GB879723A - Improvements in fuel systems for aircraft gas turbines - Google Patents

Abstract

879,723. Fluid-pressure servomotor-control systems. DOWTY FUEL SYSTEMS Ltd. Jan. 9, 1959 [Feb. 13, 1958], No. 4791/58. Class 135. [Also in Group XXVI] In a fuel system for a two-spool gas-turbine engine having a rotational speed governor for each rotor, each rotor is adjustable to select any speed throughout the rotational speed range of its associated rotor and both governors control the fuel supplied to the engine combustion chambers. A two-spool gas-turbine is defined as one having a lowpressure compressor and turbine formed as one rotor and a highpressure compressor and turbine formed as another rotor. A twospool gas-turbine comprises a lowpressure rotor having a compressor 1, a turbine 2 and a shaft 3 extending between them, a high-pressure rotor comprising a compressor 4, turbine 5 and a tubular shaft 6 connecting the two together and coaxially arranged around the shaft 3, and combustion chambers 7 extending between the compressor 4 and turbine 5. Part of the air for the compressor 1 may be fed directly to the turbine 2 or to the jet pipe 8. The combustion chambers 7 are fed with fuel by spill spray nozzles 9 to which fuel is supplied by conduit 11 by a pump 14 driven by the high-pressure rotor. The fuel supplied to the combustion chambers 7 is controlled by a spill valve 13. The speed governor associated with the high-pressure rotor 4, 5, 6 is as described in Specification 692,202, in which the speed of the rotor is sensed by the pressure drop occurring between the supply and return pipes 11, 12 of the nozzles 9. The fuel control comprises a spill valve unit 16 and a governor control unit 17. The spill valve 13 controlling the flow back to the suction of pump 14 is controlled by a servo piston 23 operating in a cylinder 24 to which are connected a pair of servo vent pipe lines 27, 28. The vent 27 is carried by a pivoted lever 31 loaded at one end by a pair of bellows 33, 32. Bellows 32 are connected to the discharge of the compressor and the bellows 33 are evacuated. A spring 35 acts between the other end of the lever 31 and the spill valve. The arrangement is such that excessive fuel is not supplied to the engine during acceleration. The vent pipe 28 is controlled by the governor unit 17. A halfball valve 39 is carried by a pivoted lever 38 loaded at one end by a pair of pressure pads 41, 42 connected to the supply and return pipes 11, 12, and at the other end by a spring 46 the loading of which is adjustable by the throttle lever 62 acting through links 61, 48 and cam 47. Since the pressure difference between the pads 41, 42 is proportional to the high-pressure spool speed, the arrangement is used to control this speed in accordance with the throttle lever setting. To maintain the desired engine speed with increase in altitude, a pair of bellows 49, 51 are provided which act on the lever 38. The bellows 49 are evacuated and the bellows 51 connected to ambient pressure. A flyweight governor 53 responsive to the speed of the low-pressure rotor 1, 2, 3 is arranged to control a half-ball valve 59. If the low-pressure rotor overspeeds, the valve 59 opens and allows a flow through pipe 43. Due to the restrictor 45, this causes the pressure on the pad 41 to fall. The half-ball valve 39 then opens, which causes the spill valve 13 to open further and reduce the fuel supply to the engine. The spring loading of the governor 53 is adjustable by the throttle lever 62 through links 61, 58 and cam 57. In a modification, Fig. 4 (not shown), the governor 53 acts directly on the servomotor controlling the spill valve 13. The high-pressure rotor governor may control over the whole speed range and the low-pressure rotor governor set at a higher equivalent speed which governor only comes into operation if anything happens to the high-pressure rotor governor to cause it to allow over-speeding of the low-pressure rotor governor. The speed settings of the two governors need not be adjusted from a single control.