Patent GB-812135-A (GB)
Title: Improvements in or relating to fuel control systems for aircraft propulsion plants
Inventor: HOOKER STANLEY GEORGE
IPC: F02K3/00
Filed: 1955-05-10
Abstract: 812,135. Gas-turbine plant. BRISTOL AERO-ENGINES Ltd. Aug. 10, 1956 [May 10, 1955], No. 13540/55. Class 110 (3). [Also in Group XXIX] A fuel control system for a two-spool turbo-jet engine comprising a fuel metering means adjustable by a throttle lever has a first speed governor responsive to the speed of rotation of the high-pressure spool to act on the fuel metering means to reduce the fuel supply when the speed of the high-pressure spool approaches its take-off value, a second speed governor responsive to the speed of rotation of the low-pressure spool to act on the fuel metering means to reduce the fuel supply when the speed of the lowpressure spool approaches its cruise value and overriding means to prevent a reduction of the fuel supply by the second governor when the throttle lever is in a position corresponding to a power in excess of the cruise power rating of the engine. A two-spool engine is defined as one in which a low-pressure compressor, a highpressure compressor, heating means, a highpressure turbine, coupled to the high-pressure compressor and a low-pressure turbine, coupled to drive the low-pressure compressor, are coaxially arranged and in sequence. The low and high-pressure compressors absorb substantially the whole of the power output of the low and high-pressure turbines respectively. In the arrangement shown, the pumps 18, 19 which are of the type described in Specification 577,016, [Group XXIX], are driven from the low and high-pressure turbine rotor shafts respectively. The output of each of the pumps is controlled by a spring-loaded servo-piston 23 the position of which is determined by the bleed 38. The bleed 38 is actuated by a diaphragm 37 loaded on one side by a spring 40 and the pressure generated by the fuel flow through a radial bore 33 in the pump rotor 31 and on the other side by the inlet pressure of the pump. When the pump and its associated rotor reaches a predetermined speed the bleed 38 is opened and the output of the pump reduced. The cylinders 24 of the two pumps are connected by conduits 59 and 61. The two pumps discharge through non-return valves 48, 49 and conduits 50, 51, 52 to a control unit 53 comprising a throttle valve and a barometric control. The barometric control comprises a capsule enclosed in a chamber which is connected by a pipe to the engine intake. This capsule controls a bleed opening on the righthand side of the pistons 23 through pipe 59, 60, 61. Opening or closing of the throttle valve by the throttle lever 54 varies the discharge pressure of the pumps which pressure acts on the pistons 23 to vary the delivery of the pumps. The diaphragm 37 of the pump 18 is adjusted by means of the spring 40 to operate its bleed 38 when the pressure in the space 36 attains a value corresponding to the cruise rotational speed of the low-pressure rotor. The diaphragm 37 of the pump 19 is adjusted so that its bleed opens when the pressure in the space 36 attains a value corresponding to the take-off rotational speed of the highpressure rotor. Actuation of either of the bleeds 38 causes a resetting of both pumps. To prevent operation of the diaphragm 37 of the pump 18 when. the throttle lever is in a setting corresponding to a power output in excess of the cruise power rating of the engine, a valve 65 operated by a cam 66 is provided. The valve is opened by the cam so that the space 36 is connected to the inlet of the pumps so that the pressure in the space 36 is relieved. A constriction 70 is provided to prevent build up of pressure in the space 36 when the low-pressure rotor approaches its take-off rotational speed. In a modification, arrangements are made to prevent the low-pressure rotor exceeding its take-off rotational speed. In this arrangement, Fig. 3 (not shown), the pressure rise across the rotor of the low-pressure pump 18 is applied to two diaphragms both of which control leak-off valves from the cylinders 24. One of these diaphragms is set to actuate its leak-off valve when the low-pressure rotor rotational speed approaches that corresponding to cruising. The other operates when this speed approaches that corresponding to take-off. A valve is provided which shuts off the first-mentioned leak-off valve. This valve is actuated by a cam connected to the pilot's control lever. Reference has been directed by the Comptroller to Specification 620,625.