Abstract:
A piezo actuator for a fuel injector, including: a piezo layer stack having a longitudinal extension; and an insulation layer surrounding the piezo layer stack. The insulation layer has an insulation layer outer surface, facing away from the piezo layer stack, which defines an outer diameter of the insulation layer. In addition, a preloading device for preloading the piezo layer stack is also provided along the longitudinal extension, wherein the preloading device has a preloading device inner surface, facing towards the piezo layer stack, which defines an inner diameter of the preloading device. In a non-assembled state, the outer diameter of the insulation layer is greater that the inner diameter of the preloading device, such that, in an assembled state, the insulation layer is compressed in the preloading device. The invention also relates to a fuel injector having said piezo actuator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase Application of PCT International Application No. PCT/EP2015/063021, filed Jun. 11, 2015, which claims priority to German Patent Application No. 10 2014 215 327.1, filed Aug. 4, 2014, the contents of such applications being incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a piezo actuator for a fuel injector, and to a fuel injector which has said piezo actuator. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fuel injectors having piezo actuators are used, for example, in internal combustion engines for metering fuel into a combustion chamber. Precise metering of the fuel by means of a fuel injector is important with regard to exacting requirements demanded of internal combustion engines which are arranged in motor vehicles, such as, for example, in respect of a highly specific power setting or the meeting of strict pollutant emission regulations. 
         [0004]    For such fuel injectors, use is made in addition to solenoid drives of piezo actuators for injecting fuel. Said piezo actuators are used in particular in diesel internal combustion engines since, in the case of diesel, the fuel which is to be metered is frequently supplied to the fuel injector at a very high pressure of approximately 2000 bar to 2500 bar and is then metered into the respective combustion chamber of the internal combustion engine by means of the fuel injector. 
         [0005]    In order to improve the efficiency of piezo actuators which are used in fuel injectors, said piezo actuators are preloaded with a force which is dependent on the cross section of a piezo layer stack arranged in the piezo actuator. By means of the preloading, adequate endurance capability is also achieved. Furthermore, it is advantageous if the piezo actuator is protected against contact with fuel since the fuel could damage the insulation of the piezo actuator and electrical contact connections. 
         [0006]      FIG. 11  and  FIG. 12  show a known solution for preloading and sealing a piezo actuator. 
         [0007]      FIG. 11  shows a partial region from a fuel injector  10 , wherein a piezo layer stack  14  which is closed by a baseplate  16  is arranged in an injector body  12 . For the sealing, a membrane  18  is provided which is shown individually in the lower illustration in  FIG. 11  and which, as illustrated by the arrow P, is welded onto the injector body  12  in such a manner that a bore  20  in which the piezo layer stack  14  is arranged is sealed off from an environment  22 . 
         [0008]    The baseplate  16  and the piezo layer stack  14  are surrounded by a tube spring  24 , illustrated in  FIG. 12 , which is fixedly connected to the baseplate  16  and, opposite the baseplate  16 , to a head plate  26  (not shown). 
         [0009]    In the known fuel injector arrangement according to  FIG. 11  and  FIG. 12 , the two functions of preloading and sealing are accordingly realized by two separate components. The preloading takes place by means of the tube spring  24 , while the sealing takes place with the membrane  18  which is welded to the injector body  12  and to the baseplate  16 . 
         [0010]    However, in the event of large actuator strokes, as are necessary, for example, in the case of fuel injectors having a directly driven nozzle needle, the load-bearing capacity limit of said membranes is exceeded. In particular, in the case of fuel injectors having hydraulic play compensation, loading occurs as a quasi-static stroke because of thermal length change differences between a piezo actuator and the injector body. 
         [0011]    Furthermore, injection systems which carry out up to ten injections per operating cycle will be required in future. This gives rise to high electrical losses which allow the temperatures in the piezo actuator to rise. However, it is important to keep the temperature at the surface of the piezo layer stack and in the piezo actuator below a maximum permissible temperature. 
