Abstract:
A spark plug includes a partially cylindrical insulator element, including a bore hole and a central electrode located in an opening in the base part of the insulator element. The insulator element and the central electrode are connected by at least one metallic soldered connection and/or one metallic welded connection and/or one friction-locked connection aligned in the radial direction.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a spark plug which includes a partially cylindrical insulator element having a bore hole. A central electrode is located in the bore hole of the insulator element on the side of a base part of the insulator element (referred to hereinafter in short as base part). The present invention also relates to a corresponding method for production of the spark plug.  
         BACKGROUND INFORMATION  
         [0002]    In conventional spark plugs, the central electrode is, for example, cast into the insulator element with the aid of a glass melt and/or with the aid of a contact or compound. The glass melt simultaneously acts as a damping resistor to damp the spark discharge during operation of the spark plug.  
           [0003]    It is an object of the present invention to provide a compact spark plug and a corresponding method for its production, with the metal electrode being attachable in the insulator element in a simple manner.  
         SUMMARY  
         [0004]    The above and other beneficial objects of the present invention are achieved by providing a spark plug and a method for its production as described herein.  
           [0005]    The present invention is based on the concept that a glass melt and/or an adhesive, contact or compound are only suitable for attachment of the metal electrode on or in the insulator element under certain conditions. Therefore, in the spark plug according to the present invention, the insulator element and central electrode are connected by at least one metallic soldered connection or one metallic welded connection. The soldered connection may be produced by hard soldering or active soldering, i.e., through a soldering procedure at a temperature over 450° C. The soldered connection may be produced through fusion welding, friction welding, or, for example, inductive welding.  
           [0006]    In one example embodiment of the present invention, the connection is at an end of the central electrode further from the combustion chamber. The outer diameter of the central electrode is slightly smaller in the region of the connection than the inner diameter of the insulator element at the same distance from the free end of the base part. The connection extends along the circumference of the central electrode and closes the gap between the central electrode and the insulator element. In this manner, the connection performs a double function—it connects and seals gas-tight.  
           [0007]    In an alternative example embodiment of the present invention, the inner diameter of the insulator element in the region of the connection is slightly smaller than the outer diameter of the central electrode, not including the surrounding insulator core, at the same distance from the free end of the base part. In the assembled state, the central electrode is pressed together by a surrounding insulator core. A very solid connection is produced due to the differing coefficients of thermal expansion.  
           [0008]    To produce the friction-locked connection, the metallic central electrode is, for example, cooled down and inserted in its contracted state into the insulator element. As it warms up, the central electrode expands and is pressed against the inner wall of the insulator element.  
           [0009]    In a further example embodiment of the spark plug according to the present invention, a force is exerted on the central electrode in the axial direction with the aid of a spring-loaded element, for example, with the aid of a contact pin. The force exerted by the contact pin counteracts forces which arise in the combustion chamber during operation of the spark plug and are transmitted to the central electrode. The connection between the central electrode and the insulator element is stressed less by the use of the contact pin than if the contact pin is not there. In this manner, the connection between the central electrode and the insulator element may be configured smaller or may be stressed more while remaining the same size.  
           [0010]    In a further example embodiment of the present invention, the contact pin is buckled at at least one position. The spring effect may be increased by buckling the contact pin. Excess mechanical stress is prevented in the insulator element when a terminal stud is being screwed in if the contact pin buckles when specific stresses are exceeded.  
           [0011]    In the spark plug according to the present invention, the insulator element includes ceramic. The surface of the ceramic is treated in the region of the connection so that the load capacity of the connection is enhanced. Roughening of the surface and/or applying a metallic topcoat may be suitable methods. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIGS. 1A and 1B illustrate a compact spark plug with a damping resistor made of a solidified glass melt.  
         [0013]    [0013]FIGS. 2A and 2B illustrate a compact spark plug without a damping resistor.  
         [0014]    [0014]FIGS. 3A and 3B illustrate a compact spark plug with a nondestructively replaceable damping resistor. 
     
    
     DETAILED DESCRIPTION  
       [0015]    [0015]FIG. 1A illustrates a compact spark plug  10  in a partial cross-sectional view. Spark plug  10  includes a cylindrical insulator  12  which tapers at its end toward an insulator base  14 . Insulator  12  is penetrated along its longitudinal axis  16  by a through hole  18 , the diameter in the region of a central electrode  20  of which is somewhat smaller than along the rest of insulator  12 . The half of insulator  12  containing insulator base  14  is almost completely surrounded by a housing  22 . Viewed from insulator base  14  outwardly, housing  22  includes, in this sequence, a ground electrode  24 , a threaded sleeve  26  having, for example, M14 external thread  28 , a peripheral groove  30  for a sealing ring which provides a seal in the conical seal seat, a central part  32 , and a double hex insertion nut  34 . Housing  22  is screwed into an engine block of the vehicle and is connected with the ground electrode. Insulator  12 , which is made of ceramic, insulates housing  22  and central electrode  20  as well as additional elements for current conduction located in through hole  18 .  
