BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a stroke-controlled fuel metering valve for an injection system of an internal combustion engine.  
           [0003]    2. Description of the Prior Art  
           [0004]    In designing stroke-controlled fuel metering valves for modern injection systems, there is a conflict of purpose in terms of the choice of the valve needle speed. For optimal system performance, high opening and closing speeds are advantageous, since in this way a large proportion of the fuel to be injected is pumped without throttling at the valve seat. However, for metering very small injection quantities, in which the valve needle is not completely opened (“ballistic mode”), a slow valve motion is advantageous, since the metering precision increases as the valve speed drops.  
         OBJECT AND SUMMARY OF THE INVENTION  
         [0005]    It is the object of the invention to make suitable provisions for more-precise metering of small injection quantities, which are typical for a preinjection, for stroke-controlled injection systems, yet at the same time as much as possible to reduce power losses caused by throttling in the valve seat.  
           [0006]    Preferably, the coupling body is disposed relative to the valve needle in such a way that it is not actuated by the valve needle until after the valve needle has already executed part of its opening stroke motion.  
           [0007]    The fundamental concept of the invention accordingly is a two-stage opening of the valve needle of the metering valve by means of a coupling body of great mass, which the valve needle upon opening strikes after a slight stroke, after which it continues its opening motion jointly with the coupling body. The valve needle is braked by its impact with the coupling body. The valve needle remains in the region of the seat throttling for a relatively long period, and the time available for metering a small quantity accordingly increases markedly, compared to the length of time that the valve needle is unbraked. The influence of the speed of motion of the valve needle on the preinjection quantity decreases, and markedly more-precise metering of the preinjection quantity is made possible.  
           [0008]    Any sacrifices in performance in metering large injection quantities can be kept slight, since only the opening behavior of the valve needle has to be influenced by the coupling body. Because of the greater mass inertia of the coupling body, it is easy to disconnect the valve needle from the coupling body upon closure of the valve needle, thus enabling the valve needle to execute a very fast closing motion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings, in which:  
         [0010]    [0010]FIG. 1 shows one embodiment of a direct controlled so-called 3/2-way valve, and  
         [0011]    [0011]FIG. 2 shows one embodiment of a common rail injector. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]    In FIG. 1, reference numeral  10  generally designates a valve body, which comprises three parts  11 ,  12  and  13  connected axially in line with one another. The valve body parts  11 ,  12  and  13  are joined to one another by screw bolts  14 ,  15  and  16 ,  17  and are sealed off from one another by O-ring seals  18 ,  19 .  
         [0013]    An electromagnet  20  with a coil winding  21 , a magnet armature  22 , and a current lead  23  is received in the upper valve body part  11 . The current lead  23  for the electromagnet  20  is contained in a closure part  24 , which is secured to the upper valve body part  11  by means of the screw bolts  16 ,  17 , and is sealed off from it by an O-ring  25 .  
         [0014]    A valve needle, identified overall by reference numeral  26 , is guided axially movably in a bore  27 , serving as a valve needle guide, in the middle valve body part  12 .  
         [0015]    A conduit  29  in the middle valve body part  12 , discharging into a first pressure chamber  28  annularly surrounding the valve needle  26 , serves to provide high-pressure supply to the 3/2-way valve, from a so-called common rail (not shown). A similarly annularly embodied second pressure chamber  30  is located below the first pressure chamber  28 , and from it, a conduit  31  leading to the injection nozzle (not shown) begins. The valve needle  26  has a sealing edge at  32 , which cooperates with a valve seat  33  embodied above the second pressure chamber  30 .  
         [0016]    A cup-shaped insert  34 , which has a recess  35 , is screwed—from the direction of the back—into the lower valve body part  13 . The upper end face  36  of the cup-shaped insert  34  comes to rest on a stepped guide bush  37 , which is disposed—above the cup-shaped insert  34 —partly in a recess  38  in the lower valve body part  13  and partly—below the pressure chamber  30 —in the guide bore  27 . The guide bush  37  has a recess  39 , which in a certain sense forms the upper continuation of the recess  35  of the cup-shaped insert  34 . The two recesses  35 ,  39  serve to receive a valve compression spring  40 , which is braced on one end (at the bottom) on the bottom of the recess  35  in the cup-shaped insert  34  and on the other (at the top) via a disk  41  on the valve needle  26 , urging it with force in the direction of the arrow  42 . Thus by means of the valve compression spring  40  (when the electromagnet  20  is without current), the valve needle  26  is held in the closing position visible in FIG. 1.  
