Abstract:
A method of forming a spark plug for an internal combustion engine is provided, the method including the steps of: separately securing a ground electrode to a ground shield, the ground shield having an elongated base section being configured to substantially surround a first insulator section of an insulator configured to substantially surround a center electrode, the insulator having a substantially cylindrical body with at least the first insulator section and a second insulator section, the first and second insulator sections having first and second diameters respectively and being separated by an insulator shoulder; and the elongated center electrode having a center electrode tip at one end and a terminal proximate another end of the center electrode, wherein the ground shield has a frustoconical flange protruding from a first end of the elongated base section, the frustoconical flange being configured to engage the insulator shoulder, and wherein the ground electrode extends from a second end of the elongated base section to define a spark gap with respect to the center electrode tip; and securing the ground shield to the spark plug after the ground electrode has been separately secured to the ground shield.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 12/360,492 filed Jan. 27, 2009, which claims the benefit of the following U.S. Provisional Patent application Ser. No. 61/024,054 filed Jan. 28, 2008, the contents of which are incorporated herein by reference thereto. 
     
    
     BACKGROUND 
       [0002]    This application relates generally to spark plugs for internal combustion engines and, more particularly, to a ground shield for a spark plug having an annular threaded portion for engaging the engine with a spark plug seat that is located between the spark gap and the threaded portion. 
         [0003]    Traditional spark plug construction includes an annular metal casing having threads near one end and a ceramic insulator extending from the threaded end through the metal casing and beyond the opposite end. A central electrode is exposed near the threaded end and is electrically connected through the insulator interior to a terminal which extends from the opposite insulator end to which a spark plug ignition wire attaches. A “J” shaped ground electrode extends from one edge of the threaded end of the metal casing into axial alignment with the central electrode to define a spark gap therebetween. The force applied to seal the spark plug in the head is the result of torque transmitted by the threaded metal casing; hence, the threaded portion of the metal casing must be sturdy and of substantial size. A portion of the metal casing is formed to be engaged by a socket tool to provide torque to the threaded portion. The threaded portion is located away from the portion which is engaged by the socket tool. 
         [0004]    To facilitate the controlled and efficient exhaust of gases from a combustion chamber, the valves are sometimes increased in size. This may necessitate a decrease in the size of the spark plug, a reduction in the size and sturdiness of the threaded metal casing end, and, in particular, a decrease in the inside diameter of the metal bore of the spark plug and in the combustion chamber wall area available to threadedly receive the spark plug. 
         [0005]    The decrease in the inside diameter of the metal bore of the spark plug reduces the ability of the spark plug to resist carbon build up and similar deposits reducing ignition efficiency. Various designs for spark plugs that reduce the deleterious effect of reducing the spark plug size by having an insulator with a cylindrical body that surrounds a central electrode are taught in U.S. Pat. Nos. 5,091,672, 5,697,334, 5,918,571, and 6,104,130, the contents for each incorporated herein by reference. In these designs, the cylindrical body is provided with a first diameter section separated from a second diameter section by a shoulder that provides a surface for sealing to the engine cylinder head. A shield that surrounds the second diameter has a base portion that is positioned a fixed distance from the tip to the center electrode by the engagement of a flange on the shield with the shoulder on the cylindrical body. The shield is formed with a ground electrode that integrally extends from the base portion. A shell portion surrounds the first diameter section of the cylindrical body and contains a threaded section positioned higher than the cylinder head seating surface along the cylindrical body. A radial tab extends from an end of the shell and aligns with the flange within the head to establish uniform positioning of the base portion. A separate end or retainer nut extends from the opposing end of the shell to locate and position the spark plug within the combustion chamber. 
         [0006]    Particularly suited for high-compression, high-performing engines, these various high-thread spark plug designs can provide more power by allowing for more space to optimize engine design, a superior cylinder head-seating position, a more compressive seal, improved heat transfer, and a more stable spark plug operating temperature for a more focused ignition, as well as a longer service life and increased corrosion protection. Nevertheless, to maintain the sparking gap between the center electrode and the ground electrode, the ground shield must be manufactured from an expensive, proprietary nickel alloy material. 
         [0007]    Accordingly, the inventor herein has recognized that it is desirable to provide a cost effective ground shield for use in a high-thread spark plug structure. 
