Abstract:
A fuel injector, particularly a fuel injector for fuel injection systems of internal combustion engines, has a magnetic coil, an armature acted upon in a closing direction by a resetting spring and a valve needle connected to the armature by force-locking for operating a valve-closure member, which forms a sealing seat together with a valve-seat surface. The armature has a pot-shaped axial extension, in which at least one cutout is formed.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a fuel injector. 
   BACKGROUND INFORMATION 
   German Published Patent Application No. 196 26 576 an electromagnetically operable fuel injector describes in which, for the electromagnetic actuation, an armature cooperates with an electrically energizable magnetic coil, and the lift of the armature is transmitted to a valve-closure member via a valve needle. The valve-closure member interacts with a valve-seat surface to form a sealing seat. Several fuel channels are provided in the armature. The armature is reset by a resetting spring. 
   An electromagnetically operable fuel injector is also described in German Published Patent Application No. 195 03 821, in which an armature also cooperates with an electrically energizable magnetic coil. The lift of the armature is transmitted to a valve-closure member by a valve needle. 
   What is disadvantageous about the fuel injectors known from the above-named documents is particularly the lack of free flow space for the fuel, which is caused by the positioning of the valve needle in a hollow recess in the armature. This leads to big pressure differences between the upper and the lower sides of the armature, particularly during movement of the armature, since pressure equalization is hindered. The diameter of borings in the armature, put there to make it possible for the fuel to pass through, is limited because of the necessary armature pole surface and the low space availability. 
   It is also disadvantageous that the hydraulic pressure force of the fuel on the armature leads especially to longer valve opening times, which has a corresponding effect on the quantity of fuel metered in. On the other hand, due to fluctuations in the pressure difference, for example, in the case of different temperatures of the fuel injector, and viscosity differences resulting from this, variations in the switching time of the fuel injector are caused, which, in addition to the increased length of the switching times, lead to metering in irregular quantities of fuel. 
   SUMMARY OF THE INVENTION 
   By contrast, the fuel injector according to the present invention has the advantage that fuel can flow in an unhindered way through a large armature boring as well as through the openings arranged in a pot-shaped extension of the armature. Ideally, the armature boring should have the same diameter as an inner longitudinal recess of the internal pole of the magnetic coil. Thereby, the pressure difference between the armature upper side and lower side can be reduced to any low value desired. In addition, because of the bigger armature boring, the effective armature surface can be made smaller, and thus the remaining pressure force acting on the armature can be reduced. This leads to shorter valve opening times and to a reduction in the variation of switching times because of fluctuations in the pressure difference. 
   The pot-shaped extension of the armature can be designed in one piece with it, or it can be made as a separate part. 
   The extension preferably has at least two openings, which aids the uniform flow through the extension. However, it is also possible to have several or only one opening. Accordingly, the openings are separated from one another by an equal number of circular segments of the hollow cylindrically designed extension. 
   Of special advantage is the connection of the measures according to the present invention to the so-called prestroke principle, which also makes possible abbreviated opening times. 
   Advantageously, the component parts corresponding to the adaptation of this principle are all arranged in the downstream direction after the armature, whereby the flow through the armature is not impaired. 
   Particularly advantageous is the use of a hollow cylindrical valve needle, which is axially movable in the extension of the armature, and has fuel flowing through it. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a schematic section through a first exemplary embodiment of a fuel injector according to the present invention. 
       FIG. 1B  shows a section along the line IB—IB in FIG.  1 . 
       FIG. 2  shows a schematic section through a second exemplary embodiment of a fuel injector according to the present invention. 
       FIG. 3  shows a schematic section through a third exemplary embodiment of a fuel injector according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIGS. 1A and 1B  show a longitudinal section through a first exemplary embodiment of a fuel injector  1  according to the present invention, as a segment of a very much schematic sectional representation. 
   Fuel injector  1  has a magnetic coil  2  which acts together with an armature  3 . Magnetic coil  2  acts together with an internal pole  4  and an external pole. External pole  5  continues on the downstream side in a valve housing  6 . 