       SUMMARY OF THE INVENTION 
       [0012]    Therefore, an aspect of the invention proposes an improved piezo actuator which meets the abovementioned requirements. 
         [0013]    A piezo actuator for a fuel injector has a piezo layer stack having a longitudinal extension, an insulation layer surrounding the piezo layer stack and having an insulation layer outer surface which faces away from the piezo layer stack and defines an outer diameter of the insulation layer, and a preloading device for preloading the piezo layer stack along the longitudinal extension, wherein the preloading device has a preloading device inner surface which faces the piezo layer stack and defines an inner diameter of the preloading device. In a non-assembled state, the outer diameter of the insulation layer is greater than the inner diameter of the preloading device, and therefore, in an assembled state, the insulation layer is compressed in the preloading device. 
         [0014]    By means of this arrangement, the preloading device and the insulation layer come into tight contact with each other, and therefore working heat arising during the operation of the piezo actuator can be removed via the contact of insulation layer and preloading device to an environment. As a result of the fact that the outer diameter of the insulation layer is greater than the inner diameter of the preloading device, the fitting of the piezo layer stack leads to a defined compression of the insulation layer and therefore to a prescribed ratio between the preloading device inner surface and the insulation layer outer surface, which results in a defined heat flow. 
         [0015]    In addition, such a defined compression of the insulation layer in the preloading device has the advantage that the piezo layer stack is automatically centered in the preloading device. 
         [0016]    The preloading device preferably has a first end region and a second end region and also an extension region between the first and second end region, wherein the inner diameter of the preloading device is greater at least in one of the two end regions than in the extension region. In particular, the inner diameter of the preloading device is greater in the end region than in the extension region which, during the production of the piezo actuator, forms the side from which the piezo layer stack is introduced into the preloading device. Scraping of the insulation layer during the fitting together of preloading device and piezo layer stack with insulation layer is thus advantageously avoided. 
         [0017]    For this reason, it is particularly advantageous if the preloading device has rounded edges at least on the end region having the greater inner diameter. However, the preloading device particularly preferably has rounded edges in all regions which come into contact with the insulation layer during the fitting. 
         [0018]    For sealing the piezo actuator from an environment, it is advantageous if the preloading device is fixedly connected to the other elements of the piezo actuator. For this purpose, at a first end the piezo layer stack advantageously has a head place closing said piezo layer stack off and at a second end has a baseplate closing said piezo layer stack off, wherein the preloading device is preferably welded to head plate and baseplate in order to seal the piezo layer stack and the insulation layer from the environment. 
         [0019]    In a particularly advantageous refinement, a three-dimensional surface structure is arranged on the insulation layer outer surface. The effect that can be advantageously achieved in a particularly simple manner by the three-dimensional surface structure is a shaping of the insulation layer such that the insulation layer has an outer diameter greater than the inner diameter of the preloading device. The fitting of piezo layer stacks with insulation layer and preloading device is preferably significantly simplified if advantageously only predetermined regions of the three-dimensional surface structure, rather than the entire insulation layer outer surface, have an outer diameter greater than the inner diameter of the preloading device. This is because, if the outer diameter of the insulation layer as a whole were to be greater than the inner diameter of the preloading device, a very high fitting force would be produced which would have to be overcome first in order to fit the piezo layer stack with the insulation layer surrounding the latter into the preloading device. 
         [0020]    For example, the three-dimensional surface structure may be realized by a ribbed structure on the insulation layer. Advantageous examples of a ribbed structure are longitudinal ribs which are particularly preferably arranged distributed uniformly on the circumference of the insulation layer. For example, three longitudinal ribs or four longitudinal ribs can be provided. 
         [0021]    Alternatively or additionally, however, one or more helical ribs running around the surface of the insulation layer may also be provided. Alternatively or additionally, it is also conceivable to provide an insulation layer formed as a polygon in the cross section perpendicular to the longitudinal extension. Examples here include a hexagonal, octagonal, pentagonal or star-shaped cross-sectional form. 
