Patent GB-806734-A (GB)
Title: Improvements in gas turbine engine fuel systems
IPC: F02C7/22
Filed: 1955-05-23
Abstract: 806,734. Gas turbine plants. HOBSON Ltd., H. M., GREENLAND, L. S., and MoCOURTY, W. D. March 14, 1956 [May 23, 1955], No. 14792/55. Class 110 (3). [Also in Group XXIX] The fuel supply to a gas turbine engine comprising spill type burners fed by an enginedriven centrifugal pump has two spill valves arranged in parallel in the spill line between the burners and the inlet of the pump, one of which is controlled by a speed governor which is adjustable by the pilot to select a desired engine speed and the other by an altitude responsive device. During normal running of the engine, fuel is supplied by an engine-driven centrifugal pump 20 along a delivery line 58 to a delivery gallery 21 and burners 22. Fuel is returned to the inlet of the pump through a spill gallery 23, a spill line 24, spill valves 25, 26 arranged in parallel and a non-return valve 27. The valve 25 is controlled by the speed governor 15 and the valve 26 by an evacuated capsule 42 subject to ram pressure. The rotational speed of the engine is preset by the pilot's control lever 10. The lever 10 operates a spring-loaded valve 12 which controls the flow of fuel between the ports 14, 16, and thus controls the hydraulic pressure on the servo-piston 17 which loads the spring 18. When the pilot moves the lever 10 to select a higher speed, the piston 17 moves to the right and increases the loading of the spring 18. The spring 18 causes the valve 33 to close and thus increases the pressure on the upper surface of the servo-piston 28 which causes the valve 25 to close and increase the delivery of fuel to the engine. The bobweights 34 actuate the valve 33 to maintain the preset engine speed. To prevent compressor stall during acceleration at low engine speeds, a shaped needle 35 which moves in an orifice 36 is attached to the servo-piston 17. The profile of the needle 35 is such that the rate of movement of the piston 17 depends upon its actual position at any instant. By suitable choice of the size of the orifice 37 and valve 25 or a stop for the servo-piston 28 so that the valve cannot spill the whole of the output of the pump 20, the use of a minimum flow valve is eliminated. The valve 26 is actuated by a servopiston 38 controlled by a valve 41 connected to the evacuated capsule 42. When there is a reduction of ram pressure the capsule 42 expands and increases the opening of the valve 41 which reduces the pressure above the piston 38. The piston 38 then moves upwards and opens the valve 26 until the loading on the spring 40 is balanced by the pressure differential across the piston. The fuel system also includes three cocks 45, 46 and 47 mechanically connected to the pilot's control lever 10. When the lever 10 is in the shut-off position, the delivery and spill galleries 21, 23 are shut off from the system by the cocks 45, 46 and connected together by a drain-pipe 48. The valve 47 is also in the closed position. To start the engine the booster pump (not shown) is energized or pressurized fuel tanks connected to the inlet duct 49. The fuel enters the system through the priming valve 50 and expels air through the de-aerating valve 51. As soon as fuel flows through the restrictor 52, the pressure drop across the restrictor increases and acts on the diaphragm 53 to close the valve 51. The pilot then moves the lever 10 to the ground idling position. The valves 45, 46 are then moved to the position shown with the valve 47 remaining in the closed position. Fuel is then supplied to the gallery 21 and the engine-driven gear pump 54 is then brought into action. The valve 55 operates to limit the amount of fuel delivered to the engine. With the pilot's lever in this position the speed selected by the governor is less than that determined by the valve 55 so that the valve 25 is in the fully open position. This allows of the maximum rate of fuel circulation through the pump 20 and galleries 21, 23. The lever is then moved to the flight idling position shown, in which the valve 47 is opened. As the pilot moves the lever forward from this position, the setting of the governor 15 is varied to increase the fuel supply to the engine. In a modification, the functions of the flow control valve 55 and relief valve 56 of the gear pump 54 are replaced by a single valve. Specification 764,072 is referred to.