Patent GB-751724-A (GB)
Title: Improvements in or relating to fuel systems for internal combustion engines
Inventor: DAVIES DAVID OMRI
IPC: F02M69/00
Filed: 1952-08-11
Abstract: 751,724. Centrifugal compressors; gas turbine plant. ROLLS-ROYCE, Ltd. Nov. 10, 1953 [Aug. 11, 1952], No. 20217/52. Classes 110(1) and 110(3) [Also in Groups XI and XXIX] In a fuel control system for gas turbines in which a fixed-capacity positive displacement pump 13 driven at a speed proportional to engine speed delivers fuel along a first conduit 14 past a sharp-edged restricting orifice 15, the pressure difference on opposite sides of which is applied to a member such as the diaphragm 19 to effect a control operation and in which the liquid fuel used is of the kind of which the viscosity is a unique function of the density, means are provided to compensate for variations in the fuel density such means comprising of a source of fuel under higher pressure than that delivered by the pump 13, a second conduit 24 which connects the source of higher pressure to the first conduit 14 at a point between the pump 13 and the sharp-edged orifice 15 and a viscosity sensitive restrictor 28 in the conduit 24. Thus any decrease in the pressure drop across the orifice 15 due to a decrease in fuel density is made up at least in part by increased flow past the restrictor 28. By suitable choice of the restrictor 28, it can be arranged that the total flow through the orifice 15 varies inversely as the square root of the fuel density so that the pressure drop across the orifice 15 is constant for a given engine-rotational speed N or a given value of N/ /T, where T is the engine inlet temperature. Fig. 1 shows the source of higher pressure as the main enginedriven pump 10 which draws fuel from the same pipe 11 as the pump 13 and delivers it by a pipe 12 to the fuel injectors. A relief valve 25 controls the pressure upstream of the restrictor 28 and a further restrictor 27 limits the flow through the relief valve. The orifice 15 may have' a fixed area or it may be controlled by a pressure responsive device, or by a capsule 17 connected to a temperature responsive element 18. The relief valve may be loaded in the closing direction by the delivery pressure of the pump 13. In a modified arrangement, the relief valve 25 is dispensed with and the pressure drop across the viscosity sensitive restrictor 28 is maintained constant by a spring-loaded diaphragm-actuated lift valve in the conduit 24. Fig. 3 shows a practical embodiment of the arrangement shown in Fig. 1. The pump 10 is of the multi-cylinder swash-plate type, the swash-plate mechanism 44 being adjusted by a spring-loaded servo-piston 47 controlled by adjustment of the bleed flow through a pipe 55. The fixed capacity pump 13 which has an adjustable by-pass 60 is fed from the chamber 58 of the pump 10 with fuel which is pressurized to a value dependent on the engine rotational speed, the chamber 58 being supplied through radial passages 57 in the pump rotor 40. The orifice 15, the diaphragm 19, and the viscosity sensitive restrictor 28 are embodied in a single control unit 62 which also includes a servo-mechanism adapted to adjust a fuel throttle or the variable angle swirl vanes of the engine compressor. The latter mechanism comprises a differential piston 63 connected by'a link 69 to the member to be adjusted and having a smaller effective area connected by a passage 70 and the conduit 24 to the delivery of the pump 10 and a larger effective area connected to passage 70 through a restrictor 72 and a filter 71. Pressure in the space 65 corresponding to the larger piston area is controlled by a leak-off valve 74 loaded to open by a spring 76 and to close by springs 77, 78. The spring 78 acts on the diaphragm 19 and the spring 77 on a, thrust member 79 and also on the piston 63 so that its load depends on the position of the piston. Fuel from the pump 13 passes into a duct 81 and then past the orifice 15 into a duct 85 leading back to the main pump inlet via a conduit 16. The effective area of the orifice 15 is determined by a needle valve element 83. The diaphragm 19 which acts on the valve 74 is exposed to the, pressures on opposite sides of the orifice 15. The viscosity responsive restrictor 28 is connected with the upstream side of the orifice 15 by ducts 88, 90. The restrictor 28 is supplied from the conduit 24 though a spring-loaded valve 31 operated by a diaphragm 34 open to the pressures on opposite sides of the restrictor 28.