         [0016]    In through hole  18  there are, in sequence from central electrode  20  to a terminal stud  36  screwed onto insulator  12  for connection of an ignition cable, an electrically conducting contact or compound  38 , a glass melt  40 , which forms a damping resistor, an electrically conducting contact or compound  42 , and an electrode  44 . Electrode  44  tapers toward insulator base  14  and forms a section  46  having a somewhat smaller diameter than the main part of electrode  44 .  
         [0017]    Housing  22  is connected to insulator  10  by a welded connection  48 . Welded connection  48  extends longitudinally up into threaded sleeve  26  from the end of housing  22  further from the base part. Welded connection  48  extends completely around the circumference arranged transverse to the longitudinal direction. A gap between insertion nut  34  and insulator  12  is completely closed by welded connection  48 . A gap formed between the end of threaded sleeve  26  further from the base part and insulator  10  is also completely closed by welded connection  48 .  
         [0018]    [0018]FIG. 1B illustrates a connection  48   b , in which a housing  22   b , constructed like housing  22 , of a spark plug  10   b  having an insulator  12   b  is only welded in a region  50  which extends along the half of a threaded sleeve  26   b  further from the base part. Region  50  extends, for example, 10=10 mm in the longitudinal direction, i.e., in the direction of a longitudinal axis  16   b  of insulator  12   b . Welded connection  48   b  extends along the lateral surface of insulator  12   b  in region  50 .  
         [0019]    In the region of a insertion nut  34   b  constructed like insertion nut  34 , a peripheral gap  52  is disposed between insulator  12   b  and insertion nut  34   b . Otherwise, spark plug  10   b  is constructed like spark plug  10 .  
         [0020]    Due to welded connection  48  or  48   b , spark plug  10  may be made very compact. The largest diameter D of insulator  12  is, for example, 10.4 mm. Diameter D remains constant in the main part of insulator  12  and therefore substantially determines the overall space necessary for the installation of spark plug  10 . Insertion nut  34  is configured as a double hex nut, e.g., for a width  14  across flats. This is only possible because insulator  12  has no projections in the region of insertion nut  34 .  
         [0021]    In other example embodiments of the present invention, an interlayer may be used, in place of welded connection  48  or  48   b , which is welded or soldered onto insulator  12  or  12   b  and onto housing  22  or  22   b . The welded or soldered connections, respectively, between the interlayer and insulator  12  and between the interlayer and housing  22  are in the region of central part  32  and threaded sleeve  26  and in the region of insertion nut  34 . Alternatively, there are connections between the interlayer and insulator  12   b  both in the region of threaded sleeve  26   b  and in the region of insertion nut  34   b . In the alternative, a connection exists between the interlayer and housing  22   b  only in the region of threaded sleeve  26   b . A gap is disposed between the interlayer and insertion nut  34   b  in the region of insertion nut  34   b.    
         [0022]    [0022]FIG. 2A illustrates, in a partial cross-sectional view, a compact spark plug  10   c  which has no damping resistor. Functional elements illustrated in FIG. 2A which are constructed substantially like those described with reference to FIG. 1A have the same reference numbers in FIG. 2A but are suffixed with the lowercase letter c. This particularly applies to reference numbers  12   c  to  36   c.  Central electrode  20   c  has a diameter in its main part which is smaller than the diameter of central electrode  20 . This arrangement allows the diameter of through hole  18   c  and outer diameter Dc of insulator  10   c  to be reduced. Central electrode  20   c  is coated with a hard solder paste and then inserted through through hole  18   c  into insulator  12   c . A contact pin  100 , made of, for example, a brass alloy, is inserted into through hole  18   c . When terminal stud  36   c  is screwed in, contact pin  100  is compressed and buckles at multiple buckling positions.  
         [0023]    Central electrode  20   c  is secured by contact pin  100 . Insulator  10  is transported through a high vacuum furnace at a temperature of a magnitude between 600° C. and 900° C., for example, 800° C. The hard solder paste melts and connects central electrode  20   c  firmly and permanently with insulator  12   c . This connection is also gas-tight. The hard solder paste is, for example, applied in the region of a shoulder  102 , at which the inner diameter of through hole  18   c  decreases. Alternatively, central electrode  20   c  may be coated substantially completely with hard solder paste, so that central electrode  20   c  and insulator  10   c  are also connected in the region of insulator base  14   c.    
