Abstract:
A fuel injector, particularly an injector for a fuel injection system of an internal combustion engine, includes a piezoelectric or magnetostrictive actuator and a valve closure member which is actuable by the actuator with the aid of a valve needle and which is cooperable with a valve-seat surface to form a sealing seat. An electromagnetic damping device is provided for damping the movement of the valve needle.

Description:
FIELD OF THE INVENTION 
     The present invention concerns a fuel injector. 
     BACKGROUND INFORMATION 
     A fuel injector is discussed in International Patent Application No. WO895/0478. The damping device discussed in this document is made of a pot-shaped damping element, a weak compression spring having a low spring coefficient and a strong compression spring having a high spring coefficient. The two compression springs are staggered axially relative to each other and surround the valve needle by sections. The pot-shaped damping element is situated between the two compression springs which act on the pot-shaped damping element in opposite direction and which, on the side facing away from the pot-shaped damping element, are in each case braced against support elements applied to the valve needle. The weak compression spring counteracts the closing of the fuel injector, the strong compression spring counteracts the opening of the fuel injector. Formed between the edge of the damping element and the inner wall of the valve housing is a narrow, circumferential gap, extending in the axial direction, which is filled with fuel. Therefore, in response to movement of the damping element, a shear force develops in the fuel liquid between the edge of the damping element and the inner wall of the valve housing, the shear force producing a frictional force which counteracts the movement of the damping element. In cooperation with the compression springs, damping of the valve needle is thus achieved. 
     The known fuel injector has the following disadvantages: The damping force is permanently predefined by the spring force and the shear force, and therefore cannot adapt to the performance quantities of the internal combustion engine; in particular, it is not variably adjustable as a function of time. Since the fuel inflow in the direction of the sealing seat is influenced by the damping plate, flow eddies occur in the fuel, thus causing the moldability of the fuel discharge to deteriorate. A fuel inlet below the damping plate, suggested in WO 89/10478 as an alternative, is believed to be impractical, since it may markedly increase the size of the valve housing on the discharge side. Furthermore, due to the additional mechanical components, the fuel injector is more susceptible to wear, especially since the damping force is dependent on the width of the gap formed between the edge of the damping element and the inner wall of the valve housing. 
     In U.S. Pat. No. 5,236,173 is discussed a clamping spring in the form of a disc spring between the valve-seat member and a valve-seat support on which the valve-seat member is mounted, so that the valve-closure member strikes gently against the valve-seat surface formed on the valve-seat member. However, the disadvantage of this type of damping is that, after the valve-closure member has struck, the valve-seat member swings through in the spray direction, while the valve-closure member either comes to a standstill, or, because of the reversal in impetus, moves back from the valve-seat member contrary to the spray direction. For this reason, valve bouncing can even occur in increased measure with this fuel injector design, so that this type of damping has not proven to be very worthwhile. 
     SUMMARY OF THE INVENTION 
     The fuel injector of an exemplary embodiment of the present invention is believed to have the advantage that the fuel injector is debounced in a satisfactory manner. In addition, the electromagnetic damping device requires no mechanically stressed components such as compression springs and disc springs, and needs no damping fluid. Furthermore, the damping device is temperature-stable and permits a variable damping force. 
     The damping device advantageously has an excitation coil for generating a magnetic field, and at least one electroconductive induction loop arranged on the valve needle. In this manner, the electromagnetic field necessary for the damping can be generated in a simple manner. In addition, the damping force can act directly on the valve needle. 
     It is advantageous that the excitation coil is wound onto a valve housing of the fuel injector, the valve housing having a circumferential groove for this purpose. This results in an accommodation of the excitation coil which is simple from a standpoint of production engineering, and in which the excitation coil is well protected and can easily be replaced. 
     Another advantage is that the electric conductivity of the induction loop is greater than the electric conductivity of the valve needle. A loop voltage induced in the induction loop thereby produces an electrical induction current conducted in the induction loop. 
     It is also advantageous that the induction loop is electrically insulated from the valve needle. The electromotive force is thus particularly well utilized. 
     A further advantage is that the induction loop is sleeve-shaped and surrounds the valve needle by sections. This results in an induction-loop form which is adapted to the geometry of the injector valve, and which also permits simple mounting on the valve needle. 
     The axial length of the induction loop along the valve axis is advantageously less than the axial length of the excitation coil along the valve axis. A greater loop voltage is thereby induced in the sleeve. 
     A control unit advantageously has a current regulation for the current-regulated driving of the excitation coil and/or the actuator. This permits an exact, quickly reacting control of the damping force acting on the valve needle. 
