Abstract:
A fuel injection device for an internal combustion engine, having two valve elements each having a hydraulic control surface acting in the closing direction and associated with a hydraulic control chamber. A control valve influences the pressure in the control chamber, and loading devices act on the valve elements in the opening direction. The valve elements react at different hydraulic opening pressures prevailing in the control chamber. The control valve is able to set at least three different pressure levels in the control chamber: all of the valve elements are closed at a comparatively high pressure level; one valve element is open at a medium pressure level; and all of the valve elements are open at a comparatively low pressure level.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 2004/001201 filed on Jun. 9, 2004. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a fuel injection device for an internal combustion engine, having at least two valve elements, each of which has a hydraulic control surface acting in the closing direction that is associated with a hydraulic control chamber, having a control valve that influences the pressure in the control chamber, and having loading devices that are able to act on the valve elements in the opening direction, in which the valve elements react at different hydraulic opening pressures prevailing in the control chamber, and to a method for operating a fuel injection device of this kind. 
   2. Description of the Prior Art 
   A fuel injection device of the type mentioned at the beginning is known from DE 101 22 241 A1, which discloses an injection nozzle for internal combustion engines having two valve elements situated coaxially relative to each other. Both of the valve elements are stroke-controlled, i.e. they open when the pressure of a hydraulic fluid in a control chamber is reduced. The force of the valve elements acting in the opening direction is generated by an injection pressure acting on a corresponding pressure surface. As a result, the outer valve element opens first, followed by the inner valve element. If only the outer valve element is to be opened, then the pressure reduction in the control chamber must be terminated promptly and the pressure must be increased again. 
   Fuel injection devices are provided with several valve elements for the following reasons: 
   In particular in diesel internal combustion engines, in order to reduce emissions and increase efficiency, it is necessary to inject the fuel in as finely atomized a form as possible into the corresponding combustion chambers of the engine. This can be achieved if the fuel travels into the fuel injection device at a high injection pressure. 
   Using several valve elements, each of which controls a certain number of fuel outlet openings, makes it possible, even if a small fuel quantity is to be injected, to achieve a sufficiently long injection duration with a good atomization quality without simultaneously having to accept an excessively long injection duration and/or an excessively high injection pressure if a large fuel quantity is to be injected. 
   The object of the present invention is to modify a fuel injection device of the type mentioned at the beginning so that it can be triggered in as simple a fashion as possible and nevertheless functions reliably. At the same time, its use should enable a good emissions and fuel consumption behavior of the associated internal combustion engine. A further object of the present invention is to provide a method of operation of a valve of the type mentioned at the beginning so that even if only one valve element is to be actuated, this occurs as needed in the fastest possible way. 
   The first object mentioned above is attained in a fuel injection device of this type in that the control valve is able to set at least three different pressure levels in the control chamber; all of the valve elements are closed at a comparatively high pressure level; one valve element is open at a medium pressure level; and all of the valve elements are open at a comparatively low pressure level. 
   The second object mentioned above is attained in a method of operation of the valve by virtue of the fact that in a fuel injection device of this type, in order to open only one valve element, the control chamber is first connected to a low-pressure connection and then, is simultaneously connected to the low-pressure connection and a high-pressure connection. 
   SUMMARY AND ADVANTAGES OF THE INVENTION 
   With the fuel injection device according to the invention, the control chamber can be set to an additional medium pressure level at which the one valve element is already open, but the other valve element remains closed. In this way, it is possible to achieve even longer injection durations with only one open valve element, which, particularly in the partial load range, yields a favorable emissions and fuel consumption behavior of an internal combustion engine into which the fuel injection device according to the invention is incorporated. At the same time, the device is simply designed since it is not necessary to execute separate triggering actions for the valve elements with separate control chambers. It is also possible for the fuel injection device to contain only a single control chamber. 
   The advantage of the method proposed according to the invention lies in the fact that initially, through the connection of the control chamber to only the low-pressure connection, the pressure in the control chamber is reduced very quickly, but through the subsequent additional connection of the control chamber to the high-pressure connection, this pressure reduction is limited, namely to the level of a corresponding intermediate pressure. The second process step advantageously occurs before the valve element has reached an open end position. 