         [0022]    In order to configure the fitting of piezo layer stacks to surrounding insulation layer and preloading device in a particularly advantageous manner, the three-dimensional surface structure is preferably formed tapering in the cross section perpendicular to the longitudinal extension. This means that said surface structure advantageously tapers away from the piezo layer stack toward the preloading device. 
         [0023]    The material of the insulation layer preferably has a greater coefficient of thermal expansion than the material of the preloading device. The preloading device is particularly advantageously formed from steel. During operation, the insulation layer therefore preferably expands to a greater extent than the preloading device, which advantageously results in an enlarged contact surface between the insulation layer outer surface and preloading device inner surface, as a result of which an advantageous improved transport of heat is possible. 
         [0024]    By way of example, the insulation layer is formed using an elastomer, for example using silicone. 
         [0025]    The insulation layer is preferably formed using a thermally conductive and electrically insulating material. For this purpose, for example, thermally conductive particles are embedded in an electrically insulating elastomer. 
         [0026]    It is also possible by way of example to form an insulation layer from an electrically nonconductive material and to fit a three-dimensional surface structure which is thermally conductive, for example by being mixed with thermally conductive particles, thereon. In this case, the electrically nonconductive insulation layer advantageously prevents an electric sparkover by means of an undesirable contact of the particles with inner electrodes of the piezo layer stack. The electrically nonconductive insulation layer preferably has a significantly lower layer thickness than the three-dimensional surface structure in order to avoid an accumulation of heat. 
         [0027]    Advantageously, the difference from outer diameter of the insulation layer to inner diameter of the preloading device is selected in such a manner that a compression force between insulation layer and preloading device lies within a range of 1 N to 25 N, in particular 3 N to 20 N, more particularly 5 N to 10 N. Within this range of forces, the fitting of piezo layer stack having the surrounding insulation layer and preloading device is preferably possible without an excessive production of force occurring, which could result in damage of individual or of a plurality of elements of the piezo actuator. 
         [0028]    A surface of the preloading device inner surface overlapped by the three-dimensional surface 
         [0029]    structure is preferably at maximum 50%, preferably 15% to 35%, of the preloading device inner surface. Said ranges are preferred particularly during the production process of the piezo actuator at room temperature. This is because, if heating and therefore expansion of the materials occur during the operation of the piezo actuator, the degree of filling and therefore the overlapped surface are increased. In order advantageously to avoid damage of the piezo actuator, the overlapped surface is therefore selected during the production in such a manner that overfilling is advantageously avoided during operation. 
         [0030]    In a particularly preferred refinement, the preloading device is formed by a zigzag spring having a profile which is sinuous in the direction of the longitudinal extension. In this case, the zigzag spring is in particular a tubular zigzag spring surrounding the piezo stack and the insulation layer. The production of such a zigzag spring is described, for example, in DE 10 2012 212 264 A1, which is incorporated by reference, the disclosure of which is incorporated here. A tubular zigzag spring advantageously permits particularly good sealing from the environment, if said tubular zigzag spring is welded to the baseplate and to the head plate, and at the same time good preloading of the piezo stack along the longitudinal extension thereof. 
         [0031]    The zigzag spring here has at least one first zigzag peak facing the piezo layer stack and at least one second zigzag peak facing away from the piezo layer stack, wherein the inner diameter of the zigzag spring as preloading device is defined by the first zigzag peak. 
         [0032]    A fuel injector has an injector needle and a piezo actuator driving the injector needle. The piezo actuator is formed here as described above. 
         [0033]    The injector needle is preferably driven here directly by the piezo actuator, that is to say without a hydraulic servo arrangement inbetween. 
