Abstract:
A spark plug for igniting gases in an internal combustion engine is disclosed. The spark plug has a center electrode, an insulator, a one-piece shell, and a terminal. The center electrode is in communication with an energy source. The insulator surrounds the center electrode. The one-piece shell surrounds and contacts the insulator for securing the insulator within the shell, wherein the shell has a plurality of threads near a first end and a ground electrode attached to the shell and aligned with a tip of the center electrode at a second end to define a spark gap. Further, a seat is formed in the shell between the plurality of threads and the ground electrode for sealing the shell against the engine. The terminal has a first end in communication with the center electrode and a second end which has a connector portion for connecting to the energy source.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to U.S. Provisional Patent Application Ser. No. 60/821,343, filed Aug. 3, 2006, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to spark plugs, and more particularly to spark plugs having an extended shell and insulator. 
     2. Related Art 
     Spark plugs have been used for many years to provide a means to ignite the fuel air mixture in the combustion chambers of an internal combustion engine. Spark plugs have taken on many forms to adapt to the particular engine design and environment. Generally, spark plugs have a center electrode surrounded by an insulator wherein the insulator is disposed in and captured by a metal housing or shell. The shell typically has a plurality of threads which are matched to the bore threads in the engine block. The threads allow the spark plug to be screwed into the bore using a conventional tool. Further, the shell includes a ground electrode extending from an end of the shell proximate the center electrode. The ground electrode together with the center electrode define a spark gap. The shell also acts as a ground shield to provide an electrical ground path from the spark gap to the engine block. 
     The spark plug seats or seals against the engine cylinder head to seal the combustion chamber and prevent combustion gases from escaping through the spark plug hole in the cylinder head. Commonly, the seat is located above the threads and is combined with a sealing gasket that has an interference fit with respect to the threads so as to retain the gasket during installation of the sparkplug. 
     Increasingly, engine designs employing multiple valves, fuel injection points, coil on plug ignition systems, combustion related sensors and other features have placed increasing demands on the space in the cylinder head immediately adjacent to the combustion chamber, particularly the space above the combustion chamber, which have in turn made it desirable to minimize the space envelope needed for the spark plug, particularly in the lower portions of the spark plug proximate the spark gap where the spark plug is exposed to the combustion chamber and combustion gases. 
     In addition to restrictions on the space envelope available for the spark plug on the sparking end, in applications where space is restricted, there is also a trend toward higher engine operating temperatures which increases the temperatures to which the spark plugs operating in this restricted space envelope are exposed, making it desirable to improve the ability of the spark plug to remove the heat resulting from operation of the spark plug and the associated combustion processes (i.e., the need for colder spark plugs). 
     Another common requirement for spark plugs is that they be able to operate without replacement for extended periods of engine and vehicle operation, such as 50,000 or even 100,000 miles of operation. 
     These space restrictions have led to the use of spark plugs having smaller diameters (e.g., 12 mm, 10 mm and smaller) to achieve the necessary space envelope and heat removal properties, but the manufacture of smaller diameter spark plugs presents other challenges associated with the performance and manufacture of the various spark plug components, such as the insulators and electrode materials. 
     Another approach has been to extend the spark plug shell maintaining a larger upper portion (e.g., 16 mm), since there is frequently still space available in the head away from the combustion chamber to receive the larger diameter, while reducing the diameter and extending the shell to reach the combustion chamber so as to meet the restricted space envelope requirements. One such spark plug configuration is described in U.S. Pat. No. 5,918,571 to Below which describes an extended shell spark plug where the shell is of two-piece construction of a retainer for the insulator and a ground shield. Below describes the construction by teaching that the insulator and its included center electrode are axially passed into the cylindrical shell ground shield. The flared frustoconical flange of the ground shield engages the insulator shoulder and the cylindrical shell retainer is then passed over the insulator from the opposite end and its interior frustoconical ledge engages a second shoulder of the insulator. A portion of the retainer is then radially collapsed about the flange to secure the ground shield and retainer together with the insulator captured therebetween. The formed portion also serves as the seat for the spark plug. While Below is not specific as to the material of construction, commercial products having the configuration and construction of Below have been observed to utilize a steel retainer and a higher temperature alloy for the ground shield, such as Inconel 600. The two-piece construction has attendant reliability concerns associated concerns when using standard reliability analysis such as Failure Modes Effects Analysis (FMEA) associated with the presence of the additional mechanical compression joint in the spark plug, which has an associated probability of failure. Further, it is believed that placement of the spark plug seat on a formed part which is subject to manufacturing variances associated with two parts may provide an attendant variability of the seat that has a possibility to affect the performance of the seat and the spark plug, as well as the performance of the engine in which it is installed. 
