Patent Publication Number: US-10309357-B2

Title: Fluid injector

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     Present disclosure relates to a fluid injector which is in particular operable to inject fuel into a combustion engine, especially in a motor vehicle. 
     A fuel injector for injecting fuel into a combustion engine comprises a valve assembly for controlling a flow of fuel into the engine and an actuator for operating the valve assembly. The actuator is of the solenoid type and comprises a coil that is wound around a longitudinal axis of the injector and an armature that is axially movable with respect to the coil. When the coil is energized by an electrical current, a magnetic field is generated that moves the armature in an axial direction. In response to the movement, the valve assembly opens and permits a predetermined flow of fuel into the engine. 
     Due to imperfections of the magnetic field, the force exerted onto the armature is not purely axial but may also have a radial component. The radial force may push the armature against an encasement where friction is generated. Among the disadvantages that come with such friction are an early wear, an increase of the time the valve assembly is opened, lowered injection repeatability, a lowered maximum operative pressure, a spray instability or static and dynamic flow shift over lifetime. 
     To overcome these problems, narrow tolerances may be used to prevent a radial movement of the armature. Alternatively, a radial air gap between armature and encasement may be introduced to reduce the fluctuations of the magnetic force. However, narrow tolerances may lead to high production cost and the radial air gap may not be sufficient to stabilize the armature, especially when the engine is coming through heavy vibrations as may be experienced under normal operating conditions. In addition, the air gap will lose its effect once the armature is moved by a certain amount in a radial direction. 
     U.S. Pat. No. 4,313,571 A shows an electromagnetically actuated injector for an internal combustion engine. A diamagnetic material is used between adjacent elements of the actuator as a ware-resistant material. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of present invention to provide an injector with reduced radial forces onto the axially movable armature of an actuator of the solenoid type. This object is achieved by a fluid injector having the features of the independent claim. Advantageous embodiments and developments of the fluid injector are specified in the dependent claims, in the following description and in the figures. 
     According to the invention, a fuel injector for a combustion engine comprises a tubular body. The tubular body in particular hydraulically connects a fluid inlet end of the injector to a fluid outlet end of the injector. For example, the tubular body is a valve body of the injector. 
     The fuel injector further comprises a magnetic core affixed inside the body. In particular, the magnetic core is affixed to the tubular body by means of a friction-fit connection with the tubular body. 
     In addition, the fuel injector comprises a solenoid on the outside of the tubular body. The solenoid may comprise a bobbin around which the turns of the solenoid are wound. Additionally, an axially moveable armature is arranged inside the tubular body. 
     The fuel injector has a valve assembly for controlling a fluid flow, in particular an axial flow, of fuel through the tubular body and comprising a valve needle. The valve needle is configured to be operated by the armature. It interacts in particular with a valve seat at the fluid outlet end of the fluid injector to control the fluid flow. The valve seat is preferably comprised by the tubular body or by a seat element which is inserted into an opening of the tubular body at the fluid outlet end. 
     Further, the fuel injector comprises a sleeve of diamagnetic material. The sleeve is located radially between the armature and the body. Preferably, the sleeve and the armature overlap axially. 
     A diamagnetic material has the property to create a magnetic field in opposition to an externally applied magnetic field. Mounted in a radial direction of the armature, the diamagnetic sleeve may reduce the radial forces of the magnetic field created by the solenoid. This way, the armature may move more freely in an axial direction, i.e. friction and/or wear may be particularly small. This way, the injector may have an increased lifetime, production cost may be lowered as allowable tolerances may be increased, the repeatability of the opening and closing characteristics of the valve assembly may be increased, the flow spray stability may be improved, the injector may be operated at a higher fuel pressure, and/or static and dynamic flow shift over lifetime may be reduced. 
     In contrast to other means for centering the armature, the diamagnetic sleeve will create an increasing force biasing the armature away from the tubular body, the closer the armature comes to the body. Therefore, a stable equilibrium is created where the armature is particularly well centred in the middle of the sleeve. 