         [0034]    A space into which fuel is advantageously guided during operation is provided in the fuel injector, preferably between an injector body and the preloading device of the piezo actuator. The fuel is particularly preferably guided here in a low pressure region. As a result, it is possible to fill air gaps, for example located, governed by the design, between the injector body and the piezo actuator, with a material, namely fuel, and therefore to realize an advantageous thermal connection which likewise contributes to dissipating working heat of the piezo actuator. 
         [0035]    The piezo actuator is produced in such a manner that the piezo layer stack is first of all covered with a thin passivation layer of a thickness of approximately 2 μm to approximately 10 μm. Said passivation layer can act, for example, as an adhesion promoter and is formed, for example, from silicone. The piezo layer stack passivated in this manner is subsequently placed into an injection mold, preferably a two-part injection mold, which is designed in such a manner that it predetermines the three-dimensional surface structure. The injection mold is closed and sprayed, for example, with silicone or the insulation layer materials already described above. By means of the special shaping of the injection mold, a three-dimensional surface structure is produced on the insulation layer outer surface, for example a ribbed structure as described above or a polygon shape, which is formed in cross section perpendicular to the longitudinal extension, of the insulation layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    Advantageous refinements of the invention are explained in more detail below with reference to the attached drawings, in which: 
           [0037]      FIG. 1  shows a fuel injector having an injector needle driven by a piezo actuator; 
           [0038]      FIG. 2  shows a perspective view of a first end region of the piezo actuator from  FIG. 1 , which has a piezo layer stack and an insulation layer and also a surrounding preloading device; 
           [0039]      FIG. 3  shows a first embodiment of the piezo layer stack with insulation layer from  FIG. 2 ; 
           [0040]      FIG. 4  shows a second embodiment of the piezo layer stack with insulation layer from  FIG. 2 ; 
           [0041]      FIG. 5  shows a third embodiment of the piezo layer stack with insulation layer from  FIG. 2 ; 
           [0042]      FIG. 6  shows a schematic view of a cross section through the insulation layer perpendicular to the longitudinal extension of the piezo actuator from  FIG. 1  with a hexagonal cross-sectional shape; 
           [0043]      FIG. 7  shows a schematic view of a cross section through the insulation layer perpendicular to the longitudinal extension of the piezo actuator from  FIG. 1  with an octagonal cross-sectional shape; 
           [0044]      FIG. 8  shows a schematic view of a cross section through the insulation layer perpendicular to the longitudinal extension of the piezo actuator from  FIG. 1  with a pentagonal cross-sectional shape; 
           [0045]      FIG. 9  shows a schematic view of a cross section through the insulation layer perpendicular to the longitudinal extension of the piezo actuator from  FIG. 1  with a star-shaped cross-sectional shape; 
           [0046]      FIG. 10  shows a view from the front of the piezo actuator from  FIG. 1 , wherein the preloading device has been removed in the right region; 
           [0047]      FIG. 11  shows a partial region of a fuel injector according to the prior art; and 
           [0048]      FIG. 12  shows a tube spring for preloading a piezo actuator according to the prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0049]      FIG. 1  shows a fuel injector  10  having an injector needle  28  which is driven by a piezo actuator  30 . The piezo actuator  30  is accommodated in an injector body  12  of the fuel injector  10  and extends with a longitudinal extension  32  through the injector body  12 . 
         [0050]    Parallel to the piezo actuator  30 , a bore  34  is arranged in the injector body  12 , through which bore fuel is guided to the injector needle  28  in order to be injected into a combustion chamber (not illustrated) of an internal combustion engine during opening of the injector needle  28 . 
         [0051]    The interior of the piezo actuator  30  has a piezo layer stack  14  in which a multiplicity of piezo-electrically active layers are stacked one above another in an alternating manner with inner electrode layers. If a voltage is applied to the piezo layer stack  14  via the inner electrode layers, the expansion of the piezo-electric layers changes, which results in a change of length of the piezo actuator  30  along the longitudinal extension  32  thereof. This gives rise to a stroke which is either transmitted hydraulically or directly to the injector needle  28  and opens the latter such that the fuel supplied via the bore  34  can be injected into the combustion chamber of an internal combustion engine. 
