Abstract:
A fuel injector, e.g., for fuel-injection systems of internal combustion engines in motor vehicles, includes a valve housing including a sleeve-shaped housing section; and a solenoid, situated on the housing section, having a coil brace, magnetic coil and magnetic cup. To protect the magnetic coil from salt fog that arises under specific environmental conditions and penetrates along the contact areas of solenoid and housing section, the coil brace with wound magnetic coil is completely enclosed on all sides by a one-piece plastic coat.

Description:
FIELD OF THE INVENTION 
   The present invention is based on a fuel injector, in particular for fuel injection systems of internal combustion engines of motor vehicles. 
   BACKGROUND INFORMATION 
   In a conventional fuel injector the magnetic cup with coil brace and magnetic coil wound thereon is slipped over the sleeve-shaped housing section and placed on a lower housing part via its cup opening, the lower housing part fixing a valve-seat support in place on the sleeve-shaped housing section. Via the inner cylinder wall of its support body, the coil brace sits directly on the sleeve-shaped housing section and is surrounded by the cup wall of the magnetic cup with air clearance. The magnetic cup has a cup nipple, which extends axially along the cup base, is integrally formed with the cup base and encloses the sleeve-shaped housing part. Using machining, a circumferential labyrinth is cut into the outside of the cup nipple. A second labyrinth is introduced on the sleeve-shaped housing part by machining, with axial clearance from the cup nipple. The sleeve-shaped housing part and the cup nipple are enveloped by a plastic-extrusion coat on which a plug has been premolded to connect the magnetic coil to a mating plug. Due to the two labyrinths onto which the plastic is shrink-fitted, the magnetic coil is sealed from environmental influences, so that salt fog, which forms under certain environmental conditions, will not penetrate the transitions between the plastic of the plastic-extrusion coat and the metal of the sleeve-shaped housing part and travel along the metal walls to the magnetic coil to cause electrical damage there. Since the labyrinths are able to be produced only by machining, the production of these labyrinths is very expensive, which is reflected to a considerable extent in the production costs of the fuel injector. 
   SUMMARY 
   Example embodiments of the present invention may achieve uniformly excellent sealing of the magnetic coil from damaging environmental influences at low production expense an may provide that the outer diameter of the solenoid is able to be kept smaller with no change in the magnetic output. The labyrinth premolded on the coil shell in the fuel injector is easy to produce from a standpoint of production engineering and is already premolded during production of the coil brace. The die mold for producing the coil brace may have a simpler design since the labyrinth is omitted, and the overall axial height of the solenoid is able to be reduced as well with no change in the magnetic capacity. By appropriate adaptation of the material of the coil brace and the plastic of the plastic-extrusion coat and of the injection-molding parameters of the plastic, a homogenous connection between the coil brace and the plastic on the contact surfaces is achieved in the injection-molding operation. 
   The magnetic cup, the magnetic coil brace embedded in the magnetic cup by the plastic-extrusion coat and carrying the magnetic coil, the yoke integrated in the plastic coat and used for closing the magnetic circuit, as well as the plug premolded on the plastic coat for contacting the magnetic coil may form a prefabricated assembly unit. This assembly unit may be produced and delivered outside of the production line for the fuel injector, for instance by a supplier. In this manner, the cycle time required for the injection-molding operation of the plastic coat is not linked to the clock cycle of the production line of the fuel injector. 
   Example embodiments of the present invention are explained in greater detail in the following description with reference to the appended Figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cutaway view of a longitudinal section of a fuel injector. 
       FIG. 2  is an enlarged view of the cutaway portion II illustrated in  FIG. 1 , with a modification of the fuel injector. 
       FIGS. 3 and 4  are semi-longitudinal cross-sectional views of a fuel injector according to exemplary embodiments. 
