Patent Publication Number: US-8523624-B2

Title: Spark plug for internal combustion engine and manufacturing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Division of application Ser. No. 11/336,876, filed Jan. 23, 2006, which is based on and claims priority from Japanese Patent Application No. 2005-18013, filed on Jan. 26, 2005, the content of which is hereby incorporated by reference into this application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to spark plugs for use in internal combustion engines of automotive vehicles and cogeneration systems. 
     More particularly, the invention relates to a spark plug for an internal combustion engine which has a capability to reliably induce and detect an ion current within a combustion chamber of the engine, and a method of manufacturing such a spark plug. 
     2. Description of the Related Art 
       FIG. 12  shows an existing spark plug  9  for use in an internal combustion engine, for example, of an automotive vehicle. As shown in the figure, the spark plug  9  includes an insulator  92 , a cylindrical center electrode  93 , a tubular metal shell  94 , and a pair of ground electrodes  95 . 
     The center electrode  93  is secured in the insulator  92  and has a protruding portion  932  that protrudes from an end  921  of the insulator  92 . The insulator  92  is retained in the metal shell  94  such that the end  921  thereof protrudes from an end  941  of the metal shell  94 . Each of the ground electrodes  95  has a base end  952 , which is joined to the end  941  of the metal shell  94  by, for example, welding, and a tip end  951  that faces the side surface of an end portion  931  of the protruding portion  932  of the center electrode  93  through a spark gap  91  in the radial direction of the center electrode  93 . 
     Such a spark plug  9  is generally used to ignite the air/fuel mixture within a combustion chamber of the internal combustion engine. In addition, the spark plug  9  may also be used, as disclosed in Japanese Patent First Publications No. 2000-34969 and No. 2004-22450, to induce and detect an ion current within the combustion chamber of the engine for the purpose of increasing the output and improving the fuel economy of the engine. 
     Specifically, during combustion of the air/fuel mixture within the combustion chamber of the engine, positive and negative ions are created due to ionization of the air/fuel mixture. The positive and negative ions are absorbed by the corresponding electrodes of the spark plug  9 , thereby inducing the ion current that flows between the center electrode  93  and the ground electrodes  95 . Through detecting the ion current, it is possible to determine the combustion pressure, the occurrence of a misfire, and other parameters and events relating to the combustion. 
     However, with running of the engine, the surface of the protruding portion  932  of the center electrode  93  is gradually fouled with combustion residues, such as carbon, thereby reducing the “effective area” of the surface. The effective area of the surface here denotes an area of the surface which is not fouled with combustion residues and thus can absorb ions to induce the ion current. 
     To enable the effective area of the surface of the protruding portion  932  of the center electrode  93  to be sufficiently large, the Japanese Patent First Publication No. 2004-22450 discloses a spark plug, in which the end face area of the tip end  951  of each of the ground electrodes  95  is specified to be in a given range, so as to effectively burn-off, by spark discharges, the combustion residues that have deposited on the surface of the protruding portion  932 . 
     However, with such a spark plug, it is still difficult to effectively burn-off in a sufficiently wide range the combustion residues, and thus it is still difficult to reliably induce and detect the ion current within the combustion chamber of the engine. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to overcome the above-mentioned problems accompanying the existing spark plugs. 
     It is another object of the present invention to provide a spark plug for an internal combustion engine, which has a capability to effectively burn-off the combustion residues that have deposited on a center electrode of the spark plug, and a manufacturing method thereof. 
     It is a further object of the present invention to provide a spark plug for an internal combustion engine, which has a capability to reliably induce and detect an ion current within a combustion chamber of the engine, and a manufacturing method thereof. 
     According to one aspect of the present invention, a spark plug is provided which includes a tubular metal shell, an insulator, a cylindrical center electrode, and at least one ground electrode. 
     The insulator is retained in the metal shell such that an end thereof protrudes from an end of the metal shell. 
     The center electrode is secured in the insulator and has a protruding portion that protrudes from the end of the insulator. 
     The ground electrode has an inclined portion, a straight portion, and a bend between the inclined and straight portions. The inclined portion extends obliquely with respect to the axial direction of the center electrode from a base end of the ground electrode, which is joined to the end of the metal shell, to the bend that is positioned closer to the center electrode in the radial direction of the center electrode than the base end. The straight portion extends substantially parallel to the axial direction of the center electrode from the bend to a tip end of the ground electrode. The straight portion has an inner side surface that faces the side surface of an end portion of the protruding portion of the center electrode through a spark gap in the radial direction of the center electrode. 
     Preferably, the spark plug further includes a noble metal chip that has a length with a base end joined to the inner side surface of the straight portion of the ground electrode and a tip end facing the side surface of the end portion of the protruding portion of the center electrode through the spark gap in the radial direction of the center electrode. Further, in the spark plug, the end portion of the protruding portion of the center electrode has a cross-sectional area perpendicular to the axial direction of the center electrode in a range of 0.79 to 4.9 mm 2 ; the protruding portion of the center electrode has a surface area in a rage of 10 to 30 mm 2 ; the noble metal chip has a cross-sectional area perpendicular to the lengthwise direction of the noble metal chip in a range of 0.12 to 1.13 mm 2  and a protruding length, which represents a distance from the inner side surface of the straight portion of the ground electrode to the tip end of the noble metal chip in the radial direction of the center electrode, in a range of 0.3 to 1.5 mm. 
     Further, in the spark plug, the end portion of the protruding portion of the center electrode preferably includes on the side surface thereof a noble metal portion that faces the noble metal chip through the spark gap in the radial direction of the center electrode. 
     Both the noble metal chip and the noble metal portion of the center electrode may be made of a Pt-based alloy that includes Pt in an amount of not less than 50% by weight and at least one additive selected from Ir, Rh, Ni, W, Pd, Ru, Re, Al, Al 2 O 3 , Y, and Y 2 O 3 . Otherwise, those may also be made of an Ir-based alloy that includes Ir in an amount of not less than 50% by weight and at least one additive selected from Pt, Rh, Ni, W, Pd, Ru, Re, Al, Al 2 O 3 , Y, and Y 2 O 3 . 
     Moreover, in the spark plug, the following dimensional relationship is preferably defined:
 
