Abstract:
A method for producing a fuel injector and a fuel injector for internal combustion engines, having a control valve with an electromagnetically actuatable valve needle; a coil is received in a magnet cup made of magnetizable material. At least one clamping sleeve, with a slot extending between the face ends of the clamping sleeve, is received in the magnet cup.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based on German Patent Application 10 2004 028 523.3 filed Jun. 11, 2004, upon which priority is claimed. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an improved fuel injector for internal combustion engines, and to a method of producing the fuel injector. 
   2. Description of the Prior Art 
   For supplying fuel to internal combustion engines, fuel injection valves are often used at present. For meeting emissions limits and reducing fuel consumption, the fuel quantity injected into the individual cylinders must be dimensioned precisely. This requires extremely short opening and closing times of the injection valve. Moreover, the valve needle must have the same stroke each time it opens, to assure that the same quantity of fuel will be injected into the cylinder each time. This is attained by means of a stroke limitation. 
   In the fuel injectors that are available on the market, the limitation of the stroke of the valve needle is done in various ways. For instance, the stroke limitation is realized by using a stop disk, which is struck by the valve needle. The stop disk rests on the magnet cup, so that a direct impact of the valve needle on the magnet cup is avoided. The stroke required for operation of the fuel injector and the remanent air gap are adjusted by grinding protrusions or steps into the valve needle. 
   Another possible way of attaining the stroke limitation is for the valve needle to strike a sleeve press-fitted into the magnet cup. In this variant, the stroke and the remanent air gap are adjusted by adjusting disks, whose thickness is adapted to the stroke into the remanent air gap. 
   Another possible way of attaining the stroke limitation is to use a holding-down device with a sleevelike stop that is surrounded by the magnet cup. One such sleevelike stop is known from German Patent Disclosure DE 102 49 161 B3. In it, the stroke is adjusted by an adjusting disk between the valve housing and the holding-down device, and the remanent air gap is adjusted by grinding the sleeve, embodied as a stop, down to the appropriate length. 
   To avoid eddy currents in the magnet cup, which slow down the switching of the fuel injector, the magnet cup is made from a metal-polymer composite material, as described in Bosch Research Info, 3/2001. To that end, fine iron particles are sheathed in plastic, compacted, and sintered to make a workpiece. However, this material is very brittle and therefore vulnerable to impacts. For this reason, the valve needle must be prevented from striking the magnet cup. 
   If the valve needle strikes the magnet cup, parts can break off, causing the magnetic properties of the magnet cup to change. The broken-off particles may also cause increased wear and thus lead to the failure of the fuel injector. 
   OBJECT AND SUMMARY OF THE INVENTION 
   In the fuel injector embodied according to the invention, which is triggered via a control valve with an electromagnetically actuatable valve needle, and with a coil received in a magnet cup made of magnetizable material, to prevent the valve needle from striking the magnet cup a tubular clamping sleeve with a slot extending between the face ends is received in the magnet cup. 
   For assembly, the clamping sleeve is prestressed and inserted into a bore, which is located, preferably centered, in the magnet cup. The prestressing creates a spring force in the clamping sleeve, by which the clamping sleeve is pressed against the wall of the bore and is thus held in the magnet cup. The spring force is dimensioned such that the clamping sleeve is not released as a result of the jarring stress occurring during operation or of its own mass. The forces required to expel the clamping sleeve from the magnet cup are preferably in the range from 100 to 500 N. The term “expulsion force” is understood to mean the force required to release the clamping sleeve from the magnet cup. 
   In a preferred embodiment, the clamping sleeve is made from a nonmagnetizable material. A sleeve of magnetizable material has the disadvantage that even after the delivery of current to the coil ends, the magnetization is preserved, and the valve needle is thus initially kept in the open position by the clamping sleeve and does not close until after a delay. Moreover, the material from which the clamping sleeve is made is preferably not deformable by the impact of the valve needle. A plastic deformation of the clamping sleeve would cause the needle stroke to lengthen over the course of operation of the fuel injector. Lengthening of the needle stroke can for example lead to an increase in the injected fuel quantity and thus to increased fuel consumption. Carbon steel is an example of a suitable material for producing the clamping sleeve. 