       SUMMARY 
       [0008]    Exemplary embodiments of the present invention relate to a spark plug for an internal combustion engine. In one embodiment, a method of forming a spark plug for an internal combustion engine is provided, the method including the steps of: separately securing a ground electrode to a ground shield, the ground shield having an elongated base section being configured to substantially surround a first insulator section of an insulator configured to substantially surround a center electrode, the insulator having a substantially cylindrical body with at least the first insulator section and a second insulator section, the first and second insulator sections having first and second diameters respectively and being separated by an insulator shoulder; and the elongated center electrode having a center electrode tip at one end and a terminal proximate another end of the center electrode, wherein the ground shield has a frustoconical flange protruding from a first end of the elongated base section, the frustoconical flange being configured to engage the insulator shoulder, and wherein the ground electrode extends from a second end of the elongated base section to define a spark gap with respect to the center electrode tip; and securing the ground shield to the spark plug after the ground electrode has been separately secured to the ground shield. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a cross-sectional view of a spark plug in accordance with an exemplary embodiment of the present invention; 
           [0010]      FIG. 2  is a side view of the exemplary spark plug illustrated in  FIG. 1 ; 
           [0011]      FIG. 3  is a perspective view of the exemplary spark plug illustrated in  FIG. 1 ; 
           [0012]      FIG. 4  is a side view of the sparking end of the exemplary spark plug illustrated in  FIG. 1 ; 
           [0013]      FIG. 5  is a partial cross-sectional view of a sparking end of an exemplary embodiment of a spark plug in accordance with the present invention; 
           [0014]      FIGS. 6 and 7  are side views of a ground shield for a spark plug in accordance with an exemplary embodiment of the present invention; 
           [0015]      FIG. 8  shows various views of a ground electrode of the exemplary ground shield illustrated in  FIGS. 6 and 7 ; 
           [0016]      FIG. 9  shows various views of a base section of the exemplary ground shield illustrated in  FIGS. 6 and 7 ; and 
           [0017]      FIG. 10  is a side view of a ground shield for a spark plug in accordance with an exemplary embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0018]      FIGS. 1-4  illustrate an overall structure of an exemplary embodiment of a high-thread spark plug employing a ground shield in accordance with the present invention. The spark plug  10  is designed for use in internal combustion engines of automotive vehicles. The installation of spark plug  10  into an internal combustion engine is achieved by fitting it so that it protrudes into a combustion chamber (not shown) of the engine through a threaded bore provided in the engine head (not shown). Spark plug  10  includes a cylindrical center electrode  21  extending along the axial length of the spark plug, a ceramic or similarly comprised insulator  41  that concentrically surrounds center electrode  21 , and a cylindrical shell shaped ground shield  37  that concentrically surrounds insulator  41 . 
         [0019]    In the present exemplary embodiment, center electrode  21  has a cylindrical body with a tip  33  at one end, and is secured concentrically within insulator  41  to be electrically isolated from ground shield  37 . The end of center electrode  21  opposing tip  33  is electrically connected to an end of a resistive element  13  through a glass seal  15  that comprises an electrically conductive material. In exemplary embodiments, glass seal  15  can be a fired-in seal (conductive or otherwise) that coaxially surrounds resistive element  13  such that it is located between the inner surface of insulator  41  and the outer surface of the resistive element. The other end of resistive element  13  is electrically connected, through the glass sealing material  15 , to an end of a cylindrical terminal stud  23 . Glass seal  15  serves as the electrical connection between terminal stud  23  and center electrode  21 . Terminal stud  23 , in turn, is attached to a terminal nut  17 , which is configured to attach to the ignition cable (not shown) that supplies the electric current to the plug when the plug is installed. 
         [0020]    In exemplary embodiments, center electrode  21  can comprise a core  49  made of a highly heat conductive metal material such as, for example, copper, covered by a longer than conventional sheath  47  made a highly heat-resistant, corrosion-resistant metal material such as, for example, Inconel, another nickel-based alloy, or other suitable metal or metal alloy. In exemplary embodiments, center electrode  21  can include a noble metal chip  45 , such as one made from a gold, palladium, or platinum alloy in any suitable form for enabling proper spark plug functioning such as, for example, flat or finewire, that is joined to center electrode tip  33  to improve heat transfer and maintain the sparking gap. In exemplary embodiments, terminal stud  23  can comprise steel or a steel-based alloy material with a nickel-plated finish. 
         [0021]    In the present exemplary embodiment, insulator  41  has an elongated, substantially cylindrical body with first  25 , second  27 , and third  67  insulator sections having different diameters. First insulator section  25  substantially surrounds center electrode  21 . Second insulator section  27  is located intermediate first  25  and third  67  insulator sections and the diameter of the second insulator section  27  is greater than that of either of the other two insulator sections. Second insulator section  27  and narrower first insulator section  25  are separated by a shoulder  29 , and the second insulator section and narrower third insulator section  67  are separated by a shoulder  69 . In exemplary embodiments, insulator  41  can comprise a non-conducting ceramic material such as, for example, alumina ceramic so that it may fixedly retain center electrode  21  while preventing an electrical short between the center electrode and ground shield  37 . 