   Advantageous modifications of the invention are disclosed. According to a first modification, the control chamber is connected to a high-pressure connection via an inlet throttle and the control valve is connected to the control chamber on the one hand and to a low-pressure connection on the other. In a fuel injection device of this kind, the fuel injection can be completely controlled by means of a simple control valve and only two pressure connections, namely a high-pressure connection and a low-pressure connection. This embodiment is therefore inexpensive and functions reliably. 
   In a modification of this, the control valve has a switching chamber with a switching element, which rests against a first valve seat leading to the low-pressure connection in a first switched position, rests against a second valve seat leading to a bypass conduit in a second switched position, said bypass conduit being connected to the high-pressure connection, and does not rest against either the first valve seat or the second valve seat in a third switched position. A control valve of this kind is simple and therefore inexpensive. 
   The bypass conduit makes it possible to set a high, middle, or low fluid pressure in the switching chamber. This correspondingly results in the respective final pressures in the control chamber and correspondingly also results in the speeds with which the pressure in the control chamber falls. Furthermore, the connection of the switching chamber to the high-pressure connection at the end of an injection makes it possible to also connect the control chamber to the high-pressure connection via the switching chamber so that the pressure in the control chamber rises very quickly and the valve elements close quickly. This is particularly advantageous with regard to the emissions behavior. 
   In another modification of this, in the third switched position, the control valve constitutes a throttle that restricts the flow toward the low-pressure connection. This makes it possible to limit the fuel flow from the high-pressure connection directly to the low-pressure connection. As a result, it is not necessary to supply as much fuel and a smaller fuel pump can be used. 
   It is also possible for the control chamber to be connected to the high-pressure connection, for the control valve to be connected to the control chamber via at least two control conduits, and for the control valve to disconnect all of the control conduits from a low-pressure connection in a first switched position, to connect one control conduit to the low-pressure connection in a second switched position, and to connect all of the control conduits to the low-pressure connection in a third switched position. 
   Since the maximum influx of fuel from the high-pressure connection into the control chamber is limited, a higher or lower pressure level occurs in the control chamber depending on the outflow cross section, which is determined by the number of control conduits selected. This makes it possible to set an arbitrary opening time of the other valve element. Particularly under full load, both valve elements are opened directly at the start of injection. This achieves a maximum injection quantity at a given injection duration. 
   This fuel injection device is technically simple to implement and therefore particularly inexpensive. Fundamentally, it is conceivable for the control conduits to be identical and therefore when the number of control conduits being used is doubled, this doubles the available outlet cross section. However, the control conduits can also be embodied differently from each other, with an entirely specific throttle behavior associated with each control conduit. This makes it possible to set the pressure level prevailing in the control chamber in a very precise fashion. 
   Another easy-to-implement possibility for achieving different pressure levels in the control chamber is comprised in that the control chamber is connected to a high-pressure connection, the control valve connects the control chamber to a low-pressure connection in a first switched position and disconnects the control chamber from it in a second switched position, and the control valve can be continuously switched back and forth between the first switched position and the second switched position. 
   In this particularly preferred embodiment of the fuel injection device according to the invention, the setting of the different pressure levels in the control chamber requires only a simple 2/2-way relay valve. In the simplest case, the valve is closed again shortly before the valve element that opens second begins its opening movement (preferably before the valve element that opens first has reached its open end position) and is opened again shortly before the valve element that opens first has closed to such a degree that the emerging flow of fuel is throttled to an impermissible degree. The medium pressure level is thus the average value of a pulsating pressure curve caused by the opening and closing of the control valve. Alternatively, a constant, average pressure level can be set by a rapid succession of opening and closing, for example by means of a pulsed triggering. 
