Patent GB-1308304-A (GB)
Title: Fluid pressure actuated device and a gas turbine fuel control  system incorporating such device
IPC: F02C9/26
Filed: 1969-04-11
Abstract: 1308304 Gas turbine engine fuel systems JOSEPH LUCAS (INDUSTRIES) Ltd 13 July 1970 [11 April 1969] 18741/69 Heading FIG A fluid-pressure actuated device 21, Figs. 1 and 2, for a gas turbine engine fuel system comprises bellows 29, 30 subjected externally to an air pressure P3 (compressor interstage pressure) via an inlet 24, bellows 30 being evacuated while bellows 29 is subjected internally to an air pressure P4 (compressor discharge pressure) via an inlet 25. Both bellows are connected at one end to a pivoted lever 31, bellows 29 being secured at its other end to the housing 22 while bellows 30 is connected at its other end to a pivoted lever 32. A pivoted lever 33 is connected to the lever 32 by a link 35 and is engaged by an abutment 34 carried either by the lever 31 or, as shown, by the bellows 29. The effect of this arrangement is that when the ratio of P4 to P3 exceeds a certain value, the torque applied to the lever 33 is a function of P4 and P3, but when the ratio falls below that value, the lever 31 comes against an abutment 36 and thereafter the torque is a function of P3 alone. The torque is opposed by a torque applied to the lever 33 by a piston 37 acting through a connecting rod 38. The piston force is provided by the pressure drop through an orifice 39 which forms, with an orifice 40, a fuel potentiometer connected between a high pressure fuel line 14 and a low pressure fuel line 52. The effective area of the orifice 39 is dependent on the proximity of the piston 37 to the base of the bore in which it slides, the piston and the lever 33 reaching an equilibrium position in which the pressure at the junction between the orifices 39, 40 produces the required counterbalancing torque on the lever. A further pressure is derived from this pressure by a further fuel potentiometer comprising a fixed orifice 50 and a variable orifice 46, the further pressure being applied via a line 51 to one side of a spring-loaded piston 19 which is subjected on its other side to the low pressure in line 52. The piston 19 controls the fuel metering valve 10. The pressure drop across the metering valve 10 is controlled by a throttle valve 17 connected in series with it and subjected to the pressure drop and also to the action of a governor 18. The variable orifice 46 is controlled by a blade carried by a pivoted lever 44 acted on directly by a bellows 41 and indirectly by a bellows 42 via a pivoted lever 43 and a roller unit 45 carried by a longitudinally movable throttle link 26. Both bellows are subjected externally to the compressor inlet pressure P1 via an inlet 23, bellows 42 being evacuated and bellows 41 being subjected internally to the pressure P4. The lever 44 is in equilibrium for a valve of the ratio of P4 to P1 which is dependent on the position of the throttle link 26. During steady running, the fuel flow is determined mainly by the ratio of P4 to P1, whereby there is a specific engine speed, at any given altitude, for each setting of the throttle link 26. During rapid acceleration or deceleration the lever 44 engages a stop 53 or 54, respectively, and the fuel flow is then determined by the relationship between P3 and P4. The bellows 29, 30, 41, 42 may be replaced by diaphragm units or piston-and-cylinder units.