GB1151571A - Improvements in Fuel Supply Systems for Aircraft - Google Patents

Abstract

1,151,571. Gas turbine fuel systems. H.M. HOBSON Ltd. March 13, 1967 [Feb. 2, 1966], No.4661/66. Heading F1G. [Also in Division G3] A fuel supply system for an aircraft jet propulsion engine comprises a pump for supplying fuel to a main burner through variable metering orifices in series, each orifice being variable by a control mechanism to provide a different mode of control of the engine, one mode of control only being operational at any given time, and a device for varying the pressure drop across the metering orifices as a function of engine speed. A pump 30 driven from the shaft of the high-pressure compressor of the engine delivers fuel to the main burner via line 39, metering orifices 41, 42, 43, a further orifice 44 and a line 40. An acceleration control unit 34 which serves to prevent compressor surge and limit the maximum speed of the engine, comprises a governor 21 driven from the shaft of the high-pressure compressor. The governor flyweights 38 apply a force to a servopiston 17 through a spring 20 and also control the position of a servovalve 18 which controls a variable orifice 22 in series with a fixed restrictor 23 in lines 28, 45, 62 connected between the discharge and suction sides of the pump. The restrictor 23 and variable orifice 22 form a fuel potentiometer which applies a decreasing force to the underside of the piston 17 as the engine speed increases whereby the piston moves downwards to increase the opening of the metering orifice 41 at low engine speeds by means of a conical valve portion 10A. At engine speeds above ground idling speed a parallel valve portion 10B registers with the metering orifice 41 and further opening cannot occur. Above ground idling speed the fuel flow is controlled by the metering orifice 43 which is controlled by a throttle valve 13 which is operated from the pilot's lever through pinion 25 and rack 47. When the engine speed approaches its maximum value, a cut-off valve 8 carried by the piston 17, which normally has no significant effect on the fuel flow, approaches its seat 6 and reduces the flow through the metering orifice 42 to prevent further increase in speed. If the pilot's lever is moved through a throttle guard to an "emergency over-ride" position, the throttle valve 13 is further opened beyond its normal maximum opening position and, in turn, opens an overspeed valve 11. Fuel in an annulus 9 is then bled through the overspeed valve 11 and lines 45, 62 to the pump inlet, causing the fuel pressure above a piston 24 to move it downwards against the action of a spring 7, whereby the valve seat 6 is moved downwards away from the cut-off valve 8 to give a higher governed maximum speed. Compensation for altitude is provided by the orifice 44 which is controlled by a valve 14 actuated by a diaphragm 16. As the altitude increases, a barometric capsule 3 opens a needle valve 5 to increase the fraction of the pressure drop across the metering orifices 41, 42, 43 determined by the fuel potentiometer 5, 105 and applied to the right hand side of the diaphragm 16, thereby reducing the opening of the valve 14 against the action of a spring 15 to reduce the fuel flow. The altitude compensation is operative only when the throttle valve 13 is open, and to this end a valve 64 in the line 48 is opened by a cam 65 when the throttle valve is opened. When the engine is under the control of the acceleration control unit 34 (i. e. at speeds up to ground idling speed), the orifice 44 is controlled in accordance with engine speed. For this purpose, a bell-crank 12A, 12B connected to the servopiston 17 and providing a movable abutment for the spring 15 increases the opening of the valve 14 as the engine speed increases until a stop 61 is encountered at idling speed. A yieldable joint 52 allows the servopiston 17 to continue to move downward after the stop 61 has been encountered. Primary burners (not shown) are supplied via a line 67 by an engine-driven gear pump 26. A valve 19 is operated from the pilot's lever by a cam 68 so that the line 67 is closed at shut-down and above ground idling speed, the discharge of the gear pump 26 then being returned to the inlet through a relief valve 27. When linked shut-off cocks 58, 59 in the lines 40, 67 are shut, they open the lines to a dump line 70.