     To utilize the displacement current developing in response to the compression of the actuator, the excitation coil is advantageously connected in series to the actuator. Thus, the energy stored in the actuator can be used for damping the valve needle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a partial axial intersection through a first exemplary embodiment of a fuel injector according to the present invention, the fuel injector being designed to open to the inside. 
     FIG. 2 shows an exemplary embodiment of a control of a fuel injector according to the present invention. 
     FIG. 3 shows a schematic sketch for clarifying the functioning method of an exemplary embodiment of a fuel injector according to the present invention. 
     FIG. 4 shows diagrams for clarifying an exemplary embodiment of a fuel injector according to the present invention. 
     FIG. 5 shows a circuit diagram for an exemplary embodiment of a fuel injector according to the present invention. 
     FIG. 6 shows diagrams for clarifying an exemplary embodiment of a fuel injector according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1, in a partial axial sectional view, shows a fuel injector  1  according to to an exemplary embodiment of the present invention. Fuel injector  1 , as a so-called gasoline direct injection valve, is used in particular for the direct injection of fuel, especially gasoline, into a combustion chamber of a mixture-compressing internal combustion engine having externally supplied ignition. However, fuel injector  1  of an exemplary embodiment of the present invention is also suitable for other application cases. 
     Fuel injector  1  is designed to open to the inside. Fuel injector  1  has a valve housing  2  and an end cover plate  3 . Disposed in valve housing  2  is a valve-closure member  5  that can be actuated by an axially movable valve needle  4 , and that in the exemplary embodiment shown, is formed in one piece with valve needle  4 . Valve-closure member  5  tapers frustoconically in the spray direction. Valve-closure member  5  cooperates with a valve-seat surface  7 , formed on a valve-seat member  6 , to form a sealing seat. In this context, valve-seat member  6  is secured in the front part of valve housing  2 . 
     Disposed on an inner contact surface  10 , which is formed on a projection  11  of valve housing  2 , is a contact element  12 . Contact element  12  can be designed to be plastically or elastically deformable. An intermediate plate  13  is secured in interior  16  of fuel injector  1  by a screw element  14 . Intermediate plate  13  is pressed by screw element  14  against contact element  12 , thereby deforming contact element  12 . To apply the force necessary for this purpose, screw element  14  is screwed into an internal screw thread  15  formed on the inner side of valve housing  2 . 
     A piezoelectric actuator  21  abuts against end face  20  of intermediate plate  13  on the inflow side, and a compression spring  23  abuts against the end face of intermediate plate  13  on the sealing-seat side. Actuator  21  and compression spring  23  are enclosed by a tubular housing wall  24  which has openings  25   a ,  25   b  through which intermediate plate  13  projects. Tubular housing wall  24  is joined to a housing plate  25  on the inflow side and a housing plate  26  on the sealing-seat side. Together, tubular housing wall  24 , housing plate  25  on the inflow side and housing plate  26  on the sealing-seat side form an inner housing  24 ,  25 ,  26 . In this context, actuator  21  acts on inner housing  24 ,  25 ,  26  via housing plate  25  on the inflow side, and compression spring  23  acts on inner housing  24 ,  25 ,  26  via housing plate  26  on the sealing-seat side. Valve needle  4  is secured to housing plate  26  on the sealing-seat side. 
     The fuel is conducted into interior  16  of fuel injector  1  via a bore hole  30  in end cover plate  3 . From there, it is conducted via at least one bore hole  31  in intermediate plate  13  in the direction of the sealing seat formed of valve-seat surface  7  and valve-closure member  5 . Actuator  21  expands in response to actuation, thereby shifting inner housing  24 ,  25 ,  26  in the direction of end cover plate  3 , and valve-closure member  5  secured to valve needle  4  lifts off from valve-seat surface  7 , thereby opening the sealing seat. Fuel arrives, via the resulting gap between valve-seat surface  7  and valve-closure member  5 , into a spray-discharge channel  32 , through which it emerges from fuel injector  1  into a combustion chamber of an internal combustion engine. Fuel injector  1  is closed via compression spring  23 , which acts on inner housing  24 ,  25 ,  26  contrary to actuator  21 , which means inner housing  24 ,  25 ,  26  shifts in the direction of valve-seat member  6 , and valve-closure member  5  of valve needle  4  is moved onto valve-seat surface  7  of valve-seat member  6 . In this manner, the sealing seat, formed of valve-seat surface  7  and valve-closure member  5 , closes. 