     Preferably, the mass and magnetic susceptibility of the sleeve are chosen such that the radial forces on the armature cancel out—or at least essentially cancel out—when the solenoid is energized. That is, the sleeve is dimensioned such that its capacity to create a magnetic field in opposition to an externally applied magnetic field is just as large as or even larger than a radial component of the magnetic field created by the solenoid. This way, radial forces may be truly cancelled out. 
     In a preferred embodiment, the valve needle comprises an armature retainer that extends into a corresponding cavity of the core for axially guiding the valve needle. Due to the diamagnetic space ring centering the armature, the radial force transferred to the valve needle by the armature are particularly small. Thus, with advantage, the wear and/or friction in the region of the armature retainer are particularly small. 
     The material of the armature retainer may be chosen such that it glides freely on the surface of the core. Magnetic or electrical considerations may not be necessary. The bearing of the valve needle inside the injector may thus be precise and smooth. 
     In one embodiment, the valve needle extends axially through the armature, in particular through a central opening of the armature. The armature may be axially displaceable with respect to the valve needle and mechanically coupled to the valve needle by means of the armature retainer. The central opening is in particular dimensioned in such fashion that the valve needle is operable to axially guide the armature. By using the armature retainer and the cavity of the magnetic core as lateral guide, the armature need not have physical contact to the sleeve or the body. 
     The armature retainer may be shaped such that it permits a predetermined tilting of the armature with respect to the core. This may prevent a hyperstatic bearing of the core. It may also permit a certain degree of radial movement of the armature towards or away from a section of the sleeve. As mentioned, the amount of force acting between the sleeve and the armature is dependent on the distance between the two. By permitting a certain degree of tilting it may be easier for the armature to find its radial position of force equilibrium. 
     In one embodiment, the diamagnetic sleeve is affixed to the inner radial surface of the body. For example, the diamagnetic material is applied to the inner radial surface for forming the sleeve. In this case, the tubular body, the sleeve and the armature are preferably dimensioned in such fashion that there is an annular gap between the diamagnetic sleeve and the armature. The annular gap may be an air gap and serve to stabilize the armature. Also, the gap may enable a radial movement of the armature with respect to the sleeve. The term “air gap” in particular refers to the injector without the fluid which it dispenses in operation. In operation of the injector, the annular gap is in particular filled with the fluid. 
     In an alternative embodiment, the diamagnetic sleeve may be affixed to the outer radial surface of the armature. For example, the diamagnetic material is applied to the outer radial surface for forming the sleeve. In this case, the tubular body, the sleeve and the armature are preferably dimensioned in such fashion that there is an annular gap between the diamagnetic sleeve and the body. 
     In one embodiment, the sleeve comprises or consist of at least one diamagnetic material selected from the following group: bismuth, pyrolytic graphites, perovskite copper-oxides, alkali-metal tungstenates, vandanates, molybdates, titanate niobates, NaWO 3 , YBa 2 Cu 3 O 7 , TiBa 2 Cu 3 O 3 , Al x Ga 1 As and Cr, Fe selenides. 
     In one embodiment, the sleeve comprises a polymer having the diamagnetic material suspended therein. This way, characteristics of the sleeve may be designed specifically to the present requirements. 
     In one embodiment, the valve needle is in the shape of a tube which extends axially through the armature, the tube being configured to conduct the fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       An exemplary embodiment of the fluid injector will now be described in more detail with reference to the figures, in which: 
         FIG. 1  shows a longitudinal section view of a portion of a fluid injector according to an embodiment; 
         FIG. 2  shows a magnification of a part of the fluid injector of  FIG. 1 , and 
         FIG. 3  shows a schematic diagram of energy levels of the armatures of different fluid injectors. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a longitudinal section of a fluid injector according to an embodiment of the invention. The fluid injector is configured for controlling a flow of fuel into an internal combustion engine, especially a piston engine for use in a motor vehicle. In other words, the fluid injector of the present embodiment is a fuel injector  100  for an internal combustion engine. It is in particular provided for dosing fuel directly into the combustion chamber of the internal combustion engine. 
