Patent Publication Number: US-7586246-B2

Title: Spark plug designed to ensure high strength of electrode joint and production method thereof

Description:
CROSS REFERENCE TO RELATED DOCUMENT 
   The present application claims the benefit of Japanese Patent Application Nos. 2004-30907 and 2004-340406 filed on Feb. 6, 2004 and Nov. 25, 2004, respectively, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates generally to a spark plug which produces a series of sparks to ignite an air-fuel mixture and a production method thereof, and more particularly to such a spark plug designed to ensure a high strength of joint between a ground electrode and a metal shell and a production method thereof. 
   2. Background Art 
   US 2002/0063504 A1 (Japanese Patent First Publication No. 2002-222686) discloses a spark plug which has an Ir (iridium) alloy-made ground electrode joined directly to an end surface of a metal shell by laser welding. U.S. Pat. No. 6,307,307 B1 and U.S. Pat. No. 6,373,172 B1 (Japanese Patent First Publication No. 2001-210447) propose spark plugs which include an Ni base alloy-made support joined to a metal shell and an Ir alloy-made ground electrode joined to the support by laser welding. The ground electrode contains 50 Wt % or more of Ir and is embedded in the support. 
   When the ground electrode is joined to the metal shell using typical laser welding techniques, a weld area between them is usually small, which could result in a lack of the strength of the joint between the ground electrode and the metal shell. Similarly, the weld between the metal shell and the Ni base alloy-made support may lack the mechanical strength because of restriction on a weld area. Usually, the laser welding results in a considerable rise in temperature of a weld during exposure to laser beams. After the laser exposure, the weld is cooled rapidly, thus increasing the possibility of solidification cracking thereof, which results in a lack of the strength of the joint between the ground electrode and the metal shell. 
   In recent years, modern automotive vehicles have been required to meet high power, low fuel consumption, and low exhaust emissions requirements, thus resulting in an increase in temperature of burning atmosphere in the engine. Therefore, when the weld between the metal shell and the ground electrode has already experienced the solidification cracking, and the ground electrode which is usually subjected to the most intense heat in the combustion chamber of the engine rises in temperature thereof greatly, it may cause the weld to be cracked completely, thus resulting in separation of the ground electrode from the metal shell. 
   SUMMARY OF THE INVENTION 
   It is therefore a principal object of the invention to avoid the disadvantages of the prior art. 
   It is another object of the invention to provide a spark plug designed to ensure a higher strength of joint between a ground electrode and a metal shell. 
   According to one aspect of the invention, there is provided a spark plug which works to produce a sequence of sparks to ignite an air-fuel mixture and which is so designed as to provide a higher strength of joint between a ground electrode and a metal shell. The spark plug comprises: (a) a hollow cylindrical metal shell; (b) a center electrode retained in the metal shell to be insulated from the metal shell; and (c) a ground electrode having a portion facing a tip of the center electrode through a spark gap. The ground electrode is joined to the metal shell through a weld. The weld ranges from an end surface to an inner peripheral surface of the metal shell. This results in an increased weld interface between the ground electrode and the metal shell, thereby enhancing the strength of the joint between the ground electrode and the metal shell. 
   In the preferred mode of the invention, the ground electrode is jointed to the metal shell by resistance welding. The use of the resistance welding facilitates ease of welding the ground electrode to the metal shell. 
   The shell may have an extension extending from the end surface of the metal shell in a longitudinal direction of the metal shell. The ground electrode is welded to the extension. This structure permits the length of the ground electrode to be reduced by an amount equivalent to the height of the extension, thereby enhancing the heat disappearance from the ground electrode. Additionally, the extension permits the ground electrode to extend inwardly of the metal shell and perpendicular to the longitudinal center line of the metal shell. This allows the ground electrode to be minimized in length thereof, thereby enhancing the heat disappearance from the ground electrode further. 
   The ground electrode may be made of a plate member which extends straight from the weld to the metal shell toward the longitudinal center line of the metal shell. Use of such a plate member results in a decreased length of the ground electrode exposed to a combustion chamber of the engine as compared with a conventional L-shaped ground electrode, thus leading to a decreased temperature of the ground electrode during running of the engine. 
