Patent GB-985520-A (GB)
Title: Improved gas turbine engine fuel systems
Inventor: ROBINSON KEITH
IPC: F02K3/00
Filed: 1961-10-31
Abstract: 985,520. Gas turbine plant. DOWTY FUEL SYSTEMS Ltd. Oct. 22, 1962 [Oct. 31, 1961], No. 38982/61. Heading F1G. [Also in Division G3] In a gas turbine fuel system which includes an engine-driven fixed positive displacement pump which supplies fuel to a spill burner nozzle provided with a spill passage leading to a low pressure zone, an adjustable spill throttle valve in the spill passage for control of the fuel discharged from the nozzle, and means responsive to compressor delivery pressure to limit closure of the valve during engine acceleration, the closure member of the valve is so shaped in relation to its seat that when the limiting means is operative the expression Q/N is linearly related to the compressor delivery pressure. Q is the fuel flow rate entering the engine and N is the engine speed. In the fuel control unit shown the fuel flow is regulated, in accordance with the equation Q/N=K1.P2ŒK2.P1, where K1 and K2 are constants and P1 and P2 are respectively the compressor inlet and outlet pressures, to provide acceleration without stall. Fuel from a tank passes through a boost pump and a positive displacement pump to spill burner nozzles of an aircraft gas turbine engine. The spill fuel passes to the inlet 8 of the control unit and continues through a non- return valve 11, a passage 12 and an adjustable spill valve 15 to the outlet 9 leading to the inlet side of the positive displacement pump. Air is bled from the compressor delivery through passage 33a, a sharp-edged restrictor 33, a Venturi 34 and passage 34a to the engine nacelle, the pressure between the orifices 33, 34 being applied through a pipe 32 to a bellows 29. When the engine speed is sufficient to cause choking of the Venturi 34 a predetermined fraction of the compressor delivery pressure is applied to the bellows 29, arranged in opposition to an evacuated bellows 28, and applies a force through a pin 31a to a lever 23 pivoted at 27. In addition, the compressor inlet pressure is transmitted through a passage 36a to a bellows 36, opposed by an evacuated bellows 35, and applies a force to the lever 23 through a pin 40. The moment created by each set of bellows is in this case opposed to the other, giving a negative constant K2, but the bellows 35, 36 can be reversed to provide a positive constant K2. The resultant moment is balanced by the moment produced by the resultant force exerted by springs 22 and 24, the latter spring being adjustable by a screw 25. The loading in the springs is balanced by the pressure differential acting on a piston 42 which is determined by the escape flow of fuel through a servo vent 39 controlled by a half ball valve 38, which fuel is supplied from the delivery side of the positive displacement pump through a passage 43, a chamber 43a and a restrictor 44 to the vent 39. Fuel from the chamber 43a also passes through a restrictor 49 to a chamber 48 below the piston 42 and to a vent 46 which is controlled by a diaphragm 45, exposed to the pressure between the restrictor 44 and vent 39, and which leads to the inlet side of the positive displacement pump. Movement of the piston 42 to the equilibrium position moves the spill valve portion 16 which is so shaped in relation to its seat 14 that the expression Q/N is linearly related to its displacement, the latter being linearly related to the loading by suitable selection of the springs. Hence Q/P1.N is linearly related to P2 when the Venturi   34 is choked. Below engine speeds at which choking occurs, the pressure drop across the Venturi 34 is small, and the pressure in pipe 32 remains substantially at nacelle pressure. Q/P1.N is then independent of P2 and remains   constant. The Venturi 34 may be replaced by a sharp-edged restrictor which gives a non-linear relationship between Q/P1.N and P2/P1 below the choking point so as to follow the lower part of the stall curve more closely. A servo vent controlled by a centrifugal or like governor and extended from the servo vent 39 may be provided for constant speed control.