         [0052]    The change in length of the piezo actuator  30  results in the production of working heat at the piezo actuator  30 , which working heat has to be removed from the piezo actuator  30 . In the case of fuel injectors  10  which are currently on the market, as are used, for example, in the case of diesel common rail technology, the removal of heat from the piezo layer stack  14  on the injector body  12 , that is to say the actuator housing, can be achieved without special measures since the number of injections per operating cycle is relatively low and varies within the range of three injections per operating cycle. However, in future injection systems, up to ten injections per operating cycle will be realized. The electrical losses will also rise proportionally thereto, as will the temperatures to the same extent in the piezo actuator  30 . In order to keep the temperature at the surface of the piezo layer stack  14  under a maximum permissible temperature of, for example, 170° C., measures which increase the heat flow in the direction of the injector body  12  are therefore required. 
         [0053]    Therefore, a solution as illustrated in  FIG. 2  is now proposed. 
         [0054]      FIG. 2  shows a perspective view obliquely onto an end side of the piezo actuator  30  from  FIG. 1 . An insulation layer  44  is arranged around the piezo layer stack  14  and is in turn surrounded by a preloading. As shown in  FIG. 1 , the preloading device  46  is designed as a zigzag spring  48  and tubularly surrounds the piezo layer stack  14  with the insulation layer  44  surrounding the latter. As can likewise be seen in  FIG. 1 , the zigzag spring  48  is firmly welded to a head plate  26  and a baseplate  16 , which close off the piezo layer stack  14  upward and downward. As a result, reliable preloading and simultaneous sealing—together with baseplate  16  and head plate  26 —from an environment  22  is realized by the zigzag spring  48 . The zigzag spring  48  therefore fulfills two functions, namely preloading and sealing, and is therefore particularly suitable for use in the case of a limited construction space. As a result, the piezo actuator  30  illustrated in  FIG. 2  can also be used in a simple manner in an inline injector concept, in which the piezo actuator  30  is integrated in the injector body  12 , as is illustrated by way of example in  FIG. 1 . The available construction space is greatly restricted in this arrangement. 
         [0055]    In addition, in the arrangement according to  FIG. 2 , a high heat flow in the direction of the injector body  12  is realized since an outer diameter  54  of the insulation layer  44 , as can be seen in  FIG. 2 , is greater than an inner diameter  56  of the preloading device  46 . 
         [0056]    The outer diameter  54  of the insulation layer  44  is defined here by an insulation layer outer surface  58  facing away from the piezo layer stack  14 , and the inner diameter  56  of the preloading device  46  is defined by a preloading device inner surface  60  facing the piezo layer stack  14 . 
         [0057]    If the preloading device  46  is formed by a zigzag spring  48 , the zigzag spring  48  has a plurality of first wave peaks  62  facing the piezo layer stack  14  and a plurality of second wave peaks  64  facing away from the piezo layer stack  14 . In this case, the inner diameter  56  of the zigzag spring  48  is defined by the preloading device inner surface  60  in the region of the first wave peaks  62 . 
         [0058]    The arrangement shown in  FIG. 2  realizes a preloading and sealing solution which is optimum in terms of construction space and ensures as high a heat flow as possible in the direction of the injector body  12 . This is because, firstly, a combination of the preloading and sealing functions is realized by means of the zigzag spring  48  and, secondly, a maximum heat flow is achieved by optimizing a contact region K between the insulation layer  44  and the preloading device inner surface  60 , that is to say the inner side of the corrugated pipe. 
         [0059]    In order to facilitate fitting of the piezo layer stack  14  surrounded by the insulation layer  44  into the zigzag spring  48 , the insulation layer  44  does not have an outer diameter  54  greater overall than the inner diameter  56  of the zigzag spring  48 , but rather has a three-dimensional surface structure  66  on the insulation layer outer surface  58  thereof. 