     In this exemplary embodiment, the electromagnetic damping device of the present invention for damping the movement of valve needle  4  is formed of sleeves  40   a  through  40   c , and an excitation coil  41  which is wound in a circumferential groove  42  onto valve housing  2  of fuel injector  1 . 
     To restrict the opening cross-section during the opening of fuel injector  1 , the movement of valve needle  4  is usually limited by a suitable limit stop. In the exemplary embodiment shown, this limiting is depicted in a simplified manner by the striking of housing plate  25  on the inflow side against stop elements  43   a ,  43   b . Upon closing fuel injector  1 , valve-closure member  5  of valve needle  4  strikes against valve-seat surface  7  of valve-seat member  6 . Because of the kinetic momentum during the opening and closing, without a damping device  40   a  through  40   c ,  41  of an exemplary embodiment of the present invention, bouncing of valve needle  4  occurs, which means the sealing seat is not opened with a constant opening cross-section and is not closed abruptly. 
     The functional principle of the damping device according to an exemplary embodiment of the present invention is explained below in the description with respect to FIG.  3 . To boost the damping force, sleeves  40   a  through  40   c  are electrically insulated against each other and against valve needle  4 . For example, this insulation can be effected by a lacquer or an oxide layer. If the space around sleeves  40   a  through  40   c  is filled with fuel, a suitable sealing for the sleeves against the fuel can be provided. Alternatively, it is possible to manufacture sleeves  40   a  through  40   c  from a material which exhibits a higher electric conductivity than valve needle  4 . 
     FIG. 2 shows an operating diagram which, in simplified manner, depicts the wiring configuration of actuator  21  and excitation coil  41 . To apply an electrical control voltage to actuator  21 , electrical supply leads  50   a ,  50   b  are run into fuel injector  1  to actuator  21 . In addition, electrical supply leads  50   c ,  50   d  are run into fuel injector  1  to excitation coil  41 . Electrical supply leads  50   a  through  50   d  are connected to a control unit  51 . It is advantageous if control unit  51  drives coil  41  in a current-controlled fashion, since in this manner, in response to a change in current intensity I L , coil inductance  11  can be counteracted by a correspondingly high voltage of control unit  51 , which is conducted to coil  41  via electrical supply leads  50   c ,  50   d . In addition, to protect electrical supply leads  50   a  through  50   d  and the winding of coil  41  from thermal overloading, it is useful to regulate the current in a current-limited manner. Using control unit  51 , it is possible, as a function of the performance quantities of the internal combustion engine, to drive actuator  21  and coil  41  in a manner that they are adjusted to one another, in order to prevent valve needle  4  from bouncing. 
     FIG. 3 shows a schematic sketch for clarifying the functional principle of the damping device of fuel injector  1 . A current I L  flowing in excitation coil  41  generates a radially symmetric magnetic field B which is proportional to coil current I L . An inhomogeneous magnetic field B results due to the finite length I L  of coil  41  in the axial direction, a marked change in magnetic field B on coil axis  55  being present, given a location change on the order of magnitude of length I L  of coil  41 . Located in magnetic field B is an induction loop  56  which represents the edge of a not necessarily planar area A. Of the two sides of area A, one can be arbitrarily defined as the outer side, thereby predefining a direction  57  of area A. A sense of rotation  58  of induction loop  56  is given by direction  57  of area A. Magnetic field B, which penetrates area A, generates a magnetic flux Φ through induction loop  56 . Magnetic flux Φ through induction loop  56  is changed by changing coil current I L  and/or by moving induction loop  56 . According to Faraday&#39;s law of induction, a time change of magnetic flux Φ, which flows through induction loop  56 , generates in induction loop  56  an electrical loop voltage which is contrary to sense of rotation  58  of induction loop  56  and is proportional to the time change of magnetic flux Φ. In induction loop  56 , the loop voltage generates an electrical current which, in response to an increase of magnetic flux Φ, is directed oppositely to sense of rotation  58 , thereby generating a magnetic field B′. In response to a time increase (decrease) in magnetic flux Φ, magnetic field B′ is oriented in the opposite (same) direction as magnetic field B. In the case of oppositely directed magnetic fields B, B′, induction loop  56  is repulsed by excitation coil  41 ; in the case of unidirected magnetic fields B, B′, induction loop  56  is attracted by excitation coil  41 . 
     To summarize, in response to an increase in magnetic flux Φ, induction loop  56  is repulsed by excitation coil  41 , and in response to a decrease in magnetic flux Φ, induction loop  56  is attracted by excitation coil  41 . According to an exemplary embodiment of the present invention, force K 0  associated with this is utilized for damping valve needle  4 . 
