Abstract:
A spark plug is described having a partially cylindrical insulator element and a housing surrounding the insulator element on the side of a base part. The insulator element and the housing are connected with one another by at least one material bond and/or one friction-lock connection aligned in the radial direction. A compact spark plug can be produced using this type of connection. In particular, the diameter of the spark plug is smaller than the diameter of known spark plugs having the same characteristics.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a spark plug which includes a partially cylindrical insulator element and a housing which surrounds the insulator element. The present invention also relates to a corresponding method of production. The insulator element typically includes a ceramic material. In contrast, the housing is made of metal.  
         BACKGROUND INFORMATION  
         [0002]    Various methods are known for connecting the insulator element and the housing. Basically, these can be divided into hot assembly and cold assembly. In hot assembly, the insulator is inserted into the housing. The insulator is then pretensioned in the axial direction by reshaping an inwardly curved flange on the housing. The final pretension in the axial direction is achieved through a shrink fit process. During the shrink fit process, a shrinkage recess which surrounds the housing circumferentially is inductively heated to approximately 1050° C. by a current pulse. As the shrinkage recess cools, the material in the region of the shrinkage recess shrinks. The housing is thus essentially secured on a projection of the insulator element by axial forces. At the same time, the housing is axially friction-locked between two shoulders of the insulator.  
           [0003]    In cold assembly, a talcum powder packet is inserted between the flange, which is not yet curved, and the insulator element. Subsequently, the talcum powder packet is compressed by the reshaping process of the flange. In cold assembly as well, the insulator element must have a projection on which the inwardly curved flange is secured.  
           [0004]    The known spark plugs do have connections which have high mechanical strength and are gas-tight, but they require a comparatively costly reshaping process.  
         SUMMARY OF THE INVENTION  
         [0005]    An object of the present invention is the provision of a spark plug having a simple construction and a corresponding method of production, with the spark plug particularly being more compact than spark plugs, produced with typical methods of production, having similar or identical operating characteristics, e.g., with regard to thermal conductivity and with regard to electrical characteristics.  
           [0006]    The present invention is based on the consideration that reshaping is only possible if the housing has a significantly larger diameter than the insulator element at the reshaping position. In addition, a peripheral projection of the insulator element in the region of the reshaping position must secure the housing.  
           [0007]    In the spark plug according to the present invention, the insulator element and the housing are connected to one another by at least one material bond and/or one friction-lock connection aligned in the radial direction. The material bond is, e.g., a metallic soldered or welded connection and the radial friction-lock is a shrink fit connection.  
           [0008]    This connection forms at least a significant portion of the cohesion of the housing and the insulator element. If the material bond and/or the friction-lock connection aligned in the radial direction absorb a part, e.g., approximately half, of the forces which act between housing and insulator element, reshaping can be reduced or even avoided completely, because the cohesion of insulator element and housing is achieved in another way. In addition, the peripheral projection on the insulator element can be designed smaller or even be dispensed with completely. If the other properties are unchanged, the spark plugs according to the present invention are more compact than comparable typical spark plugs, because the diameter of the housing selected can be smaller. Spark plugs according to the present invention have smaller internal thread diameters and smaller screw-in devices than known spark plugs having the same thermal value. For example, the outer diameter of the internal thread can be reduced from M14 to M12. Spark plugs produced until now with M8 threads can now be produced with M6 threads.  
           [0009]    In a refinement of the spark plug according to the present invention, the diameter of the insulator core remains approximately the same or increases as the distance to the free end of the base part of the insulator (referred to in short in the following as base part) increases in the entire region surrounded by the housing. For example, the insulator core tapers in a stepped shape toward the free end of the base part. In other words, the insulator core does not have a projection in the region of the housing used to secure the housing and is therefore more compact than comparable known insulator elements.  