     While such prior art spark plug designs having extended shells and insulators have achieved their intended purposes. Therefore, a need exists to for spark plugs configured meet the space envelope restrictions while effectively dissipating excessive heat and durable enough to withstand the harsh environments of an internal combustion engine. 
     SUMMARY OF THE INVENTION 
     A spark plug for igniting gases in an internal combustion engine is provided. The spark plug has a center electrode in communication with an energy source, an insulator surrounding the center electrode, a one-piece shell surrounding and in contact with the insulator for securing the insulator within the shell, wherein the shell has a plurality of threads near a middle portion and a ground electrode attached to the shell and aligned with a tip of the center electrode at a second end to define a spark gap. A seat is formed in the shell between the plurality of threads and the ground electrode for sealing the shell against the engine. Further, the terminal has a first end in communication with the center electrode and a second end having a connector portion for connecting to the energy source. 
     In one aspect of the invention, the spark plug includes a center electrode assembly comprising a terminal at one end and a center electrode with a sparking surface at an opposite end; a generally tubular insulator surrounding the center electrode assembly; and a one-piece extended shell surrounding the insulator and having along its length a formed shoulder on a first end, an attachment portion, a threaded portion, a body portion having at an end away from the formed shoulder a tapered seat, a barrel extension and a ground electrode at a second end which is attached to the barrel extension and spaced from the sparking surface to form a spark gap, the ground electrode having a thermally conductive core, wherein the spark plug has an IMEP heat rating greater than about 200. In another aspect of the present invention the insulator has a conical surface near a first end proximate the spark gap. 
     In yet another aspect of the present invention the insulator has a plurality of sections each having a different diameter. 
     In yet another aspect of the present invention the section of the insulator disposed between the seat and the tip of the center electrode is in contact with the shell over substantially its entire length. 
     In yet another aspect of the present invention a gap is defined between the insulator and the shell proximate to the tip of the center electrode. 
     In yet another aspect of the present invention the seat has a frustoconical shape. 
     In yet another aspect of the present invention the shell has a hex head formed at the first end for engaging a tool. 
     In yet another aspect of the present invention an annular groove in the shell defines a narrow wall, wherein the annular groove is disposed between the seat and the plurality of threads. 
     In yet another aspect of the present invention a section of the insulator is disposed outside of the shell. 
     In yet another aspect of the present invention the connector has a height that is equal to or less than a third of the height of the section of the insulator that is disposed outside of the shell. 
     In yet another aspect of the present invention a hot lock seal is formed from said body portion and located between the body portion and the insulator. 
     In yet another aspect of the present invention the insulator has a distance between the rolled shoulder of their shell and said terminal of at least 0.90 inches. 
     In yet another aspect of the present invention, the ground electrode includes an Ni alloy and the thermally conductive core includes a Cu alloy. 
     In yet another aspect of the present invention, the center electrode includes a thermally conductive core. 
     In yet another aspect of the present invention, the center electrode includes an Ni alloy and the thermally conductive core includes a Cu alloy. 
     In yet another aspect of the present invention, the center electrode and the ground electrode further include a sparking tip. 
     In yet another aspect of the present invention, the sparking tip includes one of gold, a gold alloy, a platinum group metal or a tungsten alloy. 
     In yet another aspect of the present invention, the platinum group metal includes at least one element selected from the group consisting of platinum, iridium, rhodium, palladium, ruthenium and rhenium. 