   

   DETAILED DESCRIPTION 
   The fuel injector schematically illustrated in  FIG. 1  in longitudinal cross-section may be used in fuel-injection systems of internal combustion engines in motor vehicles. It has a valve housing  11  having a thin-walled, sleeve-shaped upper housing section  12  whose free end forms a connection nipple  34  for the fuel feed, and a lower housing section  13 , which is integrally joined thereto and arranged as valve-seat support having a valve opening. As an alternative, the valve-seat support may also be inserted in lower housing section  13  as a separate component. Connection nipple  34  encloses a fuel-intake duct  36 , which is sealed by a fuel filter  34  and continues through lower housing section  13  up to the valve opening. A solenoid  14  is situated on sleeve-shaped upper housing section  12 . Solenoid  14  includes a coil brace  15 , a magnetic coil  16  wound thereon, a magnetic cup  17  in which coil brace  15  is accommodated, and a sleeve-shaped solenoid core  18 , which is inserted into sleeve-shaped, thin-walled upper housing section  12  and affixed therein so as to reduce the magnetic resistance and to form an air gap with respect to a magnetic armature  19  in the magnetic circuit of solenoid  14 . Magnetic armature  19  lying opposite solenoid core  18  with an axial gap clearance is guided in valve housing  11  in a displaceable manner and permanently connected to a valve needle  20 . To place solenoid  14  on upper housing section  12 , cup base  172  of magnetic cup  17  is provided with a central base opening  173 , so that cup base  172  rests against upper housing section  12 . The magnetic circuit of solenoid  14  is closed by a yoke  22 , which at least sectionally rests against the inner surface of cup wall  171  on the one side and against upper housing section  12  on the other side. Disposed in solenoid core  18  is a valve closure spring  37 , which is braced on valve needle  20  and on an adjustment sleeve  38  pressed into solenoid core  18 , and which presses valve needle  20  with its valve top onto the valve seat in the valve-seat support. The magnitude of the spring resilience of valve-closure spring  37  is defined by the insertion depth of adjustment sleeve  28 . 
   Coil brace  15  has a hollow-cylindrical support body  151  and two support flanges  152  which delimit support body  151  at the front side. Magnetic coil  16  is wound onto support body  151  and axially fixed in place by support flanges  152 . Premolded on both support flanges  152  is an axially projecting labyrinth  23 , which is made up of a plurality of concentric bars  231 . Also premolded on upper support flange  152  is an insulation dome  24 , which encloses two electrical connector pins  25  for magnetic coil  16 . Connector pins  25  are guided out of insulation dome  24  at its free end and configured for contacting the contact sockets  26  of a connector plug  27 . Premolded on lower support flange  152  is a spacer  28 , which creates a defined axial clearance of coil brace  15  with respect to cup base  172  of magnetic cup  17 . Labyrinths  23 , spacer  28  and insulation dome  24  are already premolded during the production process of coil brace  15  made of plastic. 
   Coil brace  15  having magnetic coil  16  is situated in magnetic cup  17  such that the inner wall of support body  151  facing away from magnetic coil  16  is able to be slipped directly onto sleeve-shaped upper housing section  12 . Coil brace  15  lies inside magnetic cup  17  with radial clearance from cup wall  171  and with an axial clearance from cup base  172  that is predefined by a spacer  28 . The free space between coil brace  15  having wound magnetic coil  16 , and magnetic cup  17  is filled with injection-molded plastic material, which also coats upper support flange  152 , so that coil brace  15  is enveloped by a plastic coat  29  that ends at sleeve-shaped upper housing section  12 . When coil brace  15  is injection-molded, yoke  22  is integrated in plastic coat  29  on the one hand, and a plug  30  is premolded on plastic coat  29  on the other hand. Plug  30  has a recess  31  that terminates freely at its free end faces and is used to slide connector plug  27  on. Recess  31  is configured such that insulation dome  24  protrudes somewhat into recess  31 , beyond its base. When sliding connector plug  27  onto plug  30 , contact sockets  26  of connector plug  27  slide onto connector pins  25  projecting from insulation dome  24 . During plastic-extrusion-coating, the plastic shrinks onto labyrinths  23  on support flanges  152  and seals magnetic coil  16  from sleeve-shaped upper housing section  12 . The salt fogs that occur under certain environmental conditions and which penetrate between sleeve-shaped upper housing section  12  and coil brace  15  are then unable to reach magnetic coil  16  and damage it in the long term. Since insulation dome  24  projects beyond the base of recess  31  in plug  30 , it is also prevented that the salt fog is able to reach magnetic coil  16  via the magnetic-coil connection. 
