Patent GB-1100339-A (GB)
Title: Gas turbine engine fuel system
Inventor: JOHNSON CHRISTOPHER LINLEY
IPC: F02C9/16
Filed: 1964-08-05
Abstract: 1,100,339. Fluid-pressure servomotor systems. ROLLS ROYCE Ltd. July 29, 1965 [Aug. 5, 1964], No. 31916/64. Heading G3P. [Also in Division F1] A gas turbine engine fuel system comprises a plurality of measuring means for measuring a plurality of engine variables and acting on control means to control the engine fuel supply according to the ratio of the dynamic pressures at first and second spaced apart points in the engine having substantially the same gas mass flow-rate, and according to the value of at least one other gas variable at the first point, said gas variable being chosen so that the gas temperature at the second point is functionally dependent solely upon said ratio and said variable. In a first embodiment, Figs. 2, 3, for a jet propulsion engine, the measuring means sense the air density and dynamic pressure at the compressor inlet 12 and the static pressure at the compressor outlet, which static pressure is a measure of the dynamic pressure at the nozzle guide vanes 15 when the vanes are choked. A function involving the ratio of these dynamic pressures and density is maintained constant during steady running by varying the fuel supply to thus maintain the flame temperature at the guide vanes constant. In this embodiment fuel is supplied to the burners 20 along conduit 23 from a swash plate pump 24. The position of a piston 30 controlling the angle of the swash-plate 26 and the output of the pump is determined by the degree of opening of a leak-off valve 41 co-operating with a valve seat 40 at the end of a conduit 36 which communicates with one side of the piston 30. Valve 41 is at one end of a bell-crank 42, 43 which pivots spring 51 to a force proportional to the square of the compressor outlet pressure p 3  and an opposing force proportional to the dynamic pressure at the inlet to the compressor sensed as the differential between the total and static pressure Pi, P 0 , respectively, at the inlet applied to opposite sides of a diaphragm 133 and transmitted to arm 42 of the bell-crank by a rod 132. Roller fulcrum 44 is movable and its position is adjusted in accordance with variations in density of the air at the compressor inlet measured by sealed bellows 123 within a chamber 124. The combined measurements of inlet air density and dynamic pressure give a measurement proportional to the air mass flow. The mass flow may also be measured electronically. Chamber 124 is mounted in the compressor intake 12 and the bellows 123 contains a reference gas, such as hydrogen, and is fixed at one end 125 to the chamber. A diaphragm 122 seals the other end of the bellows, the periphery of the diaphragm being connected to a sleeve 97 carrying a piston 106 slidable in a cylinder 105. Opposite sides of piston 106 are supplied with high-pressure fuel from conduit 67 through orifices 107, 108 respectively. The centre of diaphragm 122 is connected through a thrust bearing 120 with a servovalve 111 slidable within sleeve 97 and having an annular recess 114 communicating with a vent passage 113. In the equilibrium position recess 114 is positioned intermediate discharge ports 115, 116 in sleeve 97 but does not communicate with either set. If conditions change so that the compressor inlet pressure is not the same as the pressure of the reference gas within bellows 123, diaphragm 122 moves servovalve 111 to allow fuel to leak off from one side of the piston 106 through recess 114 and passage 113. Piston 106 thus moves until recess 114 is once again between and out of communication with ports 115, 116, the movement being transmitted to expand or contract bellows 123 and thus equalize the pressure therein with the inlet pressure. The position of diaphragm 122 is therefore representative of the density of the compressor inlet air and is communicated to movable fulcrum 44 by way of a lever 95 pivoted to sleeve 97 at 96. The desired flame temperature T 4  is set by the pilot's lever 102 which has a roller 101 running in a slot 100 in lever 95, movement of lever 102 causing movement of fulcrum 44. The compressor outlet pressure P 3  is applied to the exterior of a bellows 80 within a chamber 81. A conduit 91 communicates with the interior of bellows 80 and by way of restrictions 86, 87 respectively with the compressor outlet and the static pressure at the compressor inlet such that the pressure P 3 ' within the bellows is about half P 3 . Restriction 86 is arranged to unchoke at the same, or a lower, pressure ratio as the nozzle guide vanes 15 so that adequate fuel flow can be obtained at starting together with a low flame temperature during acceleration. Bellows 80 is anchored at 82 and is connected through bearing 83 to a servovalve 70 slidable within a sleeve 61 integral with a piston 62. Servovalve 70 and piston 62 function in the same manner as valve 111 and piston 106 of the density measuring unit. Both valves 70, 111 are continuously rotated to reduce friction. Movement of piston 62 and sleeve 61 as a result of changes in P 3  is transmitted to a cam 54 by way of an arm 60 on sleeve 61 and linkage 56, 57 so that the force of spring 51 is proportional to P 3 <SP>2</SP>. In a modification, Fig. 4, the flame temperature T 4  at the nozzle guide vanes 155 is maintained constant during steady running by regulating the fuel supply so as to maintain constant a function involving the compressor outlet temperature T 3 , the dynamic pressure at the nozzle guide vanes 155 and the dynamic pressure at a point 198 immediately upstream of the combustion zone. Fuel is supplied to the burners 160 by a centrifugal pump 164, the output of which is controlled by an inlet throttle valve 166 positioned by a piston 170. The position of piston 170 is determined by the leak off of fuel through a variable restriction 81 obturated by a slidable valve rod 182 moved by arm 183 of a bellcrank 182, 184. Rod 182 is continuously rotated to reduce friction. Diaphragm 193 acts on bellcrank 183, 184 to give a measure of the dynamic pressure at guide vanes 155, whereas diaphragm 194 gives a measure of the dynamic pressure at point 198. A number of bleed ducts 205 receive air from compressor outlet, each duct having a throat 206 through which the flow is sonic at most engine speeds. A thermocouple 207 at the throat transmits a signal corresponding to the air temperature T 3  to a device 211 which includes means for advancing the phase of the signal in accordance with the rate of change of T 3 . The degree of phase advance is also controlled in dependence upon the rate of change of engine speed, sensed at 213. Device 211 has a push rod 214 connected by a lever 215 to a push rod 216 which thus positions roller fulcrum 192 in accordance with the phase advance signal. The pilot's lever 224 operates a rack 222 having a roller 220 engaging a slot in lever 215 so as to set the desired flame temperature T 4 . Stops 217, 218 on push rod 216 limit the effect of device 211 on the position of fulcrum 192 during acceleration or deceleration.