   According to another advantageous embodiment of the fuel injection device according to the invention, the valve elements are coaxial to each other and an axial boundary surface of the control chamber has a circumferential sealing region which, in an open end position of the outer valve element, subdivides the control chamber into an outer region connected to the high-pressure connection and an inner region connected to the control valve. The coaxial design makes the fuel injection device very compact. In the open end position of the outer valve element, the sealing region disconnects the control chamber region associated with the control surface of the inner valve element from the influx of highly pressurized fuel. The pressure in this control chamber region therefore falls particularly quickly so that the inner valve element opens with a corresponding rapidity. This reduces emissions. 
   In all of the fuel injection devices mentioned above, it is desirable for the control valve to switch very quickly. This can be achieved in a very simple fashion if the control valve includes a piezoelectric actuator. 
   In a modification of this, the control valve includes a valve body that is hydraulically coupled to the piezoelectric actuator; leakage fuel emerging from a guide of at least one valve element is used as the hydraulic fluid. The hydraulic coupling makes it possible to amplify the comparatively small stroke of the piezoelectric actuator by means of a hydraulic boosting. A corresponding valve body of the control valve is therefore able to open up a sufficient flow cross section when it opens, without needing to be large in size. By using the leakage fuel, which is present anyway, for the hydraulic coupling, it is possible to eliminate an additional fluid supply. This fuel injection device is therefore compact and comparatively inexpensive. 
   An additional advantageous embodiment of the fuel injection device according to the invention is distinguished in that one valve element has a catch that acts on the other valve element in the opening direction. This assures that the later-opening valve element opens precisely when the initially opening valve element has traveled a particular stroke distance. In certain load/speed situations in the internal combustion engine, this produces an injection curve in which particularly low emissions are generated. Depending on the pressure in the control chamber, however, the force that the catch exerts on the later-opening valve element may not be sufficient to open it. In this case, the catch functions as a stop that limits the stroke of the initially opening valve element. This makes it possible to inject extremely small fuel quantities. 
   In a modification of this, the catch is embodied so that it strikes the other valve element shortly before the one valve element reaches its maximum stroke. This assures that on the one hand, only the one valve element can be open so long as it has not yet reached its maximum stroke and on the other hand, the second valve element opens reliably by virtue of the first valve element being moved to the maximum stroke. 
   In a particularly preferred embodiment of the fuel injection device according to the invention, the loading device, which acts in the opening direction of the other valve element, and the hydraulic control surface of the other valve element are matched to each other so that this valve element opens only if the catch of the one valve element exerts an additional force acting in the opening direction. In order for the second valve element to open, it is therefore necessary not only for a reduction of the pressure in the control chamber to occur, but also for the driving action to be exerted by the valve element that opens first. This makes it possible to embody the control surfaces and the loading devices so that the opening pressures of the valve elements differ quite significantly from each other, which increases the operational reliability of the fuel injection device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Particularly preferable exemplary embodiments of the present invention will be explained in detail below, in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows a partial sectional view of regions of a first exemplary embodiment of a fuel injection device with two coaxial valve elements; 
       FIG. 2  is a schematic depiction of the fuel injection device from  FIG. 1  with the valve elements closed; 
       FIG. 3  is a schematic depiction similar to  FIG. 2  during an opening process for opening both valve elements; 
       FIG. 4  is a schematic depiction similar to  FIG. 2  with the valve elements open; 
       FIG. 5  is a schematic depiction similar to  FIG. 2  with only one valve element open; 
       FIG. 6  is a graph depicting a pressure curve in a control chamber of the fuel injection device from  FIG. 2  during the opening and closing process depicted in  FIGS. 3 and 4 ; 
       FIG. 7  is a graph similar to  FIG. 6  for the case depicted in  FIG. 5 ; 
