1,041,214. Gas turbine engine fuel systems. PLESSEY CO. Ltd. July 18, 1963 [May 9, 1962], No. 17904/62. Heading FIG. [Also in Division G3] In a fuel system for a gas turbine engine in which fuel is delivered by an engine driven fixed displacement pump at a rate proportional to the engine speed and is arranged to pass on its way to the combustion chamber, so long as the engine speed remains below a pre-determined value, through an orifice of constant area, means being provided for diverting the excess fuel at a suitable part of the circuit, at such a rate as to keep the pressure drop across this aperture proportional to the difference between compressor outlet and inlet pressures, the crosssectional area of the orifice being increased when the predetermined speed is exceeded. In Fig. 2 fuel is supplied from a source thereof through duct 1 to an engine driven gear pump 2 from which it is delivered into chamber 4. A fixed bush 10 is provided in the fuel control unit and within a bore in the bush is axially slidably and rotatably mounted on hollow sleeve 8, the sleeve having apertures 9 at its lower end through which fuel may flow from the chamber 4, and apertures 14 at its upper end which co-operate with an annular port 11 in the fixed bush 10, the port 11 communicating with bores 13 which in turn communicate with the fuel discharge duct 12, 12a. A governor device comprising fly-weights 6 is mounted on the platform 5 carried by the engine driven shaft 3, the fly-weight levers acting to move the sleeve 8 upwardly against the loading of a control spring 16, the loading of the spring being adjustable by means of a trim device 20. At low engine speeds, the spring 16 acts to keep the sleeve 8 in its lowermost position against the stop 15 so that only a small area of the apertures 14 will be open for fuel flow. A by-pass duct 21 controlled by a variable restrictor 22 is provided so as to allow a minimum fuel flow to the combustion chamber. A valve member 31 is mounted for axial and rotational movement in a bore 25 and has a port 23 which co-operates with a port 24 in the control unit so as to vary the effective area of a by-pass duct 26 by which fuel may pass from the chamber 4 back to the inlet duct 1. The valve member 31 is acted upon at its lower end by the pressure of fuel within the chamber 4 and at its upper end by pressure of fuel downstream of the variable orifice 11, 14 so that its axial position and so the effective area of the by-pass orifice 23, 24 is dependent on the pressure drop across the orifice 11, 14. The axial position of the valve member 31 is also dependent on the pressure rise in the engine compressor i.e. on (P 2 -P 1 ) by means of a diaphragm 28 which is subject at its upper face to compressor discharge pressure P 2 and at its lower face to compressor intake pressure Pi. At low engine speeds the fuel flow will be proportional to A 1 #P 2 -P 1 where A 1 is the combined cross-sectional area of the variable orifice 11, 14 and of the by-pass duct 21. When the engine speed reaches such a value that the centrifugal effect of the fly-weights 6 is balanced by the loading of the spring 16, the effective area of the orifice 11, 14 will be a maximum and the fuel flow is now proportional to A 2 #P 2 -P 1 where A 2 is the combined crosssectional area of the orifice 11, 14, i.e. the maximum value thereof, and of the by-pass duct 21. As the normal speed is approached, the variable orifice 11, 14 will begin to close and the fuel flow will be reduced so that at normal speed fuel flow is just sufficient to maintain that speed. In Fig. 3 (not shown) the sleeve 8a is extended and the ports 14 at the upper end thereof co-operate with a port 37 in a cylindrical member 38 to form a variable area metering orifice. The member 38 is urged upwardly against a stop member 39 by a spring 40 but it may be moved downwardly by means of a diaphragm 42 which may be loaded at its upper face by pressure of fuel within chamber 4 acting through duct 46. The lower face of the diaphragm 42 is subject to fuel pressure within the chamber 41 i.e. the fuel pressure downstream of the orifice 14, 37. The upper face of the diaphragm 42 may alternatively be subject to this last-mentioned pressure by way of the duct 44, the diaphragm 42 then being ineffective and the unit operating similarly to the previous embodiment, the member 38 being in fact fixed. When the diaphragm 42 is effective and the member 38 axially movable, the unit operates as an isochronous fuel governor. The sleeve 8a is formed with a central land 49 and with axially-extending grooves 50 and 51 at either side thereof. When operation as an isochronous governor is required the set-screw 47 is opened and set-screw 45 closed. When the governor weights 6 are in the normal position, the sleeve 8a is in the axial position shown in which the land 49 closes the bores 52 which communicate with the duct 46 by way of the annular groove 48. If the engine speed then increases the sleeve 8a moves upwardly and fuel can then pass from chamber 4 through grooves 50, bores 52, groove 48 and duct 46 to the chamber 43 at the upper side of the diaphragm 42 so that the member 38 moves downwardly so that the combined upward movement of the sleeve 8a and so of the aperture 14 and downward movement of the member 38 and aperture 37 will act to reduce the fuel flow to the outlet duct 12.