Abstract:
A fuel injector for a fuel injection system of an internal combustion engine includes an energizable actuator having a valve closure element actuable by the actuator by a valve needle, which coacts with a valve seating surface to form a sealing fit and which is held in the closed position by a return spring. The actuator acts on the valve needle via a sleeve-shaped needle driver that is separate from the valve needle, which is arranged in an axially movable fashion with respect to the needle driver, and a collar of the needle driver is engageable behind a needle collar of the valve needle at the end facing away from the return spring.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a fuel injector. 
     BACKGROUND INFORMATION 
     A fuel injector is discussed in German Published Patent Application No. 195 34 445. The fuel injector includes a valve housing, configured in two parts, in which a valve needle is guided in axially movable fashion. The valve housing has at one end a fuel connector through which fuel is conveyed to the fuel injector. At the other end, the valve needle coacts with the valve housing to constitute a sealing fit, the valve needle being held in the closed position by way of a return spring. To actuate the valve needle, the latter is equipped at the inflow end with a pressure shoulder which coacts with a piezoelectric actuator and is immovably joined to the valve needle. Upon actuation of the valve needle, the actuator acts against the force of the return spring. 
     Some disadvantages that may be associated with the above fuel injector are as follows: 
     Since the valve needle is immovably joined to the pressure shoulder, and the valve needle is guided at the spray discharge end, and the pressure shoulder at the inflow end, movably in the valve body, a large inert mass, made up of the mass of the valve needle and the mass of the pressure shoulder, must be actuated in order for the fuel injector to be opened and closed by the actuator and the return spring, respectively. In addition, the two guides provided for axially movable guidance of the valve needle—for the valve needle in the spray-discharge end and on the pressure shoulder at the inflow end—must be matched to one another, the result being that production of the fuel injector is relatively complex, and the fuel injector is susceptible to warping or distortion of the valve needle and/or the valve housing. 
     Since the return spring also returns the actuator in order to close the fuel injector, the closing motion of the valve needle is not decoupled from the closing motion of the actuator. 
     As a result of the large mass (made up of the mass of the valve needle and the mass of the pressure shoulder) to be actuated by the return spring, bouncing and therefore unintentional additional spray discharge of fuel occur upon closure of the fuel injector. Another result of this is increased wear on the fuel injector, and thus a shorter service life. 
     In addition, the fact that guidance of the valve needle is rigid and permanently defined means that the position of the valve needle in the sealing fit is permanently defined, the result being that the valve needle cannot center itself on a sealing fit that deviates from the ideal position as a result of production factors or wear. This results in inhomogeneous and increased wear on the valve needle in the region of the sealing fit, in particular in a degradation in the sealing of the sealing fit of the fuel injector in the closed position, and a change in the geometry of the discharged stream of fuel. 
     SUMMARY OF THE INVENTION 
     The fuel injector according to an exemplary embodiment of the present invention is believed to have the advantages that it yields a low-wear, reduced-friction design. The fuel injector is moreover almost bounce-free, so that upon actuation of the fuel injector, the duration of the spray discharge operation and the quantity of fuel discharged can be specified in defined fashion. 
     In an exemplary embodiment, the valve needle is guided in axially movable fashion by a valve needle guide at only one point. In particularly advantageous fashion, the valve needle is small and has low mass. 
     In an exemplary embodiment, the valve needle rests at one of its end faces against a swirl disk. As a result, the valve needle is guided coaxially with respect to the axis of the fuel injector, thus resulting in homogeneous energy transfer by the valve needle onto the sealing fit, and homogeneous wear in the region of the sealing fit. 
     In an exemplary embodiment, the valve needle guide and/or the swirl disk have orifices for the passage of fuel. This yields a simple physical design for passage of the fuel. 
     In an exemplary embodiment, a gap that widens in the radial direction toward the valve axis is formed between the needle collar of the valve needle and the collar of the needle driver. A liquid cushion formed between the collar of the needle driver and the needle collar can thereby be quickly displaced, the result being that the liquid cushion has no influence on switching time and that, in particular, shorter switching times are made possible. 
