CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation of copending International Application No. PCT/DE01/04089 filed Oct. 29, 2001, which designates the United States, and claims priority to German application number 10060939.2 filed Dec. 7, 2000, and German application number 10054182.8 filed Nov. 2, 2000. 


   BACKGROUND OF THE INVENTION 
   The present invention relates to a fluid dosing device for a pressurized liquid with a chamber arranged in a housing, which is supplied with pressurized fluid by means of a liquid supply line and with a valve needle, which is guided through the chamber, the first end section of said valve needle being able to be lifted outside the chamber and the second end section thereof forming a valve which is connected to the housing, in conjunction with a valve seat provided on the housing. 
   Various sealing or leadthrough elements for fluid dosing devices are known in the prior art. In cases where pressurized fuel at a pressure of up to 300 bar for example and a working temperature of −40° C. to +150° C. is dosed, special requirements are set for mass-produced products. In particular exacting requirements must be complied with in respect of embrittlement, wear and reliability. The fatigue strength of the O-ring seals used up to now does not comply with the above requirements. Diaphragm seals such as for example metal beads, etc. can also be used in place of O-ring seals. When such diaphragms are used as the leadthrough element for a valve needle through a pressurized chamber however the requirements relating to high axial flexibility are not complied with when the compression strength is adequate. 
   The valve needle can also continue to be effected by means of a clearance fit of the needle in a cylindrical hole in the housing as in diesel injectors. A disadvantage of this is the unavoidable leakage along the needle leadthrough. The higher level of hydraulic loss also reduces the overall efficiency of the motor. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide a tight leadthrough for the valve needle in a generic fluid dosing device in particular, which achieves the required fatigue strength. 
   According to the invention this is achieved with a fluid dosing device for a pressurized fluid comprising a chamber located in a housing, to which the pressurized liquid is guided through a liquid supply line, a valve needle guided through the chamber, wherein a stroke can be applied to a first end section thereof outside of the chamber and the second end section thereof forming, in conjunction with a valve seat disposed on the housing, a valve which is connected to the chamber, and a flexible leadthrough element being provided for the first end section of the valve needle from the chamber outwards, which seals the chamber in said region in a tight manner, wherein at least one throttle point is provided circumferentially between the valve needle and the inner wall of the chamber in the section of the chamber between the leadthrough element and the mouth of the liquid supply line into the chamber, with a gap representing the throttle point being a few μm wide. 
   The object can also be achieved by a fluid dosing device for a pressurized fluid comprising a chamber located in a housing, to which the pressurized liquid is guided through a liquid supply line, a valve needle guided through the chamber having a first end section outside of the chamber and a second end section which forms in conjunction with a valve seat disposed on the housing a valve which is connected to the chamber, and a flexible leadthrough element being provided for the first end section of the valve needle, which seals the chamber in said region in a tight manner, wherein at least one throttle point is provided circumferentially between the valve needle and the inner wall of the chamber in the section of the chamber between the leadthrough element and the mouth of the liquid supply line into the chamber, wherein the throttle point is formed by a gap having a width of a few μm. 
   The fluid dosing device may further comprise bellows, in particular metal bellows, as the leadthrough element. The metal bellows may have a wall thickness of 25 to 500 μm. The leadthrough element may be attached to an assembly sleeve, in particular by means of a welded connection. The throttle point may be created in the chamber by the assembly sleeve. An upper valve needle guide can be provided and the throttle point can be created in the chamber by the upper valve needle guide. The free cross-section between the valve needle and the inner wall of the chamber can be changed abruptly in the region of the throttle point. The gap in the region of the throttle point may be a few μm wide. Fuel can be used as the liquid and the fuel pressure may be in the range of between 1 and 500 bar. The diameter of a clearance fit of the valve needle can correspond to a hydraulically effective diameter of the metal bellows. 
   According to the invention, at least one throttle point is arranged circumferentially between the valve needle and the inner wall of the chamber in the chamber section between the leadthrough element and the mouth of the liquid supply line into the chamber. Measurements have shown that metal bellows designed as leadthrough elements for use in high pressure injection valves, for example in vehicle engineering, can withstand static pressure loads up to approx. 200 bar without any problems. A much higher compression resistance can also be achieved by increasing the wall thickness. Further tests on moving metal bellows seals also showed that metal bellows subjected to high pressure do not suffer degradation during execution of an axial movement of up to 50 μm with a frequency of 50 Hz typical of the injection valves. Using metal bellows thus means that the fuel chamber is hermetically sealed with adequate compression strength. 