         [0060]    Said three-dimensional surface structure  66  can be formed, for example, by a ribbed structure  68  as is shown, for example, in cross section in  FIG. 2  and in a view from the front in  FIG. 3  and  FIG. 4 . A plurality of longitudinal ribs  70 , but, for example, also one or more helical ribs  72  can be arranged here on the insulation layer  44 . A combination of the three-dimensional surface structures  66  mentioned is also possible. 
         [0061]    Alternatively or additionally, the insulation layer  44  may, however, also be formed as a polygon  74  in the cross section perpendicular to the longitudinal extension  32  of the piezo actuator  30 . This is shown in a top view in  FIG. 5 . Examples of a polygonal cross-sectional shape of the insulation layer  44  are shown in  FIG. 6  to  FIG. 9 .  FIG. 6  shows a hexagonal cross-sectional shape,  FIG. 7  shows an octagonal cross-sectional shape,  FIG. 8  shows a pentagonal cross-sectional shape and  FIG. 9  shows a star-shaped cross-sectional shape. 
         [0062]    Therefore, the obtaining of a maximum heat flow is achieved by forming the insulation layer  44  on the piezo layer stack  14  in the shape such that the insulation layer  44  has, for example on the surface  76  thereof, that is to say on the circumference thereof, a ribbed structure  68 , the outer diameter  54  of which is greater than the inner diameter  56  of the zigzag spring  48 . This has the effect that, when the piezo layer stack  14  is fitted into the zigzag spring  48 , there is a defined compression of the insulation layer  44  and therefore a prescribed ratio between the zigzag spring inner surface  60  and the contact surface with respect to the insulation layer  44 . 
         [0063]    As can furthermore be seen in  FIG. 2 , the three-dimensional surface structure  66  tapers away from the piezo layer stack  14  toward the preloading device  46  in the cross section perpendicular to the longitudinal extension  32 . The fitting of piezo layer stack  14  with insulation layer  44  and preloading device  46  is thereby facilitated. 
         [0064]    The three-dimensional surface structure  66  is provided in such a manner that a surface  78  of the preloading device inner surface  60  overlapped by the three-dimensional surface structure  66  is at maximum 50%. An advantageous range lies between 15% and 35% of the preloading device inner surface  60 . 
         [0065]    The difference of the outer diameter  54  to the inner diameter  56  is selected in such a manner that a compression force  80  between insulation layer  44  and preloading device  46  lies within a range of 1 N to 25 N, in particular within a range of 3 N to 20 N. A range of 5 N to 10 N is particularly advantageous here. 
         [0066]    By means of the defined values of the overlapped surface  78  and the compression force  80 , the fitting of piezo layer stack  14  and insulation layer  44  to the preloading device  46  is firstly facilitated and, secondly, in the event of an elevated operating temperature of the piezo actuator  30 , damage of the individual elements of the piezo actuator  30  by an excessive action of force from the insulation layer  44  onto the preloading device  46  is prevented. 
         [0067]    In order further to facilitate the fitting of piezo layer stack  14  and insulation layer into the preloading device  46 , it is advantageous if the preloading device has a greater inner diameter  56  on a first end region  82  and/or on a second end region  84  than in an extension region  86  which lies between the end regions  82 ,  84 . That is to say, when the zigzag spring  48  has an enlarged inner diameter  56  on the side from which the piezo layer stack  14  is introduced, scraping of the insulation layer  44  during the fitting can be prevented. 
         [0068]    In the case of the zigzag spring  48 , damage of the insulation layer  44  by the zigzag peaks  62 ,  64  also does not occur since the latter do not have any sharp edges, as is known, for example, in the case of the punched tube springs  24  from the prior art. Therefore, it is also advantageous if there are only rounded edges  88  at least in one of the end regions  82 ,  84 . 