   The tip of the center electrode may project outside the end surface of the metal shell in a longitudinal direction of the metal shell, thereby enhancing the spread of the flame kernel in the combustion chamber of the engine to improve the ignitability of an air-fuel mixture. The extension may extend outside the tip of the center electrode in the longitudinal direction of the metal shell, thereby allowing the ground electrode to be decreased in length to enhance the heat disappearance from the ground electrode. 
   The weld is formed by melting the ground electrode and the end surface of the metal shell and sinking a portion of the end surface of the metal shell into the ground electrode in the longitudinal direction of the metal shell. A sinking depth of the metal shell is preferably 0.4 mm or more, thereby ensuring the strength of the joint between the ground electrode and the metal shell sufficient for practical use. The sinking depth is preferably 1.0 mm or less, thereby decreasing weld burrs to a practical allowable size. 
   A ratio of the sinking depth of the metal shell to a thickness of a major body of the ground electrode is within a range of 0.2 to 0.7, thereby ensuring a desired mechanical strength of the ground electrode without sacrificing the strength of the joint between the ground electrode and the metal shell. 
   The extension may have a length which extends in a circumferential direction of the end surface of the metal shell and is greater than a width of the ground electrode, thereby ensuring a weld interface between the ground electrode and the metal shell sufficient for achieving the heat disappearance from the ground electrode without sacrificing the thermal resistance of the ground electrode. 
   The length of the extension preferably ranges cover one-second or less of the circumference of the end surface of the metal shell, thereby minimizing obstruction of the extension to the spread of the flame kernel in the combustion chamber of the engine. 
   The ground electrode may contain 50 Wt % or more of one of Ni and Fe. 
   According to the second aspect of the invention, there is provided a production method of a spark plug composing the steps of: (a) preparing a hollow cylindrical metal shell which has an inner shoulder formed therein and an end surface with an opening; (b) preparing a cylindrical first electrode and a second electrode, the second electrode being contoured to an inner shape of the metal shell and including an electrode contact and a stopper; (c) disposing the second electrode within the metal shell with the electrode contact abutting the inner shoulder of the metal shell and the stopper exposed to the opening of the end surface of the metal shell; (d) placing a ground electrode on the end surface of the metal shell; and (e) moving the first electrode to press the ground electrode against the end surface of the metal shell until the ground electrode abuts the stopper of the second electrode while supplying an electrical current between the first and second electrodes to resistance-weld the ground electrode to the end of the metal shell. Specifically, the welding of the ground electrode to the metal shell is achieved while the ground electrode is being nipped firmly between the first electrode and an end surface of the stopper of the second electrode. The location and orientation of the ground electrode relative to the metal shell is, thus, determined by the end surface of the stopper, thus ensuring a desired degree of parallelization of a discharging surface of the ground electrode  40  to the end surface of the metal shell. This eliminates the need for adjusting the orientation of the ground electrode after being welded to the metal shell and assures a high strength of joint between the ground electrode and the metal shell. 
   In the preferred mode of the invention, the second electrode may be made of a cylindrical member having the electrode contact and the stopper formed integrally with each other. This facilitates the ease of insertion of the second electrode into the metal shell. 
   Each of the first and second electrodes may be made of a copper alloy. 
   The ground electrode may be resistance-welded to the end of the metal shell under a constant pressure. 