     In yet another aspect of the present invention, the platinum group metal further includes at least one element selected from the group consisting of nickel, chromium, iron, manganese, copper, aluminum, cobalt, tungsten, yttrium, zirconium, hafnium, lanthanum, cerium and neodymium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein: 
         FIG. 1  is a cross-section view of the spark plug in accordance with an embodiment of the present invention; 
         FIG. 2  is a cross-section view of an insulator in accordance with an embodiment of the present invention; 
         FIG. 3  is a cross-section view of a shell prior to attachment of a ground electrode in accordance with an embodiment of the present invention; 
         FIG. 4  is a front view of a shell after attachment of a ground electrode in accordance with an embodiment of the present invention; 
         FIG. 5  is a section view of a terminal in accordance with an embodiment of the present invention; 
         FIG. 6  is a front view of a center electrode in accordance with an embodiment of the present invention; 
         FIG. 7  is a front view of a center electrode with a sparking tip attached to a sparking end thereof in accordance with an embodiment of the present invention; 
         FIG. 8  is an enlarged view of the sparking tip of  FIG. 7 ; 
         FIG. 9  is a partial cross-section view of a ground electrode and barrel portion of the shell in accordance with an embodiment of the present invention; and 
         FIG. 10  is a cross-section view of a insulator and terminal assembly in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the FIGS., wherein like numerals indicate like or corresponding parts throughout the several views, a spark plug according to the subject invention is generally shown at  10  in  FIG. 1 . Spark plug  10  includes an insulator shown generally at  12 , an extended shell shown generally at  24 , and a center electrode assembly shown generally at  16 . Extended shell  24  is preferably made of an alloy of steel (i.e.,  1215  steel) or similar material and is configured, as will be described in further detail below, to retain or capture insulator  12  and center electrode assembly  16 . Insulator  12  is a generally cylindrical elongated member made of alumina or similar material. Shell  24  has a section that includes a ground electrode  26  extending therefrom as described further below.  FIG. 1  illustrates spark plug  10  in a nearly completely assembled condition, but prior to hot locking the shell and insulator together as described herein. In a fully assembled condition after hot locking as described herein, the buckle zone  32  of shell  24  at least partially collapses in response to heating of this element coupled with application of compressive force which urges the portions of shell  24  above and below this element into pressing engagement with insulator  12 . Generally speaking, the description of the elements below, particularly with regard to the engagement of portions of insulator  12  and shell  24  are given in the fully assembled condition (i.e., as if the hot locking operation had been performed). 
     Referring to  FIGS. 1 and 2 , the spark plug  10  includes a tubular ceramic insulator, generally indicated at  12 , which is preferably made from aluminum oxide or other suitable material having a specified dielectric strength, high mechanical strength, high thermal conductivity, and excellent resistance to thermal shock. The insulator  12  may be molded dry under extreme pressure and then sintered at high temperature using well-known processes. The insulator  12  has an outer surface which may include a partially exposed upper mast portion  14  to which an elastomeric spark plug boot (not shown) surrounds and grips to maintain an operative electrical connection with the ignition system. The exposed mast portion  14 , as shown in  FIG. 1 , may include a series of ribs (not shown) for the purpose of providing added protection against spark or secondary voltage “flashover” and to improve the grip with an elastomeric spark plug boot. The insulator  12  is of generally tubular or annular construction, including a central passage  18 , extending longitudinally between an upper portion  19  proximate terminal end  20  and a lower portion  21  proximate core nose end  22 . The central passage  18  is of varying cross-sectional area, generally greatest at or adjacent the terminal end  20  and smallest at or adjacent the core nose end  22 . Referring again to  FIGS. 1 and 2 , generally tubular insulator  12  surrounds center electrode assembly  16  described below. Insulator  12  includes generally a continuous series of tubular sections  60  of varying diameter. These sections include a first insulator section  62  which surrounds the stud portion  41  of terminal stud  40 . This first insulator section  62  transitions to a first insulator shoulder  63  which is in pressing engagement with the formed shoulder  30  of shell  24  described herein and in turn transitions to a second insulator section  64 . Second insulator section  64  has a diameter which is greater than the diameter of the first insulator section  62  and is housed within the first shell section  72  as described herein. A second insulator shoulder  65  is in pressing engagement with the first shell shoulder  73  and transitions to a third insulator section  66 . The third insulator section  66  has a diameter less than the diameter of the second insulator section  64 , and preferably less than the diameter of the first insulator section  62 , and is housed within the second shell section  74 . A third insulator shoulder  67  is in pressing engagement with the second shell shoulder  28  and transitions to a fourth insulator section  68 . Fourth insulator section  68  has a diameter which is less than the diameter of the third insulator section. Fourth insulator section  68  is housed within the third shell section  76  and includes a tapered core nose section  69 . Fourth insulator section  68  and its core nose section  69  house most of center electrode  48 . Electrode extends from the barrel extension  35  of shell  24  and is proximate the spark gap  54 . The fourth insulator section  68  and barrel extension play an important role in removing heat from the spark plug and the heat transfer characteristics of these components play a significant role in establishing the operating temperature of the spark plug and its IMEP rating as described herein. The fourth insulator section  68  and the third shell section  76  are sufficiently closely spaced and operative for removal of heat from the fourth insulator section through the third shell section as described herein. Insulator  10  also preferably includes a pocket  80  which is adapted to receive a portion of buckle zone  32  when the insulator  12  and shell  48  are hot locked as described herein. 