         [0022]    Ground shield  37 , which surrounds first insulator section  25 , includes a frustoconical section  31  at one end that is juxtaposed with insulator shoulder  29 , a generally U-shaped ground electrode strap  39  that extends from and diametrically spans the ground shield near the opposing end, and a cylindrical base portion  43  axially extending between frustoconical section  31  and ground electrode strap  39 . Base portion  43  concentrically surrounds first insulator section  25 . Ground electrode strap  39  includes a free end  55  that faces and is axially spaced from a center electrode tip  33 . In exemplary embodiments, free end  55  can include a noble metal chip  57 , such as one made from a gold, palladium, or platinum alloy in finewire form, that is joined to ground electrode strap  39  to improve heat transfer and enhance durability. In exemplary embodiments in which noble metal chips  45 ,  57  are joined to center electrode tip  33  and ground electrode strap  39  respectively, the noble metal chips define the spark gap and serve as the sparking surfaces of the spark plug. In exemplary embodiments, noble metal chips  45 ,  57  can be joined to center electrode tip  33  and ground electrode strap  39  respectively by a joining technique such as brazing, laser welding, resistance welding, or plasma welding. 
         [0023]    As illustrated in detail in  FIG. 5 , exemplary embodiment of spark plugs in accordance with the present invention can comprise a ground electrode strap  139  that includes a free end  155  facing and axially spaced from a center electrode tip  133 . Ground electrode strap  139  thus diametrically surrounds center electrode tip  133  to define an axial spark gap  135  therebetween, between which an electrical discharge can be passed to ignite a combustible mixture. Center electrode  121  can include a noble metal chip  145 , such as one made from a gold, palladium, or platinum alloy in any suitable form for enabling proper spark plug functioning such as, for example, flat or finewire, that is joined to center electrode tip  133  to improve heat transfer and enhance durability. 
         [0024]    Referring again to the exemplary embodiment illustrated in  FIGS. 1-4 , an annular retainer  59 , such as a nut or a castle head jam screw, has a threaded portion  61  surrounding second insulator section  27 . Annular retainer  59  extends axially to integrally form a jam nut  56  at one end that surrounds a portion of third insulator section  67 . Threaded portion  61  is configured to threadedly engage the threaded portion of a generally cylindrical opening that is in communication with the combustion chamber of an internal combustion engine. With jam nut  56  being formed integrally with annular retainer  59 , spark plug  10  can be removed in a helical pattern as the jam nut is unscrewed, resulting in easy, direct removal with negligible tipping. A suitable socket tool such as, for example, a 9/16 socket wrench, can engage jam nut  56  of annular retainer  59  for screwing spark plug  10  into and out of the engine bore. 
         [0025]    Annular retainer  59  includes a frustoconical portion  63  that is situated below threaded section  61  and overlaps frustoconical section  31  of ground shield  37  in juxtaposed alignment with insulator shoulder  29 . At this juncture, ground shield  37  and retainer  59  are secured together, with the insulator  41  being captured therewithin. Annular retainer  59  also includes a frustoconical portion  71  axially extending between threaded portion  61  and jam nut  56  that engages insulator shoulder  69 . Third insulator section  67  protrudes from annular retainer  59  beyond jam nut  56 . In exemplary embodiments, annular retainer  59  can comprise a conductive metal material such as a nickel-plated, low-carbon steel-based alloy. In exemplary embodiments, threaded section  61  can have an outer thread diameter of about 16 mm or less; for example, the threaded section can have an outer diameter of about 10 mm to allow for a greater amount of engine space. The shape, size, and particular construction of annular retainer  59  may, of course, vary greatly from one design to another; hence, the dimensional attributes of the annular retainer are provided in  FIGS. 1-3  only as an exemplary embodiment. 