   The metal shell may have a protrusion formed on an inner edge portion of the end surface. In this case, the ground electrode is placed on the protrusion on the end surface of the metal shell. The electrical current applied between the first and second electrodes is first concentrated on the protrusion so that the protrusion is softened or melted to facilitate welding of the ground electrode to the metal shell. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
       FIG. 1  is a partially longitudinal sectional view which shows a spark plug according to the first embodiment of the invention; 
       FIG. 2  is a partially sectional view which shows a weld between a ground electrode and a metal shell of the spark plug of  FIG. 1 ; 
       FIG. 3  is a top view of  FIG. 2 ; 
       FIG. 4(   a ) is a top view which shows a spark plug before a ground electrode is welded to a metal shell; 
       FIG. 4(   b ) is a partially sectional view of  FIG. 4(   a ); 
       FIGS. 5(   a ) and  5 ( b ) are partially sectional views which show steps of welding a ground electrode to a metal shell using resistance welding techniques; 
       FIG. 6  is a graph which shows results of evaluation of joint strength of a ground electrode of spark plug samples; 
       FIG. 7  is a top view which shows a spark plug according to the second embodiment of the invention; 
       FIG. 8  is a top view which shows a spark plug according to the third embodiment of the invention; 
       FIG. 9  is a partially sectional view which shows a spark plug according to the fourth embodiment of the invention; 
       FIG. 10  is a partially sectional view which shows a spark plug according to the fifth embodiment of the invention; 
       FIG. 11  is a partially sectional view which shows a spark plug according to the sixth embodiment of the invention; 
       FIG. 12  is a partially sectional view which shows a spark plug according to the seventh embodiment of the invention; 
       FIG. 13  is a partially sectional view which shows a spark plug according to the eighth embodiment of the invention; 
       FIG. 14  is a top view of  FIG. 13 ; 
       FIG. 15  is a partially sectional view which shows a spark plug according to the ninth embodiment of the invention; 
       FIG. 16  is a top view of  FIG. 15 ; 
       FIG. 17  is a partially sectional view which shows a spark plug according to the tenth embodiment of the invention; 
       FIG. 18(   a ) is a top view which shows a modified form of a spark plug in which a protrusion is formed directly on an end surface of a metal shell; and 
       FIG. 18(   b ) is a partially longitudinal sectional view of  FIG. 18(   a ). 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to  FIG. 1 , there is shown a spark plug  100  which may be used ignite an air-fuel mixture in internal combustion engines. 
   The spark plug  100  includes a cylindrical metal housing or shell  10 , a porcelain insulator  20 , a center electrode  30 , and a ground electrode  40 . The metal shell  10  is made of, for example, a carbon steel and has cut therein a thread  11  for mounting the spark plug  100  in a cylinder block of the engine (not shown). The metal shell  10  has an end surface  12  which will be referred to as a reference surface below) and an extension  13  extending form the reference surface  12  in parallel to a longitudinal center line C (i.e., an axis) of the metal shell  12 . 
   The porcelain insulator  20  made of an alumina ceramic (Al 2 O 3 ) is retained within the metal shell  10 . The porcelain insulator  20  has formed therein an axial bore  21  within which the center electrode  30  is retained to be electrically insulated from the metal shell  10 . The center electrode  30  has a tip portion which is exposed outside the insulator porcelain  20  and protrudes outside the reference surface  12  of the metal shell  10  in the longitudinal direction of the metal shell  10 . The extension  13  of the metal shell  10  protrudes from the tip end of the center electrode  30  in the longitudinal direction of the metal shell  10 . 
   The center electrode  30 , as shown in  FIG. 2 , consists of a body  31  and an Ir alloy chip  32 . The body  31  is made of a cylindrical member which consists of a core portion made of a metallic material such as Cu having a higher thermal conductivity and an external portion made of a metallic material such as an Ni-based alloy having higher thermal and corrosion resistances. The Ir alloy chip  32  is of a disc shape and joined to an end of the body  31  by laser welding or arc welding. 
   The ground electrode  40 , as shown in  FIGS. 2 and 3 , includes a plate body  41  made of an Ni-based alloy such as Inconel (trade mark) and a chip  42  joined to the plate body  41  by laser welding or arc welding. The chip  42  is made of a Pt alloy plate. The plate body  41  is joined to the extension  13  of the metal shell  10  by resistance welding and extends straight from the extension  13  so as to traverse perpendicular to the longitudinal center line C. The chip  42  faces the chip  32  of the center electrode  42  through the spark gap. 