     As depicted generally in  FIGS. 1 ,  3  and  4 , an electrically conductive, preferably metallic, extended shell is generally indicated at  24 . By extended, for a 16 mm spark plug example, it is meant that the shell  24  may have an overall length on the order of about 1.2 inches or more. Extended shell  24  may be made from any suitable metal, including various coated and uncoated steel alloys, such as 1215 steel. Shell  24  may be coated by plating or otherwise with protective coatings such as Ni or Ni alloys. The extended shell  24  has a generally annular interior surface or bore  70  which surrounds and is adapted for pressing and sealing engagement with the exterior surface of insulator  12  as described herein and includes at least one attached ground electrode  26 . The shell  24  surrounds the lower sections, including second  64 , third  66  and fourth  68  insulator sections of the insulator  12  and includes at least one ground electrode  26 . While the ground electrode  26  is depicted before bending  FIG. 4  and in the traditional single L-shaped style in  FIG. 1 , it will be appreciated that multiple ground electrodes of L-shape, straight or bent configuration can be substituted depending upon the desired ground electrode configuration and the intended application for the spark plug  10 . 
     Extended shell  24  has a generally tubular or annular bore  70  in its body section and includes an internal lower compression flange or second shoulder  28  adapted to bear in pressing contact against third insulator shoulder  67  of the insulator  12 . Extended shell  24  further includes an upper compression flange or formed shoulder  30  which is crimped or formed over during the assembly operation to bear in pressing contact against first insulator shoulder  63  of insulator  12 . This is formed from a shoulder portion  29  which is shown in  FIGS. 3 and 4  prior to deformation to create formed shoulder  30 . Extended shell may also include a deformable zone  32  which is designed and adapted to collapse axially and radially inwardly in response to heating of deformable zone  32  and associated application of an overwhelming axial compressive force during or subsequent to the deformation of formed shoulder  30  in order to hold extended shell  24  in a fixed axial position with respect to insulator  12  and form a gas tight radial seal between insulator  12  and extended shell  24 . Gaskets, cement, or other sealing compounds can be interposed between the insulator  12  and shell  24  to perfect a gas-tight seal and improve the structural integrity of the assembled spark plug  10 . 
     The shell  24  is provided with an attachment portion  34 , such as a tool receiving hexagon  34  or other feature for removal and installation of the spark plug in a combustion chamber opening. The feature size will preferably conform with an industry standard tool size of this type for the related application. The hex size complies with industry standards for the related application. Of course, some applications may call for a tool receiving interface other than a hexagon, such as slots to receive a standard spanner wrench, or other features such as are known in racing spark plug and other applications and in other environments. A threaded portion  36  is formed below the attachment portion  34  to be used for engagement with a threaded bore in the cylinder head of an engine. Immediately below threaded portion  36  is body portion  37 . Body portion  37  has at the end located away from formed shoulder  30  a sealing seat  38 . The seat  38  may be a squared shoulder paired with a gasket (not shown) to provide a suitable interface against which the spark plug  10  seats in the cylinder head and provides a hot gas seal of the space between the outer surface of the shell  24  and the threaded bore in the combustion chamber opening (not shown). Alternatively and preferably, the sealing seat  38  may be designed with a tapered seat located along the lower end of body portion  37  of the shell  24  to provide a close tolerance and self-sealing installation in a cylinder head which is also typically designed with a mating taper for this style of spark plug. Disposed below sealing seat  38  is barrel extension  35 . Barrel extension  35  may be on the order of 0.85 inches in length with an outer diameter of generally less than about 0.40 inches and a wall thickness of about 0.060 inches and permits spark plug  10  to satisfy the reduced space envelope requirements proximate the combustion chamber while also providing the necessary interface with the other components of spark plug  10 . Attached to the free end of barrel extension  35  is ground electrode  26 . 