       FIG. 8  is a graph depicting the curves of the switched positions of the valve elements for the pressure curve depicted in  FIG. 6 ; 
       FIG. 9  is a graph similar to  FIG. 8  for the pressure curve shown in  FIG. 7 ; 
       FIG. 10  is a schematic depiction similar to  FIG. 2  of a second exemplary embodiment of a fuel injection device; 
       FIG. 11  is a graph depicting the position of a control valve and an outer valve element plotted over time in a first triggering variant; 
       FIG. 12  is a graph depicting the position of a control valve and an outer valve element plotted over time in a second triggering variant; 
       FIG. 13  is a partly schematic partial section through a region of a third exemplary embodiment of a fuel injection device; 
       FIG. 14  shows a subregion of a modified embodiment form of the fuel injection device from  FIG. 13 ; and 
       FIG. 15  shows a subregion of a further modified embodiment form of the fuel injection device from  FIG. 13 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a fuel injection device as a whole is labeled with the reference numeral  10 . It includes a housing  12  that is comprised, among other things, of a nozzle body  14 . This nozzle body contains two valve elements  16  and  18  situated coaxially relative to each other. At their ends oriented toward the bottom in  FIG. 1 , each of the two valve elements  16  and  18  has a conical pressure surface  20 ,  22  that rests against a corresponding sealing edge  24 ,  26  on the housing when the valve element  16 ,  18  is closed. A number of fuel outlet conduits  28  that are distributed around the circumference of the nozzle body  14  lead outward from an annular chamber (unnumbered) situated between the two sealing edges  24  and  26 . Fuel outlet conduits  30  that are also distributed around the circumference of the nozzle body  14  lead outward from a blind hole (unnumbered) provided at the lower end of the nozzle body  14 . 
   The end of the inner valve element  16  toward the top in  FIG. 1  is embodied in the form of a push rod with a circular end surface  32 . If the two valve elements  16  and  18  are resting against the corresponding sealing edges  24  and  26 , then a corresponding annular control surface  34  of a push rod of the outer valve element  18  is situated at approximately the same height as the control surface  32  of the inner valve element  16 . Part of the annular control surface  34  is conical and is delimited toward the radial inside by a sealing region  36  whose function will be explained in greater detail below. The control surfaces  32  and  34  delimit a shared hydraulic control chamber  38  that is also encompassed by the nozzle body  14  and a counterpart piece  40 . A valve spring  41  acts on the outer valve element  18  in the closing direction. 
   The fuel injection device  10  also has a high-pressure connection  42 , depicted only symbolically in  FIG. 1 , which is usually connected to a fuel accumulator (not shown) of a common rail injection system during operation of the fuel injection device  10 . A conduit  44  that extends largely in the longitudinal direction of the fuel injection device  10  leads from the high-pressure connection  42  to an annular pressure chamber  46  at the lower end of the fuel injection device  10 , which pressure chamber  46 , when the outer valve element  18  is closed, is delimited by the region of the pressure surface  22  of the outer valve element  18  situated radially outside the sealing edge  26 . 
   A housing part  48  situated above the counterpart piece  40  in  FIG. 1  has an annular groove  50  let into its end surface oriented toward the counterpart piece  40 , which groove  50  is connected to the conduit  44  via a branch conduit  52 . The counterpart piece  40  contains a high-pressure conduit  54  that connects the annular groove  50  to the control chamber  38 . The high-pressure conduit  54  contains an inlet throttle  56 . 
   The fuel injection device  10  also has a low-pressure connection  58  that is only depicted in schematic form in  FIG. 1 . During operation of the fuel injection device  10 , this low-pressure connection  58  is usually connected to a return line (not shown) that leads back to a fuel tank. During operation of the fuel injection device  10 , therefore, approximately atmospheric pressure prevails in the low-pressure connection  58 , whereas a very high pressure of up to 2000 bar prevails in the high-pressure connection  42 . 
   The low-pressure connection  58  leads to a switching chamber  60  that will be discussed in further detail below. In the counterpart piece  40 , a control conduit  62  leads from the switching chamber  60  to the control chamber  38 . An outlet throttle  64  is provided in the control conduit  62 . A bypass conduit  68  also leads from the switching chamber  60 , through a throttle restriction  66 , to the annular groove  50  that communicates with the high-pressure connection  42 . The bypass conduit  68  is embodied by means of two bore segments  68   a  and  68   b  situated at an angle in relation to each other. 