   The joining of the plate body  41  of the ground electrode  40  to the extension  13  of the metal shell  10  is, as clearly shown in  FIG. 2 , achieved by placing the plate body  41 , as indicated by a broken line, on the end of the extension  13  and electrifying the plate body  41  under pressure to melt the surface of the plate body  41  in contact with the extension  13 , thereby sinking the end of the extension  13  into the plate body  41  to a depth, as indicated by a solid line, to form a joint of the plate body  41  with the extension  13  which continues from the end to a portion of an inner periphery of the extension  13 . 
   The distance L 1  between the tip of the chip  32  of the center electrode  30  and the reference surface  12  of the metal shell  10  in the longitudinal direction of the metal shell  10  is 0.5 mm. The distance L 2  between an interface M of the ground electrode  40  with the extension  13  and the reference surface  12  in the longitudinal direction of the metal shell  10  is 1.0 mm. 
   The joining of the ground electrode  40  to the metal shell  10  is, as described above, achieved by melting the ground electrode  40  to embed a portion of the extension  13  into the ground electrode  40 , thus improving the strength of the joint of the ground electrode  40  with the metal shell  10 . The joint ranges to the inner periphery of the extension  113  as well as the end thereof, thus resulting in an increased interface of the joint with the extension  13 , which ensures a higher degree of the strength of the joint. 
   Usually, the resistance welding is lower in temperature of a weld zone than the laser welding, so that the weld zone is cooled slowly and withstands solidification cracking. Specifically, the resistance welding is effective to enhance the reliability of strength of the joint of the ground electrode  40  with the metal shell  10 . 
   The welding of the ground electrode  40  to the extension  13  of the metal shell  10  allows the volume of the ground electrode  40  to be decreased by an amount corresponding to the length of the extension  13 , thereby enhancing the heat disappearance from the ground electrode  40 . The formation of the extension  13  on the metal shell  10  enables the ground electrode  40  to extend straight from the joint with the extension  13  perpendicular to the longitudinal center line C of the metal shell  10 . This permits the length of the ground electrode  40  to be minimized as compared with when an L-shaped ground electrode is used, thereby increasing the heat disappearance from the ground electrode  40  further. 
   The tip portion of the center electrode  30 , as described above, protrudes from the reference surface  12  of the metal shell  10 , thereby facilitating ease of expansion of the flame kernel within the combustion chamber of the engine and improving the ignitability of an air-fuel mixture. The extension  13  of the metal shell  10  protrudes from the tip of the center electrode  30 , thereby allowing the ground electrode  40  to be shortened in length to increase the heat disappearance from the ground electrode  40 . 
   The resistance welding of the ground electrode  40  to the metal shell  10  of the spark plug  100  will be described below in detail.  FIG. 4(   a ) is a top view of the metal shell  10  before the ground electrode  40  is welded to the metal shell  10 .  FIG. 4(   b ) is a partially sectional view of  FIG. 4(   a ). The extension  13 , as clearly shown in  FIG. 4(   b ), has a step or protrusion formed on the tip end thereof.  FIG. 4(   b ) emits the chip  42  to be joined to the ground electrode  40  for the brevity of illustration. 
   The metal shell  10 , as described above, has the extension  13  which is, as can be seen from  FIG. 4(   a ), of an arc-shape. The extension  13  has the protrusion  15  which is of an arc-shape and formed on an inner edge of the upper end thereof. 
     FIGS. 5(   a ) and  5 ( b ) are partially sectional views which illustrate the ground electrode  40  before and after welded to the metal shell  10 , respectively. 
   First, upper and lower electrodes  51  and  52  are prepared. The upper electrode  51  is made of, for example, a cylindrical electrode and used to press the ground electrode  40  against the extension  13  of the metal shell  10 . The lower electrode  52  is contoured to conform with the contour of an inner wall of the metal shell  10  and disposed inside the metal shell  10  with a tip portion protruding outside the reference surface  12 . Specifically, the lower electrode  52  includes an electrode contact which is to be placed in abutment with an inner shoulder  14  of the metal shell  10  and a stopper extending upward, as viewed in the drawing, from the electrode contact. The stopper is formed integrally with the electrode contact and placed so as to protrude from the reference surface  12  of the metal shell  10 . The upper and lower electrodes  51  and  52  are made of, for example, a copper alloy such as a copper-chromium alloy. 