   As an alternative, as illustrated in  FIG. 2 , insulation dome  24  may also end in front of recess  31  of plug  30 . In this case, insulation dome  24  is provided with a circumferential labyrinth  32 . In the exemplary embodiment illustrated in  FIG. 2 , labyrinth  32  is formed by three annular bars  321 , which radially project from insulation dome  24  with axial clearance from one another. 
   The afore-described solenoid  14  having magnetic cup  17 , coil brace  15 , which is embedded in magnetic cup  17  by plastic coat  29  and supports magnetic coil  16 , yoke  22  integrated in plastic coat  29 , and plug  30  premolded on plastic coat  29  as one piece form a premanufactured assembly unit, which is slipped over sleeve-shaped upper housing section  12  of the fuel injector in its entirety. A plastic-extrusion coat  33  is then applied on plastic coat  29  on the upper side facing away from cup base  172 , plastic-extrusion coat  33  enclosing connection nipple  34 . 
     FIG. 3  illustrates an exemplary embodiment for solenoid  14  inserted in the fuel injector according to  FIG. 1 . This exemplary embodiment differs from solenoid  14  described in connection with  FIG. 1  in that coil brace  15  having magnetic coil  16  is completely enveloped by a one-piece plastic coat  39 , i.e., is practically completely encapsulated in plastic. In this case, it is possible to dispense with the labyrinths on support flanges  152  and the labyrinth on insulation dome  24 . Premolded on coil brace  15 , which is inserted with radial clearance from cup wall  171  of magnetic cup  17 , is a first spacer  40 , which is braced on cup base  172 , and a second spacer  41 , which creates a radial clearance with respect to sleeve-shaped upper housing section  12  or with respect to the inner wall of plastic coat  39 . To encapsulate coil brace  15  having wound magnetic coil  16 , the free space between coil brace  15  and magnetic cup  17  on one side, and coil brace  15  and upper housing section  12  on the other side is completely filled with injection-molded plastic, which also coats support flange  152  facing away from cup base  172 , and which encloses insulation dome  24  premolded on coil brace  15 . An unmolding core, which replaces upper housing section  12 , is inserted in magnetic cup  17  when injecting plastic coat  39 . 
   In order to decrease the dimensions of solenoid  14  in the radial and axial directions without reducing the volume of magnetic coil  16  and thus the output of solenoid  14 , in the exemplary embodiment of solenoid  14  illustrated in  FIG. 4 , coil brace  15  is situated in magnetic cup  17  such that the inner wall of support body  151  facing away from magnetic coil  16  is able to be slipped over sleeve-shaped upper housing section  12 . Coil brace  15  in turn is accommodated in magnetic cup  17  with radial clearance from cup wall  171 , and rests on cup base  172  via its lower support flange  152 . Here, too, plastic is injected into the remaining free space between coil brace  15  having wound magnetic coil  16  and magnetic cup  17 , which coats support flange  152  facing away from cup base  172  up to housing section  12  and which forms a plastic coat  42  around coil brace  15 . The plastic is adapted to the material of coil brace  15 , and the injection parameters of the plastic such as temperature and injection pressure are selected such that the plastic and the material of coil brace  15  are integrally joined at contact areas  43 ,  44 , so that magnetic coil  16  in turn is sealed from the boundary surface between support body  151  and sleeve-shaped housing section  12 . 
   In an example embodiment, the pre-manufactured assembly unit is produced without magnetic cup  17 , and this assembly unit, which is then made up of coil element  15 , with magnetic coil  16 , plastic coat  29 ,  39  or  42  with integrated yoke  22  and premolded plug  30 , is installed in magnetic cup  17  on the assembly line of the fuel injector. Moreover, yoke  22  may also be removed from the assembly unit and placed on top of the cup opening of assembly cup  17  as a separate component during assembly of the fuel injector.