Patent Publication Number: US-2020300472-A1

Title: Fuel injector for a turbine engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to French Patent Application No. 1902846, filed Mar. 20, 2019, which is incorporated herein by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a fuel injector for a turbine engine, particularly for an aircraft turbojet engine or a turboprop engine. 
     PRIOR ART 
     A turbine engine conventionally comprises an annular combustion chamber with evenly distributed fuel injectors at its upstream end and means for supplying air around the injectors. There are mainly two types of injectors, namely the so-called aeromechanical injectors with two fuel circuits offering fuel flow rates adapted to different operating phases of the turbine engine, (the ignition phase, the low or full power operating phase), and the so-called aerodynamic injectors which have only one fuel circuit for all the operating phases of the turbine engine. 
     The patent application FR 2 832 492, in the name of the Applicant, describes an aeromechanical type injector, comprising a primary fuel circuit intended, for example, for an ignition and low-power phase, and a secondary circuit for subsequent medium- to high-power phases of operation, in addition to the primary circuit. 
     This type of injector comprises a body comprising pressurized fuel intake means, a shut-off valve mounted in the body downstream of the intake means and designed to open under a first determined fuel pressure and to remain open above this first pressure in order to supply a primary fuel circuit, and a metering valve mounted in the body downstream of the shut-off valve and designed to open beyond a second determined fuel pressure, higher than the first pressure, and to remain open beyond the second pressure in order to supply a secondary fuel circuit. 
     The fuel flow rate in the secondary circuit is controlled by means of metering slots provided in the metering valve, the flow cross-sections of which vary according to the position of the metering valve, i.e. according to the fuel supply pressure. The higher the fuel supply pressure, the larger the passage cross-sections of the slots. 
     The metering valve is subject to the action of an elastic return spring which tends to return the valve to a closed position. The spring is a helical spring of a conventional structure, i.e. formed by a single metal wire of a generally helical shape, the ends of which extend in a plane generally perpendicular to the spring axis. 
     Due to the buckling stress and friction generated by such a spring, the compression and expansion curves of the spring are significantly different from one another, creating a significant hysteresis phenomenon. The compression or expansion curve represents the variation of the return force exerted by the spring as a function of the length of the spring, during its compression or expansion. This leads to fuel flow rate or pressure characteristics which are different in the opening or closing phase of the metering valve, which generates heterogeneity between different injectors having the same structure within the same turbine engine, or wear by friction of the metering valve or of the elements surrounding said valve. 
     DISCLOSURE OF THE INVENTION 
     The invention aims to remedy such drawback in a simple, reliable and inexpensive way. For this purpose, the invention relates to a fuel injector for a turbine engine, comprising a body having a fuel inlet opening into an upstream chamber, and a fuel outlet connected to a downstream chamber, a metering valve being mounted between the upstream chamber and the downstream chamber, said valve being subjected to the action of an elastic return spring tending to return the valve to a closed position, the spring and the valve being designed to allow the opening of the valve and to allow the passage of fuel from the upstream chamber to the downstream chamber, above a given fuel pressure in the upstream chamber, the return spring extending along the axis of movement of the valve, characterized in that the spring has a first axial end and a second axial end which are annular and which are connected to one another by at least two helical parts which are elastically deformable in the axial direction. 
     The terms “axial” and “radial” are defined relative to the axis of movement of the valve, which corresponds to the axis of movement of the spring. 
     Such a spring structure limits the buckling force and the radial deformation of the spring during operation, thus limiting friction and the resulting hysteresis. This ensures a better operation of the injectors. 
     The number of helical parts can be equal to or greater than two, for example equal to three or four. 
     The helical parts can be evenly distributed around the circumference. 
     The helical parts may have the same diameter, which helps to distribute the forces during operation, so as to avoid uneven deformation or buckling of the spring. Such a feature also reduces the radial dimensions of the spring. The number of turns may range from 2 to 6, preferably 2 to 3. 
     The number of turns may or may not be an integer. The number of turns ranges from 3 to 4. 
     The spring can be made in one piece from a metallic material, e.g. steel, titanium or a titanium alloy. 
     This angle is dependent, among other things, on the number of turns and the axial distance of the spring. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       [ FIG. 1 ] is a cross-sectional axial view of an injector according to one embodiment of the prior art, 
       [ FIG. 2 ] is a front view of an injector according to one embodiment of the invention. 
     