   The end surface  52   a  of the lower electrode  52 , as shown in  FIG. 5(   a ), faces a discharging surface of the ground electrode  40  through an air gap. The distance between the end surface  52   a  of the lower electrode  52  in abutment of the electrode contact with the inner shoulder  14  of the metal shell  10  and a lower edge of the inner shoulder  15  of the metal shell  10  is expressed by “C” which is selected in order to position the ground electrode  40  relative to the metal shell  10 . 
   Next, the upper electrode  51  is, as shown in  FIG. 5(   b ), moved downward, as viewed in the drawing, to press the ground electrode  40  against the extension  13 . Simultaneously, the electrical current is supplied between the upper and lower electrodes  51  and  52 . The pressure to push the upper electrode  51  against the lower electrode  52  is approximately 40 kgf. The current flowing between the upper and lower electrodes  51  and  52  is approximately 2.6 kA. 
   The current first flows from the upper electrode  51  to the ground electrode  40 , to the protrusion  15 , to the extension  13 , to the body of the metal shell  10 , to the inner shoulder  14  of the metal shell  10 , and to the lower electrode  52 . Specifically, the current is concentrated at the protrusion  15  of the metal shell  10 , thus causing the protrusion  15  to melt completely. Afterwards, the extension  13  begins to melt. 
   While the protrusion  15  and the extension  13  of the metal shell  10  are melting, the ground electrode  40  is pressed by the upper electrode  51  against the metal shell  10 , so that the extension  13  partially sinks into and welded to the ground electrode  40 . The extension  13  sinks in the ground electrode  40  until the discharging surface of the ground electrode  40  abuts the end surface  52   a  of the lower electrode  52 , in other words, until the interval between the discharging surface of the ground electrode  40  and the lower edge of the inner shoulder  14  of the metal shell  10  reaches the distance C. 
   In the above manner, the ground electrode  40  is welded to the extension  13  of the metal shell  10  under a constant pressure and held at a selected position to the metal shell  10  with a desired degree of parallelization of the discharging surface of the ground electrode  40  to the reference surface  12  of the metal shell  10 . The end surface  52   a  of the lower electrode  52  has formed therein a hole (not shown) within which the chip  42  of the ground electrode  40  is placed when the ground electrode  40  is pressed on the extension  13  of the metal shell  10 . 
   In the initial stage of the resistance welding, the current is, as described above, concentrated at the protrusion  15  of the metal shell  10 , so that the protrusion  15  melts completely, thereby enhancing the welding of the protrusion  15  to a corresponding area of the ground electrode  40 , which increases the strength of joint therebetween. This minimizes a variation in tensile strength, as will be described later in detail. 
   After the ground electrode  40  is welded to the metal shell  10  in the above manner, the lower electrode  52  is removed from the metal shell  10 . Finally, the porcelain insulator  20  within which the center electrode  30  is retained is inserted into the metal shell  10 . The end of the metal shell  10  is crimped to hold the porcelain insulator  20  firmly to complete the spark plug  100 , as illustrated in  FIG. 1 . 
   Results of evaluation of the joint strength of the ground electrode  40  of the spark plug  100  will be described below. 
   The amount of depth of the extension  13  sinking into the ground electrode  40  in the longitudinal direction of the metal shell  10  is expressed by “A” in  FIG. 2 . We prepared spark plug samples and measured the tensile strength of the joint between the ground electrode  40  and the metal shell  10  for different values of the sinking depth A and evaluated the strength of the joint. 
   The sinking depth A was determined in the following manner. The thickness T of the plate body  41  of the ground electrode  40  was first measured. The thickness B of a portion of the plate body  41  abutting the tip end of the extension  13  was measured. In practice, the thickness B was measured at a point N located 1.5 mm inside the outer periphery of the extension  13 . The sinking depth A was calculated by the thickness B from the thickness T (i.e., A=T−B). 