           [0010]    In a subsequent refinement, the inner diameter of the housing in the region of the connection remains approximately the same or increases as the distance to the free end of the base part increases. In other words, the housing no longer has an edge which is curved inward. This allows the use of a housing with a smaller diameter, because reshaping of the edge is no longer necessary.  
           [0011]    In a subsequent refinement, the diameter of the insulator element at the end further from the base part in the region adjoining the region surrounded by the housing is approximately equal to the largest diameter of the insulator core in the surrounded region. The insulator element is preferably cylindrical both inside a section of the housing and outside the housing, i.e., it has a uniform diameter. The fewer the projections and constrictions that are located on the insulator element, the more crack resistant it is.  
           [0012]    In a subsequent refinement, the housing has at least one tubular section in which the diameter of the insulator core is only slightly smaller than the inner diameter of the housing lying at the same distance to the free end of the base part. The connection lies along the circumference of the insulator element in the gap between insulator element and housing. In this refinement, the connection has a double function, because it is used both for connecting insulator element and housing and for sealing the combustion chamber in which the spark plug is to be inserted.  
           [0013]    The tubular section lies close to the base part and/or further from the base part. If the section is close to the base part, it is subjected to greater mechanical load and higher temperatures. On the other hand, the insulator element is thin near the base part, so that the circumference is smaller than further away from the base part. If the connection also seals the combustion chamber gas-tight, the combustion chamber is enlarged only insignificantly if the connection is near the free end of the base part. If the connection is at a greater distance from the free end of the base part, for example at the end of the housing further from the base part, the mechanical loads and the temperature effect are less. The connection will not be loaded as strongly during operation of the spark plug. If the connection is in multiple zones, the disadvantages of one position can be avoided by the advantages of the other position.  
           [0014]    In embodiments, the connection is a soldered connection, e.g., a hard soldered connection, an active soldered connection, a welded connection, and/or an adhesive connection. For the welded connection, the known welding methods are used, e.g., friction welding or gas fusion welding. Reactive adhesives, whose components react during curing, are, for example, used as the adhesive for the adhesive connection. However, hard-setting adhesive materials whose components do not react during curing are, for example, also used.  
           [0015]    In an alternative refinement, the housing contains at least one tubular section in which the diameter of the insulator element is slightly larger than the inner diameter of the housing, when the insulator element is not in place, lying at the same distance to the free end of the base part. Therefore, this is a compression connection, for example a longitudinal compression connection or a transverse compression connection. During the production of the transverse compression connection, for example, the housing is heated. Subsequently, the insulator core is inserted into the expanded housing. As the housing cools, it shrinks and tightly surrounds the insulator element.  
           [0016]    In a refinement of the spark plug according to the present invention, insulator element and housing are connected with one another using an interlayer which was produced before housing and insulator were connected. The interlayer is produced from a material which is capable of being connected well on one side with the ceramic and on the other side with the metal of the housing. The interlayer can, for example, be formed by a thin sheet steel sleeve. However, interlayers made of other materials, e.g., plastic or glass melt, are also used. The interlayer is applied or attached to the insulator element. Thus, interlayers can be deposited directly on the insulator element. The interlayer is attached to the housing using a material bond and/or a friction-lock connection.  
           [0017]    If, in an embodiment, the interlayer also extends into regions which lie outside the connection region, the interlayer can be attached better to the insulator, because the connection surface between the insulator and the interlayer is larger.  
           [0018]    In a refinement, there is a gap between the housing and the interlayer in the region of the section lying closer to the base part. In the region of a section lying further away from the base part than this section, the interlayer is connected with the housing. In the section lying further away, the interlayer can also be connected with the insulator. However, in an alternative, there is a gap between interlayer and insulator in the section lying further away. In this refinement, a small peripheral ring of the interlayer is exposed in the gap between the insulator and housing. The ring-shaped section forms a kind of membrane which absorbs mechanical loads.  