         [0069]      FIG. 3  to  FIG. 9  show advantageous embodiments of the insulation layer  44 , namely the longitudinal ribs  70  mentioned, one or more helical ribs  72  or a polygonal cross-sectional surface  74 . The magnitude of the compression force can also be influenced through the selection of the injection molding geometry. 
         [0070]      FIG. 10  shows an illustration of the piezo actuator  30  illustrated with the zigzag spring  48  removed in the right region in order thus to open up the view of the insulation layer  44  with the three-dimensional surface structure  66 . 
         [0071]    An additional advantage of the compression of the insulation layer  44  in the zigzag spring  48  is the automatic centering of the piezo layer stack  14  in the zigzag spring  48 . 
         [0072]    Care should be taken when configuring the compression force  80  to ensure that the latter does not exceed a maximum value at room temperature since, as the temperature rises, the higher thermal expansion which the material of the insulation layer  44  customarily has, for example if said material is formed from silicone  90 , in comparison to the zigzag spring  48 , which is generally formed from steel  92 , could result in overfilling of the interior space of the zigzag spring  48 . However, the increase in the compression force as the temperature rises is, on the other hand, desirable since the maximum possible heat transport therefore increases. The gradient of increase of the temperature of the piezo layer stack  14  is thereby reduced as the temperature rises. 
         [0073]    In order to even further improve a thermal coupling of the piezo actuator  30  to the injector body  12 , the fuel injector  10  shown in  FIG. 1  has a space  94  between zigzag spring  48  and injector body  12  that is filled with fuel in the low pressure range during operation. The material used for the insulation layer  44 , as a rule a silicone elastomer, has low heat conductivity and, depending on the design of the fuel injector  10 , there are also at least two air gaps between the surface of the insulated piezo layer stack  14  and the injector body  12 , and therefore this results in an unfavorable thermal connection. For example, when multiple injection strategies are used—also at high rotational speeds and loads—this results in an impermissibly high temperature in the material of the insulation layer  44  since a sufficient heat flow cannot be achieved in the direction of the injector body  12 . If, however, the space  94  between the zigzag spring  48  and injector body  12  is filled with fuel, air gaps which may influence the heat transport in a highly disadvantageous manner during operation do not occur. 
         [0074]    Overall, the arrangement is based on the combination of the sealing and preloading function of a tube spring  24  which is formed as a zigzag spring  48  and into which the piezo layer stack  14  is inserted, the insulation layer  44  of which has a defined compression force  80  with respect to the zigzag spring  48 . The heat flow from the surface of the piezo layer stack  14  to the zigzag spring  48  is therefore increased, as a result of which, even in the event of a high number of injections per operating cycle, impermissibly high temperatures in the insulation layer  44  and the piezo layer stack  14  can be prevented. At the same time, the piezo layer stack  14  is readily centered in the zigzag spring  48 . 
       REFERENCE SIGNS 
       [0000]    
       
           10  Fuel injector 
           12  Injector body 
           14  Piezo layer stack 
           16  Baseplate 
           18  Membrane 
           20  Bore (piezo layer stack) 
           22  Environment 
           24  Tube spring 
           26  Head plate 
           28  Injector needle 
           30  Piezo actuator 
           32  Longitudinal extension 
           34  Bore (fuel) 
           44  Insulation layer 
           46  Preloading device 
           48  Zigzag spring 
           54  Outer diameter 
           56  Inner diameter 
           58  Insulation layer outer surface 
           60  Preloading device inner surface 
           62  First zigzag peak 
           64  Second zigzag peak 
           66  Three-dimensional surface structure 
           68  Ribbed structure 
           70  Longitudinal rib 
           72  Helical rib 
           74  Polygon 
           76  Surface 
           78  Overlapped surface 
           80  Compression force 
           82  First end region 
           84  Second end region 
           86  Extension region 
           88  Rounded edge 
           90  Silicone 
           92  Steel 
           94  Space 
         P arrow 
         K contact region