   The specifications of each of the spark plug samples used for the evaluation of the joint strength of the ground electrode  40  are as follows. The metal shell  10  is made of S25C. The plate body  41  of the ground electrode  40  is made of Inconel (i.e., Ni-based alloy). The thickness T of the plate body  41  is 1.6 mm. The width W of the plate body  41 , as shown in  FIG. 3 , is 3.3 mm. 
     FIG. 6  is a graph which shows the results of evaluation of the joint strength of the ground electrode  40  of the spark plug samples. The vertical axis indicates the tensile strength. The horizontal axis indicates the sinking depth A. The graph shows that when the sinking depth A is less than 0.4 mm, the tensile strength is low, resulting in separation of the ground electrode  40  from the extension  13 , while when the sinking depth A is 0.4 mm or more, it produces the tensile strength sufficient for practical use. 
   When the sinking depth A exceeds 1.0 mm, it results in an increased size of weld flashes or burrs, which causes the gap between the weld burrs and the center electrodes  30  to be decreased to induce sparks therebetween. Alternatively, when the sinking depth A is below 1.0 mm, it results in a decreased size of the weld burrs, which eliminates the possibility of sparks produced between the weld burrs and the center electrode  30 . 
   We also prepared another type of spark plug sample in which the plate body  41  of the ground electrode  40  is made of aluminum-containing Inconel, and the width W of the plate body  41  is 4.1 mm and evaluated the joint strength thereof in the same manner as described above. Results of the evaluation showed the same effects as those in the above spark plug samples. Specifically, when the sinking depth A was 0.4 mm or more, it produced the tensile strength sufficient for practical use. When the sinking depth A is less than 1.0 mm, it results in a decreased size of the weld burrs, which eliminates the possibility of sparks produced between the weld burrs and the center electrode  30 . 
   A ratio of the sinking depth A to the thickness T of the plate body  41  of the ground electrode  40  of the spark plug  100  (i.e., A/T) is selected to be within a range of 0.2 to 0.7. A smaller value of the A-T ratio represents that the amount by which the extension  13  is embedded into the ground electrode  40  is smaller. A desired strength of the joint of the ground electrode  40  to the metal shell  10  without sacrificing the mechanical strength of the ground electrode  40  itself is ensured by selecting the value of the A-T ratio within the above range. 
     FIG. 7  shows a spark plug according to the second embodiment of the invention which is different from the one of the first embodiment only in the size of the extension  13  of the metal shell  10 . Other arrangements are identical, and explanation thereof in detail will be omitted here. 
   The extension  13  is of a half circle shape and ranges over half a circumference of the end surface of the metal shell  10 . If the extension  13  occupies more than one-second of the circumference of the end surface of the metal shell  10 , it will be an obstruction to obstruct the spread of the flame kernel within the cylinder of the engine greatly. It is, thus, advisable that the extension  13  occupies one-second or less of the circumference of the end surface of the metal shell  10 . 
     FIG. 8  shows a spark plug according to the third embodiment of the invention which is different from the one of the first embodiment only in the size of the extension  13  of the metal shell  10 . Other arrangements are identical, and explanation thereof in detail will be omitted here. 
   The extension  13  has a length in a circumferential direction of the metal shell  10  which is slightly greater than the width W of the ground electrode  40 . For instance, the inner diameter d of the metal shell  10  is 9 mm. The width W of the ground electrode  40  is 3.3 mm. The extension  13  ranges over one-sixth (⅙) of the circumference of the end surface of the metal shell  10 . This ensures a sufficient area of the joint between the ground electrode  40  and the metal shell  10  for achieving a desired amount of the heat disappearance or transfer from the ground electrode  40  to the metal shell  10  to assure the heat resistance of the ground electrode  40 . 
     FIG. 9  shows a spark plug according to the fourth embodiment of the invention. The spark plug does not have the extension  13 . Specifically, the ground electrode  40  is joined directly to the reference surface  12  of the metal shell  10  by the resistance welding. The tip of the center electrode  30  is disposed inside the metal shell  10 . In other words, the tip of the center electrode  30  lies inwardly of the reference surface  12  in the longitudinal direction of the metal shell  10 . The discharging surface of the ground electrode  40  which faces the center electrode  30  also lies inside the metal shell  10 . 