         [0026]    When spark plug  10  is threaded into the engine bore, insulator  41  provides a compressive force that transmits a mechanical connection between retainer  59  and ground shield  37  by urging ground shield frustoconical portion  31  into sealing engagement with annular retainer frustoconical portion  63 . Frustoconical portion  63  will, in turn, be urged to act as the external motor seat for sealingly engaging a frustoconical sealing seat portion of the engine bore (not shown) and thus establish an electrical ground connection between ground shield  37  and the engine head while at the same time sealing the combustion chamber from the surrounding environment. The assembled annular retainer  59  and ground shield  37  thus function as a unit and may be referred to herein as the ground shield and retainer unit. In exemplary embodiments, frustoconical portion  63  of annular retainer  59  and frustoconical section  31  of ground shield  37  can also be joined to one another using a joining technique such as brazing, laser welding, resistance welding, or plasma welding, to secure the ground shield and the retainer together. 
         [0027]    Exemplary embodiments of the present invention employ a ground shield design that may represent a substantial cost savings. As illustrated in the exemplary embodiment of  FIGS. 6-7 , ground shield  237 , rather than being integrally formed as a unitary piece, is a composite of base portion  243  and ground electrode strap  239 , which are formed separately and then secured together. As shown in  FIGS. 8 and 9 , ground electrode strap  237  is formed with a pair of legs  275 , and base portion  243  is formed with axial extending slots  273  configured to fittingly receive the legs of the U-shaped ground electrode strap  239  at the end proximate to the axial electrode gap. Thus, to assemble ground shield  237 , legs  275  of ground electrode strap  239  are fit within slots  273  and joined to otherwise open-ended base portion  243 . In exemplary embodiments, legs  275  can be joined to slots  273  using a joining technique such as brazing, laser welding, resistance welding, or plasma welding, to secure the ground electrode strap to base portion  243 . 
         [0028]    Because base portion  243  and ground electrode strap  239  are formed separately, these two portions of ground shield  237  may be made from different materials. Thus, in exemplary embodiments, ground electrode strap  239  can be manufactured from an expensive, proprietary nickel alloy material such as, for example, Iconel to enhance durability between a center electrode tip (such as, for example, center electrode tip  33  depicted in the exemplary embodiment illustrated in  FIG. 1 ) and ground electrode  257 , while base portion  243  (which can comprise as much as 90% or more of the total size of ground shield  237  in exemplary embodiments) can be made from any low cost, corrosion resistant material such as any suitable metal-based alloy like stainless steel and similar steel-based alloys. Accordingly, by forming ground shield  237  by securing ground electrode strap  239  to otherwise open-ended base portion  243  as described, the need to fabricate the larger base portion from an expensive nickel alloy is avoided, thereby reducing the cost of forming the high-thread ground shield to as little as 10% or less of its former cost in exemplary embodiments. 
         [0029]    Furthermore, as shown in the alternative exemplary embodiment of a ground shield  337  illustrated in  FIG. 10 , cost can further be reduced by forming the ground shield as a composite of a base portion  343  and a generally J-shaped ground electrode strap  339  having a free end that is radially aligned with and axially spaced from a center electrode tip to form the spark gap, as illustrated in  FIG. 10 . In such an embodiment, the ground electrode strap will thus be formed with a single leg that is welded to base portion  343  in a single open slot  373 . In yet another alternative exemplary embodiment, the ground electrode strap can be formed as a generally U-shaped member having an annular opening within free end in which a center electrode tip ends within or slightly below the annular opening. 
         [0030]    The unique technique for fabricating a spark plug in accordance with exemplary embodiments of the present invention should now be clear. Referring again to the exemplary embodiment illustrated in  FIGS. 1-4 , center electrode  21  is axially into passed a bore formed within insulator  41  such that center electrode firing end or tip  33  projects from one end of the insulator, and terminal stud  23  can be passed into glass sealing material  15  of resistive element  13  to axially extend from the opposing end of the insulator. Insulator  41  and its included center electrode  21  are then axially passed into cylindrical shell ground shield  37  such that base portion  43  surrounds smaller diameter first insulator section  25 , flared frustoconical section  31  engages insulator shoulder  29 , and axial sparking gap  35  is formed between center electrode tip  33  and ground electrode tip  57 . 
         [0031]    Cylindrical annular retainer  59  is then axially passed over the insulator from the opposite end and its interior frustoconical ledge  71  engages insulator second shoulder  69  such that threaded section  61  surrounds larger diameter second insulator section  27  and jam nut  56  surrounds a portion of third insulator section  67 . Frustoconical portion  63  of annular retainer  59  is then radially collapsed about frustoconical section  31  to secure ground shield  37  and annular retainer  59  together with insulator  41  being captured therebetween. In exemplary embodiments, frustoconical portion  63  of annular retainer  59  can be “hot pressed” onto frustoconical section  31 , and jam nut  56  can be joined in a similar fashion onto third insulator section  67 . 
         [0032]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims and their legal equivalence.