0≦( W−d )≦1.5 mm,
 
where W is the width of the inner side surface of the straight portion of the ground electrode perpendicular to the axial direction of the center electrode, and d is the diameter of the noble metal chip when it is cylindrical.
 
     Furthermore, the noble metal chip is preferably joined to the inner side surface of the straight portion of the ground electrode by laser welding, so that a weld is formed between the noble metal chip and the inner side surface of the straight portion of the ground electrode over the entire circumference of the noble metal chip. 
     Preferably, in the spark plug, the inner side surface of the straight portion of the ground electrode has a length in the axial direction of the center electrode in a range of 1.8 to 3.0 mm. 
     Preferably, in the spark plug, the ratio of g/G is greater than or equal to 1.3, where g is the minimum distance between the ground electrode and the insulator, and G is the size of the spark gap which represents the minimum distance between the center electrode and the noble metal chip. 
     Preferably, the spark plug includes a plurality of the ground electrodes. 
     According to another aspect of the present invention, a method of manufacturing a spark plug for an internal combustion engine is provided which includes the steps of: 
     preparing a tubular metal shell, an assembly of an insulator and a cylindrical center electrode, and at least one ground electrode base material, the center electrode being secured in the insulator and having a protruding portion that protrudes from an end of the insulator, the ground electrode base material being rod-shaped and having a first and a second end that are opposite to each other in the lengthwise direction of the ground electrode base material; 
     joining the first end of the ground electrode base material to an end of the metal shell such that the lengthwise direction of the ground electrode base material is substantially parallel to the axial direction of the metal shell; 
     bending the ground electrode base material to move the second end thereof outward in the radial direction of the metal shell, thereby forming a bend between the first and second ends of the ground electrode base material; 
     joining a noble metal chip by laser welding to the inner side surface of a portion of the ground electrode base material between the bend and the second end; 
     fitting the assembly of the insulator and the center electrode in the metal shell such that the end of the insulator protrudes from the end of the metal shell and the axial direction of the metal shell substantially coincides with the axial direction of the center electrode; and 
     bending the ground electrode base material at the first end thereof to make the portion thereof between the bend and the second end being substantially parallel to the axial direction of the center electrode and closer to the center electrode in the radial direction of the center electrode than the first end, thereby forming a ground electrode, the ground electrode having an inclined portion, a straight portion, and a bend between the inclined and straight portions which corresponds to the bend of the ground electrode base material, the inclined portion extending obliquely with respect to the axial direction of the center electrode from a base end of the ground electrode, which corresponds to the first end of the ground electrode base material, to the bend, the straight portion extending substantially parallel to the axial direction of the center electrode from the bend to a tip end of the ground electrode which corresponds to the second end of the ground electrode base material, the straight portion having an inner side surface to which is joined the noble metal chip that faces the side surface of the protruding portion of the center electrode through a spark gap in the radial direction of the center electrode. 
     Preferably, in the method, a plurality of the ground electrodes are formed from a plurality of the ground electrode base materials. 
     Preferably, in the method, the laser welding for joining the noble metal chip to the inner side surface of the portion of the ground electrode base material is performed by irradiating a laser beam with an irradiation angle with respect to the inner side surface of the portion of the ground electrode base material in a range of 20 to 55°. 
     Consequently, through providing the above spark plug and the manufacturing method, the objects of the present invention are achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood more fully from the detailed description given hereinafter 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 accompanying drawings: 
         FIG. 1  is a partially cross-sectional view showing the overall structure of a spark plug according to the first embodiment of the invention; 
         FIG. 2  is an enlarged partially cross-sectional view showing an end portion of the spark plug of  FIG. 1 ; 
         FIG. 3  is an enlarged partially cross-sectional view illustrating dimensional parameters in the spark plug of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view illustrating a width of a ground electrode and the diameter of a noble metal chip in the spark plug of  FIG. 1 ; 
         FIG. 5  is a graphical representation showing the relationship between the protruding length of ground electrodes and the ignition capability of the spark plug of  FIG. 1 ; 
         FIG. 6  is a graphical representation showing the relationship between the effective area of a center electrode of the spark plug of  FIG. 1  and mileage; 
         FIG. 7  is a graphical representation showing the relationship between a dimensional parameter g/G and the occurrence rate of side sparks in the spark plug of  FIG. 1 ; 
         FIGS. 8A-8D  are schematic views illustrating a method, according to the second embodiment of the invention, of manufacturing the spark plug of  FIG. 1 ; 
         FIG. 9  is a schematic view illustrating an irradiation angle of a laser beam in manufacture of the spark plug of  FIG. 1 ; 
         FIG. 10  is a schematic view illustrating irradiation of a laser beam in manufacture of the spark plug of  FIG. 1 ; 
         FIGS. 11A-11D  are schematic views illustrating a method, according to the third embodiment of the invention, of manufacturing the spark plug of  FIG. 1 ; and 
         FIG. 12  is an enlarged partially cross-sectional view showing an end portion of a prior art spark plug. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described hereinafter with reference to  FIGS. 1-11 . 
     It should be noted that, for the sake of clarity and understanding, identical components having identical functions in different embodiments of the invention have been marked, where possible, with the same reference numerals in each of the figures. 
     [First Embodiment] 