   So that the valve needle will not strike the magnet cup when it opens, the diameter of the stop face of the valve needle is greater than the inner diameter of the tubular clamping sleeve and the outer diameter of the tubular clamping sleeve is preferably greater than or equal to the diameter of the stop face of the valve needle. Because the width of the slot extending between the face ends makes up at most 25% of the sleeve circumference, kinking of the valve needle upon impact is avoided, and precise opening is thus achieved. An annular stop face and hence a uniform impact of the valve needle can be attained by providing that in the magnet cup, at least two clamping sleeves are received, whose slots extending between the face ends are located at different radial positions. 
   In a further preferred embodiment, the length of the clamping sleeve is equivalent to the height of the magnet cup, so that the face ends of the clamping sleeve and the magnet cup form a smooth surface. This is attained by providing that first, the at least one clamping sleeve is press-fitted into the magnet cup, and then the face ends of the magnet cup, with the clamping sleeve press-fitted inside it, are ground flat. The remanent air gap required so that the armature of the valve needle will not adhere to the magnet cup is attained by providing that the stop face of the valve needle protrudes out of the armature of the valve needle by the height of the remanent air gap. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  shows a control valve, embodied according to the invention, of a fuel injector; 
       FIG. 2  shows a detail Z of  FIG. 1 ; 
       FIG. 3  is a plan view on the magnet cup with a clamping sleeve received in it; 
       FIG. 4  is a section through a magnet cup with a clamping sleeve received in it, before the face ends are surface-ground; 
       FIG. 5  is a section through a magnet cup with a clamping sleeve received in it, with surface-ground face ends; 
       FIGS. 6.1 ,  6 . 2  and  6 . 3  show various versions of the slot extending between the face ends, of the clamping sleeve; and 
       FIG. 7  is a plan view of a magnet cup with two clamping sleeves received in it. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a control valve  1  includes a valve body  2 , with a bore received centrally in it, in which bore a valve needle  3  is guided. On the side toward an injection valve, not shown here, the valve body  2  is adjoined by a stop plate  4 . On the side toward the stop plate  4 , the bore in the valve body  2  opens up into a first valve chamber  5 . The first valve chamber  5  is defined by an end face  6  of the stop plate  4 . The first valve chamber  5  is adjoined by a bore  10  in the stop plate  4 . The bore  10  opens into a second valve chamber  8 . By means of the flat seat  7  on the valve needle  3 , the bore  10  in the stop plate  4  can be opened or closed. 
   For the sake of installing the injection valve, not shown in  FIG. 1 , in the correct position on the control valve  1 , a guide peg  9  is embodied on the stop plate  4 . For assembly the guide peg  9  is introduced into a corresponding bore on the injection valve. In this way, it is assured that the bores for conduits passing through a plurality of components, for instance, are embodied in aligned fashion in the completely assembled fuel injector. 
   On the side facing away from the stop plate  4 , the valve needle widens into a valve needle head  11 . The valve needle head  11  is adjoined by a guide peg  12 . The guide peg  12  is surrounded by a closing element  13 , preferably embodied as a spiral spring. The closing element  13  is braced by one end against one end face  14  of the valve needle head  11  and by the other end against one end face  16  of an upper housing part  15 . The guide peg  12  prevents the closing element  13  from being able to slip on the end face  14  of the valve needle head  11 . The guide peg  12  also prevents the closing element  13  from kinking upon a stroke motion of the valve needle  3  out of the flat seat  7 . 
   The valve body  2  and the upper housing part  15  are joined together by a lock nut  17 . 
   The opening and closing operation of the valve needle  3  is controlled electromagnetically. To that end, a magnet cup  18  is located in the valve body  2  and has an annularly embodied groove, in which a coil  19  is received. The coil  19  is supplied with current via an electrical terminal  20 . As soon as a voltage is fed to the coil  19 , a magnetic field develops around the coil  19 . By means of this magnetic field, the material comprising the magnet cup  18  is magnetized. This causes an armature  21 , which is made of magnetic material and surrounds the valve needle head  11 , to be attracted by the magnet cup  18 . In this way, the valve needle  3  moves in the direction of the magnet cup  18  and thus uncovers the flat seat  7 . In the process, the closing element  13  embodied as a spiral spring is compressed. The closing element  13  is located inside a bore  22  in the magnet cup  18 , so that the magnet cup  18  surrounds the closing element  13 . 