   The distance L 1  between the tip end of the chip  32  of the center electrode  30  and the reference surface  12  of the metal shell  10  in the longitudinal direction of the metal shell  10  is 1.3 mm. The distance L 2  between the interface M of the ground electrode  40  with the metal shell  10  and the reference surface  12  in the longitudinal direction of the metal shell  10  is 0.8 mm. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
   The structure of the spark plug of this embodiment results in a decreased length of the spark plug disposed inside the combustion chamber of the engine, thus decreasing the temperature of the ground electrode  40  during running of the engine. 
     FIG. 10  shows a spark plug according to the fifth embodiment of the invention. 
   The tip of the center electrode  30  is disposed inside the metal shell  10 . In other words, the tip of the center electrode  30  lies inwardly of the reference surface  12  in the longitudinal direction of the metal shell  10 . The discharging surface of the ground electrode  40  which faces the center electrode  30  lies flush with the reference surface  12  in a lateral direction of the metal shell  10 . The distance L 1  between the tip end of the chip  32  of the center electrode  30  and the reference surface  12  in the longitudinal direction of the metal shell  10  is 0.5 mm. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIG. 11  shows a spark plug according to the sixth embodiment of the invention. The spark plug does not have the extension  13  on the metal shell  10 . The ground electrode  40 , as clearly shown in the drawing, has a crank-shape and is joined directly to the reference surface  12  of the metal shell  10  by the resistance welding. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIG. 12  shows a spark plug according to the seventh embodiment of the invention. The ground electrode  40  extends from the extension  13  at a given angle to the longitudinal center line C of the metal shell  10  other than 90°. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIGS. 13 and 14  show a spark plug according to the eighth embodiment of the invention. The spark plug is a two-ground electrode plug equipped with two L-shaped ground electrodes  40 . The metal shell  10  has two arc-shaped extensions  13 . Each of the L-shaped ground electrodes  40  is joined to one of the extensions  13  by the resistance welding. Specifically, each of the ground electrodes  40  is made up of a horizontal strip  40   a  and a vertical strip  40   b . The horizontal strip  40   a  extends perpendicular to the longitudinal center line C of the metal shell  10  and connects with the extension  13 . The vertical strap  40   b  extends from the horizontal strip  40   a  in parallel to the longitudinal center line C and has a discharging surface facing the chip  32  of the center electrode  30 . Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIGS. 15 and 16  show a spark plug according to the ninth embodiment of the invention. The spark plug has two extensions  13  formed on the metal shell  10  and the ground electrode  40  joined at ends thereof to the extensions  13  by the resistance welding. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
     FIG. 17  shows a spark plug according to the tenth embodiment of the invention. The spark plug has two extensions  13  formed on the metal shell  10  and the ground electrode  40  with a central bulge. The ground electrode  40  is jointed at ends thereof to the extensions  13  by the resistance welding. Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here. 
   The ground electrode(s)  40 , as used in the above embodiments, may be made of a material containing 50 Wt % or more of Ni or 50 Wt % or more of Fe. 
   While the spark plug  100  of the first embodiment has the protrusion  15 , as illustrated in  FIGS. 4(   a ) and  4 ( b ), in order to ensure the stability of joint strength of the ground electrode  40 , it may be used in other embodiments. Alternatively, the spark plug  100  of the first embodiment may not have the protrusion  15 . 
   In a case where the metal shell  10 , like the one in the fourth embodiment, does not have the extension  13 , the protrusion  15  is preferably formed on at least a portion of the metal shell  10  to which the ground electrode  40  is to be welded.  FIGS. 18(   a ) and  18 ( b ) show an example wherein the protrusion  15  is formed directly on the reference surface  12  of the metal shell  10 . Specifically, the protrusion  15  extends on a portion of an inner edge of the reference surface  12  to which the ground electrode  40  is to be welded. Other arrangements are identical with those in the first embodiment. 
   While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.