       FIG. 1  shows the overall structure of a spark plug  1  according to the first embodiment of the invention. 
     The spark plug  1  is designed for use in an internal combustion engine of an automotive vehicle or a cogeneration system. Specifically, the spark plug  1  is designed to perform two different functions in the engine. One function is to ignite the air/fuel mixture within a combustion chamber of the engine; the other is to induce and detect an ion current within the combustion chamber of the engine. 
     As shown in  FIG. 1 , the spark plug  1  includes an insulator  2 , a cylindrical center electrode  3 , a tubular metal shell  4 , and a pair of ground electrodes  5 . 
     Referring to  FIG. 2 , the tubular metal shell  4  has a male threaded portion  41  on an outer periphery thereof, through which the spark plug  1  is installed in the combustion chamber of the engine. The metal shell  4  is made of a conductive metal material, such as low-carbon steel. 
     The insulator  2  is retained in the metal shell  4  such that an end  21  thereof protrudes from an end  42  of the metal shell  4 . The insulator  2  is made, for example, of alumina (Al 2 O 3 ). 
     The cylindrical center electrode  3  is secured in the insulator  2 , so that it is electrically isolated from the metal shell  4 . The center electrode  3  has a protruding portion  32  that protrudes from the end  21  of the insulator  2 . The center electrode  3  may be made of a highly heat conductive metal material such as Cu as the core material and a highly heat-resistant, corrosion-resistant metal material such as a Ni (Nickel)-based alloy as the cladding material. 
     The two ground electrodes  5  are disposed on the same diameter line of the metal shell  4 , with the center electrode  3  interposed therebetween. 
     Each of the ground electrodes  5  has an inclined portion  52 , a straight portion  54 , and a bend  53  between the inclined and straight portions  52  and  54 . The inclined portion  52  extends obliquely with respect to the axial direction of the center electrode  3  from a base end  51  of the ground electrode  5 , which is joined to the end  42  of the metal shell  4  by, for example, resistance welding, to the bend  53  that is poisoned closer to the center electrode  3  in the radial direction of the center electrode  3  than the base end  51 . 
     The straight portion  54  extends substantially parallel to the axial direction of the center electrode  3  from the bend  53  to a tip end  56  of the ground electrode  5 . The straight portion  54  has an inner side surface  541  that faces the side surface  34  of an end portion  31  of the center electrode  3  in the radial direction of the center electrode  3 . Each of the ground electrodes  5  is made, for example, of a Ni-based alloy. 
     Further, in the spark plug  1 , there is provided a cylindrical noble metal chip  55  on the inner side surface  541  of the straight portion  54  of each of the ground electrodes  5 . Specifically, the noble metal chip  55  has base end  55   a , which is joined to the inner side surface  541  of the straight portion  54  of the ground electrode  5 , and a tip end  55   b  that faces the side surface  34  of the end portion  31  of the center electrode  3  through a spark gap  11  in the radial direction of the center electrode  3 . 
     As to be described in detail in the following embodiments of the invention, the noble metal chip  55  is joined to the inner side surface  541  of the straight portion  54  of the ground electrode  5  by laser welding, so that a weld  55   c  (as indicated in  FIGS. 9 and 10 ) is formed between the noble metal chip  55  and the inner side surface  541  of the straight portion  54  of the ground electrode  5  over the entire circumference of the noble metal chip  55 . 
     Furthermore, in the spark plug  1 , the center electrode  3  includes a pair of noble metal portions  33 , which are formed by, for example, swaging on the side surface  34  of the end portion  31  of the center electrode  3  and faces the corresponding inner side surfaces  541  of the straight portions  54  of the ground electrodes  5 . 
     It should be noted that instead of providing such noble metal portions  33 , the entire end portion  31  of the center electrode  3  may be made of a noble metal or its alloy. 
     Preferably, the noble metal chips  55  and the noble metal portions  33  of the center electrode  3  are made of a Pt-based alloy that includes Pt in an amount of not less than 50% by weight and at least one additive selected from Ir, Rh, Ni, W, Pd, Ru, Re, Al, Al 2 O 3 , Y, and Y 2 O 3 . Otherwise, those may also be made of an Ir-based alloy that includes Ir in an amount of not less than 50% by weight and at least one additive selected from Pt, Rh, Ni, W, Pd, Ru, Re, Al, Al 2 O 3 , Y, and Y 2 O 3 . 
     Using such materials, it is possible to suppress wear of the noble metal chips  55  and the center electrode  3 , thereby securing a long service life of the spark plug  1 . 
     Having described the overall structure of the spark plug  1 , the following dimensional parameters, which are critical to the performance of the spark plug  1 , will be defined and specified hereinafter with reference to  FIGS. 3 and 4 . 
     In the spark plug  1 , a cross-sectional area S 31  of the end portion  31  of the center electrode  3  perpendicular to the axial direction of the center electrode  3  is in the range of 0.79 to 4.9 mm 2 . 
     The surface area S 32  of the protruding portion  32  of the center electrode  3  is in the range of 10 to 30 mm 2 . 
     The cross-sectional area S 55  of each of the noble metal chips  55  perpendicular to the lengthwise direction of the noble metal chip  55  is in the range of 0.12 to 1.13 mm 2 . 
     A protruding length t of each of the noble metal chips  55 , which is defined as the distance from the inner side surface  541  of the straight portion  54  of the corresponding ground electrode  5  to the tip end  55   b  of the noble metal chip  55  in the radial direction of the center electrode  3 , is in the range of 0.3 to 1.5 mm. 
     The length A of the inner side surface  541  of the straight portion  54  of each of the ground electrodes  5  in the axial direction of the center electrode  3  is in the range of 1.8 to 3.0 mm. 
     The ratio of g/G is greater than or equal to 1.3, where g is the minimum distance between the ground electrodes  5  and the insulator  2 , and G is the size of the spark gaps  11  which represents the minimum distance between the noble metal chips  55  and the center electrode  3 . 
     The width W of the inner side surfaces  541  of the straight portions  54  of the ground electrodes  5  perpendicular to the axial direction of the center electrode  3  and the diameter d of the noble metal chips  55  have the following relationship:
 