   The switching chamber  60  contains a cylindrical switching element  70  of a 3/3-way relay valve  72 . A valve spring  74  presses the switching element  70  against a first valve seat  76  situated at the end of the switching chamber  60  oriented toward the low-pressure connection  58 . The switching element  70  is coupled to an actuating rod  78  that can be actuated by a piezoelectric actuator  80 . In this manner, the switching element  70  can be pressed counter to the force of the valve spring  74 , against a second valve seat  82  situated at the end of the switching chamber  60  oriented toward the bypass conduit  68 . 
   The fuel injection device  10  functions as follows: 
     FIGS. 1 and 2  depict an operating state of the fuel injection device  10  in which the 3/3-way relay valve  72  is in a first switched position  84  in which the switching element  70  is resting against the first valve seat  76  and is lifted away from the second valve seat  82 . In this instance, the high fuel pressure in the high-pressure connection  42  is conveyed into the control chamber  38  on the one hand via the high-pressure conduit  54  and on the other hand, via the annular groove  50 , the bypass conduit  68 , the switching chamber  60 , and the control conduit  62 . As a result, the high fuel pressure that is present in the high-pressure connection  42  is also present in the control chamber  38 . Correspondingly, hydraulic forces act on the control surfaces  32  and  34  in the closing direction of the valve elements  16  and  18 . In addition, the valve spring  41  also acts on the outer valve element  18  in the closing direction. The control surfaces  32  and  34  are dimensioned so that the inner valve element  16  is held securely in the closed position in opposition to the combustion chamber pressure and the outer valve element  18  is held securely in the closed position in opposition to both the combustion chamber pressure and the high fuel pressure acting on the pressure surface  22 . 
   The procedure for opening the two valve elements  16  and  18  will now be described (see  FIGS. 3 and 4  and  FIGS. 6 and 8 ): 
   To accomplish this, the 3/3-way relay valve  72  is brought into a second switched position  86  in which it rests against the second valve seat  82 . This disconnects the switching chamber  60  from the high-pressure connection  42  and instead connects the switching chamber  60  and therefore also the control conduit  62  to the low-pressure connection  58 . As a result, fuel can now flow out of the control chamber  38 , through the outlet throttle  64 , and to the low-pressure connection  58 . 
   The presence of the inlet throttle  56  causes a pressure drop in the control chamber  38 . This is indicated by the reference numeral  88  in  FIG. 6 . As soon as the pressure drops below the opening pressure of the outer valve element  18 , which is higher than the opening pressure of the inner valve element  16  in the current fuel injection device  10 , the hydraulic force acting on the pressure surface  22  causes the outer valve element  18  to lift away from the sealing edge  26  counter to the force of the valve spring  41  (reference numeral  89  in  FIG. 8 ) so that the fuel can exit the pressure chamber  46  via the fuel outlet conduits  28 . 
   When the sealing region  36  of the valve element  18  comes into contact with the counterpart piece  40 , (reference numeral  90  in  FIG. 6 ), the region of the control chamber  38  situated inside the sealing edge  36  is disconnected from the influx of new fuel via the high-pressure conduit  54  or else at least restricts this influx. The pressure in this radially inner region of the control chamber  38 , which continues to be connected to the low-pressure connection  58  via the control conduit  62 , therefore falls further until the pressure surface  20  of the inner valve element  16  also lifts away from the sealing edge  24  (reference numeral  92  in  FIG. 6 and 93  in  FIG. 8 ). Now, fuel can also exit via the fuel outlet conduits  30 . This is shown in  FIG. 4 . 
     FIG. 6  shows that the pressure in the control chamber  38  as a whole drops to approximately one third of its original value. This value is set by a corresponding dimensioning of the inlet throttle  56  and the outlet throttle  64 . As a result, the outer valve element  18  continues to remain securely in the open position since the sealing region or sealing edge  36  is spaced slightly apart from the radially inner edge of the control surface  34  so that the region of the control surface  34  situated radially inside the sealing edge  36  is once again subjected to a very low control pressure. Furthermore, the sealing edge  36  can be embodied so that the seal between the radially outer and radially inner region of the control chamber  38  is not absolute, i.e. fuel can continue to flow out of the radially outer region of the control chamber  38 , thus assuring a corresponding pressure drop therein. 