    
    
       FIG. 1  shows an injector  1  according to the prior art. The latter comprises a body  2  comprising a main part  2   a  extending along an axis X and intended to be fixed by means of screws  3  to a stationary part  4  of the turbine engine, and an injection part  2   b  extending perpendicularly to the axis X, from a so-called lower end of the main part  2   a.  The terms high and low are defined in relation to  FIG. 1  and are not necessarily related to the actual orientation of the injector  1  in the turbine engine. 
     The main part  2   a  of the body  2  comprises a recess delimiting an upstream chamber  5  into which a fuel inlet  6  opens, and a downstream chamber  7  connected to a fuel flow channel  8  opening at a fuel outlet  9 . 
     The fuel flow channel  8  has an axial part  8   a  opening upwards into the downstream chamber  7  and a radial part  8   b  extending substantially perpendicularly to the axis X, opening outwards from the injector at the outlet  9  so as to form an injection nozzle. 
     The upstream chamber  5  and the downstream chamber  7  are separated from one another by a metering valve  10  which is movable between a closed position (visible in  FIG. 1 ) in which it is biased upwards by a helical compression spring  11 , and an open position in which it is moved downwards when the fuel pressure in the upstream chamber  5  is higher than a determined pressure. In this case, the pressure in the upstream chamber  5  exerts an axial force in the downward direction onto the metering valve  10  against the axial return force exerted by the spring  11 . 
     The spring  11  has a first, lower, axial end  11   a  and a second, upper, axial end  11   b.  The first axial end  11   a  rests on a stationary element  12  mounted in the body  2  and forming the seat of the valve  10 , in particular of the lower end  13  of the valve  10 . The second axial end  11   b  of the spring  11  is supported by an annular collar  14  of the valve  10 . The spring  11  consists of a single metal wire which is generally helical in shape and whose axial ends  11   a,    11   b  generally extend in radial planes. 
     The lower part of the valve  10  has slots  15  whose geometries are such that the passage sections between the surfaces delimiting the slots  15  and the element  12  vary according to the axial position of the metering valve  10 . 
     As mentioned above, the structure of such a helical compression spring  11  generates buckling stress and friction creating a hysteresis phenomenon that should be avoided. 
     For this purpose, the invention proposes to replace the spring  11  described with reference to  FIG. 1  by a spring  11  whose structure is shown in  FIG. 2 . This spring has a first axial end  11   a  and a second axial end  11   b  which are annular and are connected to one another by at least two helical parts  11   c,    11   d  which are elastically deformable in the axial direction. In the case of  FIG. 2 , the number of helical parts  11   c,    11   d  is equal to two. 
     The helical parts  11   c,    11   d  are evenly distributed around the circumference, i.e. diametrically opposed in pairs in the case of an even number of helical parts. 
     The number of turns of each helical part  11   c,    11   d  is between 2 and 6, e.g. 3.5 turns in the case of  FIG. 2 , preferably between 2 and 3. The spring  11  is made in one piece from a metallic material, e.g. steel, titanium or a titanium alloy. 
     Each helical part  11   c,    11   d  may have a round or polygonal, e.g. rectangular or square cross-section. 
     The different helical parts  11   c,    11   d  have the same diameter. 
     The axial ends  11   e  of the helical parts  11   c,    11   d  can be connected to the annular ends  11   a,    11   b  of the spring  11  by areas with no break of slope, e.g. by inclined areas  16  or by curved areas  17 , in order to locally limit the mechanical stresses. 
     Such a structure reduces buckling forces and friction so as to reduce hysteresis during operation and thus improve the operation and service life of the injector  1 .