Abstract:
An actuator unit which is suitable for actuating a fuel injection valve of an injection system for internal combustion engines includes a piezoelectric actuator and a hollow body embodied as a spring. The service life of the actuator unit can be improved by designing the hollow body to eliminate or minimize bending stresses on the piezoelectric actuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 2004/000569 filed on Mar. 19, 2004. 
     BACKGROUND OF THE INVENTION 
     1.Field of the Invention 
     The invention relates to an improved actuator unit comprising a piezoelectric actuator. 
     2. Description of the Prior Art 
     Actuator units of the type with which this invention is concerned are used, among other ways, in fuel injection systems and particularly in fuel injection valves, since the switching times of such actuator units are very fast. The short switching times allow more-exact dimensioning of the injected fuel quantity and enable improved shaping of the course of the injection over time. The overall term “fuel injection valve” is understood in conjunction with the invention to mean all types of fuel injection valves, such as injectors for common rail injection systems or injection nozzles of conventional fuel injection systems. A fuel injection valve with a piezoelectric actuator is actuated by subjecting the piezoelectric actuator to an electrical voltage, and as a result the piezoelectric actuator, because of known physical effects of the piezoceramic, expands rapidly and lifts the valve closing member from a valve seat. The piezoelectric actuator has a certain mass, which is accelerated. If the voltage applied to the actuator is reduced, the actuator has the tendency to contract. Because of the mass inertia of the previously accelerated mass of the actuator, the result, depending on the triggering speed, is tensile forces in the actuator, which cause damage to the piezoelectric actuator, especially cracks in the soldered connections between individual layers of the piezoelectric actuator. To avoid such damage, the procedure has changed to prestressing the piezoelectric actuator in the axial direction by means of a cylindrical hollow body embodied as a spring. One such arrangement is known, for instance from International Patent Disclosure WO 00/08353 (Siemens). This hollow body is bent from a flat metal sheet and is welded at the first seam thus created. The first seam extends parallel to the longitudinal axis of the hollow body.  
     Welding the first seam has the following disadvantages, among others: The welding causes what as a rule is an unwanted alteration in the microstructure of the hollow body in the immediate vicinity of the weld seam. A second problem is the splashes that occur during welding, which can cause difficulties in assembling the actuator unit or even lead to functional failures of the fuel injection valve, if one or more such splashes come loose during operation. A third problem is the subsidence of the weld seam (seam drop) at the beginning and end of the weld seam, and the resultant notch effect and increase in stress. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     In the actuator unit of the invention, having a piezoelectric actuator disposed in a hollow body, in which the hollow body is embodied elastically and prestressed the actuator, and the hollow body is provided with recesses and has a first seam, extending parallel to a longitudinal axis, it is provided according to the invention that at least on a first end of the hollow body, a second seam is provided, located diametrically opposite the first seam. 
     In the design of the hollow body according to the invention, it is possible to dispense with welding the first seam. This also dispenses with the problems caused by welding. Moreover, the production costs are reduced. 
     One disadvantage of actuator units with an open parting seam on the hollow body is that the spring stiffness of the hollow body in the axial direction is not constant over the circumference. As a rule, because of the recesses in the hollow body, the spring stiffness is reduced in the region of the parting seam. As a result, the upper cover plate and the lower cover plate are acted upon by lateral forces during operation of the actuator unit. This results in an unequal load on the piezoelectric actuator by unwanted forces and bending moments. 
     Because according to the invention a second seam located diametrically opposite the first seam is placed in the hollow body, the spring rate of the hollow body on the side diametrically opposite the first seam is also reduced. As a result, the prestressing force introduced into the piezoelectric actuator by the upper cover plate and the lower cover plate acts precisely in the direction of the longitudinal axis of the piezoelectric actuator. Such loads are highly favorable for the piezoelectric actuator, and hence the service life of the actuator units equipped with a hollow body according to the invention can be increased markedly. 