     In an exemplary embodiment, the needle driver has at least one orifice or bore for the passage of fuel. The interior of the needle driver can thereby serve as a fuel conduit, the fuel being directed out of the interior of the needle driver through the orifice toward the sealing fit. 
     In an exemplary embodiment, the orifice is formed by at least one slit in the needle driver extending in the axial direction. The shape of the orifice is thereby adapted to the flow direction of the fuel. 
     In an exemplary embodiment, the needle driver has an opening with radial enlargements at its end toward the needle collar, which overlap the adjacent end surface of the needle collar to form flow-through windows. The fuel can be passed through the flow-through windows that are created. 
     Alternatively, the needle driver has a circular opening at its end toward the needle collar, and the end surface of the needle collar is of polygonal configuration, so that the end surface of the needle collar is partially overlapped by the opening of the needle driver to form flow-through openings. As a result, no further design changes to the needle driver are necessary, and flow-through openings that are arranged in a manner favorable to flow are created. In addition, any liquid cushion formed between the collar of the needle driver and the needle collar can be rapidly displaced, the result being that the liquid cushion has no influence on switching times and, in particular, that shorter switching times are made possible. 
     In an exemplary embodiment, the return spring is braced, at the end facing away from the needle collar, against an adjusting element, the adjusting element being joined to the needle driver. As a result, on the one hand the return spring can be preloaded in a defined manner that is simple in terms of production engineering. On the other hand, the return spring defines only the closing force of the fuel injector when the fuel injector is closed. The forces required to open and close the fuel injector can then be defined by the actuator and by at least one further spring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a partial axial section through an exemplary embodiment of a fuel injector according to the present invention. 
     FIG. 2 shows a partial axial section through the discharge-end region of an exemplary embodiment of a fuel injector according to the present invention. 
     FIG. 3 shows the portion labeled III of FIG.  2 . 
     FIG. 4 shows a partial axial section through a portion of the fuel injector, two bore-like flow-through openings being provided in the needle driver. 
     FIG. 5 shows a partial axial section through an exemplary embodiment of a fuel injector according to the present invention, slits allowing the passage of fuel being provided between the needle collar of the valve needle and the collar of the needle driver. 
     FIG. 6 shows a partial axial section through an exemplary embodiment of a fuel injector according to the present invention, orifices with radial enlargements being provided in the needle driver. 
     FIG. 7 shows a frontal view of the exemplary embodiment of FIG. 6, in the direction labeled VII. 
     FIG. 8 shows a partial axial section through an exemplary embodiment of the fuel injector according to the present invention, in which the needle collar is triangular in shape. 
     FIG. 9 shows a frontal view of the exemplary embodiment of FIG. 8, in the direction labeled IX. 
     FIG. 10 shows a partial axial section through another exemplary embodiment of a fuel injector according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows, in a partial axial sectioned depiction, a fuel injector  1  according to an exemplary embodiment of the present invention. Fuel injector  1  is embodied here as an inward-opening fuel injector  1 . Fuel injector  1  is used in particular for direct injection of fuel, in particular gasoline, into a combustion chamber of a mixture-compressing, spark-ignited internal combustion engine, as a so-called direct gasoline injector. Fuel injector  1  according to an exemplary embodiment of the present invention is also suitable for other applications, however. 
     Fuel injector  1  has a valve housing  2  that is made up of a front valve housing  3 , a rear valve housing  4 , and a fuel connector  5 . Located in front valve housing  3  is a valve closure element  7 , actuable by way of a valve needle  6 , that in the exemplary embodiment depicted is configured integrally with valve needle  6 . Valve closure element  7  is configured with a truncated conical shape tapering in the spray discharge direction, and coacts with a valve seat surface  9  configured on a valve seat element  8  to constitute a sealing fit. In this context, valve needle  6  is held in the closed position by way of a return spring  10  that acts on valve needle  6  via a valve needle collar  11  of valve needle  6 . Return spring  10  is centered at its end toward needle collar  11  by way of a centering element  12 . In the exemplary embodiment depicted, valve needle  6 , needle collar  11 , and centering element  12  are integrally configured. The very small and low-mass (0.1-0.5 g) valve needle  6  is guided in its axial movement by a single valve needle guide  13 . Valve needle guide  13  rests at its spray-discharge end face against a swirl disk  14 . Swirl disk  14  is mounted in the front portion of valve housing  3  and rests, at its end face opposite valve needle guide  13 , against valve seat element  8 . In order to allow fuel to flow through, valve needle guide  13  and swirl disk  14  have orifices  15   a ,  15   b ,  16   a ,  16   b , orifices  16   a ,  16   b  in swirl disk  14  being configured as swirl channels. 