         [0017]    A special feature is that below the cup-shaped insert  34 , a coupling body  43  is disposed axially displaceably in a bore  44  in the lower valve body part  13 . The coupling body  43  has a peg part  45 , which is coaxial with the valve needle  26  and which penetrates the cup-shaped insert  34  in a bore  46  and protrudes at the top into the recesses  35 ,  39 ; the peg part is concentrically surrounded by the valve compression spring  40 . The peg part  45  ends just below the valve needle  26  (which is in its closing position), in such a way that between the lower end of the valve needle and the upper end of the peg part  45 , a gap  47  is formed. The essential aspect of the coupling body  43  is considered to be that it has a substantially greater mass than the valve needle  26 . The coupling body  43  has an upper—flat-faced—stroke stop  48  and a lower—also flat-faced—stroke stop  49 . The upper stroke stop  48  of the coupling body  43  cooperates with an upper counterpart stop  50 , which is formed by the lower end face of the cup-shaped insert  34 . By means of a compression spring  51 , the upper stroke stop  48  of the coupling body  43  and the counterpart stop  50  are kept in contact—in the closing position of the valve needle  26 . The compression spring  51  is received in a recess  52  of a retaining part  53  disposed below and connected to the valve body part  13 . The retaining part  53 , on its top side  54 , forms a lower counterpart stop for the lower stroke stop  49  of the coupling body  43 .  
         [0018]    The 3/2-way valve shown in FIG. 1 and described above functions as follows.  
         [0019]    In the currentless state of the electromagnet  20 , the valve needle  26  is pressed by the valve compression spring  40  into the valve seat  33  and closes it. If current is then supplied to the electromagnet  20 , the magnetic force of it acts on the valve needle  26  and accelerates in the opening direction  55 . The valve opens, and fuel is pumped. After a short travel, that is, after bridging of the gap  47 , which is smaller than the stroke that the valve needle  26  executes during a typical preinjection, the valve needle  26  strikes the coupling body  43 . Because of the mass inertia of the coupling body  43 , the valve needle  26  is braked. Since the magnet force continues to be applied, the coupling body  43  and the valve needle  26  are moved jointly onward in the valve opening direction  55 . Depending on the duration of triggering of the electromagnet  20 , the valve needle  26  together with the coupling body  43  reaches the lower counterpart stop  54  (where the coupling body  43  comes to rest with its lower stroke stop  49 ), or begins its closing motion again even before reaching stop  49  (in the direction of the arrow  42 ).  
         [0020]    In this closing motion, the coupling body  43 , because of its greater mass inertia in comparison to the valve needle  26 , separates from the valve needle  26  and is moved by the compression spring  51 —comparatively slowly—into its outset position visible in FIG. 1, in which the upper stroke stop  48  of the coupling body  43  comes into contact with the (upper) counterpart stop  50 . The valve needle  26  thus closes much more quickly than the coupling body  43  reaches its (upper) outset position.  
         [0021]    [0021]FIG. 2 shows a comparable function of the graduated valve opening by means of a coupling mass, using a common rail injector as an example, which is a servo-hydraulically actuated fuel injection valve.  
         [0022]    Reference numeral  56  indicates a housing body, with a formed-on outlet stub  57  and a plug housing  58  with a current connection  59  for an electromagnet—identified overall by reference numeral  60 . A high-pressure connection—also communicating with the housing body  56  of the common rail injector—is identified by reference numeral  61 . It is connected to a high-pressure fuel reservoir (or so-called common rail, not shown). A multiply graduated axial recess  62  is machined into the inside of the housing body  56 , and a valve control piston  63 , coupling body  64  and valve needle  65  are disposed axially movably in it. The coupling body  64  has an axial bore  66 , which is penetrated by the valve control piston  63 . The coupling body  64  is accordingly embodied in a certain sense as a hollow body or annular body.  
         [0023]    Reference numeral  67  designates a valve control chamber, in which a magnet control valve  68  with a valve ball  69  is disposed. The valve ball  69  cooperates with a conical valve seat  70  of the magnet control valve  68 . A restoring compression spring  71  keeps the valve needle  65  in its position shown in FIG. 2, in which the valve needle  65  closes an injection nozzle  72  located on the lower end of the housing body  56 . The coupling body  64  is kept in its (lower) outset position, shown in FIG. 2, by a further restoring compression spring  73 , and in this position, a narrow gap  75  is embodied between the coupling body  64  on the one hand and a thickened portion  74  of the valve control piston  63  on the other.  