     The advantages of the invention can be increased still further if a second seam, located diametrically opposite the first seam, is also provided on a second end of the hollow body. Alternatively, the second seam can extend from the first end to the second end of the hollow body. In other words, in this exemplary embodiment, the hollow body comprises two half-shells of equal size, which surround the piezoelectric actuator. The half-shells of this hollow body are also very favorable from a manufacturing standpoint. 
     Alternatively, it is also possible to introduce recesses into the hollow body on its ends, specifically opposite the first seam. As a result, the spring rate of the hollow body is purposefully reduced at this point as well, so that the spring rate of the hollow body is rotationally symmetrical, and thus the desired axial introduction of force is achieved. 
     For being able to introduce the prestressing force in the best possible way from the hollow body to the piezoelectric actuator, it is recommended that the hollow body is joined on its first end to an upper cover plate or to an adjusting disk, and the hollow body is joined on its second end to a lower cover plate or to a coupler housing. These connections can be made by welding or crimping, for instance. 
     If only a radial fixation of the hollow body is necessary, it can be done by means of an annular groove or a shoulder in the upper and/or lower cover plate, or in the adjusting disk and the coupler housing. This may be adequate, for instance whenever the hollow body is not stressed by tension but only by compression. An especially advantageous feature of these variant embodiments is that the hollow body is centered relative to the piezoelectric actuator or the hydraulic coupler by the annular groove and the shoulder. This effect can be improved still further if the annular groove and shoulder are dimensioned such that they widen the hollow body slightly during assembly. 
     It has proved especially advantageous if the contacting of a piezoelectric actuator located in the hollow body is effected via the upper cover plate, and the upper cover plate is embodied in two parts, and a parting seam is present between the two parts of the upper cover plate. 
     As a result, the installation space required for the actuator unit and the number of components needed are reduced, and moreover, the contacting of the piezoelectric actuator, which must at least partly go along with the motions of the actuator, is also taken over by the upper cover plate. The result is simple, reliable electrical contacting of the piezoelectric actuator, which has the requisite elasticity. Moreover, by this provision, the heat produced in operation of the piezoelectric actuator is dissipated very well, which improves the operating safety and the service life of the piezoelectric actuator. 
     It has also proved advantageous if the first seam and the second seam open into the parting seam of the upper cover plate, so that the electrical separation of the two parts of the upper cover plate is not undone by the hollow body, which after all may be made for instance from spring steel. 
     In addition, the upper cover plate can be electrically insulated by means of an insulator, in particular a ceramic insulator. 
     To assure the mechanical hold of the two-part upper cover plate together, a securing clamp or securing part can additionally be provided, which holds the two-part upper cover plate together. 
     In a further augmentation of the invention, it may be provided that between the hollow body and the piezoelectric actuator, a flexible binding means, in particular plastic-bonded metal or a soft solder, is present. 
     The recesses in the hollow body may have the familiar dumbbell or bonelike shape and may extend transversely to a longitudinal axis of the hollow body. It is also advantageous if a plurality of recesses are located one behind the other in one plane; and that the plane forms a right angle with the longitudinal axis of the hollow body. It has also proved advantageous if there is an even number of recesses in one plane. 
     If a plurality of planes are provided with recesses, it is recommended the planes extend parallel to one another, and the recesses of two adjacent planes are offset from one another. It is especially advantageous if the offset of the recesses of two adjacent planes is equal to half the repeat of the recesses in one plane. The term “repeat” will be explained at length hereinafter, in conjunction with  FIG. 3 . 
     Preferably, the hollow body has a cross section in the form of a circle or of a regular polygon. 
     The hollow body of the invention may also be employed in actuator units in which the piezoelectric actuator is disposed in the hollow body; and that the piezoelectric actuator is stressed for pressure by the prestressed hollow body. This means that the hollow body itself is stressed by tension. 