   The material comprising the magnet cup  18  is preferably a sintered metal, or contains fine iron particles sheathed in plastic that are compacted to make a magnet cup  18 . This prevents eddy currents, which slow down the switching operation, from being created in the magnet cup. This material is extremely brittle and thus vulnerable to impacts. The impact of the valve needle  3  can therefore cause individual particles to break off from the magnet cup  18 . As a result, the magnetic properties change. Furthermore, the broken-off particles can cause increased wear and thus can lead to the failure of the control valve  1 . To absorb the shock load from the impact of the valve needle  3 , a clamping sleeve  23  is received in the bore  22  in the magnet cup  18 . The clamping sleeve  23  has a slot  24  extending between the face ends. The slot  24  serves to enable the clamping sleeve  23  to be press-fitted into the bore  22  in the magnet cup  18  with a defined spring force. The slot  24  makes it possible to insert the clamping sleeve  23  into the bore  22  with a prestressing force. For this reason, in the assembly of the clamping sleeve  23 , no pressing forces need to be exerted on the magnet cup  18  as would be the case with a closed sleeve. Because of the high requisite pressing forces with closed sleeves, such sleeves can burst the magnet cup  18 . This is avoided by the use of the clamping sleeve  23  of the invention, having the slot  24 . 
   An adjusting ring  25  is located between the upper housing part  15  and the magnet cup  18 , and the stroke of the valve needle  3  is adjusted by way of the thickness of this adjusting ring. 
   The clamping sleeve  23  is braced by one face end against the adjusting ring  25 . In this way, the impact forces that act on the clamping sleeve  23  when the valve needle  3  hits it in the opening operation, are transmitted to the adjusting ring  25 . 
   To prevent the clamping sleeve  23  from being magnetized by the coil  19  and thus being capable of affecting the switching operation of the valve needle  3 , the clamping sleeve  23  is preferably made from a nonmagnetizable material. The material of the clamping sleeve  23  must also transmit the shock forces, which act on the clamping sleeve  23  when the valve needle  3  strikes it, to the adjusting ring  25 , and it must not be damaged by these shock forces. For this reason, a carbon steel is preferably selected as the material for the clamping sleeve  23 . Other suitable materials for the clamping sleeve are stainless steels, for example. 
   A second electrical terminal in the control valve  1  is identified by reference numeral  26 . Via the second electrical terminal  26 , a further valve in the fuel injector may for instance be supplied with current. The supply of current to the fuel injector is effected via the contacts  27 . 
     FIG. 2  shows the detail Z of  FIG. 1 , from which it can be seen that the stop diameter  28  of the end face  14  of the valve needle head  11  is less than the outer diameter  29  of the clamping sleeve  23 . This assures that the valve needle  3  will strike against only the clamping sleeve  23 , and not against the magnet cup  18 , since striking against the magnet cup  18  could cause the magnet cup to be damaged. 
   Upon opening of the valve needle  3 , the stroke is limited by the striking of the end face  14  of the valve needle head  11  against the clamping sleeve  23 . Upon closing, the stroke of the valve needle  3  is limited by the fact that the valve needle  3  is put into the flat seat  7 , not shown in  FIG. 2 . The stroke of the valve needle  3  is represented by reference numeral  30 . 
   To prevent the armature  21  from adhering to the magnet cup  18  when the valve is open, a remanent air gap  31  is provided. The adherence of the armature  21  to the magnet cup  18  results from the fact that, because of the small component size of the fuel injector, all the surfaces have only very slight roughness. For this reason, a thin fuel film between two surfaces has an adhesive effect. 
   The adjustment of the remanent air gap  31  is effected, in the fuel injector embodied according to the invention, in such a way that the valve needle head  11  protrudes out of the armature  21  by the height of the remanent air gap  31 . 
     FIG. 3  shows a plan view of a magnet cup with a clamping sleeve received in it. 
   The plan view on the magnet cup  18  shows that in it, at least one bore  32  is received, which discharges into an annular groove  33  for receiving the coil  19 . In the embodiment shown in  FIG. 3 , two bores  32  are received in the magnet cup  18 . The bores  32  serve to receive the electrical terminal  20 , with which the coil  19  by which the magnetic field is generated is supplied, and for receiving the second electrical terminal  26 , which is used for instance to supply current to a second valve in the fuel injector. The bore  22  is received in centered fashion in the magnet cup  18 , which is preferably embodied with a circular cross section. The clamping sleeve  23  is located in the bore  22  and is pressed with a spring force against the wall of the bore  22 . For bringing the spring force to bear, the slot  24  is embodied in the clamping sleeve  23 , extending between the face ends of the clamping sleeve  23 . The spring force is brought to bear by the fact that the clamping sleeve  23  is compressed, as a result of which the width of the slot  24  is reduced and the diameter of the clamping sleeve  23  is shortened. The thus prestressed clamping sleeve  23  is introduced into the bore  22 . In the bore  22 , the prestressing is absorbed by the clamping sleeve  23 , so that the clamping sleeve opens to its original shape. This opening is interrupted by the wall of the bore  22 , so that the clamping sleeve  23  is pressed against the wall of the bore  22  with a residual spring force. The residual force is great enough that jarring stress and the mass of the clamping sleeve  23  do not cause a release of the clamping sleeve  23 . In this way, the clamping sleeve  23  is fixed (in a press fat) in the bore  22  of the magnet cup  18  as a result of the spring force. 