   The injection is terminated by bringing the switching element  70  back into contact with the first valve seat  76  (switched position  84 ). This disconnects the switching chamber  60  from the low-pressure connection  58  and reconnects it to the high-pressure connection  42  via the bypass conduit  68 . The control chamber  38  is once again connected to the high-pressure connection  42  via the control conduit  62  and the high-pressure conduit  54 , which results in a very rapid pressure increase (reference numeral  94 ) in the control chamber  38 . As a result, both of the valve elements  16  and  18  close almost simultaneously (reference numerals  96  and  98  in  FIG. 8 ). 
   If only the outer valve element  18  is to be opened, then the following procedure is executed ( FIG. 5 ): 
   The 3/3-way relay valve  72  is brought into a third switched position  100  in which its switching element  70  is situated in an intermediate position between the first valve seat  76  and the second valve seat  82 . It is therefore resting against neither of the two valve seats  76  and  82 . In this switched position  100  of the 3/3-way relay valve, the switching chamber  60  is connected to the low-pressure connection  58  on the one hand and on the other hand, is also connected to the high-pressure connection  42  via the bypass conduit  68 . As a result, a pressure is set in the switching chamber  60  that is lower than the high fuel pressure in the high-pressure connection  42 , but higher than the pressure that prevails in the switching chamber  60  in the switched position of the 3/3-way relay valve  72  depicted in  FIGS. 3 and 4 . 
   The connection of the switching chamber  60  to the control chamber  38  via the control conduit  62  also reduces the pressure in the control chamber  38  (reference numeral  88  in  FIG. 7 ), but also not as sharply as in the second switched position  86  of the 3/3-way relay valve depicted in  FIGS. 3 and 4  and  FIGS. 6 and 8 . The corresponding region of the pressure curve is labeled with the reference numeral  102  in  FIG. 7 . It is clear that the pressure falls to approximately half of the initial pressure. The pressure reduction in the control chamber  38 , however, is sharp enough for the outer valve element  18  to lift away from the sealing edge  26  due to the hydraulic force acting on the pressure surface  22  (reference numeral  89  in  FIG. 9 ) so that the fuel can travel from the pressure chamber  46  to the fuel outlet conduits  28  and flow out through them. Here, too, the valve element  18  moves until its sealing edge  36  comes into contact with the counterpart piece  40  (reference numeral  90  in  FIG. 7 ), which results in a further pressure drop in the control chamber  38 , but not so sharp that the inner valve element  16  opens. 
   In order to accelerate the opening of the outer valve element  18 , the 3/3-way relay valve  72  can also be initially brought into the second switched position  86  in which the switching element  70  rests against the second valve seat  82 . The 3/3-way relay valve  72  is then brought into the third switched position  100  before the sealing region  36  of the outer valve element  18  comes into contact with the counterpart piece  40 , which prevents the pressure in the control chamber  38  from dropping too sharply. 
   It should also be noted that the “intermediate pressure”, which prevails in the switching chamber  60  when the switching element  70  is in the intermediate position  100  between the first valve seat  76  and the second valve seat  82 , is also adjusted by means of the gap between the switching element  70  and the first valve seat  76 . This gap constitutes a throttle that restricts the flow from the switching chamber  60  to the low-pressure connection  58 . 
     FIG. 10  shows a modified embodiment form of a fuel injection device  10 . Here and in the figures that follow, elements and regions that have functions equivalent to elements and regions shown in the preceding figures are provided with the same reference numerals. They are not discussed in further detail. 