     However, the hollow body of the invention can also be used in actuator units in which the piezoelectric actuator is disposed outside the hollow body; and that the piezoelectnc actuator is stressed for pressure by the prestressed hollow body. In that case, as a rule, the hollow body is stressed in compression. 
     According to the invention, it may furthermore be provided that the hollow body, on its first end and/or on its second end, has a region that is not perforated with recesses. As a result, the spring force transmitted from the hollow body to a cover plate or some other component of the injector is evened out, since the hollow body is purposefully reinforced in the region of its ends. This means that the maximum values for the spring force decrease over the circumference of the hollow body, and the problems of transverse forces introduced into the piezoelectric actuator from the hollow body are made even less severe. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention will become apparent from the detailed description contained herein below, taken in conjunction with the drawings. in which: 
         FIG. 1  is an elevation view, in section, of a first exemplary embodiment of an actuator unit of the invention; 
         FIG. 2  shows a second exemplary embodiment of an actuator unit of the invention; 
         FIG. 3  shows one example of a flat sheet from which a hollow body is formed by bending; 
         FIG. 4  is a first exemplary embodiment of a hollow body in a perspective view, with an even number of rows (in this case 16) in the longitudinal direction; 
         FIG. 5 , the first exemplary embodiment of  FIG. 4 , from a different viewing angle; 
         FIG. 6 , a second exemplary embodiment of a hollow body of the invention, seen obliquely from below, with an odd number of rows (in this case, 17) of recesses in the longitudinal direction; 
         FIG. 7 , a third exemplary embodiment of a hollow body of the invention, with an even number of rows (in this case, 16) of recesses in the longitudinal direction; 
         FIG. 8 , one example for a radial fixation of the hollow body to the cover plates; 
         FIG. 9 , a further exemplary embodiment of an actuator unit of the invention in both a fragmentary longitudinal section and in a transverse sectional view; 
         FIG. 10 , a further exemplary embodiment of a hollow body of the invention; and 
         FIG. 11 , a schematic illustration of a fuel injection system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1 , a first exemplary embodiment of an actuator unit of the invention is shown in which actuator unit comprises a piezoelectric actuator  1 , which may be constructed of a plurality of individual piezoelectric elements (not shown) stacked one above the other. The piezoelectric actuator  1  is triggered via contact pins  2 , which are disposed along the actuator  1  and are electrically conductively connected to the actuator  1 . By applying a voltage between the contact pins  2 , a longitudinal expansion of the piezoelectric actuator  1  is generated, which is used for instance for controlling an injection valve in an internal combustion engine. The piezoelectric actuator  1  with the contact pins  2  is disposed in a hollow body  4  embodied as a tubular spring. The piezoelectric actuator  1  rests with each of its end faces on a respective cover plate  5  and  6 ; the upper cover plate  6  has ducts  61 , through which the contact pins  2  extend. The upper and lower cover plates  5 ,  6  are each joined to the hollow body  4  in form-and/or force-locking fashion, preferably by welding. The weld seams in the upper and lower cover plates  5 ,  6  and in the hollow body  4  are not shown in  FIG. 1 . Alternatively, the connection between the hollow body and the two cover plates  5 ,  6  may for instance be done with the aid of crimping, in which the crimped-over upper and lower peripheral regions of the hollow body  4  each engage the cover plates  5 ,  6  from inside (not shown). 
     The piezoelectric actuator  1  is stressed by pressure with a prestressing force by means of the hollow body  4  and the cover plates  5 ,  6 . In other words, the hollow body  4 , before being welded to the upper and lower cover plates  5 ,  6 , is prestressed and then welded. 
     The hollow body  4  is preferably made from spring steel. To enable establishing a desired spring rate for a given wall thickness “s”, many apertures or recesses  7  are made in the hollow body  4 . For the sake of simplicity, not all the recesses have been identified with reference numerals in  FIG. 1 . Since the many recesses  7  can be best produced by punching, the hollow body  4  is as a rule made from sheet metal. From this sheet, first a flat blank or sheet with the recesses  7  punched out is formed. Next, the flat sheet is bent, until it has a circular cross section, for instance, or a cross section in the form of a regular polygon. Where the two ends of the bent flat sheet meet one another, a first seam (not shown in  FIG. 1 ) is created. 