     Actuation of fuel injector  1  is effected by an actuator  17  that is embodied in piezoelectric, magnetostrictive, or electromagnetic fashion (FIG.  10 ). Actuation of actuator  17  is accomplished via an electrical control signal that is transferred via an electrical connector  18  and an electrical lead (not depicted) to actuator  17 . When actuator  17  is actuated, it expands and moves a needle driver  19 , which is of tubular configuration and passes through actuator  17  in an internal longitudinal opening, toward fuel connector  5  against the force of a preload spring  20 . Needle driver  19  engages behind needle collar  11 , and upon actuation of actuator  17  acts upon valve needle  6 , thereby moving valve needle  6  in the direction of fuel connector  5 . As a result, valve closure element  7  lifts off from valve seat surface  9  of valve seat element  8 , and disengages the sealing fit. The resulting gap between valve closure element  7  and valve seat surface  9  of valve seat element  8  allows fuel to emerge from fuel injector  1  into the combustion chamber of the internal combustion engine. 
     Needle driver  19  is returned by way of preload spring  20 , which is braced at fuel connector  5  against needle driver  19 ; preload spring  20  also brings about the return of actuator  17 . Needle driver  19  has an internal orifice  21  in which a sleeve-shaped adjusting element  22  is located. Return spring  10  is braced against adjusting element  22  at the end located opposite needle collar  11 . By displacing adjusting element  22  in internal orifice  21  of needle driver  19 , it is easy to apply a defined preload to return spring  10 . The return stroke of valve needle  6  is accomplished by return spring  10 . 
     Fuel is guided from fuel connector  5  through internal orifice  21  of needle driver  19  and an internal orifice  24  of adjusting element  22  toward needle collar  11  on valve needle  6 . In order to allow fuel to flow toward the sealing fit, flow-through openings are configured in needle driver  19 . In the exemplary embodiment depicted, the flow-through openings are created by two bores  25   a ,  25   b  extending transversely in needle driver  19 . This and other fuel passage embodiments are discussed in the description below. 
     FIG. 2 depicts, in a partial axial sectioned depiction, a detail of the spray-discharge end of fuel injector  1 . Elements already described are labeled with matching reference characters, and any repeat description of them will be dispensed with. 
     In contrast to FIG. 1, valve closure element  7  is of partially spherical configuration. This configuration is believed to be particularly advantageous in the context of the self-guidance of valve needle  6  and valve closure element  7  discussed in the description of FIG. 1. A central opening  38 , which has a greater diameter than valve needle  6  and through which the latter passes, is provided in a bottom portion  37  of needle driver  19  that engages beneath needle collar  11  and represents a collar. A circular annular gap  39  is thus formed between needle driver  19  and valve needle  6 . In addition, the outside diameter of needle collar  11  is smaller than the inside diameter of needle driver  19 , so that an annular gap  26  is formed between needle collar  11  and needle driver  19 . Needle driver  19  acts with its bottom portion  37  on a stop surface  27  of needle collar  11 . 
     If, following actuation of fuel injector  1 , needle driver  19  is returned faster than valve needle  6 , a liquid cushion forms beneath stop surface  27  between bottom portion  37  of needle driver  19  and needle collar  11 . In order to close fuel injector  1  completely, return spring  10  must displace the liquid cushion beneath stop surface  27 . In order to displace the liquid cushion as quickly as possible, the needle collar is advantageously modified. One possible embodiment is described in detail in FIG.  3 . 