0≦( W−d )≦1.5 mm.
 
     The above-described spark plug  1  according to the present embodiment has the following advantages. 
     In the spark plug  1 , each of the ground electrodes  5  includes the inclined portion  52 , the straight portion  54 , and the bend  53  between the inclined and straight portions  52  and  54 . The straight portion  54  has the inner side surface  541  that faces the side surface  34  of the end portion  31  of the center electrode  3  in the radial direction of the center electrode  3 . 
     With such a configuration, the space between the protruding portion  32  of the center electrode  3  and the ground electrodes  5  becomes small, so that sparks can be discharged in a wide range between the protruding portion  32  of the center electrode  3  and the ground electrodes  5 . 
     Consequently, when the surface of the protruding portion  32  of the center electrode  3  is fouled with combustion residues, it is possible to burn off in a sufficiently wide range the combustion residues, thereby securing a sufficiently large effective area of the surface which can absorb ions created within the combustion chamber of the engine. As a result, it becomes possible for the spark plug  1  to reliably induce and detect the ion current as described above within the combustion chamber of the engine. 
     Further, in the spark plug  1 , the cross-sectional area S 31  of the end portion  31  of the center electrode  3  is specified to the range of 0.79 to 4.9 mm 2 , the surface area S 32  of the protruding portion  32  of the center electrode  3  is specified to the range of 10 to 30 mm 2 , the cross-sectional area S 55  of the noble metal chips  55  is specified to the range of 0.12 to 1.13 mm 2 , and the protruding length t of the noble metal chips  55  is specified to the range of 0.3 to 1.5 mm. 
     With such specifications, it is possible to secure a superior capability of the spark plug  1  to reliably induce and detect the ion current within the combustion chamber of the engine as well as a superior ignition capability (i.e., the capability to ignite the air/fuel mixture within the combustion chamber of the engine) of the spark plug  1 . 
     Specifically, through specifying the cross-sectional area S 31  of the end portion  31  of the center electrode  3  and the surface area S 32  of the protruding portion  32  of the center electrode  3  to the above ranges, it becomes possible to secure a sufficiently large effective area of the surface of the protruding portion  32  even when the surface remains partially fouled with combustion residues after spark discharges. At the same time, it also becomes possible to prevent the heat capacity of the center electrode  3  from becoming too large, thereby securing a high ignition capability of the spark plug  1 . 
     If the cross-sectional area S 31  of the end portion  31  of the center electrode  3  is below 0.79 mm 2 , it may be difficult to secure a superior capability of the spark plug  1  to reliably induce and detect the ion current within the combustion chamber of the engine. On the contrary, if the cross-sectional area S 31  is above 4.9 mm 2 , it may be difficult to secure a superior ignition capability of the spark plug  1 . 
     If the surface area S 32  of the protruding portion  32  of the center electrode  3  is below 10 mm 2 , it may be difficult to secure a superior capability of the spark plug  1  to reliably induce and detect the ion current within the combustion chamber of the engine. On the contrary, if the surface area S 32  is above 30 mm 2 , it may be difficult to secure a superior ignition capability of the spark plug  1 . 
     Through specifying the cross-sectional area S 55  and the protruding length t of the noble metal chips  55  to the above ranges, it becomes possible to secure a superior heat-resistant and wear-resistant capability of the noble metal chips  55 . At the same time, it also becomes possible to prevent the heat capacity of the noble metal chips  55  from becoming too large, thereby securing a high ignition capability of the spark plug  1 . 
     If the cross-sectional area S 55  of the noble metal chips  55  is below 0.12 mm 2 , it may be difficult to secure a superior heat-resistant and wear-resistant capability of the noble metal chips  55 . On the contrary, if the cross-sectional area S 55  is above 1.13 mm 2 , it may be difficult to secure a high ignition capability of the spark plug  1 . 
     If the protruding length t of the noble metal chips  55  is below 0.3 mm, it may be difficult to secure a high ignition capability of the spark plug  1 . On the contrary, if the protruding length t is above 1.5 mm, it may be difficult to secure a superior heat-resistant and wear-resistant capability of the noble metal chips  55 . 
     In the spark plug  1 , the length A of the inner side surfaces  541  of the straight portions  54  of the ground electrodes  5  in the axial direction of the center electrode  3  is specified to the range of 1.8 to 3.0 mm. 
     With such a specification, it is possible to clean in a sufficiently wide range the surface of the protruding portion  32  of the center electrode  3 , thereby securing a sufficiently large effective area of the surface. As a result, it becomes possible to improve the capability of the spark plug  1  to reliably induce and detect the ion current within the combustion chamber of the engine. 