     In  FIG. 2 , a second exemplary embodiment of an actuator unit of the invention is shown, which is integrated with a piezoelectrically actuated injector  71 . 
     Since the present invention substantially pertains to an actuator unit and a hollow body  4  belonging to it, the injector  17  will not be described in all its details; essentially only the binding of the actuator unit to the injector  71  will be described. The other functionalities of the injector  71  are already familiar to one skilled in the field of injection technology and therefore require no further explanation. 
     The injector  71  has a high-pressure connection  73 . Via the high-pressure connection  73 , the injector  71  is supplied with fuel (not shown) that is at high pressure. If an injection into the combustion chamber (not shown) of an internal combustion engine is to be done, a nozzle needle  75  lifts from its seat, not shown, and uncovers injection ports, also not shown. The nozzle needle  75  is controlled via a control valve  77 , which is actuated via a piezoelectric actuator  79 . Located between the piezoelectric actuator  79  and the control valve  77  is a hydraulic coupler  81 , which is shown enlarged on the right-hand side of  FIG. 2 . 
     The hydraulic coupler  81  substantially comprises a valve piston  83  and a booster piston  85 , which are guided in a coupler housing  86 . Between the valve piston  83  and the booster piston  85 , there is a coupler gap  87 , which is filled with fuel (not shown). This coupler gap  87  is necessary, among other reasons, because the coefficients of thermal expansion of the piezoelectric actuator  79  and of the metal component of the injector  71  differ greatly. 
     With its valve piston  83 , the hydraulic coupler  81  actuates the control valve  77 , while the booster piston  85  rests with a protrusion  89  on the piezoelectric actuator  79 . Via a hollow body  4  of the invention that is prestressed in compression, the booster piston  85  is pressed against the piezoelectric actuator  79 , thus subjecting this piezoelectric actuator to compressive prestressing. In the process, the hollow body is braced by its first end  15  against a shoulder  91  of the coupler housing  86 . By its second end  17 , the hollow body  4  is braced against an adjusting disk  93 . Via the adjusting disk  93 , the spring force of the hollow body  4  is transmitted to the protrusion  89  of the booster piston  85  and thus to the piezoelectric actuator  79 . 
     So that the hollow body  4  is concentric with the hydraulic coupler  81  and thus also concentric with the piezoelectric actuator  79 , the diameter D 1  of the shoulder  91  is adapted to the inside diameter of the hollow body  4  in such a way that the hollow body  4  is widened slightly when it is slipped onto the shoulder  91 . Since the hollow body  4  of the invention has a first seam  31  that extends over the entire length of the hollow body, the hollow body  4  can be relatively easily widened far enough that it fits onto the shoulder  91 . 
     If, as in the exemplary embodiment of  FIG. 2 , the hollow body  4  is acted upon by a compressive prestressing, it suffices if the hollow body can be braced in the axial direction on its ends  17  and  15 . To further improve the radial fixation of the hollow body  4 , an annular groove (not shown) may alternatively or additionally be provided in the shoulder  91  and/or in the adjusting disk  93 . 
     In  FIG. 3 , a flat sheet  9  is shown, from which a hollow body  4  of the invention can be coiled. Many recesses  7  are punched out of the flat sheet  9 . For the sake of simplicity, not all the recesses  7  that have a bonelike shape in the exemplary embodiment shown in  FIG. 3  have been identified by reference numerals. The flat sheet  9  is rectangular, and two opposed edges  11  and  13  of the flat sheet  9  may be interrupted by the recesses  7 , while the diametrically opposed ends or edges  15  and  17  have a straight course and are not interrupted by the recesses  7 . 