     FIG. 3 shows the detail labeled III in FIG. 2, presenting an advantageous development of needle collar  11 . In order to allow valve needle  6  to move radially, annular gaps  26 ,  39  already described between are configured between valve needle  6  and needle collar  11 , respectively, and needle driver  19 . In this context, valve needle  6 , needle collar  11 , and centering element  12  are configured integrally. A gap  28 , which widens in the radial direction toward valve axis  23 , is configured between needle collar  11 , valve needle  6 , and bottom portion  37  of needle driver  19 . In the sectioned drawing, gap  28  therefore has a wedge-shaped configuration. Stop surface  27  is therefore reduced to a narrow annular surface. Because of the particular configuration of needle collar  11 , the liquid cushion between needle collar  11  and bottom portion  37  of needle driver  19  can be rapidly displaced, with the result that valve needle  6  is returned more rapidly to its starting position. Gap  28  can also be embodied by way of a particular configuration of bottom portion  37  of needle driver  19 . In an exemplary embodiment that is not depicted, stop surface  27  can also be inclined in the opposite fashion, so that gap  28  becomes smaller toward valve axis  23 . 
     FIG. 4 shows, in a partial axial sectioned depiction, a detail of fuel injector  1  according to the present invention. Elements already described are given matching reference characters, thereby rendering any repeat description superfluous. In the exemplary embodiment depicted, needle driver  19  has lateral bores  25   a ,  25   b  which allow fuel to flow from internal orifice  21  through bores  25   a ,  25   b  toward the sealing fit. 
     FIG. 5 shows, in a partial axial sectioned depiction, a detail of fuel injector  1  according to an exemplary embodiment of the present invention. Elements already described are given matching reference characters, thereby rendering any repeat description superfluous. In the exemplary embodiment depicted, needle driver  19  has slits  29   a ,  29   b  extending in the axial direction, through which fuel can flow out of internal orifice  21  of needle driver  19  toward the sealing fit. More than two slits  29   a ,  29   b  can also be provided, in order to make possible a greater flow of fuel. 
     FIG. 6 shows, in a partial axial sectioned depiction, a detail of fuel injector  1  according to an exemplary embodiment of the present invention. Elements already described are given matching reference characters, thereby rendering any repeat description superfluous. In the exemplary embodiment depicted, opening  38  in bottom portion  37  of needle driver  19  is embodied with radial enlargements  31   a - 31   c , only radial enlargement  31   a  being visible in this depiction. Radial enlargement  31   a  overlaps the adjacent lower stop surface  27  of needle collar  11  to form a flow-through window  33   a.    
     FIG. 7 shows a front view, labeled VII in FIG. 6, of the detail of fuel injector  1  according to an exemplary embodiment of the present invention. Needle collar  11  of valve needle  6  is located in the interior of needle driver  19 . Needle driver  19  has opening  38  with radial enlargements  31   a  through  31   c . Enlargements  31   a  through  31   c  of opening  38  overlap needle collar  11  of valve needle  6 , so that flow-through windows  33   a  through  33   c  are created. As a result of flow-through windows  33   a  through  33   c  (arranged, for example at a spacing of 120° from one another), fuel flows out of the interior of needle driver  19  toward the sealing fit of fuel injector  1 . 
     FIG. 8 shows, in a partial axial sectioned depiction, a detail of fuel injector  1  according to an exemplary embodiment of the present invention. Elements already described are given matching reference characters. In this exemplary embodiment, bottom portion  37  of needle driver  19  has a circular opening  38  that is characterized by a comparatively large inside diameter. Needle collar  11  is triangular in shape, and is supported in the region of its stop surface  27  with contact surfaces  35   a  through  35   c , only contact surface  35   a  being visible in this depiction. Circular opening  38  of needle driver  19  overlaps stop surface  27  of needle collar  11  to form flow-through window  33   a  on the side exactly opposite contact surface  35   a.    