     FIG. 4  shows a magnet cup with a clamping sleeve received in it, before the concluding surface treatment. The annular groove  33  for receiving the coil  19  is located in the magnet cup  18 . The annular groove  33  communicates with the bores  32 , so that the coil  19  in the annular groove  33  can be supplied with current via the bore  32 . This view shows that the clamping sleeve  23  protrudes out of the magnet cup  18 , forming a protrusion  35 . This shows the step in assembly when the clamping sleeve  23  is already inserted in the magnet cup  18 , but the end face  34  of the magnet cup  18  has not yet been ground flat. 
     FIG. 5  by comparison shows the magnet cup  18  with the clamping sleeve  23  inserted and with the surface-ground end face  34  of the magnet cup  18  and the surface-ground face end  36  of the clamping sleeve  23 . The advantage of the assembly process in which the clamping sleeve  23  is first inserted into the magnet cup  18  and after that the magnet cup  18  and the clamping sleeve  23  inserted in it are brought to the same height  37  is that the components need not be paired. In other words, there is no need to take care that the height  37  of the magnet cup  18  and the length of the clamping sleeve  23  match exactly. As a result, during the production process, the clamping sleeve  23  need not be introduced into the magnet cup  18  repeatedly and taken out of it again for postmachining, since the machining is done of the two parts jointly. This makes economical assembly possible. Because of the joint grinding of the magnet cup  18  and clamping sleeve  23 , an exactly plane surface  34 ,  36  is achieved. 
   In  FIGS. 6.1 ,  6 . 2  and  6 . 3 , various embodiments of the slots extending between the face ends are shown.  FIG. 6.1  shows a slot  24  in the clamping sleeve  23  that extends in the axial direction between the face ends. 
   In  FIG. 6.2 , a slot  24  is shown that extends in a spiral around the clamping sleeve  23 . The part of the slot  24  that is located on the side of the clamping sleeve  23  that projects into the plane of the drawing is represented by dashed lines. Besides the variant embodiment shown here, with a slot  24  extending all the way around the clamping sleeve  23  once, the slot  24  may also extend in the form of a spiral more than once around the clamping sleeve  23 . 
   A further embodiment of the slot  24  is shown in  FIG. 6.3 . Here, the slot  24  extends in an arc on the clamping sleeve  23 . 
   Besides the forms shown in  FIGS. 6.1 ,  6 . 2  and  6 . 3 , in which the slot  24  can extend between the face ends of the clamping sleeve  23 , any other course known to one skilled in the art is also possible. In the embodiment of the slot  24 , care must merely be taken that it extend between the face ends of the clamping sleeve  23 . 
   To prevent the valve needle  3  from becoming tilted upon striking the clamping sleeve  23 , the width of the slot  24 , in a preferred embodiment, amounts to a maximum of 25% of the circumference of the clamping sleeve  23 . 
   A further possible way of preventing tilting of the valve needle  3  upon striking the clamping sleeve  23  is shown in  FIG. 7 . Here, a second clamping sleeve  38  is braced against the clamping sleeve  23  in the bore  22  in the magnet cup  18 . The assembly of the second clamping sleeve  38  is done analogously to the assembly of the clamping sleeve  23 . To prevent the valve needle  3  from tilting upon impact and to make a uniform stop face available, a slot  39  of the second clamping sleeve  38  is offset from the slot  24  of the clamping sleeve  23 . This assures that the end face  14  of the valve needle head  11  will strike the clamping sleeves  23 ,  38  over its entire circumference. 
   It is also possible for more than two clamping sleeves  23 ,  38  to be used. In a preferred embodiment, the slots  24 ,  39  of the clamping sleeves  23 ,  38 , when more than two clamping sleeves are used, are offset from one another in such a way that the slot of each clamping sleeve is located at a different position along the circumference. 
   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.