     The flat sheet  9  is coiled up into a cylindrical or polygonal hollow body in such a way that the edges  15  and  17  form the first end  15  and second end  17  of the hollow body  4  (see  FIG. 4 ). That is, the longitudinal axis  35 , of the hollow body  4  extends parallel to the edges  11  and  13 . 
     When the flat sheet  9  is bent as described above into a cylinder or polygon, the edges  11  and  13  touch and form a first seam  31  (see  FIGS. 4 and 5 ), which extends parallel to the longitudinal axis  35  of the hollow body  4 . 
     A plurality of recesses  7  are always located one after the other in rows in the flat sheet  9  and are separated by webs  19  between the recesses. For the webs  19  as well, no attempt has been made to identify all the webs of the flat sheet  9  with reference numerals, for the sake of simplicity. When the flat sheet  9  is bent into a hollow body in the way described above, the recesses  7  located one behind the other, i.e. those located in a row before bending, are located in a common plane. For instance, in  FIG. 3 , one row of recesses  7  that are located one behind the other is indicated by a line  20 . In the exemplary embodiment of a flat sheet  9  shown in  FIG. 3 , 16 rows, each of six recesses  7 , are located between the edge  15  and the edge  17 . 
     As seen from  FIG. 3 , the recesses  7  of adjacent rows are offset relative to one another. The offset is selected such that it amounts to half the length of one recess  7  and one web  19 . This amount is represented as an example in  FIG. 3  by the double arrow  21  for one recess and two half webs  19 . This amount is also called the “repeat”. The offset between the recesses  7  of two adjacent rows of recesses is designated by reference numeral  23  in  FIG. 3 . 
     When the flat sheet  9  is rolled up to make a hollow body  4  (see  FIG. 4  or  FIG. 5 ), and this hollow body  4 , at its face ends, is subjected to a compressive force via an upper cover plate  5  (see  FIGS. 1 and 8 ) and the lower cover plate  6  (see  FIGS. 1 and 8 ), then the force F acting between the upper cover plate  5  and the edge  15  over the circumference of the hollow body  4  has the course represented qualitatively by the line  25 . The circumferential angle φ begins at the edge  13  at 0° and ends at the edge  11  at 360°. 
     It has been demonstrated that wherever a web  19  “braces” the edge  15 , a major force F, represented by the maximum values  27  along the line  25 , can be transmitted. The sole exception is where the edges  11  and  13  meet. There, the “cut-through” recess  7 , with its parts  7 ′ and  7 ″, weakens the structured of the flat sheet, so that the force F transmitted at this point between the upper cover plate  5  and the hollow body  4  is less. This subject matter is represented in  FIG. 3  by the markedly lesser value for the force F, at φ=0° and at φ=360°, compared to the maximum values  27 . 
     The situation is similar at the edge  17 . As seen in  FIG. 3 , in the immediate vicinity of the edge  17 , at φ=0° and 360°, there is a cut-into recess, comprising the parts  7 ′ and  7 ″, while in the immediate vicinity of the edge  15 , at φ=0° and 360°, there is a cut-apart web  19 , which has the halves  19 ′ and  19 ″. The result is a somewhat different course of forces over the circumference of the edge  17 . 
     As can be seen from the lower F-φ graph in  FIG. 3 , there are four maximum values there, and two further local maximum values  29  in the vicinity of the edges  11  and  13  at the angles φ=30° and 330°, which are markedly less than the maximum values  27 . 
     Because of this unequal transmission of force over the circumference between the upper cover plate  6  and the edge  15 , on the one hand, and between the lower cover plate  5  and the edge  17  on the other, a bending moment acting on the upper cover plate  6  and the lower cover plate  5  is generated by the hollow body  4  when the hollow body  4  is secured with prestressing to the upper and lower cover plates  6 ,  5 . This bending moment is intrinsically transmitted to the piezoelectric actuator  1  as well, which has an unfavorable effect on its operating safety and service life. Moreover, this bending moment is unwanted at the hydraulic valve members that are actuated by the actuator unit. 