   It was however surprisingly established that the metal bellows fail after approx. 10 min when used operationally in a high-pressure injection valve at a static pressure load of 200 bar. The reason for this is that during the opening and closing of the injection valve or injector, pressure waves are triggered in the fuel chamber of the injector, which fluctuate about the basic pressure set with an amplitude of up to ±50% of the fuel pressure set and a frequency of approx. 500 Hz–10 Hz, typically in the range of approx. 500–800 Hz, depending on the opening and closing times of the injector. The occurrence of such pressure oscillations results in failure of the metal bellows seal when pressure waves are triggered. The throttle points provided according to the invention protect the metal bellows from the destructive effect of these pressure oscillations. 
   To summarize, therefore, according to the invention adequate tightness of the fuel chamber is achieved by means of the metal bellows, with the metal bellows seal being protected from pressure waves occurring during operation, thereby achieving a typical fatigue strength for vehicle engineering of at least 10 9  load cycles (approx. 2000 operating hours). 
   Advantageously the metal bellows have a wall thickness of 25 to 500 μm. These low wall strength levels have proven totally adequate at high pressures of for example 300 bar. Tests have shown that a configuration of the metal bellows in the form of semi-circular segments ranged adjacent to each other—visible in the longitudinal cross-section—offers particular advantages. These semi-circular segments can be supplemented by intermediate straight sections. 
   According to a preferred embodiment the flexible leadthrough element is attached to an assembly sleeve, in particular by means of a welded connection. This is particularly favorable for manufacturing purposes, as metal bellows in particular can only be attached directly to the valve needle at relatively high cost. The assembly sleeve provides an element by means of which a precisely dimensioned throttle point can be achieved in the fuel chamber in a simple manner. 
   In order to be able to create a suitable throttle point in the fuel chamber, an upper guide sleeve is configured as an alternative to or in addition to the appropriately dimensioned assembly sleeve, so that a narrow and as long as possible a clearance fit is achieved through this valve needle guide. As the upper valve needle guide is provided anyway in the fuel injector, additional components can be dispensed with. 
   If both the assembly sleeve and upper valve needle guide throttle points are created at the same time in the fluid dosing device, the respective throttle gaps can be larger and/or shorter in the axial direction, without having a negative impact on the protective effect of the throttle points for the metal bellows. Also fitting errors are avoided, which may result in the valve needle jamming. However this also applies if the throttle point created by the assembly sleeves is dispensed with, with the throttle point created by the upper guide sleeve being designed accordingly. 
   In order to prevent or significantly restrict propagation of the pressure waves in the fuel chamber in the direction of the metal bellows, the free cross-section between the valve needle and the inner wall of the chamber is changed abruptly in the region of the throttle point. This results in the required reflection of the pressure waves off the section of the inner wall of the chamber extending perpendicular to the direction of propagation of the pressure waves. 
   The gap width of the throttle point is selected on the basis of the position of the throttle point in the fuel chamber and the length of the throttle gap taking into account the static and dynamic pressure conditions. A few μm have proved to be a typical value for the gap width of the throttle point in the fuel chamber of a high-pressure fuel injector. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Four embodiments of the fluid dosing device according to the invention are described below using diagrammatic representations. These show: 
       FIG. 1   a  a longitudinal section of the first embodiment of the fluid dosing device, 
       FIG. 1   b  two cross-sectional representations along the lines A—A and B—B in  FIG. 1   a,    
       FIG. 2  a longitudinal section of the second embodiment, 
       FIG. 3   a  a longitudinal section of the third embodiment of the fluid dosing device and 
       FIG. 3   b  two cross-sectional representations along the lines A—A and B—B in  FIG. 3   a.    
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The actuator unit generally known per se is not shown for the purposes of simplicity in an injection value  1  shown diagrammatically in  FIGS. 1   a, b  according to a first embodiment. The fuel injection valve  1  has a housing  3  with a central hole, in which a valve body  5  is mounted. A valve needle  9  is guided in an axially displaceable manner in a valve body hole  7  of the valve body. To this end a lower or front and upper or rear guide sleeve  11 ,  13  is attached to the valve body  5  in the upper and lower end sections of the valve body hole  7  and these guide sleeves create corresponding valve needle guides. The resulting narrow points are designed so that they do not impede or throttle a flow of liquid when the valve  1  opens and closes. To this end the valve needle  9  has a circumferentially projecting, rounded square cross-section according to  FIGS. 1   a, b  (section A—A and section B—B) at both the level of the lower and upper guide sleeves  11 ,  13  or the two valve needle guides. The valve needle  9  with the rounded edge areas  14  is inserted into the two guide sleeves  11 ,  13  with a clearance of less than 2 μm. The free gap between the four side surfaces of the square of the valve needle  9  and the cylindrical inner wall of the guide sleeves  11 ,  13  is configured so that it is significantly larger to avoid any throttle effect. 