     If the length A is below 1.8 mm, it may be difficult to clean in a sufficiently wide range the surface of the protruding portion  32  of the center electrode  3 . As a result, it would become difficult to prevent the effective area of the surface from decreasing with running of the engine. On the contrary, if the length A is above 3.0 mm, “side sparks” may occur instead of “normal sparks”, so that the induced combustion would become unstable and thus could not create a sufficient number of ions necessary for inducing the ion current. 
     Hereinafter, the side sparks denote sparks which move from the center electrode  3  along the end  21  of the insulator  2  and jump to the inclined portions  52  of the ground electrodes  5 . In comparison, normal sparks denote sparks which are discharged across the spark gaps  11 . 
     In the spark plug  1 , the ratio of g/G is specified to be greater than or equal to 1.3. 
     Specifically, in the spark plug  1 , it is easier to make the ground electrodes  5  too close to the insulator  2  than in the existing spark plug  9  shown in  FIG. 12 . However, through specifying the ratio of g/G as above, it becomes possible to effectively suppress occurrence of side sparks, thereby securing the combustion efficiency within the combustion chamber of the engine. 
     In the spark plug  1 , the width W of the inner side surfaces  541  of the straight portions  54  of the ground electrodes  5  and the diameter d of the noble metal chips  55  are specified to have the relationship of 0≦(W−d)≦1.5 mm. 
     Through specifying such a relationship, in laser welding of the noble chips  55  to the corresponding inner side surfaces  541 , a laser beam can be easily and reliably irradiated to the joining portions thereof, thereby securing high welding quality. 
     If (W−d)&gt;1.5 mm, it may be difficult to irradiate the laser beam with a suitable irradiation angle to the joining portions over the entire circumference thereof. On the contrary, if (W−d)&lt;0, the noble metal chips  55  would protrude from the corresponding inner side surfaces  541 . In both the cases, it would be difficult to secure high welding quality. 
     In the spark plug  1 , there is formed the weld  55   c  between each of the noble metal chips  55  and the inner side surface  541  of the straight portion  54  of the corresponding ground electrode  5  over the entire circumference of the noble metal chip  5 . 
     Consequently, the weld strength between the noble metal chips  55  and the corresponding inner side surfaces  541  are secured. 
     In the spark plug  1 , there is provided more than one ground electrode  5 . With the increased number of ground electrodes  5 , it becomes possible to more reliably clean the surface of the protruding portion  32  of the center electrode  3 , thereby improving the capability of the spark plug  1  to induce and detect the ion current within the combustion chamber of the engine. 
     The above-described advantages of the spark plug  1  have been confirmed through the experiments to be described below. 
     Experiment 1 
     This experiment was conducted to determine the effect of the protruding length t of the noble metal chips  55  on the ignition capability of the spark plug  1 . 
     In the experiment, four different types A, B, C, and D of sample spark plugs were used, and two different protruding lengths t of 0.3 mm and 0.8 mm were used for each of the four types. 
     Specifically, in sample spark plugs of the type A, the diameter of the center electrode  3  was 2.5 mm, the cross-sectional area S 31  of the end portion  31  of the center electrode  3  was 4.9 mm 2 , the surface area S 32  of the protruding portion  32  of the center electrode  3  was 30 mm 2 , the diameter d of the noble metal chips  55  was 1.2 mm, and the cross-sectional area S 55  of the noble metal chips  55  was 1.13 mm 2 . 
     In sample spark plugs of the type B, the diameter of the center electrode  3  was 2.5 mm, the cross-sectional area S 31  of the end portion  31  of the center electrode  3  was 4.9 mm 2 , the surface area S 32  of the protruding portion  32  of the center electrode  3  was 30 mm 2 , the diameter d of the noble metal chips  55  was 0.4 mm, and the cross-sectional area S 55  of the noble metal chips  55  was 0.12 mm 2 . 
     In sample spark plugs of the type C, the diameter of the center electrode  3  was 2.0 mm, the cross-sectional area S 31  of the end portion  31  of the center electrode  3  was 3.1 mm 2 , the surface area S 32  of the protruding portion  32  of the center electrode  3  was 21 mm 2 , the diameter d of the noble metal chips  55  was 1.2 mm, and the cross-sectional area S 55  of the noble metal chips  55  was 1.13 mm 2 . 
     In sample spark plugs of the type D, the diameter of the center electrode  3  was 2.0 mm, the cross-sectional area S 31  of the end portion  31  of the center electrode  3  was 3.1 mm 2 , the surface area S 32  of the protruding portion  32  of the center electrode  3  was 21 mm 2 , the diameter d of the noble metal chips  55  was 0.4 mm, and the cross-sectional area S 55  of the noble metal chips  55  was 0.12 mm 2 . 