         [0024]    A pressure conduit  76  also extends inside the housing body  56 ; it communicates hydraulically with the high-pressure connection  61  and serves to supply fuel to the injection nozzle  72 . Embodied above the valve control piston  63  is a control chamber  77 , which communicates hydraulically with the pressure conduit  76  via an inlet throttle  78  and with the valve control chamber  67  and a fuel return  80  via an outlet throttle  79 . Thus from the high-pressure connection  61 , the fuel is carried via the pressure conduit  76  to the injection nozzle  72  and via the inlet throttle  78  into the control chamber  77 . The hydraulic communication of the control chamber  77  with the fuel return  80  can be established—via the outlet throttle  79 —by opening the magnet control valve  68 .  
         [0025]    In the closed state of the outlet throttle  79 , the hydraulic force acting on the valve control piston  63  from the control chamber  77  predominates over the hydraulic force that is exerted on a pressure step  82  of the valve needle  65  by the fuel located in the high-pressure conduit  76 , via a pressure chamber  81 . As a consequence, the valve needle  65  is pressed into its seat at  72  and closes the high-pressure conduit  76  tightly off from the combustion chamber (not shown) of the engine. Thus no fuel can reach the combustion chamber.  
         [0026]    If the coil marked  83  of the electromagnet  60  is now supplied with current, then a force in the direction of the arrow  85  is exerted on the magnet armature  84  that actuates the magnet control valve  68 , and by this force, the magnet control valve  68  and thus also the outlet throttle  79  are opened. As a result, the pressure in the control chamber  77  drops, and the hydraulic force on the valve piston  63  decreases accordingly. As soon as the hydraulic force acting on the valve control piston  63  in the direction of the arrow  86  from the control chamber  77  becomes less than the force exerted on the valve needle  65  from the pressure chamber  81  via the pressure step  82 , the valve needle  65  moves in the direction of the arrow  85  and uncovers the injection nozzle  72 . Fuel from the high-pressure conduit  76  can now flow through the injection nozzle  72  to reach the combustion chamber of the engine.  
         [0027]    The operation described above involves an indirect triggering of the valve needle  65  via a hydraulic force booster system. This system is used because the forces required for comparatively fast opening of the valve needle  65  cannot be generated by the magnet valve  68  directly. The so-called control quantity required in addition to the injected fuel quantity reaches the fuel return  80  via the throttles  78 ,  79  of the control chamber  77 .  
         [0028]    The special feature now is that the valve control piston  63 , in the above-described opening motion, in which it is actuated by the valve needle  65 , moving in the direction of the arrow  85 , via a pressure piece  87 , strikes the coupling body  64  after only a short travel distance, namely after overcoming the width of the gap  75 . Because of its comparatively great mass and the resultant mass inertia force (which acts in the direction of the arrow  86 ), the valve control piston  63  and thus also the valve needle  65  are braked in their opening direction (direction of the arrow  85 ).  
         [0029]    The closing motion of the valve needle  65  (in the direction of the arrow  86 ) is initiated by switching off the current to the electromagnet  60 . A compression spring  88 , acting on the magnet armature  84  in the direction of the arrow  86 , can now actuate the magnet control valve  68  accordingly, until the valve ball  69  closes the valve seat  70  and thus the outlet throttle  79 . The high pressure prevailing in the high-pressure conduit  76  now builds up—via the inlet throttle  88 —in the valve control chamber  77 . The same pressure also prevails in the chamber volume (pressure chamber  81 ) of the valve needle  65 . The forces exerted by the high rail pressure on the end faces of the valve control piston  63  and the restoring compression spring  71 —acting in the direction of the arrow  86 —keep the valve needle  65  closed, counter to the opening force which engages the pressure step  82  of the valve needle  65 .  
         [0030]    Because of the lesser mass of the system comprising the valve control piston  63  and valve needle  65 , compared to the mass of the coupling body  64 , in the closing motion as described above a decoupling of the system  63 / 65  from the coupling body  64  takes place, so that the closing motion of the valve needle  65  can ensue quickly, and without being braked by the mass inertia forces of the coupling body  64 . The coupling body is acted upon by force in the direction of the arrow  86  by the restoring compression spring  73  and moved into its outset position—visible in FIG. 2.  
         [0031]    The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.
A stroke-controlled valve as a fuel metering device of an injection system for internal combustion engines has a valve body with a valve seat and a valve needle which is actuatable in the valve body counter to the resistance of a valve needle restoring spring and which has a sealing edge that cooperates with the valve seat. A coupling body with a greater mass than the valve needle is disposed in the valve body, in the axial extension of the valve needle, and is movable coaxially to the valve needle and is actuatable by the valve needle during the opening stroke of the valve needle.