   Armature  3  has an extension  7  which is formed as a hollow cylinder and is positioned at the downstream side  34  of armature  3 . Extension  7  has a bottom portion  24 , which closes off extension  7  on the downstream side. In an inner recess  8 , which is developed in armature  3  and extension  7 , there is a resetting spring  9 . Resetting spring  9  is prestressed by adjusting sleeve  10  pushed into internal pole  4  in a hollow recess  11  of internal pole  4 . 
   A valve needle  13  is supported at a downstream end  12  of the extension  7 . Valve needle  13  is preferably welded to the bottom portion  24  of extension  7 . At a downstream end  37 , valve needle  13  has valve-closure member  14 , which collaborates with a valve-seat surface  16  formed in a valve-seat element  15  to form a sealing seat. 
   Fuel injector  1  shown in  FIG. 1A  is a fuel injector  1  opening toward the inside. In valve seat element  15  a spray-discharge opening  17  is formed. Fuel is let in via a central fuel supply  18 , flows through hollow section  11  of internal pole  4  as well as through recess  8  of extension  7  and leaves extension  7  through openings  20  marked more clearly in FIG.  1 B. Thereafter, the fuel flows through valve housing  6  to the sealing seat. 
   When fuel injector  1  is at rest, valve-closure member  14  is held in sealing contact to valve-seat surface  16  by the stress of resetting spring  9 . Fuel injector  1  is thus closed. If an energizing current is supplied to magnetic coil  2 , armature  3  is drawn, counter to the force of the resetting spring  9 , in the direction of internal pole  4 , after sufficient build-up of the magnetic field. After passing through an armature lift predefined by the size of a working gap  19 , armature  3  strikes with its inlet-side armature endface  21  against an armature stop  22  developed in internal pole  4 . Fuel flows from central fuel supply  18  through hollow recesses  11  and  8 , as well as openings  20  in the direction of the sealing seat. 
   If the current energizing magnetic coil  2  is switched off, after sufficient fall-off in the magnetic field, armature  3  falls away from internal pole  4  because of the force of resetting spring  9 , which causes valve needle  13  to move in the downstream direction, valve-closure member  14  to move onto valve-seat surface  16 , and fuel injector  1  to be closed. 
   In an extracted schematic sectional illustration,  FIG. 1B  shows a section through extension  7 , along line IB—IB of FIG.  1 A. 
   The basic shape of extension  7  is hollow cylindrical and it is made up of several segments  23 , preferably at least two, between which, in the circumferential direction, there is a corresponding number of openings  20 . Segments  23  form a casing portion of extension  7 , and are preferably made as one piece with the bottom portion  24  of extension  7 . Resetting spring  9  is supported on bottom portion  24 . On the side opposite bottom portion  24  from resetting spring  9 , valve needle  13  is supported, as shown in detail in FIG.  1 A. The fuel, which flows in centrally, flows through inner recess  8  of extension  7  and out of extension  7  through openings  20 . By the size of inner recess  8  and openings  20  between segments  23  it is ensured that the fuel can flow through fuel injector  1  without being significantly dammed up at armature  3 . 
   Fuel injector  1  according to the present invention is advantageously operated when the so-called prestroke principle is used. In this connection, armature  3  is pre-accelerated and runs through a partial lift, during which valve needle  13  is not yet carried along. Only when a first armature stop is reached is the valve needle carried along via suitable devices and against the force of a second resetting spring. 
   If, in addition, fuel injector  1  is constructed in such a way that the additional component parts, making possible the partial lift, are arranged in the downstream direction after armature  3 , the magnetic circuit remains uninfluenced by the partial lift. That is why, among other things, the diameter of internal pole  4  can be selected to be smaller, whereby the effective pole surface, and thus the effectively working magnetic force is increased. 
   Two exemplary embodiments of fuel injector  1  according to the present invention, in conjunction with the prestroke principle, are described in more detail in the light of  FIGS. 2 and 3 . In  FIGS. 2 and 3 , corresponding component parts are giving corresponding reference numerals to those in FIG.  1 A. 
   In a partial sectional illustration, slightly enlarged over  FIG. 1A ,  FIG. 2  shows a second exemplary embodiment of fuel injector  1  according to the present invention. 