   The fuel injection device  10  shown in  FIG. 10  differs from the above-described fuel injection device only in the embodiment of the relay valve  72 : instead of being embodied as a 3/3-way relay valve, it is now embodied as a 3/2-way relay valve. As such, in a first switched position  84 , it can connect the high-pressure connection  42  directly to the control chamber  38  via the annular groove  50 , the bypass conduit  68 , and the control conduit  62 . In this switched position, therefore, the maximum pressure prevails in the control chamber  38 , which corresponds to the pressure prevailing in the high-pressure connection  42 . In the second switched position  86 , however, the control chamber  38  is connected to the low-pressure connection  58  via the outlet throttle  64  and the control conduit  62 . In this switched position, therefore, a comparatively low pressure prevails in the control chamber  38 , which depends on how the outlet throttle  64  and the inlet throttle  56  are embodied. 
   As has already been explained above in connection with the exemplary embodiment shown in  FIGS. 1 through 9 , when high pressure prevails in the control chamber  38 , both of the valve elements  16  and  18  are closed. At a low pressure, both of the valve elements  16  and  18  are opened. If only the outer valve element  18  is to be opened, then the control chamber  38  must be set to a medium pressure level. In the fuel injection device  10  shown in  FIG. 10 , a medium pressure level of this kind is achieved through a successive and continuous opening and closing of the relay valve  72 . 
   As is also clear from  FIGS. 11 and 12 , this means that the relay valve  72  is first brought into the open switched position  86  (curve  96  in  FIG. 11 ) so that the pressure in the control chamber  38  drops, which initially causes the outer needle  18  to open (curve  98  in  FIG. 11 ). Shortly before or precisely at the moment that the outer valve element  18  reaches its open end position in which it comes into contact with the counterpart piece  40  (dashed horizontal line in  FIG. 11 ), the relay valve  72  is brought back into the closed switched position  84 . As a result, the pressure in the control chamber  38  rises again and the outer valve element  18  begins to execute a closing motion. But before the outer valve element  18  has closed enough to restrict the flow between the sealing edge  26  and the pressure surface  22  (see  FIG. 1 ), the relay valve  72  is brought back into the open switched position  86 . In this way, the control chamber  38  is set to a medium pressure level in that the outer valve element  18  opens, but the inner valve element  16  is still closed. 
   In an exemplary embodiment that is not shown, in lieu of the 3/2-way relay valve  72  depicted in  FIG. 10 , a 2/2-way relay valve is used. It is then possible for the corresponding fuel injection device not to have a bypass conduit so that in the closed switched position of the 2/2-way relay valve, the control conduit  62  is simply closed. 
   As is clear from  FIG. 12 , it is also possible for the relay valve  72  to be opened and closed with a very rapid switching frequency (curve  96  in  FIG. 12 ), for example by means of a pulsed triggering. The flow cannot follow this switching action rapidly enough to yield a powerful fluctuation of the control pressure in the control chamber, but instead yields a relatively constant average pressure. As a result, the outer valve element assumes a relatively constant middle position (curve  98 ) close to the stop (dashed horizontal line). 
     FIG. 13  shows another possible embodiment form of a fuel injection device  10 . It also has a 3/3-way relay valve  72 , but does not have a bypass conduit. Instead, two parallel control conduits  62   a  and  62   b  lead from the switching chamber  60  to the control chamber  38 . The one control conduit  62   a  is connected to the switching chamber  60  at the second valve seat  82 . When the relay valve  72  is open, this control conduit  62   a  is thus closed. The second control conduit  62   b  is connected to the switching chamber  60  lateral to the switching element  70 . The two control conduits  62   a  and  62   b  contain outlet throttles  64   a  and  64   b  that have different throttling actions. 
   Furthermore, in the fuel injection device  10  shown in  FIG. 13 , the switching element  70  is not coupled to the piezoelectric actuator  80  directly, but by means of a hydraulic booster  104 . This booster has a booster chamber  106  into which a cylindrical booster element  108  protrudes on one side, which is connected to the switching element  70  by means of the actuating rod  78 . A boosting body  110  coupled to the piezoelectric actuator  80  likewise protrudes into the booster chamber  106 . The diameter of the boosting body  110  is greater than that of the booster element  108 . 