     All the sample spark plugs were tested using an internal combustion engine, which has a displacement of 1.8 L and four in-line cylinders, under idling condition. The ignition capability of the sample spark plugs was evaluated in terms of lean limit air/fuel ratio. 
       FIG. 5  shows the test results, where the plots of “◯” indicate the results with the sample spark plugs of type A, the plots of “⋄” indicate the results with the sample spark plugs of type B, the plots of “●” indicate the results with the sample spark plugs of type C, and the plots of “♦” indicate the results with the sample spark plugs of type D. 
     It can be seen from  FIG. 5  that when the protruding length t of the noble metal chips  55  was greater than or equal to 0.3 mm, the lean limit air/fuel ratio was greater than or equal to 17.4 for all the sample spark plug types. 
     In other words, a high ignition capability of the spark plug  1  is secured through specifying the protruding length t of the noble metal chips  55  to be not less than 0.3 mm. 
     Experiment 2 
     This experiment was conducted to investigate the change in the effective area of the surface of the protruding portion  32  of the center electrode  3  with mileage. 
     Four different types A, B, C, and D of sample spark plugs were used in the experiment. Specifically, sample spark plugs of the types A-C were fabricated according to the present invention, in each of which the cross-sectional area S 31  of the end portion  31  of the center electrode  3  was 3.1 mm 2 , the surface area S 32  of the protruding portion  32  of the center electrode  3  was 25 mm 2 , the cross-sectional area S 55  of the noble metal chips  55  was 0.38 mm 2 , and the protruding length t of the noble metal chips  55  was 0.8 mm. The length A of the inner side surfaces  541  of the straight portions  54  of the ground electrodes  5  was 1.3 mm for the sample spark plug of type A, 1.8 mm for that of type B, and 2.3 mm for that of type C. On the other hand, the type D was the existing one as shown in  FIG. 12 . 
     All the sample spark plugs were tested using an internal combustion engine of an automotive vehicle, which has a displacement of 2 L and six in-line cylinders. 
       FIG. 6  shows the test results, where the plots of “⋄” indicate the results with the sample spark plug of type A, the plots of “●” indicate the results with the sample spark plug of type B, the plots of “◯” indicate the results with the sample spark plug of type C, and the plots of “Δ” indicate the results with the sample spark plug of type D. 
     It can be seen from  FIG. 6  that with increase in mileage, the effective area of the surface of the protruding portion  32  of the center electrode  3  decreased much more slowly in the sample spark plugs of types A-C than in the sample spark plug of type D. Especially, in the case of types B and C, the effective area of the surface decreased only in early stage of the running and kept almost constant thereafter. 
     In other words, through specifying the length A of the inner side surfaces  541  of the straight portions  54  of the ground electrodes  5  to be not less than 1.8 mm, a sufficiently large effective area of the surface is secured in the spark plug  1 , thus ensuring a superior capability of the spark plug  1  to induce and detect the ion current. 
     Experiment 3 
     This experiment was conducted to determine the effect of the ratio of g/G on the occurrence rate of side sparks in the spark plug  1 . 
     Five different sample spark plugs were used in the experiment, in which the ratios of g/G were 1.0, 1.1, 1.2, 1.3, and 1.8, respectively. 
     All the sample spark plugs were tested using an internal combustion engine, which has a displacement of 1.8 L and four in-line cylinders, under idling condition. 
     During the test, the determination as to whether a discharged spark is a normal or a side spark was made based on the waveform of the discharged spark. Further, the occurrence rate of side sparks for a sample spark plug was determined as the ratio of the number of occurrence of side sparks to the total number of occurrence of normal and side sparks in that sample spark plug. 
       FIG. 7  shows the test results, where the horizontal axis represents the ratio of g/G, while the vertical one represents the occurrence rate of side sparks. 
     It can be seen from  FIG. 7  that the occurrence rate of side sparks decreased with increase in the ratio of g/G, and occurrence of side sparks was completely suppressed with the ratio of g/G being greater than or equal to 1.3. 
     In other words, through specifying the ratio of g/G to be greater than or equal to 1.3 mm, side sparks are prevented from occurring in the spark plug  1 . 
     [Second Embodiment] 
     This embodiment illustrates a method of manufacturing the spark plug  1  described in the previous embodiment. 
     According to the method, the metal shell  4  and a pair of ground electrode base materials  50  for forming the ground electrodes  5  are first prepared. 
     Each of the ground electrode base materials  50  is rod-shaped and has a first end  50   a  and a second end  50   b  which are opposite to each other in the lengthwise direction of the ground electrode base material  50 . 
     Secondly, as shown in  FIG. 8A , the first end  50   a  of each of the ground electrode base materials  50  is joined by, for example, resistance welding to the end  42  of the metal shell  4 . 