   In the basic state a valve disk  15  configured at the front end section of the valve needle  9  seals a valve seat  16  on the valve body  5 . A valve body fuel supply line  17  is provided in the valve body and this opens into the valve body hole  7  with a mouth  19  between the lower and upper guide sleeves  11 ,  13  when viewed in the axial direction. A housing fuel supply line  21  is also correspondingly provided in the valve housing  3 . At the upper end section of the valve needle  9  a spring plate  23  is attached to this. A nozzle spring  25  presses against this and is braced on the housing side, thereby tensioning the valve needle  9  in the closing direction. Above the upper guide sleeve  13  an outer assembly sleeve  27  is attached in the central hole of the valve housing  3 . The outer assembly sleeve  27  has a sleeve collar  44  at its lower end and this rests on a ring-shaped contact surface  45  on the housing  3 . The sleeve collar has an outer surface  46 , which is assigned to an inner wall  47  of the housing  3 . A sealing element  48  in the form of a sealing ring is inserted between the outer surface  46  and the inner wall  47 . The sleeve collar  44  is welded tightly to the inner wall  47  with a ring-shaped circumferential weld seam  49 . This creates a needle leadthrough through an opening in a sleeve base  29 , the leadthrough being sealed as described below. In a partial section of the outer assembly sleeve  27  restricted in the axial direction its inner wall forms a narrow point described in more detail below with the outer wall of an inner assembly sleeve  31 , which is in turn attached to the valve needle  9 . Cylindrical metal bellows  33  are welded to the outer and inner assembly sleeves  27 ,  31 , the valve needle  9  being guided outwards by said bellows. The metal bellows  33  serve to seal the fuel chamber  35  off hermetically from an unpressurized, air-filled intermediate space  36 . The metal bellows  33  are preferably in the region of the opening on the sleeve base  29  and attached to a surface of the inner assembly sleeve  31 , which is turned towards the sleeve base  29 . 
   Using the metal bellows  33  in the needle leadthrough allows the high-pressure area in the chamber  35  of the injection valve  1  to be sealed off totally, permanently and reliably from the intermediate space  36  with the drive area (not shown). Despite a low level of wall thickness of for example 50 to 500 μm the metal bellows  33  can withstand very high pressures due to their very high level of radial rigidity, without suffering irreversible deformation. The metal bellows  33  can also be designed so that high mechanical flexibility, i.e. a small spring constant in the direction of movement of the valve needle or the axial direction, is achieved. This means that deflection of the valve needle  9  is not impaired and that the forces induced in the valve needle due to length changes in the needle leadthrough caused by temperature are kept as small as possible. Furthermore the use of the metal bellows  33  in the needle leadthrough means that fuel leakage can be prevented with a high level of reliability. 
   The needle leadthrough sealed with the metal bellows in the outer assembly sleeve  27  can also be configured so that the forces caused by pressure and acting on the valve needle  9  mutually offset each other. This means that the valve needle  9  is generally kept pressure-free. For this the hydraulically effective diameter of the metal bellows is selected so that it corresponds exactly to the diameter of the valve seat  16  (not shown). As a result the pressure force triggered by the pressurized fuel acting on the valve needle  9  and the valve disk  15  and the force induced due to pressure by the metal bellows  33  in the valve needle mutually offset each other. This means there is no pressure force component acting on the valve needle  9  as a result. This ensures that the injection valve  1  exhibits a switching response which is almost completely independent of the fuel pressure, as the opening and closing forces are only determined by the actuator element, for example by piezo-actuators pretensioned in a spring tube, and the force of the pretensioned nozzle spring  25 . The metal bellows  33  also have a broad operating temperature range with the same level of functionality due to their metal material. Even thermal length changes in the metal bellows  33  only result in negligibly small changes of force at the valve needle  9  in the axial direction due to the low level of axial spring constant of the metal bellows. The metal bellows can also partially or wholly replace the nozzle spring  25  due to their mechanical spring effect in the axial direction. 
   The outer sleeve housing  27  is configured according to  FIG. 1   a  so that it creates a narrow and as long as possible a clearance fit with the inner assembly sleeve  31 . The clearance here is only a few μm. The throttle effect of this long cylindrical fit means that rapid pressure changes in the fuel chamber  35  are kept away from the metal bellows  33 , while static pressures can act unhindered on the bellows wall. Also the pressure waves in the region of the cross-section change of the first throttle point  37  are reflected off the chamber wall section perpendicular to the axial direction or the front face of the sleeve, so that only a pressure wave with a greatly reduced pressure amplitude continues into the ring-shaped gap created by the first throttle point  37 . 