     As illustrated in  FIGS. 3 and 4 , extended shell  24  has an annular bore  70  with sections of varying diameters which are progressively reduced from the formed shoulder  30  to the free end of barrel extension  35 . They include a first shell section  72  associated with formed shoulder  30  and attachment portion  34 . Extending from first shell section  72  is first shell shoulder  73  which is adapted for pressing engagement with second insulator shoulder  65  and in turn transitions to a second shell section  74 . Second shell section  74  is associated with threaded portion  36  and an end of said body portion  37  located toward formed shoulder  30 . Extending from second shell section  74  is second shell shoulder  28  which is adapted for pressing engagement with third insulator shoulder  67 . Second shell shoulder  28  transitions to third shell section  76  which is associated with said end of said body portion away  37  from the formed shoulder  30  and with barrel extension  35 . 
     As shown in  FIG. 1 , an electrically conductive terminal stud  40  is partially disposed in the central passage  18  of the insulator  12  and extends longitudinally from an exposed top post  39  to a bottom end  41  embedded partway down the central passage  18 . The top post  39  may be a bantam post having a reduced height of about 0.35 inches or may have a more conventional height. It is adapted for connection to an ignition wire terminal (not shown) and receives timed discharges of high voltage electricity required to fire or operate the spark plug  10  by generating a spark in spark gap  54 . 
     The bottom end  41  of the terminal stud  40  is embedded within a conductive glass seal  42 , forming the top layer of a composite three layer suppressor-seal pack. The conductive glass seal  42  functions to seal the bottom end  41  of the terminal stud  40  and electrically connect it to a resistor layer  44 . This resistor layer  44 , which comprises the center layer of the three-layer suppressor-seal pack  43 , can be made from any suitable composition. Depending upon the recommended installation and the type of ignition system used, such resistor layers  44  may be designed to function as a more traditional resistor suppressor or, in the alternative, as a low resistance. Immediately below the resistor layer  44 , another conductive glass seal  46  establishes the bottom or lower layer of the suppressor-seal pack  43  and electrically connects terminal stud  40  and suppressor-seal pack  43  to the center electrode  48 . Top layer  42  and bottom layer  46  may be made from the same conductive material or different conductive materials. Many other configurations of glass and other seals and EMI supressors are well-known and may also be used in accordance with the invention. Accordingly, electricity from the ignition system travels through the bottom end  41  of the terminal stud  40  to the top portion of conductive glass seal  42 , through the resistor layer  44 , and into the lower conductive glass seal layer  46 . 
     As shown in  FIG. 1 , conductive center electrode  48  is partially disposed in the central passage  18  and extends longitudinally from its head which is encased in the lower glass seal layer  46  to its exposed sparking end  50  proximate the ground electrode  26 . The suppressor-seal pack  43  electrically interconnects the terminal stud  40  and the center electrode  48 , while simultaneously sealing the central passage  18  from combustion gas leakage and also suppressing radio frequency noise emissions from the spark plug  10 . As shown, the center electrode  48  is preferably a one-piece unitary structure extending continuously and uninterrupted between its head and its sparking end  50 . Conductive center electrode  48  is preferably formed from an electrically conductive material which combines high thermal conductivity with high temperature strength and corrosion resistance. Among suitable materials for conductive center electrode  48  are various Ni-based alloys, including various nickel-chromium-iron alloys, such as those designated generally by UNS N06600 and sold under the trademarks Inconel 600®, Nicrofer 7615®, and Ferrochronin 600®, as well as various dilute nickel alloys, such as those comprising at least 92% by weight of nickel; and at least one element from the group consisting of aluminum, silicon, chromium, titanium and manganese. These alloys may also include rare earth alloying additions to improve certain high temperature properties of the alloys, such as at least one rare earth element selected from the group consisting of yttrium, hafnium, lanthanum, cerium and neodymium. They may also incorporate small amounts of zirconium and boron to further enhance their high temperature properties as described in commonly assigned, co-pending U.S. patent applications Ser. Nos. 11/764,517 and 11/764,528 filed on Jun. 18, 2007 which are hereby incorporated herein by reference in their entirety. 
     Either one or both of the ground electrode  26  and center electrode  48  can also be provided with a thermally conductive core. This core  27  is shown in the case of ground electrode  26  in  FIGS. 1 and 9 . In the case of center electrode  48 , it is shown as core  49  in  FIGS. 7 and 8 . Thermally conductive core is made from a material of high thermal conductivity (e.g., ≧250 W/M*° K.) such as copper or silver or various alloys of either of them. Highly thermally conductive cores serve as heat sinks and help to draw heat away from the spark gap  54  region during operation of the spark plug  10  and the associated combustion processes, thereby lowering the operating temperature of the electrodes in this region and further improving their performance and resistance to the degradation processes described herein. 