     FIG. 9 shows a front view, labeled IX in FIG. 8, of the detail of fuel injector  1 . Needle driver  19  has a circular opening  38  at its end toward the needle collar, which partially overlaps needle collar  11 , configured in triangular fashion, of valve needle  6  to form flow-through windows  33   a  through  33   c . Needle driver  19  acts via contact surfaces  35   a  through  35   c  on needle collar  11  of valve needle  6 . The fact that the total contact area resulting from contact surfaces  35   a  through  35   c  is relatively small results in the advantage that the liquid cushion, explained in the description of FIGS. 2 and 3, between needle driver  19  and needle collar  11  beneath contact surfaces  35   a  through  35   c  can be quickly displaced by return spring  10 , with the result that the liquid cushion has little influence on the switching time of fuel injector  1 . 
     FIG. 10 shows, in a partial axial sectioned depiction, a further exemplary embodiment of a fuel injector  1  according to the present invention. Elements already described are given matching reference characters, thereby rendering any repeat description superfluous. 
     In the exemplary embodiment depicted, front valve housing  3  is mounted onto rear valve housing  4  by way of a threaded joint  40 . Sealing of this join is provided by a sealing ring  41  that is placed in a circumferential groove  42  of front valve housing  3 . A stroke adjustment disk  43  is provided between an internal projection  44  of rear valve housing  4  and front valve housing  3  in order to adjust a stroke of valve needle  6 . In the exemplary embodiment depicted, preload spring  20  is braced against an adjusting element  45 ; the preload of preload spring  20  can be adjusted by way of the axial position of adjusting element  45 . Preload spring  20  acts on a magnet armature  46 , thus causing needle driver  19  to be impinged upon by a preload force in the direction of the sealing fit. As described with reference to FIG. 1, valve closure element  7  of valve needle  6  is thereby pressed into valve seating surface  9  of valve seat element  8 , thereby forming a sealing fit. Guidance of valve needle  6  is accomplished in this context by valve needle guide  13 . A swirl disk  14  is arranged downstream from valve needle guide  13 . 
     In this exemplary embodiment, actuation of fuel injector  1  is provided by an electromagnetically actuable actuator  46 ,  47 , which includes a magnet coil  47  and magnet armature  46 . Actuator  46 ,  47  is actuated by an electrical control signal that is guided via an electrical supply lead  48  to magnet coil  47  and is connected, in connector  18  of fuel injector  1 , to a contact  49 . 
     Upon actuation of magnet coil  47 , magnet armature  46  is moved in opening direction  50  as far as a stop that is defined by a stop surface  51 . Needle driver  19  is immovably joined to magnet armature  46 , so that the latter also moves in opening direction  50 . Since needle driver  19  engages behind needle collar  11  of valve needle  6  with its collar-shaped bottom portion  37 , this motion causes valve needle  6  to be moved in opening direction  50 ; as a result, valve closure element  7  of valve needle  6  lifts off from valve seating surface  9  of valve sealing element  8 , and the sealing fit is disengaged. As a result of the gap created between valve closure element  7  and valve seating surface  9 , fuel emerges into spray discharge channel  52  of valve seat element  8 , so that fuel is sprayed out of fuel injector  1  into the combustion chamber of the internal combustion engine. 
     After magnet coil  47  is deactivated, magnet armature  46  is moved by preload spring  20  opposite to opening direction  50 , as a result of which needle driver  19  is returned in the direction of the sealing fit. As described with reference to FIG. 1, valve needle  6  is impinged upon, by way of return spring  10 , by a return force in the direction of valve seat element  8 , so that the sealing fit formed by valve closure element  7  and valve seating surface  9  closes. 
     The exemplary embodiments described in FIGS. 2 through 9 can be transferred without limitation to fuel injector  1  described in FIG.  10 . 
     Present invention are not limited to the exemplary embodiments described. In particular, fuel injector  1  can also be embodied as an outward-opening fuel injector  1 . In addition, needle driver  19  need not be configured in the interior of actuator  17 , and return spring  10  need not be arranged in internal orifice  21  of needle driver  19 .