   In order to be able to apply the prestroke principle, extension  7  of armature  3  has an opening  25  in bottom portion  24  which is penetrated by valve needle  13 . At its fuel inlet end  36 , valve needle  13  has a flange  26  having a projecting collar  27 . Valve needle is preferably welded to flange  26 , but can also be made as one piece with it. First resetting spring  9  is supported on collar  27  of flange  26 . Between collar  27  and bottom portion  24 , a second resetting spring  28  is clamped in. With regard to this, the spring constant of second resetting spring  28  is substantially smaller than the spring constant of first resetting spring  9 , in order to make possible the movement of armature  3  without valve needle  13 . 
   In the state of rest of fuel injector  1 , first resetting spring  9  presses valve needle  13  onto the sealing seat via collar  27  of flange  26 . During this time, armature  3  rests upon an armature seat  29  which is formed ring-shaped in valve housing  6 . If a current is made to flow through magnetic coil  2 , not shown in detail in  FIG. 2 , armature  3  moves in the direction of internal pole  4 . At this point in time, armature  3  has to move only against the force of second resetting spring  28 , since the spring constant of second resetting spring  28  is so small that armature  3  is not substantially impeded in its motion, valve needle  13 , however, still remaining at rest. After running through a prestroke corresponding to the height of prestroke gap  30  between bottom portion  24  of extension  7  and flange  26  of valve needle  13 , bottom portion  24  of extension  7  strikes flange  26 , and armature  3 , via flange  26 , takes valve needle  13  along with it in the lift direction in opposition to the force of first resetting spring  9 , which opens fuel injector  1 . 
   As soon as working gap  19  is closed, armature endface  21  on the fuel inlet side of armature  3  strikes armature stop  22  of internal pole  4 . As long as current is running through magnetic coil  2 , fuel injector  1  remains in the open position. If the coil current is switched off, armature  3 , because of the force of first resetting spring  9 , falls away from internal pole  4 , together with flange  26  and valve needle  13  connected to flange  26  by force-locking. The closing motion takes place in one move over the total lift, whereby fuel injector  1  may be rapidly closed. 
     FIG. 3  shows in an extract the schematic section illustration of a third exemplary embodiment of fuel injector  1  according to the present invention in conjunction with the prestroke principle. 
   In contrast to the exemplary embodiment shown in  FIG. 2 , valve needle  13  in this present exemplary embodiment is designed as a hollow cylinder, and thereby it assumes the function of extension  7  which is now designed in rudimentary fashion. Valve needle  13  has transversely running discharge ports  31 . Extension  7  of armature  3  in the present exemplary embodiment is formed without bottom portion  24 , but is instead welded to a sleeve  32  which is penetrated by valve needle  13 . 
   At its fuel inlet end, valve needle  13  has a collar  33  which is pressed against the downstream side of armature endface  34  by second resetting spring  28 , which is clamped in between sleeve  32  and collar  33 . First resetting spring  9  is set in recess  8  of armature  3 , and it is supported on fuel inlet side end  36  of valve needle  13 . The sum of the cross-sectional areas of the transversely running discharge ports  31  of valve needle  13  should be greater than, or at least equal to the cross-sectional area of recess  8  of armature  3 . 
   If a current is run through magnetic coil  2 , just the same as in the exemplary embodiment in  FIG. 2 , armature  3  goes through a prestroke lift corresponding to the height of prestroke lift gap  30  between sleeve  32  and collar  33  of valve needle  13 . As soon as sleeve  32  strikes collar  33 , armature  3  moves valve needle  13  along with it counter to the force of first resetting spring  9 . After running through the prestroke lift and the closing of working gap  19  between fuel inlet side armature endface  21  and armature stop  22  of internal pole  4 , armature  3  strikes internal pole  4 . As long as the magnetic coil has current running through it, fuel injector  1  remains in the open position. 
   If the current energizing magnetic coil  2  is switched off, after a sufficient reduction of the magnetic field, armature  3  falls away from internal pole  4  because of the force of first resetting spring  9 , and the fuel injector is closed. 
   An inner recess  35  of valve needle  13  is given a diameter slightly smaller than recess  11  of internal pole  4  and recess  8  of armature  3 . That is why a slight ram pressure can form on collar  33 , which supports the functioning of fuel injector  1  by making a minor contribution to the closing force. 
   The present invention is not limited to the exemplary embodiments shown, and can also be used, for example, for fuel injectors  1  opening outwards.