     A firing tip  52  may optionally be located at the sparking end  50  of the center electrode  48 , as shown in  FIGS. 1 ,  7  and  8 . The firing tip  52  provides a sparking surface  53  for the emission of electrons across a spark gap  54 . The firing tip  52  for the center electrode  48  can be made according to any of the known techniques, including loose piece formation and subsequent attachment by various combinations of resistance welding, laser welding, or combinations thereof, of a pad-like, wire-like or rivet-like member made from any of the known precious metal or high performance alloys including, but not limited to, gold, a gold alloy, a platinum group metal or a tungsten alloy. Gold alloys, including Au—Pd alloys, such as Au-40Pd (in weight percent) alloys. Platinum group metals, include: platinum, iridium, rhodium, palladium, ruthenium and rhenium, and various alloys thereof in any combination. For purposes of this application, rhenium is also included within the definition of platinum group metals based on its high melting point and other high temperature characteristics similar to those of certain of the platinum group metals. Firing tips  52  may also be made from various tungsten alloys, including W—Ni, W—Cu and W—Ni—Cu alloys. Additional alloying elements for use in firing tips  52  may include, but are not limited to, nickel, chromium, iron, manganese, copper, aluminum, cobalt, tungsten, zirconium, and rare earth elements including yttrium, lanthanum, cerium, and neodymium. In fact, any material that provides good erosion and corrosion performance in the combustion environment may be suitable for use in the material composition of the firing tip  52 . Further, firing tip  52  may be a composite firing tip  52  having a free end portion located away from the center electrode  48  that includes the sparking surface  53 , which is a precious metal or high performance alloy, such as those described above, and a base end portion which is attached to the center electrode  48  on a base end and on the other end to the free end portion. The base end portion may be any material suitable for attachment to the free end portion, such as the Ni-based electrode materials described herein. The free end portion and base end portion may be joined together by any suitable joining method, such as various forms of welding. Depending on the materials selected for use as the free end portion and the base end portion and the joining method employed, the composite sparking tip  52  will also have joint between them. The joint may have a coefficient of thermal expansion (CTE) that is between the CTE&#39;s of the materials used for the free end portion and the base end portion, or may fall outside this range, depending on the materials selected for free end portion and the base end portion and the method used to form the joint. This composite or multi-layer sparking tip structure may be formed as a wire or headed rivet. The tip structures and methods of making and using them are explained further in commonly assigned, co-pending U.S. patent applications Ser. Nos. 11/602,028; 11/602,146; and 11/602,169 filed on Nov. 20, 2006 which are hereby incorporated herein by reference in their entirety. These sparking tips have numerous advantages, including reduced materials costs as compared to all precious metal or high performance alloy tips. They are also more easily welded to the center or grounds electrodes because the base end may be formed from the same or similar alloys used to make the electrodes, such as various nickel-based alloys. Because they may be made from the same or similar alloys as the electrodes themselves, they also have a significantly reduced CTE mismatch, which improves the resistance to thermal stress and cycling induced cracking and fracture of the interface between the base portion of the sparking tip and the electrode. 
     As perhaps best shown in  FIG. 1 , the ground electrode  26  extends from an anchored end  56  adjacent the shell  24  to a distal end  58  adjacent the sparking gap  54 . The ground electrode  26  may be of the typical rectangular cross-section, including an nickel-based alloy jacket surrounding a copper or other thermally conductive material core (see  FIGS. 1 and 9 ). 
     Spark plug  10  has demonstrated an industry standard IMEP rating of about  212 , it is believed that spark plugs of this construction can routinely achieve an IMEP rating of 200 or more, particularly by the incorporation of cored center and ground electrodes of the types described above. Spark plugs  10  also avoid two-piece shell construction and the potential limitations associated therewith described herein, including the need for the use of high temperature alloys for a portion of the shell. These are believed to offer significant reliability and cost advantages. 
     Generally, the elements of terminal assembly  16  are assembled in insulator to form an insulator and terminal assembly  17  as described herein. Insulator and terminal assembly  17  is inserted into the formable section  29  at the end of shell  24  and is captured therein as described herein. This has the advantage of insertion and assembly from a single end in contrast to assembly methods used when two-piece shells are employed, where separate shell portion must be inserted over opposite ends of the insulator and joined together to form the spark plug shell. 
     The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.