbine plant. BENDIX AVIATION CORPORATION. March 29, 1956 [April 14, 1955], No. 10004/56. Class 110 (3). To maintain a minimum fuel flow to the engine irrespective of wide variations in operating conditions, a gas turbine engine fuel control system comprises conduit means for conducting fuel to the engine which includes parallel branch passages each controlled by a valve one of the valves being controlled by means responsive to the fuel flow downstream of the junction of the passages so that when the fuel flow controlled by the other valve falls below a predetermined value it is opened to increase the flow. Fuel from a pump 22 passes through a conduit 24 into a chamber 52 where it is divided. Part passes through ports 54, 56 and chamber 62 to the inlet of the pump and part passes through a conduit 64 and a metering valve 72 to the burners. The ports 54, 56 are controlled by a valve 60 to maintain a constant pressure difference across the metering valve 72. A minimum flow control valve 88 is provided to control a by-pass around the metering valve. The metering valve comprises a square port 102 which registers with a square port 104 formed in a sleeve 68 surrounding the valve. The Y dimension of the port is controlled by the control 106 in response to engine speed and the position of the pilot controlled lever 42 and to an acceleration control responsive to engine speed and compressor inlet temperature. An adjustable minimum flow stop 110 is provided to limit axial movement of the valve. The X dimension of the port is controlled by bellows 116 responsive to the compressor discharge pressure. The valve 60 is controlled by a regulator 61 which comprises a flexible diaphragm 122 and a bellows 126. Variations in pressure across the metering valve act on the diaphragm 122 and cause it to move the pilot valve 140 against the action of a spring 156. The pilot valve 140 is connected to the valve 60 and moves this valve to restore the valve of the pressure drop. To maintain the length of the spring 156 constant irrespective of the position of the valve 60, the pilot valve 140 is provided with a land 142 which controls the flow of fuel through the passages 144, ports 136, 138, passage 148 and passage 152. Movement of the pilot valve 140 varies the relationship between the flow out of the ports 136, 138 below the land 142 into the chamber 128 to the flow into the ports 136, 138 above the land 142 out of the chamber 128 and thus varies the pressure in the chamber 128. This, in turn, causes the bellows 126 to expand or contract until the flow into and out of the ports 136, 138 is again in balance and so restore the spring 156 to its original length. A stack of bimetallic discs 158 is provided to vary the length of the spring 156 in accordance with fuel temperature so that the quantity of fuel metered by the port 102 is unaffected by variations in fuel temperature. The minimum flow control valve 88 is carried by one arm of a pivoted lever 162 the other arm of which is connected to a diaphragm 168 responsive to the pressure difference across a variable area minimum flow restriction 83. During normal running the pressure drop across the restriction 83 is sufficiently high to overbalance the pressure on the face of the valve member 86. At low fuel flows, the pressure on this face opens the valve and allows additional fuel to pass to the engine through the by-pass 84, 98 and 100 around the metering valve. The area of the restriction 83 may be controlled by an engine operating condition such as engine speed or compressor discharge pressure. A spring-loaded check valve 180 is provided to control a by-pass around the restriction 83 to limit the maximum differential pressure across the restriction.