         [0024]    There is an interlayer  104  on insulator  10   c  which is less than, for example, 1 mm thick. Interlayer  104  is connected to insulator  10   c  via, for example, a hard soldered connection, in the region of a step  106  of insulator  10   c , which is approximately, e.g., 11=12 mm long. At the end of step  106  further from the base part, interlayer  104  conforms to the shape of insulator  10   c , which widens. In a section  108 , however, interlayer  104  forms a tubular section having a larger inner diameter than outer diameter Dc of insulator  10   c . Thus, there is a gap  110  in the region of section  108  between interlayer  104  and insulator  10   c . In section  108 , interlayer  104  is connected on its outer side with the inner side of insertion nut  34   c , for example, by a soldered or welded connection. In the region of step  106 , the outer side of interlayer  104  is not connected with housing  22   c , so that in this region a gap  111  is disposed between interlayer  104  and housing  22   c.    
         [0025]    Through the shaping and nature of the attachment of interlayer  104 , forces which arise in housing  22   c  as spark plug  10   c  is screwed in may not be transmitted directly to insulator  10   c . Interlayer  104  absorbs these forces in the transition region between step  106  and section  108 .  
         [0026]    [0026]FIG. 2B illustrates a spark plug  10   d  constructed similarly to spark plug  10   c . There are differences only in the region of an interlayer  104   d , which is used in place of interlayer  104 . Interlayer  104   d  is connected in the region of a step  106   d  with an insulator  12   d . In a transition region  112 , interlayer  104   d  widens conically in correspondence with the shape of insulator  12   d . In transition region  112 , as well as in an adjacent section  114 , the inner side of interlayer  104   d  is also connected with insulator  12   d , for example, with the aid of a soldered or welded connection.  
         [0027]    The outer side of interlayer  104   d  is exposed in the region of step  106   d , so that a gap  110   d  is formed between interlayer  104   d  and housing  22   d . The outer side of interlayer  104   d  is connected to housing  22   d  in the region of section  114 , for example, by soldering or welding. The connection has a length of, e.g., 12=8 mm along a longitudinal axis  16   d.    
         [0028]    Mechanical stresses which arise in the region of a groove  30   d  as spark plug  10   d  is screwed in may not be directly transmitted to insulator  12   d  due to gap  110   d . The force lines first extend into housing  22   d  and only enter insulator core  12   d  in section  114 . The forces are, however, less at this point than in the region of groove  30   d.    
         [0029]    A sealing ring is located in the region of groove  30   d  which forms a seal in the flat sealing seat between the engine block and a central part  32   d . Otherwise, spark plug  10   d  is constructed like spark plug  10   c.    
         [0030]    [0030]FIG. 3A is a partial cross-sectional view of a compact spark plug  10   e  which is constructed similarly to spark plug  10   c  illustrated in FIG. 2A. Elements with reference numbers  12   e  to  36   e  correspond in their configuration and function to the elements  12   c  to  36   c  explained above with reference to FIG. 2A.  
         [0031]    Central electrode  20   e  is inserted first into through hole  18   e.  Subsequently, a replaceable damping resistor  120  is inserted, which has a shape resembling a conventional fuse. Only then is a contact pin  122  inserted, which buckles at multiple buckling positions as terminal stud  36 e is screwed in. Insulator  12   e , which was screwed on in this manner, is in turn heated to approximately 800° C., with a soldering paste applied to central electrode  20   e  melting and central electrode  20   e  connecting with insulator  12   e.    
         [0032]    An interlayer  124  corresponds to interlayer  104  in its configuration, function, and type of attachment to insulator  12   e  and housing  22   e  describe above with reference to FIG. 2A.  
         [0033]    [0033]FIG. 3B illustrates a part of a spark plug  10   f,  which is configured like spark plug  10   e  as described above with reference to FIG. 3A. An interlayer  126   f  is soldered onto insulator  12   f  of spark plug  10   f  in a section  130 . Section  130  is arranged within threaded sleeve  26   f . The inner diameter of interlayer  126   f  and the diameter of insulator  12   f  increase uniformly within a transition section  132 . In the region of a section  134  arranged within insertion nut  34   f , the inner diameter of the sleeve formed by interlayer  126   f  remains constant. The diameter of insulator  12   f  also remains constant within section  134 . In section  134 , interlayer  126   f  is soldered to both insulator  12   f  and housing  22   f . In contrast, in the region of section  130  and in the region of transition section  132 , a gap  136  is disposed between housing  22   f  and insulator  12   f.