     The fuel injector  100  comprises a tubular body  105  that extends along a longitudinal axis  110  for hydraulically connecting a fluid inlet end of the injector  100  to a fluid outlet end of the injector. 
     The fuel injector  100  comprises an actuator assembly comprising a coil which is in particular in the shape of a solenoid  115 , a magnetic core  120  and a moveable armature  125 . The solenoid  115  is arranged radially subsequent to the tubular body  105  on the outside of the tubular body  105 . The solenoid generally comprises a number of turns wound around the longitudinal axis  110 . The solenoid  115  may be affixed to the outside of the body  105 . The magnetic core  120  is arranged inside the body  105  so that it faces the solenoid  115 . The core  120  is magnetic—i.e. in particular it is made from a magnetic material such as a ferromagnetic material, for example from a ferritic steel—and, thus, may help channelling or controlling the magnetic field which is generated when the solenoid  115  is energized by supplying an electrical current that flows through the turns of the solenoid  115 . The armature is arranged inside the tubular body  105  axially adjacent to the magnetic core  120  and in particular downstream of the magnetic core  120 . The armature  125  is axially displaceable in reciprocating fashion along the longitudinal axis  110  with respect to the tubular body  105  and the magnetic core  120  which is positionally fix with respect to the latter. The armature  125  is also made of a magnetic material such as a ferritic steel so that it will be attracted by the magnetic core  120  when the solenoid  115  creates a magnetic field. 
     The fuel injector further comprises a valve assembly  130 . The valve assembly  130  comprises a valve needle  135 . Expediently, it further comprises a valve seat (not shown in the figures) which cooperates with the valve needle to prevent fluid flow from the fluid injector in a closing position of the valve needle  135  and enables dispensing of fluid from the fluid injector through one or more injection holes in further positions of the valve needle. Such a valve assembly is also useful for any other embodiment of the fluid injector. 
     The armature  125  is connected to a valve assembly  130  via the valve needle  135 . In particular, the armature  125  is mechanically coupled to the valve needle so that it is operable to displace the valve needle  135  away from the closing position. It is preferred that the valve needle  135  is hollow such as to permit a flow of fuel parallel to the longitudinal axis  110  towards the valve assembly  130 . The valve needle  135  may especially include a tube that runs axially through the armature  125 . 
     In the present exemplary embodiment, the armature  125  is axially displaceable with respect to the valve needle  135 . Relative axial displacement of the armature  125  and the valve needle  135  is limited by an armature retainer  140  which is comprised by the valve needle  135 . The armature retainer  140  may be fixed to the tubular shaft of the valve needle  135  as in the present embodiment. Alternatively, the armature retainer  140  may be in one piece with the shaft of the valve needle. By means of interaction with the armature retainer  140 , the armature  125  is operable to take the valve needle  135  with it when moving in axial direction towards the magnetic core  120 . 
     The armature retainer  140  extends into a corresponding cavity  145  of the magnetic core  120  in the present embodiment. The member  140  will be discussed in more detail below with respect to  FIG. 2 . 
     It is furthermore preferred that a first elastic member  150  is configured to press the valve needle  135  in a direction away from the core  120 , which is in particular equivalent with an axial direction towards the valve seat. In other words, the first elastic member  150  is configured to bias the valve needle  135  towards the closing position. By means of mechanical interaction via the armature retainer  140 , the armature  125  is also biased in axial direction away from the magnetic core  120  by the first elastic member  150 . Thus, the armature  125  may move away from the core  120  when the solenoid  115  is not energized. In one embodiment, a second elastic member  155  exerts an opposing force from the opposite side of armature  125  to force the armature against the armature retainer  140  and/or to decelerate a movement of the armature with respect to the valve needle  135  in direction away from the magnetic core  120 . 
     The injector  100  may be configured for a fuel flow that starts in an upper part of  FIG. 1  and extends along the longitudinal axis  110  into the core  120 , through the first elastic member  150 , into the valve needle  135  and to the valve assembly  130 . From there, the fuel may be injected into a combustion engine when a current flows through the solenoid  115 , so that the armature  125  is moved up axially against the core  120 , thereby opening the valve assembly  130  through a valve needle  135 . 