     FIG. 4 shows diagrams, by which the functioning method of the damping device according to fuel injector  1  is clarified. In this context, time t is plotted in each case on the abscissa, and the various performance quantities of fuel injector  1  are plotted on the ordinates. For the sake of simplicity, in the following only the closing operation of fuel injector  1  is considered. The functioning method of the damping device of fuel injector  1  can be transferred correspondingly to the opening operation. In the open state of fuel injector  1 , an electrical actuator voltage U A  is applied to actuator  21  up to point of time t a . Since actuator voltage U A  is constant up to point of time t a , the position of valve needle  4  also remains unchanged, which corresponds to a constant valve-needle lift h. To close fuel injector  1 , actuator voltage U A  is reduced at point of time t a  up to point of time t b , starting from value U A  (t a ) to value U A  (t b )=0. As of point of time t b , actuator  21  is de-energized. From point of time t a  up to point of time t b , fuel injector  1  is being closed, which means lift h of valve needle  4  decreases. In the case of an undamped fuel injector  1 , bouncing of valve needle  4  occurs thereby raising valve needle  4  from the sealing seat, which corresponds to additional lift movements  60   a  through  60   d.    
     At point of time t 1 , which can also be less than or equal to point of time t a , current i I  of excitation coil  41  is switched on for damping valve needle  4 . In this context, the effective damping force upon switching on excitation coil  41  is negligible compared to the actuator force of actuator  21 . During time interval t 23 , a constant magnetic field U is generated by excitation coil  41 , so that valve needle  4  is damped by the movement of induction loop  56  which is formed by sleeves  40   a  through  40   c . Therefore, for lift h′ of damped valve needle  4 , a closing operation results which proceeds free of bouncing, and thus exhibits no additional valve-needle lifts  60   a  through  60   d . During time interval t 34 , current I L  of excitation coil  41  is slowly reduced, in order not to transfer any power peaks to the valve needle. 
     FIG. 5 shows an alternative circuit arrangement for the wiring of fuel injector  1  according to exemplary embodiment of the present invention, in which excitation coil  41  is connected in series to actuator  21  in order utilize the displacement current developing during the compression of actuator  21 . A substitute circuit diagram made up of a non-dissipative inductance L and a dissipative resistance R L  is shown for excitation coil  41 , while a substitute circuit diagram made up of a non-dissipative capacitance C and a dissipative resistance R A  is shown for actuator  21 . 
     It is noteworthy that, upon feeding an external voltage U through inductance L of excitation coil  41 , at first only a part of external voltage U acts on actuator  21 , and the voltage dropping at actuator  21  only gradually approximates external voltage U. The time scale of the approximation is given from the quotient of inductance L of excitation coil  41  and the sum of actuator resistance R A  and coil resistance R L . In response to the compression of actuator  21 , the capacitance of actuator  21  changes, thereby changing the charge applied to actuator  21 , and thus an electrical displacement current develops. Since the currents occurring in response to a change in the length of actuator  21  are usually very great, correspondingly great magnetic fields can be generated with excitation coil  41 . 
     The functioning of the damping device according to the invention when it is interconnected as in FIG. 5 is depicted by the diagrams shown in FIG.  6 . The opening operation of fuel injector  1  is considered for this exemplary embodiment. However, the functional principle can be transferred to the closing of fuel injector  1 , as well. In each case, time t is plotted on the abscissa in the diagrams. 
     To open fuel injector  1 , voltage U is increased at point of time t 1 , up to point of time t 2 . Because of this, lift h of valve needle  4  increases. In the case of an undamped fuel injector  1 , valve needle  4  bounces after fuel injector  1  has opened, whereby additional valve-needle lifts  60   a  through  60   c  occur. The induction current is thereby generated in induction loop  56 , i.e. in sleeves  40   a  through  40   c . Valve needle  4  is damped by induction current I ind . The time profile of lift h′ of valve needle  4  therefore exhibits no additional valve-needle lifts  60   a  through  60   c  which result from the bouncing of valve needle  4 . Consequently, at most a weak additional valve-needle lift  60   e  occurs. 
     The present invention is not limited to the exemplary embodiments described. In particular, the present invention is also suitable for a fuel injector  1  opening to the outside. The damping device does not necessarily have to act directly on valve needle  4 , and can also be arranged differently in fuel injector  1 . Instead of an induction loop  56 , it is also possible to arrange on valve needle  4  a permanent magnet which, together with excitation coil  41 , forms an electromagnetic damping device. Induction loop  56  can also be formed by a wound coil instead of by sleeves  40   a - 40   c.