   With a fuel injection valve  1  according to the second embodiment only one modification is made in  FIG. 2  in the region of the first throttle point  37  compared with the valve  1  according to the first embodiment, to the effect that the free internal diameter of the sleeve collar  44  of the outer assembly sleeve  27  is reduced for the same throttle gap dimensions in favor of the external diameter of the inner assembly sleeve  31 . As in the valve according to the first embodiment the throttle gap between inner and outer assembly sleeves  27 ,  31  is selected to be so small and long that an adequate throttle effect is achieved. The pressure waves triggered during the opening and closing of the valve  1  in the fuel chamber  35  cannot or can only slightly impact on the metal bellows  33  due to the short distance between the inner and outer assembly sleeves  27 ,  31 . 
   A fuel injection valve  1  according to the third embodiment shown in  FIGS. 3   a, b  has a second throttle point  39  in the region of the upper valve needle guide or the upper guide sleeve  13  as an alternative in place of the first throttle point according to the first two embodiments. As the fuel supply line  17  opens below the upper valve needle guide  13  into the space between the valve needle  9  and the valve body  5  or the fuel chamber  35 , the fuel to be injected into this does not have to pass the upper valve needle guide  13 . Therefore the upper valve needle guide can even be configured as a narrow, long cylindrical clearance fit of the valve needle  9  in the upper guide sleeve  13 , as shown in section B—B in  FIG. 3   b . Unlike the lower valve needle guide (section A—A) the valve needle  9  here is not configured as a square but is cylindrical (section B—B). The pressure waves triggered during opening and closing processes are reflected off this second throttle point  39  and a dynamic volume exchange is throttled significantly in the direction of the metal bellows  33 . Integration of the throttle point  39  in the valve needle guide means that multifits can be avoided. The throttle effect of the upper valve needle guide  13  splits the fuel chamber  35  into two sub-volumes, namely a first and a second chamber sub-volume  41 ,  43 . Although dynamic pressure changes of great amplitude are generated in the lower first sub-volume  41  of the fuel chamber  35  by the opening and closing of the injection nozzle, the action of these in the upper second sub-volume  43  of the fuel chamber  35 , where the metal bellows needle leadthrough is located, can be greatly reduced by the dynamic sealing effect of the second throttle point  39 . The metal bellows  33  are protected from dynamic pressure changes as a result. 
   According to the fourth embodiment of a fuel injection valve (not shown) the throttle points  37 ,  39  shown in  FIGS. 1  or  2  and  3  are combined in one valve. The first throttle point  37  is created by the inner and outer assembly sleeves  27 ,  31  and the second throttle point  39  is created by the upper guide sleeve  13  or the upper valve needle guide. 
   In the embodiments disclosed bellows in the form of a metal bellows were disclosed as a flexible leadthrough element. The invention is however not limited to this type of flexible leadthrough element but can also be used with other types of flexible leadthrough elements such as for example a diaphragm or a flexible plastic or rubber sleeve. The diaphragm is preferably made of metal. The diaphragm and the sleeve are stuck or welded in the same way as the disclosed metal bellows to the inner and outer assembly sleeve  27 ,  31 . 
   In general the pressure in the second chamber sub-volume  43  can be adjusted by appropriate selection of the diameter of the clearance fit of the valve needle  9  in relation to the hydraulically effective diameter of the metal bellows  33 . Adjusting the diameter of the clearance fit to be bigger (or smaller) than the hydraulically effective diameter of the metal bellows  33  means that the pressure in the second chamber sub-volume  43  drops (or increases) when the injection valve is opened. It is particularly advantageous if the diameter of the clearance fit corresponds to the hydraulically effective diameter of the metal bellows  33 , because in this way the pressure in the second chamber sub-volume  43  remains essentially constant when the injection valve is opened; the metal bellows  33  are then only exposed to a constant pressure load in all operating states.
A fluid dosing device for a pressurized liquid is disclosed, which comprises a chamber ( 35) which is supplied with pressurized liquid by means of a liquid supply line ( 17, 19); a valve needle ( 9) which is guided through the chamber ( 35), the first end section of said valve needle being able to be lifted and the second end section thereof forming a valve in conjunction with a valve seat disposed on the housing ( 3). Metal bellows ( 33) are provided as a leadthrough element for the first end section of the valve needle ( 9). The metal bellows seal the chamber in said region in a tight manner. A throttle point ( 37, 39) is provided between the valve needle ( 9) and the inner wall of the chamber between the metal bellows ( 33) and the mouth ( 18) of the liquid supply line ( 17) leading into the chamber.