           [0019]    In refinements of the spark plug, the insulator element includes ceramic. The surface of the ceramic is treated in the region of the connection in such a way that the load capacity of the connection is enhanced. Roughening of the surface and/or applying a metallic topcoat are suitable methods. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1A is a first illustration of a compact spark plug with a damping resistor made of a solidified glass melt.  
         [0021]    [0021]FIG. 1B is a second illustration of the compact spark plug shown in FIG. 1A.  
         [0022]    [0022]FIG. 2A is a first illustration of a compact spark plug without a damping resistor.  
         [0023]    [0023]FIG. 2B is a second illustration of the compact spark plug shown in FIG. 2A.  
         [0024]    [0024]FIG. 3A is a first illustration of a compact spark plug with a nondestructively replaceable damping resistor.  
         [0025]    [0025]FIG. 3B is a second illustration of the compact spark plug shown in FIG. 3B. 
     
    
     DETAILED DESCRIPTION  
       [0026]    [0026]FIG. 1A shows a compact spark plug  10  in a partial section 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 , whose diameter in the region of a central electrode  20  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  outward, 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 further elements for current conduction located in through hole  18 .  
         [0027]    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  38 , a glass melt  40 , which forms a damping resistor, an electrically conducting contact  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 .  
         [0028]    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 lying 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 .  
         [0029]    [0029]FIG. 1B shows 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 .  
         [0030]    In the region of a insertion nut  34   b  constructed like insertion nut  34 , a peripheral gap  52  remains between insulator  12   b  and insertion nut  34   b . Otherwise, spark plug  10   b  is constructed like spark plug  10 .  
         [0031]    Due to welded connection  48  or  48   b , spark plug  10  can 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 essentially determines the overall space for the installation of spark plug  10 . Insertion nut  34  is implemented 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 .  
         [0032]    In other exemplary embodiments, an interlayer is 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 remains between the interlayer and insertion nut  34   b  in the region of insertion nut  34   b.    
         [0033]    [0033]FIG. 2A shows, in a partial section view, a compact spark plug  10   c  which has no damping resistor. Functional elements shown in FIG. 2A which are constructed essentially 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 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 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.  
         [0034]    Central electrode  20   c  is secured by contact pin  100 . Insulator  10  is then 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  can be coated almost 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.    
         [0035]    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  lies between interlayer  104  and housing  22   c.    
         [0036]    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 cannot be transmitted directly to insulator  10   c . Interlayer  104  absorbs these forces in the transition region between step  106  and section  108 .  
         [0037]    [0037]FIG. 2B shows 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.  
         [0038]    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.    
         [0039]    Mechanical stresses which arise in the region of a groove  30   d  as spark plug  10   d  is screwed in cannot be directly transmitted to insulator  12   d  due to gap  110   d . The force lines first run into housing  22   d  and only enter insulator core  12   d  in section  114 . The forces are, however, already less at this point than in the region of groove  30   d.    
         [0040]    A sealing ring, not shown, 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.    
         [0041]    [0041]FIG. 3A shows a partial section view of a compact spark plug  10   e  which is constructed similarly to spark plug  10   c , see FIG. 2A. Elements with reference numbers  12   e  to  36   e  correspond in their design and function to the elements  12   c  to  36   c  which were explained with reference to FIG. 2A.  
         [0042]    Central electrode  20   e  is again inserted first into through hole  18   e . Subsequently, a replaceable damping resistor  120  is inserted, which has a shape resembling a known 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 way, 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.    
         [0043]    An interlayer  124  corresponds to interlayer  104  in its design, function, and type of attachment to insulator  12   e  and housing  22   e , see FIG. 2A.  
         [0044]    [0044]FIG. 3B shows a part of a spark plug  10   f , which is designed like spark plug  10   e , see FIG. 3A. An interlayer  126   f  is soldered onto insulator  12   f  of spark plug  10   f  in a section  130 . Section  130  lies 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  lying 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  lies between housing  22   f  and insulator  12   f.