     A rectangle with broken line shows an area of  FIG. 1  that is presented magnified in  FIG. 2 . 
     In an upper area of  FIG. 2  it can be seen that the armature retainer  140  fits snugly in the cavity  145  of core  120 . In this way, the armature retainer  140  cooperates with the magnetic core  120  to guide the valve needle  135  axially. The tube of the valve needle  135 —which extends through a central opening in the armature  125 —may in turn cooperate mechanically with the armature  125  for axially guiding the armature  125 . 
     It is preferred that friction between the member  140  and the core  120  is low. Materials, especially of member  140 , may be chosen accordingly. It is furthermore preferred that a radially outer surface of member  140  is spaced from the cavity  145  so that a certain degree of tilting between the valve needle  135 —and consequently the armature  125 —and the core  120  may take place. 
     A sleeve  205  is mounted radially between the tubular body  105  and the armature  125 . Preferably, the sleeve  205  extends at least partly into the area of the solenoid  115 . In other words, the sleeve  205  or a portion of the sleeve  205  may be circumferentially enclosed by the solenoid  115 . The sleeve  205  comprises or consists of a diamagnetic material, the diamagnetic material being for example selected from the group consisting of bismuth, pyrolytic graphites, perovskite copper-oxides, alkali-metal tungstenates, vandanates, molybdates, titanate niobates, NaWO 3 , YBa 2 Cu 3 O 7 , TiBa 2 Cu 3 O 3 , Al x Ga 1 As and Cr, Fe selenides. The sleeve  205  may also comprise a polymer having a diamagnetic material as one of those mentioned above suspended therein. 
     The diamagnetic sleeve  205  per definition has a magnetic susceptibility that is negative. In reaction to an external magnetic field, the diamagnetic material of sleeve  205  generates another magnetic field of opposite direction. As the sleeve  205  is disposed laterally to the armature  125 , i.e. it extends circumferentially around the armature  125 , it may help to reduce or cancel out a radial portion of the magnetic field generated by the solenoid  115  in the region of the armature  125 . 
     When the solenoid  115  is energized, its magnetic field generates an axial force  210  which pulls the armature  125  along longitudinal axis  110  towards the magnetic core  120  which sometimes is also denoted as a “pole piece”. However, a portion of the magnetic field may induce a first radial force  215 . The radial force may act in a radial direction which may not be predictable at the time of assembling the injector and may vary from injection event to injection event, and therefore may be hard to balance. Thus, wear and/or friction may be caused in conventional injectors by this radial force. 
     However, in case of the injector  100  according to the present embodiment, the same radial component of the magnetic field passes through the sleeve  205  in which an opposing magnetic field is created, exerting a second radial force  220  onto the armature  125  in opposite radial direction. Ideally, the radial forces  215  and  220  cancel themselves out. 
       FIG. 3  shows a schematic diagram  300  of energy levels of the armatures  125  of different fuel injectors. In a horizontal direction, a displacement of armature  125  in a radial direction x is displayed. In a vertical direction, energy E of the armature  125  is shown. The higher the energy of armature  125  is, the stronger a residual force onto armature  125  in a radial direction may be. 
     A first point C symbolizes the conditions in a standard injector in which no further means are taken for radial stabilization of the armature  125 . It can be seen that the armature  125  is in an unstable equilibrium state. A small displacement may lead to effective forces that increase the displacement. 
     A second point A shows circumstances on a conventional injector  100  with radial air gap. For small radial displacements of armature  125  the energy level remains constant. However, if the armature  125  is moved in a positive x-direction far enough, the movement is increased. Point A represents an indifferent equilibrium state. 
     In contrast, point B represents a stable equilibrium state. This represents the configuration of the injector  100  discussed above with respect to  FIGS. 1 and 2 . Through the use of diamagnetic sleeve  205 , both a positive and a negative displacement of armature  125  in a radial direction will lead to an increasing counterforce that moves it back onto longitudinal axis  110 . Thus, the radial position of armature  125  is kept stable.