   The booster chamber  106  is filled with fuel. To accomplish this, the booster chamber  106  is connected to a leakage line  116  via a branch line  112  that contains a check valve  114 . This leakage line  116  leads to the low-pressure connection  58 . A corresponding branch line  118  also leads to the relay valve  72  and to an annular chamber  120 , which contains the compression spring  41  and into which leakage fluid can flow via a leakage conduit  122 , which leakage fluid flows out of the control chamber  38  through the gap between the upper regions of the two valve elements  16  and  18 . In this manner, the booster chamber  106  is supplied with the leakage fluid flowing from the control valve  72  and from the annular chamber  120 . 
   Because of the differing diameters of the booster element  108  and the boosting body  110 , a change in the length of the piezoelectric actuator  80  produces a stroke of the switching element  70  that is greater than the change in length of the piezoelectric actuator  80 . If the switching element  70  is resting against the first valve seat  76 , then this disconnects the two control conduits  62   a  and  62   b  from the low-pressure connection  58 . As a result, a high pressure prevails in the control chamber  38  and the two valve elements  16  and  18  are closed. 
   If the relay valve  72  is opened so that the switching element  70  is positioned between the first valve seat  76  and the second valve seat  82 , then fuel can flow out of the control chamber  38  to the low-pressure connection  58  via both of the control conduits  62   a  and  62   b . As a result, the pressure in the control chamber  38  drops sharply so that both valve elements  16  and  18  open. 
   But if the switching element  70  is brought into a position in which it rests against the second valve seat  82 , then the control conduit  62   a  is closed. Fuel can flow from the control chamber  38  to the low-pressure connection  58  only via the control conduit  62   b . The outlet throttle  64   b  and the inlet throttle the  56  are matched to each other so that in this case, the control chamber  38  is set to a medium pressure level at which the outer valve element  18  does open, but the inner valve element  16  remains closed. 
     FIG. 14  shows a further modified embodiment form. The differences relate to the end regions of the valve elements  16  and  18 . It is clear from the drawing that the inner valve element  16  is provided with an annular collar  124  that is positioned in a recess  126  in the end region of the outer valve element  18 . In the neutral position when both of the valve elements  16  and  18  are closed, the axial end surfaces of the recess  126  are spaced slightly apart from the annular collar. 
   The fuel injection device shown in  FIG. 14  functions in a manner similar to the one shown in  FIG. 13 . But if the outer valve element  18  is opened, the edge surface of the recess  126  toward the bottom in  FIG. 14  comes into contact with the annular collar  124 . The resulting additional force that the outer valve element  18  exerts on the inner valve element  16  in the opening direction causes the inner valve element  16  to also then open. The limit surface of the recess  126  on the outer valve element  18 , which surface is situated toward the bottom in  FIG. 14 , therefore functions as a catch that drives the inner valve element  16 . 
   The axial positions of the annular collar  124  and the recess  126  are matched to each other so that the lower edge of the recess  126  only strikes the annular collar  124  of the inner valve element  16  shortly before the outer valve element  18  reaches its maximum stroke. This permits the achievement of a stepped injection rate (“boot injection”), which makes it possible to reduce emissions of the internal combustion engine in which the fuel injection device  10  is used. The control surface  32  of the inner valve element  16  is also designed so that even when both control conduits  62   a  and  62   b  are “activated”, i.e. when the minimum possible pressure is present in the control chamber  38 , the inner valve element  16  only opens after the recess  126  has struck the annular collar  124 . 
     FIG. 15  shows a further modified embodiment form of the fuel injection device  10 . In this embodiment form, the valve elements  16  and  18  are each embodied of one piece. The control chamber  38  is delimited radially not by the housing  12 , but by a sleeve  128 , which has a sealing edge (unnumbered) at its edge toward the top in  FIG. 15 . The compression spring  41  presses this sealing edge against the housing surface (unnumbered) opposite from the control surfaces  32  and  34  of the valve elements  16  and  18 . 
   The foregoing relates to a preferred exemplary embodiment 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.