     Thirdly, as shown in  FIG. 8B , each of the ground electrode base materials  50  is bent to move the second end  50   b  outward in the radial direction of the metal shell  4 , thereby forming the bend  53  between the first end  50   a  and the second end  50   b.    
     Fourthly, as shown in  FIG. 8C , the noble metal chip  55  is joined by laser welding to the inner side surface  541  of a portion  50   c  of each of the ground electrode base materials  50  between the bend  53  and the second end  50   b.    
     Fifthly, an assembly of the insulator  2  and the center electrode  3  is prepared and fitted in the metal shell  4 . 
     Sixthly, each of the ground electrode base materials  50  is bent at the first end  50   a  to make the portion  50   c  being parallel to the axial direction of the center electrode  3  and closer to the center electrode  3  in the radial direction of the center electrode  3  than the first end  50   a , thereby forming the ground electrode  5 . 
     As a result, the spark plug  1  according to the previous embodiment is finally obtained, as shown in  FIG. 8D . It should be noted that in the above method, the fifth step of fitting the assembly of the insulator  2  and the center electrode  3  in the metal shell  4  may be arranged after the sixth step of forming the ground electrodes  5 . In addition, the noble metal chips  55  may be temporally joined by, for example, resistance welding to the corresponding ground electrode base materials  50  before the fourth step of laser welding. 
     In the fourth step of the above method, referring to  FIG. 9 , a laser beam L is irradiated with an irradiation angle θ with respect to the inner side surface  541  of the ground electrode base material  50  in the range of 20 to 55°. 
     Referring further to  FIG. 10 , the laser beam L is irradiated to the joining portions of the noble metal chip  55  and the ground electrode base material  50  over the entire circumference of the noble metal chip  55 . 
     In this case, however, there exists a dead zone Z, in which the metal shell  4  hinders irradiating the laser beam L perpendicularly with respect to the circumference of the noble metal chip  55  to the welding point P that is closest to the metal shell  4 . Therefore, to the welding point P, the laser beam L is irradiated obliquely with respect to the circumference of the noble metal chip  55 . 
     The above-described method of manufacturing the spark plug  1  has the following advantages. 
     According to the method, the fourth step of laser welding is arranged after the third step of bending the ground electrode base materials  50  and before the fifth step of fitting the assembly of the insulator  2  and the center electrode  3  in the metal shell  4 . 
     With such an arrangement, the laser welding of each of the noble metal chips  55  to a corresponding one of the ground electrode base materials  50  will not be hindered by the metal shell  4 , the center electrode  3 , the insulator  2 , and the other ground electrode base material  50 . Consequently, it becomes easy to perform the laser welding over the entire circumference of the noble metal chip  55 . 
     Further, according to the method, it becomes easy to adjust the size G of the spark gaps  11  due to the arrangement of the sixth step of forming the ground electrodes  5  after the fourth step of laser welding and the fifth step of fitting. 
     Furthermore, according to the method, in the fourth step of laser welding, the irradiation angle θ of the laser beam L with respect to the inner side surface  541  of the ground electrode base material  50  is specified to the range of 20 to 55°. 
     With such a specification, it becomes possible to easily and reliably irradiate the laser beam L to the joining portions of the noble metal chip  55  and the inner side surface  541  of the ground electrode base material  50 , thereby enhancing the strength of the laser welding. 
     If the irradiation angle θ of the laser beam L is below 20°, it may be difficult to prevent the ground electrode base material  50  from being damaged by the laser beam L and to sufficiently melt the joining portion of the ground electrode base material  50 . On the contrary, if the irradiation angle θ of the laser beam L is above 55°, it may be difficult to prevent the noble metal chip  55  from being damaged by the laser beam L and to sufficiently melt the joining portion of the noble metal chip  55 . 
     The above range of the irradiation angle θ of the laser beam L has been determined through the experiment to be described below. 
     Experiment 4 
     This experiment was conducted to determine the effects of the irradiation angle θ of the laser beam L and the width W of the inner side surfaces  541  of the ground electrodes  5  on the quality of the laser welding. 
     In the experiment, the irradiation angle θ was varied in the range of 10 to 60° and the width W was varied in the range of 1.4 to 2.6 mm, as shown in TABLE 1. Moreover, the diameter d of the noble metal chips  55  was 0.7 mm, and the distance from the end  42  of the metal shell  4  to the centers of the noble metal chips  55  was 3.8 mm. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 W (mm) 
                   
