Patent GB-1084925-A (GB)
Title: Gas turbine fuel control systems
IPC: G05D13/00
Filed: 1966-06-30
Abstract: 1,084,925. Fluid-pressure servomotor systems. GENERAL MOTORS CORPORATION. June 30, 1966 [July 26, 1965], No.29379/66. Heading G3P. [Also in Division F1] A fuel control system for a gas turbine engine comprises an inlet for fuel under pressure, an outlet for fuel to the engine, a speed governorcontrolled fuel regulating valve, a fuel limiting valve connected in series with the regulating valve between the inlet and the outlet, there being means for maintaining a constant pressure drop across the said regulating and limiting valves, also means responsive to a condition indicative of flow through the regulating valve as an indication of engine speed for variably supplementing fuel flow to the engine to maintain isochronous operation. In Fig. 1 fuel is supplied from the tank 5 by pump 6 though line 23 in which are disposed a governor valve 7 and a flow limiting valve 8, the fuel passing through line 10 to the engine. The governor valve 7 is controlled by engine speed responsive means N and the limiting valve by means responsive to compressor discharge pressure CDP, the valve 7 controlling the fuel supply during steady state operation, and the valve 8 during acceleration. The pressure drop across the valves 7 and 8 is maintained constant by pressure regulating valve 21. A fuel flow meter 29 of Venturi type is disposed in the line 27 between the governor valve 7 and the regulating valve 8, pressure at the inlet of the Venturi being communicated through line 30 to the left hand side of diaphragm 37 and that at the throat of the Venturi being communicated through line 33 to the right hand side of diaphragm 38, the diaphragms forming part of a speed error detector 31 which comprises also a pilot spool valve 35 mounted on a rod connected at its ends to the diaphragms. An adjustable spring 39 also bears against the right hand side of the diaphragm 38. The spool valve 35 controls flow of fuel used as servofluid from line 40 to either of the lines 43, 44 which lead to an integrator 45. For example movement of the spool valve 35 to the right will cause servofluid to pass through line 43 to the chamber at the left hand side of diaphragm 51 of the integrator while fluid in the chamber at the right hand side of diaphragm 52 will pass through line 44 to the chamber at the right hand side of diaphragm 37 and thence through line 49 back to the tank 5. The integrator 45 embodies a fuel metering valve 57 connected in parallel with the metering valves 7 and 8. The chambers at the inner sides of the diaphragms 51 and 52 are connected by line 54 to the outlet of the pump 6. The metering valve 57 is formed by a contoured portion of rod 55 which connects the two diaphragms 51, 52, the valve controlling flow of fuel from the line 54 into the chamber 61 and thence through line 62 into line 10 and to the engine. Valve 63 in the line 62 is normally open but is closed during starting and acceleration. In operation fuel flow is measured by the Venturi 29 and the error detector 31 moves the spool valve 35 in response to any flow error and thus in response to any speed error. Thus movement of the spool valve to the right causes the stem 55 to move to the right thus opening the valve 57 to cause an increase in fuel flow to the engine. The supply of fuel through the correcting valve 57 is thus that which is required to keep the engine on speed and the valve 57 provides isochronous governing as a supplement to the proportional governor valve 7. In Fig.2 the flowmeter 29 and the speed error detector 31 are dispensed with, fuel upstream of the governor valve 7 being supplied to the integrator 45 through line 41 to the inner side of the diaphragm 51 also to the valve 57, while fuel from downstream of the governor valve is supplied through line 30 to the outer side of diaphragm 51 also to the inner side of diaphragm 52. The line 62 containing the normally open valve 63 is connected to the outer side of the diaphragm 52 also to the chamber 61. The valve stem 55 is thus biased to the left by the pressure drop across the governor valve 7 and to the right by the pressure drop across the compressor discharge pressure controlled valve 8. If the engine runs over or under-speed, the opening of the valve 7 will be varied so that the diaphragms 51, 52 will move the valve 57 to connect fuel flow to the engine. The pressure drop across the valves gives a measure of fuel flow to the engine. In Fig. 3 a governor controlled valve 7 is provided in series flow with a CDP responsive valve 8 also a normally closed valve 70, a normally open valve 72 and a fixed orifice 73 being provided in parallel with the valve 8 and normally closed valve 70 but in series with the governor valve 7. The pressure differential across the governor valve 7 is applied to the diaphragm 78 of the integrator 75, the diaphragm being connected to a movable valve member 77 which is also supported at its opposite end by a diaphragm 79. The pressure differential across the CDP valve 8 and valve 70 or across the valve 72 and fixed orifice 73 through whichever fuel flows, is communicated to the diaphragm 83, the diaphragm being spring-loaded at its inner side and carrying a stem 81 which has a contoured portion 86 which co-operates with wall 87 of the movable valve member 77 to constitute an integrating valve. Fuel at pump discharge pressure in line 23 passes through line 41 into the chamber 89, through openings 90, valve 86, 87, openings 91 thence through line 62 containing normally open valve 63 to line 10 and thus to the engine. The pressure differential across the fixed orifice 73 acts to position the valve stem 81 as a direct function of fuel flow through the governor valve 7 while the diaphragms 78 and 79 are responsive to the differential of the pressure drops across the governor valve 7 and the fixed orifice 73 to position the valve body 77 to a position to balance the pressure drops. The pressure drop across the fixed orifice 73 is arranged to be equal to that across the governor valve 7 at normal speed. The effective area of the diaphragm 78 is twice that of the diaphragm 79.