Patent GB-1372967-A (GB)
Title: Fuel controls for engines
IPC: F02C9/32
Filed: 1972-01-28
Abstract: 1372967 Gas turbine engine fuel systems CHANDLER EVANS Inc 28 Jan 1972 [1 Feb 1971] 4206/72 Heading FIG An engine fuel control comprises a fuel supply line; a metering valve for controlling the flow of fuel through the supply line, the metering head across the valve being determined by first and second pressures respectively upstream and downstream of the valve; a speed sensor to generate a signal representative of the actual engine speed; a computer receiving the speed signal and generating a third pressure within a cavity and having an inlet referred to a fourth pressure and an outlet referred to the first pressure; a first differential pressure regulator connected to the computer for maintaining a substantially constant differential between the fourth and first pressures; and a second differential pressure regulator connected to the fuel supply line for maintaining a substantially constant differential between the third and second pressures. A pressure drop P 1 -BF in the main fuel line 70, fig. 2 is imposed by a spring-loaded valve 88, a variable portion SS-BF of this pressure drop, increasing with the gas producer speed N 1 , being tapped off by a hydraulic potentiometer comprising a fixed restrictor 246 and a valve 238 in series, the valve 238 forming part of a hydraulic speed sensor 170. The pressure SS is communicated via a line 278, Figs. 2 and 3, to one side of a diaphragm 404 which is subjected on its other side to a pressure FG occuring downstream of a valve 400 actuated by the diaphragm. Movement of the diaphragm is opposed by a spring 398 and fueltemperature compensating discs 394, 396, the spring abutment 424 being positioned in accordance with the desired value of the gas producer speed N 1  by a hand lever 14. The valve 400 forms, in conjuction with a restrictor 166, Fig. 2, a hydraulic potentiometer connected via lines 416, 418, 165 in parallel with the valve 88, whereby a variable proportion FG-BF of the pressure drop SS-BF occurring across the valve 88 is tapped off and applied to the diaphragm 138 of a main fuel metering valve 90, the pressure differential FG-BF being indicative of the discrepancy between the actual and desired values of the gas producer speed N 1 . The spool 112 of the metering valve 90 is acted on, via a spring 120, by an evacuated capsule 96 subjected externally to the compressor discharge pressure P c  the pre-load on the spring 120 being such that the spring acts as a solid link during acceleration to position the spool 112 in accordance with the pressure P c . During acceleration, the spool 112 is not influenced by the diaphragm 138, which is then out of contact with a spring 145. After acceleration, when the spool 112 has risen sufficiently, under the influence of the increase in the pressure P c  occuring during acceleration, to bring the spring 145 into contact with the diaphragm 138, the pre-load on the spring 120 is overcome and the position of the spool 112 is governed by the diaphragm 138 to hold the gas generator speed N 1  at the value set by the lever 14. During deceleration, when the actual value of the speed N  1  is much greater than the value set by the lever 14, the pressure differential FG-BF is sufficiently high to bring a stop screw 154 against a flange 156, and the spool 112 is again controlled by the diaphragm 96 in accordance with the pressure P c . After flowing through the main metering valve 9D, the fuel enters the power turbine governing system at 504, Fig. 4, a pressure drop AF-BG being imposed by a valve 508. A hydaulic speed sensor 534 forms, in conjuction with a restrictor 540, a hydraulic. potentiometer to tap off a variable portion SS-BG of the pressure drop AF-BG. This pressure differential SS-BG, which increases with the power turbine speed N2, is applied to a diaphragm 552 to ieduce the opening of an overspeed limiting valve 514 in the event of overspeeding of the power turbine. After passing through the valve 514, the fuel flows via a line 516 at a pressure AL which is normally (i.e., when the valve 514 is fully open) equal to the pressure BG, and thence through a governor valve 522 and a deceleration valve 524 in parallel to a duct 520 at a pressure AG. The pressure drop AL-AG (normally equal to BG-AG) across the valves 522, 524 is maintained constant by a bypass valve 530 actuated by a diaphragm 570 subjected to the pressure drop BG-AG. After leaving the valves 522, 524 the fuel passes through an orifice 528 which imposes a pressure drop AG-AR'. The space above a diaphragm 628is connected to the duct 520 via an adjustable restrictor 632, the pressure AD in the space being equal to the pressure AG under stable conditions. A diaphragm 598 urging the spool 584 of the governor valve 522 to a position of reduced opening is acted on by a pressure differential SS'-AL, which is a measure of the power turbine speed N 2 . The spool 584 is acted on in the opposite direction by a spring 622 having an abutment 620 which is positioned by a hand lever 16 in accordance with the desired value of N 2 , and also by a spring 624 having an abutment winch is positioned by the diaphragm 628 in accordance with the pressure differential AD-AR' which is normally equal to the pressure drop AG-AR' through the orifice 528 and is therefore a measure of the fuel flow. This latter control in accordance with fuel flow permits isochronous operation of the governor 522. In conjuction with the chamber above the diaphragm 628, the adjustable restrictor 632 provides an adjustable time constant for varying the dynamic characteristics of the governor 522. During deceleration, the governor valve 522 closes, and  the fuel flow is then controlled by the deceleration valve 524, which is actuated by an evacuated bellows 640 subjected externally to the pressure P c , changes in fuel density being compensated for by bimetallic discs 649. The deceleration valve 524 may be omitted if the governor valve 522 has a minimum flow stop capable of preventing flameout. Since the gas producer control and power turbine control are in series, the lower of the fuel flows commanded by the controls is the actual fuel flow to the engine. When the engine is starting, the valve 88 has not yet opened, the flow being bypassed around this valve by a circuit comprising a start regulator valve 176, a conduit 178, a starting valve 262, an enrichment valve 260 and a conduit 182. The total pressure drop BR-BF across the valves 262 and 260 is maintained constant by the valve 176, which is actuated by a diaphragm 198 subjected to that pressure drop. The pressure signal SS-BF produced by the speed sensor 170 and indicative of the gas producer speed N 1  is applied via lines 278, 282 and 182 to a diaphragm 276 to position the valves 260, 262, the total pressure drop AR-AF across the valves 262 and 90 being maintained constant by a bypass valve 368 actuated by a diaphragm 362 subjected to that pressure drop. Initially, a spring 308 connecting the spool 288 of the enrichment valve 260 and the valve disc 310 of the starting valve 262 acts as a rigid link. As the disc 310 moves downwards, it initially decreases the restriction to flow through an orifice 274 and thereafter increases the restriction to flow through an orifice 320, whereby the flow is initially increased and then decreased as the gas generator speed increases in the low speed region. Meanwhile, a cylindrical portion 302 of the enrichment valve spool 288 moving in an orifice 272 maintains a constant restriction. When the gas generator attains a sufficient speed, a valve 264 in parallel with the starting valve 262 is opened manually to increase the pressure AR and thereby cause the bypass valve 368 to open further to reduce the flow through the metering valve 90 to forestall an overtemperature condition. Fuel enrichment is produced when the hourglass-shaped portion 300 of the enrichment valve spool 288 moves into the orifice 272 as further increase of speed occurs, during acceleration from idle to maximum speed.