Patent GB-996001-A (GB)
Title: Improvements in or relating to fuel delivery control systems for gas turbine, engines, more particularly for turbojet engines
Inventor: TURNER HORACE GEORGE
IPC: F02C9/46
Filed: 1960-10-25
Abstract: 996,001. Gas turbine engine fuel systems. PLESSEY CO. Ltd. Oct. 12, 1961 [Oct. 25, 1960], No. 36577/60. Heading F1G. A fuel system for an engine such as a gas turbine jet engine comprises an engine-driven positive-displacement fuel supply pump, an adjustable throttle through which fuel passes on its way from the pump to the combustion chamber of the engine, means being provided for maintaining the pressure drop across the throttle constant by diverting the excess amount of fuel supplied by the pump to a point at low pressure. The system shown comprises a gear type main fuel pump 4 which receives fuel from a tank by way of a centrifugal type backing pump 65, the main pump delivering fuel through a throttle and spill valve unit 5 to a starting fuel burner 2 and a main burner 1. The throttle valve comprises a valve member 7 disposed within a sleeve 8 the valve member being adjusted by means of a control lever 6 through a rack and pinion drive 9. The valve member 7 is provided with tapered grooves 10 which co-operate with port 11 to which fuel is delivered by the pump 4 through line 34. The valve member 7 is shown in the position corresponding to the normal maximum rating of the engine in which fuel flows past the pressurising valve 26 to the main burner 1, fuel supply to the starting burner being cut off by the land 19 of the valve member. The spill valve 12 controls discharge of fuel through a dump port 18, the valve member being urged in the closing direction by means of a spring 13 and being controlled also by means of a diaphragm 14 which is subject to the pressure drop across the fuel metering orifice 10, 11, the space above the diaphragm being subject to fuel pressure upstream of the orifice by way of duct 15, and the space below the diaphragm being subject to fuel pressure downstream of the orifice by way of a duct 16 extending through the throttle valve member 7. If fuel pressure in the line 34 rises above a pre-determined value, the valve 12 opens and excess fuel is discharged through port 18. In the upper or shut-off position of the valve member 7 the port 11 is closed off by the land 19 and the space above the valve member 7 communicates with a dump line 21 by way of ducts 16, 20. As the valve member 7 is opened the annular recess 22 first opens communication between the port 11 and the port 23 whereby fuel flows to the starting jet 2. Continued opening movement of the valve member 7 attains an idling position in which the grooves 10 open to allow fuel to pass to the main burner 1 and simultaneously the supply to the starting burner 2 is cut off by the land 19. Also, the longitudinal port 24 establishes communication between the signal line 25 and the dump port 18. Further movement of the valve member allows increased fuel flow to the main burner 1, maximum normal flow being provided when the valve reaches the position shown. Under emergency conditions however, the valve member may be moved downwardly beyond the position shown, increasing flow through the metering orifice 10, 11. At the same time, the passage 24 is disconnected from the dump passage 18, but the duct 20 now communicates with the signal line 25 whereby the fuel pressure downstream of the metering orifice 10, 11 is communicated to line 25. The maximum-speed control of the fuel supply is effected by a governor unit comprising centrifugal weights 28 pivotally mounted on a sleeve 29, the weights acting through a rod 31 to control movement of a piston type slide valve 32 so as to progressively uncover ports 33 whereby fuel from the supply line 34 passes to a chamber 35 which communicates by way of duct 36 with the suction side of the main fuel pump 4. The opening action of the governor weights is assisted by a pair of springs 37, 38 but is resisted by a spring 63 acting on the valve 32. One end of a pivoted lever 39 is disposed between the adjacent ends of the springs 37, 38, the other end of the lever bearing on a piston 41. In the normal speed range of the engine, the space to the left of the piston 41 communicates by way of line 25 with the dump outlet 18 and no pressure acts on the piston. The governor weights allow excess fuel to pass through ports 33 to chamber 35 when the engine reaches its normal maximum speed. If however the control lever 6 is moved to the emergency position, the main burner pressure is communicated to the left-hand side of the piston 41 by way of the line 25. The piston therefore acts to move the lever 39 in a clockwise direction so compressing spring 37 and reducing the loading of the spring 38. The spring 63 will therefore move the piston valve 32 to close off the ports 33 so preventing escape of fuel from the line 34 to chamber 35 and the ports will only be re-opened when the emergency maximum speed of the engine has been reached. The system also comprises an acceleration control unit 45, an adjustable restriction 46 being disposed in the fuel supply line 34, the pressure upstream of the restriction being applied to a thrust member 51, and the pressure downstream of the restriction being applied to a piston valve member 47. The thrust member 51 is additionally loaded towards the right by a spring 52 and transmits its thrust to piston valve 47 by means of a thrust bearing unit 53. The bore in which the piston valve is disposed has two ports 54, 55, the port 54 communicating with the duct 34 and the port 55 communicating with a chamber 56 at one end of a freely movable link valve 57, the link valve having ports 59 by which fuel may pass from the chamber 56 to the chamber 35 which communicates with the suction side of the main fuel pump 4. The opposite side of the link valve is subject to the pressure in the supply duct 34 and so the link valve takes up a position such that the pressure in the chamber 56 is maintained equal to that in the duct 34. The pressure drop across the ports 54, 55 will thus be equal and therefore the ratio at which the fuel delivered by the pump 4 is distributed between duct 34 and the return to the pump inlet through the port 55 corresponds to the relative size of the apertures provided by the co-operation of the ports 54, 55 with their complementary apertures in the valve member 47. The pressure drop across the restrictor 46 and thus the displacement of the valve 47 will be a pre-determined function of engine speed. The valve member 47 and the link valve are continuously rotated by an engine-driven shaft 50, the governor sleeve 29 also being driven from the shaft 50, the gear 58 on the link valve acting as an intermediate gear.