     In  FIG. 4 , a hollow body  4  that has been made from a flat sheet  9  shown in  FIG. 3  is shown in perspective. The rows of recesses  7 , not individually identified by reference numerals in  FIG. 4 , form  16  planes E 1  through E 16 , which extend perpendicular to the longitudinal axis  35  of the hollow body  4 . For the sake of illustration, one plane E 2  is symbolically shown in  FIG. 4 . The wall thickness s of the hollow body  4  is also shown in  FIG. 4 . 
     According to the invention, a second seam  33  is therefore provided at the edges  15  and  17 , at the angle φ=180°. This second seam  33  will be described below in conjunction with  FIG. 5 . The hollow body  4  shown in these figures has been wound from the flat sheet  9  shown in  FIG. 1 . The edge  17  forms a first end of the hollow body  4 , while the edge  15  forms a second end of the hollow body  4 . 
     In  FIGS. 4 and 5 , it can be clearly seen that the edges  11  and  13  of the flat sheet  9  (see  FIG. 3 ) of the hollow body  4  are in opposed, abutting relation to one another. They are not welded together, so that the changes described in conjunction with  FIG. 3  (transverse force, bending moment) in the axial direction of the hollow body  4  take place at the first seam  31  formed by the edges  11  and  13 . 
     According to the invention, one or two second seams  33  (see  FIGS. 5 and 6 ) are now provided in the hollow body  4 , offset from the first seam  31  by 180°. The second seams  33 , in the exemplary embodiments of  FIGS. 5 and 6 , are only long enough that they reach one recess  7 . As a result, the hollow body  4  is likewise weakened at a circumferential angle φ of 180°, and the maximum value  27  at circumferential angle φ of 180° is decreased markedly. As a result, this provision means that the spring force exerted by the hollow body  4  on the piezoelectric actuator  1 ,  79  extends solely in the axial direction. Bending moments or forces in the radial direction are not introduced into the piezoelectric actuator  1 ,  79 . 
     In  FIG. 6 , a second exemplary embodiment of a hollow body of the invention is shown. The essential distinction is that an odd number of rows of recesses  7 , namely 17 rows, are provided. The termination of the first seam  31  is therefore the same at the edge  17  and at the edge  15 . As a result, the force course is the same at both the edge  15  and the edge  17  and is equivalent to the force course shown in  FIG. 3  in conjunction with the edge  17 . Since the force course is symmetrical at the two ends  15  and  17  of the hollow body  4 , the spring behavior of the hollow body  4  of the invention is further improved. The 17 rows of recesses  7 , which are not individually designated by reference numeral in  FIG. 6 , form  17  planes E 1  through E 17 , which extend perpendicular to the longitudinal axis of the hollow body  4 . 
     In the exemplary embodiment of  FIG. 7 , the second seam  33  extends from the edge  17  to the edge  15 , so that the hollow body  4  comprises two cylindrical half-shells  4   a  and  4   b . As a result, the force can be transmitted over the circumference between the edges  17  and  15  and the upper cover plate  6  and the lower cover plate  5  (not shown) is made still more uniform. 
     In  FIG. 8 , an actuator unit equipped with a hollow body as in  FIG. 6  or  FIG. 7  is shown, highly simplified, in fragmentary longitudinal section. So that the hollow body  4 , of which only one half  4   a  is shown in section, is fixed in the radial direction, the upper cover plate  6  has a shoulder  37 . By means of this shoulder  37 , the hollow body  4  is radially fixed. In the lower cover plate  5 , an annular groove  39  is provided for the sake of radial fixation of the hollow body  4 . It is understood that an annular groove (not shown) may also be provided in the upper cover plate  6 . In the actuator unit shown in  FIG. 8 , not only two-part hollow bodies  4 , but other hollow bodies of the invention, of the kind shown for instance in  FIGS. 3 through 6 , can be installed. 
