Patent GB-651599-A (GB)
Title: Improvements in or relating to fuel-systems for gas turbine engines
Inventor: JUBB ALBERT
IPC: F02C9/28
Filed: 1948-08-23
Abstract: 651,599. Gas turbine plant fuel systems. ROLLS-ROYCE, Ltd. Aug. 23, 1948, No. 22223.. [Class 110(iii)] ] [Also -in Groups XI and XX IX] A gas turbine engine fuel system, having means to control the fuel flow to the burners at a selected datum for steady running of the engine and means to pre-set this datum, is provided with means to control the engine during acceleration comprising a fuel flow restriction in the fuel supply conduit to the burners which is not varied by the datum-setting mechanism and means responsive to a load dependent on the pressure drop across the restriction and to a load dependent on the engine compressor delivery pressure, the loads being arranged to act in opposition on means for varying the fuel flow so that the instantaneous fuel flow is limited as a function of the instantaneous compressor delivery pressure. The fuel supply to the burners 14, Fig. 2, of a gas turbine engine from a variable capacity pump 20 is controlled by a manually-operated throttle 53, an atmospheric pressure control 21 and an acceleration control 22. The atmospheric pressure control comprises a body 44 divided into two chambers 45, 46 by a diaphragm 47 carrying a lever 48 which projects into the two chambers. The chamber 45 is connected to the spring-loaded side of the piston 29, controlling the swash plate mechanism 25 of the fuel pump 20, by a bleed pipe 49, the flow through which is controlled by a half-ball valve 50 carried by the lever 48. The lever. 48 is loaded by an evacuated expansible capsule 56, responsive to changes in atmospheric pressure, an adjustable spring 51 and a tappet member 55 responsive to the pressure upstream of the throttle 53, the arrangement being such that a decrease of atmospheric pressure or an increase of the fuel pressure upstream of the throttle above a selected value will cause the valve 50 to open and allow fuel to pass from one side of the piston 29 and thus reduce the output of the pump 20. The atmospheric pressure may be the static atmospheric pressure, the static pressure as modified by the velocity head due to the forward speed of the aircraft or by conditions in the intake of the compressor or by both forward speed and conditions in the intake. The acceleration control 22 comprises a valve body 60 and a pressure responsive unit 71 which is used to regulate the output of the pump 20 by controlling the leak off from one side of the pump control piston 29. The valve body 60 has a spring-loaded valve 62 which causes a pressure drop in the fuel supply conduit 28 directly proportional to the fuel flow and a second spring-loaded valve 65 controlling a passage byepassing the valve 62. The dimensions of the valve 65 and the rating of the spring 68 are so selected that when the pressure drop across the valve 62 reaches a predetermined value the valve 65 opens and-the pressure drop across the valve body 60 remains substantially constant irrespective of increase in fuel flow. The unit 71 is divided into two chambers 72, 73 by a diaphragm 74 carrying a lever 75 which controls through a halfball valve 76 the flow through a pipe line 77 from one side of the pump control piston 29. The lever 75 is controlled as to its rocking movement by a load dependent in part at least on the absolute compressor delivery pressure, a load dependent on the pressure drop across the valve body 60 and a spring load. The load dependent upon the absolute compressor delivery pressure is applied by connecting the chamber 72 with the compressor delivery and forming. part of the wall of the chamber 72 by a diaphragm 79 which is loaded by an evacuated capsule 82 contained in a chamber 80 open to an atmospheric pressure. In operation, if the effective areas of the diaphragm 79 and capsule 82 are selected to be equal then the load applied by a spring 82a to the lever 75 is decreased as the absolute compressor delivery pressure is increased.. If the effective area of the diaphragm and capsule are unequal then the load applied by the spring 82a will be in part determined by the absolute compressor delivery pressure and in part by an atmospheric pressure. The load dependent on the pressure drop across the valve body 60 is applied to the lever 75 through a spring- loaded diaphragm 85. A tappet member 84 is provided to compensate for the difference in effective areas of the two sides of the diaphragm 85. The combined spring load applied by the springs 82a and 90 is arranged to tend to keep the valve 76 closed. The arrangement is such that for a given value of the compressor delivery pressure there is a limit to the amount of fuel which can flow to the burners through the valve 62, above that limit the stroke of the fuel pump is automatically reduced. This control continues to operate until the valve 65 opens. A valve 97 is provided to render the acceleration control, if desired, inoperative. This valve may be operated manually or automatically in response to a selected engine speed or to a predetermined compressor ratio in the compressor. A modification in which the lever 75 is loaded in accordance with the absolute compressor delivery pressure by connecting the chambers on opposite sides of the diaphragm 79 to the inlet and throat of a choked venturi connected to the compressor delivery and omitting the capsule 80 is also described. In'a further modification, the atmospheric pressure control 21 and acceleration control 22 are combined in a single unit but operate independently on the valve controlling the lead off from the spring-loaded side of the pump control piston 29. In this arrangement, the atmospheric control controls the pressure drop across the throttle 53 instead of the pressure upstream of the throttle. The control described, if chosen to give satisfactory acceleration conditions up to compression ratios of 3:1, may tend to starve the engine of fuel at high altitudes and high rotational speeds. To avoid this difficulty, the arrangement shown in Fig. 6, may be provided to increase the effective load due to the absolute compressor delivery pressure on the locking lever 175 of the acceleration control. A pivoted beam 213 is connected at each end to evacuated capsules 214, 218' accommodated in chambers 216, 220 open to the compressor delivery and suction pressures respectively. As the compression ratio of the compressor increases the beam rotates anti-clockwise until, at a predetermined compression ratio, the electrical contacts 211, 212 engage, the solenoid 208 is energized, and the valve 206 operated to connect the conduit 204 to the compressor discharge instead of the compressor intake. The resulting pressure drop across the diaphragm 201 acts through the tappet 200 and the diaphragm 179 on to the rocking lever 175. The means for setting and maintaining the datum for steady running may be a manually-operated throttle or means arranged to select and maintain the engine rotational speed or a temperature at a point in the engine. The throttle 53 may be located in a body 98, Fig. 4, incorporating also a shut-off cock 99 and a pressurizing valve 105 which is spring-loaded on to its seat 106 to control the fuel flow through conduit 107 to the main jet of a double jet burner 14 of known form. During slow running, fuel passes only through a conduit 107a to the slow running jet of the burner but when the fuel pressure reaches a predetermined value, the valve 105 lifts and permits fuel to pass to the main jets of the burners. Specification 577,016, [Group XXIX], is referred to.