               
            
           
           
               
               
               
               
               
            
               
                 θ (°) 
                 1.4 
                 1.8 
                 2.2 
                 2.6 
               
               
                   
               
               
                 10 
                 Δ 
                 x 
                 x 
                 x 
               
               
                 20 
                 ∘ 
                 ∘ 
                 ∘ 
                 Δ 
               
               
                 45 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                 50 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                 55 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                 60 
                 x 
                 x 
                 x 
                 x 
               
               
                   
               
            
           
         
       
     
     The experiment results are also shown in TABLE 1, where the plots of “◯” indicate good welding, the plots of “Δ” indicate somewhat defective welding, and the plots of “x” indicate defective welding. The defective welding includes, for example, occurrence of damage to the noble metal chip  55  or to the ground electrode  5  and irradiation of the laser beam L to only one of the two mating members. 
     It can bee seen from TABLE 1 that when the irradiation angle θ was in the range of 20 to 55° and the width W was not greater than 2.2 mm, the resultant welding was good. 
     However, in the case of the irradiation angle θ being equal to 60°, the resultant welding was defective due to occurrence of damage to the noble metal chip  55  or irradiation of the laser beam L only to the ground electrode  5 . On the contrary, in the case of the irradiation angle θ being equal to 10°, the resultant welding was defective due to occurrence of damage to the ground electrode  5  or irradiation of the laser beam L only to the noble metal chip  55 . 
     The above defectivenesses in the laser welding can be considered as being caused by unsuitable values of (W−d). Accordingly, based on such a consideration, a suitable range of (W−d) for achieving good welding can be determined as not greater than 1.5 mm (i.e., (2.2-0.7) mm). 
     Consequently, it has been made obvious from the experiment that good welding can be achieved through specifying the irradiation angle θ to the range of 20 to 55° and the difference of (W−d) to be not greater than 1.5 mm. 
     Third Embodiment 
     Though the spark plug  1  can be advantageously manufactured by the method of the previous embodiment, it can also be manufactured by other methods such as the one to be described below. 
     According to the method of this embodiment, the metal shell  4  and a pair of ground electrode base materials  50  for forming the ground electrodes  5  are first prepared. 
     Each of the ground electrode base materials  50  is rod-shaped and has a first end  50   a  and a second end  50   b  which are opposite to each other in the lengthwise direction of the ground electrode base material  50 . 
     Secondly, as shown in  FIG. 11A , the first end  50   a  of each of the ground electrode base materials  50  is joined by, for example, resistance welding to the end  42  of the metal shell  4 . 
     Thirdly, as shown in  FIG. 11B , the noble metal chip  55  is joined by laser welding to the inner side surface of each of the ground electrode base materials  50  close to the second end  50   b.    
     Fourthly, as shown in  FIG. 11C , each of the ground electrode base materials  50  is bent at the first end  50   a  inward in the radial direction of the metal shell  4  to form the inclined portion  52  and at a position corresponding to the bend  53  to form the straight portion  54 . Consequently, the ground electrodes  5  are obtained. 
     Fifthly, an assembly of the insulator  2  and the center electrode  3  is prepared and fitted in the metal shell  4 . 
     As a result, the spark plug  1  is finally obtained, as shown in  FIG. 11D . 
     The method of the present embodiment has the advantage of including only one bending step, thereby simplifying the manufacturing process of the spark plug  1 . 
     While the above particular embodiments of the invention have been shown and described, it will be understood by those who practice the invention and those skilled in the art that various modifications, changes, and improvements may be made to the invention without departing from the spirit of the disclosed concept. 
     Such modifications, changes, and improvements within the skill of the art are intended to be covered by the appended claims.