     In  FIG. 9 , a further exemplary embodiment of an actuator unit of the invention, which is equipped with a two-part hollow body  4 , is shown both in fragmentary longitudinal section and in a view from above. In this exemplary embodiment, the upper cover plate  6  is also embodied in two parts. The two parts of the upper cover plate  6  are identified by reference numerals  6   a  and  6   b  in  FIG. 9 . Each of the two contact pins  2  is electrically conductively connected to a respective part  6   a ,  6   b  of the upper cover plate  6 . Via the upper cover plate  6   a  and  6   b , the actuator  1  is electrically connected. The details of the electrical connection between the actuator  1  and the parts  6   a  and  6   b  of the upper cover plate are not shown in  FIG. 9 . However, how such contacting will be done is familiar to one skilled in the field of construction and in the manufacture of electrical actuators. In the exemplary embodiment shown in  FIG. 9 , the part  4   a  of the hollow body  4  is welded to the part  6   a  of the upper cover plate, as represented by a stylized weld seam  41 . The part  4   b  of the hollow body is likewise welded to the part  6   b  of the upper cover plate  6  by means of a weld seam  41 . The weld seam  41  also takes on the task of radial fixation of the hollow body  4   a ,  4   b.    
     Between the parts  6   a  and  6   b  of the upper cover plate, an insulator  43 , which is preferably made from ceramic, is provided, in order to disconnect the contact pins  2  as well as the parts  6   a  and  6   b  of the upper cover plate  6  electrically from one another. A securing clamp  45  is also shown in the exemplary embodiment of  FIG. 9  and assures the holding together of the insulator  43  as well as the parts  6   a  and  6   b  of the upper cover plate. 
     Between the hollow body  4   a ,  4   b  and the actuator  1 , there is a flexible binding means  47 , which is preferably of plastic-bonded metal or a soft solder. 
     In  FIG. 9 , the view shown in the upper portion of the figure is located along section A-A of the lower portion. 
     In  FIGS. 10   a  and  10   b , two further versions of hollow bodies  4  of the invention are shown in perspective. In these version, unlike the versions described above, an avoidance of the transverse force of the hollow body on the side diametrically opposite the first seam  31  (where φ is approximately equal to 180°) is achieved by providing a recess  51  there. The recess  51  results in a purposeful weakening of the hollow body  4  on the side diametrically opposite the first seam  31 ,  50  that as a result, the hollow body  4  exerts a force solely in the axial direction on the piezoelectric actuator  1 ,  79 . The differences between the versions of  FIGS. 10   a  and  10   b  are the shape of the recesses  51  and the location of the recesses  51  relative to the recesses  7 . In the version of  FIG. 10   a , the recess  51  takes the form of a segment of a circle, while in the version of  FIG. 10   b  the recess  51  is rectangular. The precise definition of the shape of the recesses  51  and of the dimensioning can readily be done for each particular application by one skilled in the art of FEM calculation. 
     In some cases, it has proved advantageous if recesses  53  are provided in the region of the first seam  31  as well. 
     The first ends  15  of the hollow body  4 , not shown in  FIGS. 10   a  and  10   b , may be embodied identically to the second ends  17 . 
     In conjunction with  FIG. 11 , it will now be explained how the fuel injection valve  116  of the invention is integrated into a fuel injection system  102  of an internal combustion engine. The fuel injection system  102  includes a fuel tank  104 , from which fuel  106  is pumped by an electric or mechanical fuel pump  108 . Via a low-pressure fuel line  110 , the fuel  106  is pumped to a high-pressure fuel pump  111 . From the high-pressure fuel pump  111 , via a high-pressure fuel line  112 , the fuel  106  reaches a common rail  114 . A plurality of fuel injection valves  116  are connected to the common rail and inject the fuel  106  directly into combustion chambers  118  of an internal combustion engine, not shown. 
     It is understood that each of the characteristics described in the specification, shown in the drawings, or recited in the claims may be essential to the invention either individually or in combination with other characteristics. 
     The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.