Patent Publication Number: US-10320158-B2

Title: Spark plug

Description:
This application claims the benefit of Japanese Patent Application No. 2016-064995, filed Mar. 29, 2016, which is incorporated herein in its entirety by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a spark plug. 
     BACKGROUND OF THE INVENTION 
     Conventionally, in order to improve durability and ignition performance of a spark plug, a noble metal tip has been joined to a center electrode or a ground electrode (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2015-159000). 
     Problems to be Solved by the Invention 
     However, in the case where a spark plug having a noble metal tip is used for an internal combustion engine in which the flow velocity of an air-fuel mixture in a cylinder is high, such as a highly supercharged internal combustion engine or a direct injection internal combustion engine, sparks may fly directly to a center electrode or a ground electrode (hereinafter, collectively referred to as an “electrode component”), rather than to the noble metal tip, due to flowing of sparks. If sparks fly directly to the electrode component, the electrode component may be worn out, leading to decrease in durability of the spark plug. Accordingly, there has been a need for a technique applied to a spark plug having a noble metal tip so as to restrain flying of sparks directly to an electrode component of the spark plug. 
     SUMMARY OF THE INVENTION 
     Means for Solving the Problems 
     The present invention has been accomplished in order to solve the above-mentioned problem, and can be realized as the following modes. 
     (1) A spark plug according to one mode of the present invention comprises an insulator having an axial hole extending along an axial line of the spark plug; a center electrode disposed in the axial hole; a tubular metallic shell surrounding the insulator; a ground electrode whose proximal end is fixed to the metallic shell; and a cylindrical noble metal tip which is fixed to the ground electrode via an intermediate tip so as to form a gap between the noble metal tip and the center electrode, wherein a weld portion is provided between the noble metal tip and the intermediate tip. In the spark plug, the weld portion has a diameter larger than that of a portion of the intermediate tip which is adjacent to the weld portion, and the noble metal tip is located inside a virtual tapered cylindrical surface which extends from a peripheral edge of a gap-side end of the center electrode so as to contact an outer circumferential edge of the weld portion. According to the spark plug of this mode, the weld portion between the noble metal tip and the intermediate tip has a large diameter. As a result, flying of sparks directly to the ground electrode, which is an electrode component, can be restrained.
 
(2) In the spark plug of the above-described mode, over the entirety of a circumference of the weld portion, the diameter of the weld portion may be larger than the diameter of the portion of the intermediate tip which is adjacent to the weld portion. In this case, flying of sparks directly to the ground electrode is more effectively reduced.
 
(3) In the spark plug of the above-described mode, the intermediate tip may include a flange which is joined to the ground electrode, and a distance between a surface of the flange which faces toward the noble metal tip and the weld portion may be 10% or more of a distance between the surface of the flange and a gap-side end of the noble metal tip. In this case, a sufficient distance is provided between the flange of the intermediate tip and the weld portion. By using that portion of the intermediate tip, the intermediate tip can be easily joined to the ground electrode.
 
(4) A spark plug according to another mode of the present invention comprises an insulator having an axial hole extending along an axial line; a center electrode disposed in the axial hole; a tubular metallic shell surrounding the insulator; a ground electrode whose proximal end is fixed to the metallic shell; and a first noble metal tip which is fixed to a forward end of the center electrode so as to form a gap between the noble metal tip and the ground electrode, wherein a weld portion is provided between the noble metal tip and the center electrode. In the spark plug, the weld portion has a diameter larger than that of a portion of the center electrode which is adjacent to the weld portion. According to the spark plug of this mode, the weld portion between the noble metal tip and the center electrode has a large diameter. As a result, flying of sparks directly to the center electrode, which is an electrode component, can be restrained.
 
(5) In the spark plug of the above-described mode, a second noble metal tip may be disposed on the ground electrode, and the first noble metal tip may be located inside a virtual tapered cylindrical surface which extends from a peripheral edge of a gap-side end of the second noble metal tip so as to contact an outer circumferential edge of the weld portion. In this case, flying of sparks directly to the center electrode is more effectively reduced.
 
(6) In the spark plug of the above-described mode, over the entirety of a circumference of the weld portion, the diameter of the weld portion may be larger than the diameter of the portion of the center electrode which is adjacent to weld portion. In this case, flying of sparks directly to the center electrode is more effectively reduced.
 
     The present invention can be realized in other various forms other than the spark plug. For example, the present invention can be realized as a method of manufacturing a spark plug. 
    
    
     
       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 like designations denote like elements in the various views, and wherein: 
         FIG. 1  is a partial cross sectional view of a spark plug according to a first embodiment. 
         FIG. 2  is an enlarged view of a noble metal tip and a center electrode. 
         FIG. 3  is an enlarged view of a center electrode according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. First Embodiment 
       FIG. 1  is a partial cross sectional view of a spark plug  100  according to a first embodiment of the present invention. The spark plug  100  has an elongated shape along an axial line O. In  FIG. 1 , the right side of the axial line O, which is indicated by the long dashed short dashed line, shows an external front view, and the left side of the axial line O shows a cross sectional view taken along the axial line O. In the following description, the lower side of the  FIG. 1  is referred to as a forward end side of the spark plug  100 , and the upper side of  FIG. 1  is referred to as a rear end side. 
     The spark plug  100  includes an insulator  10  having an axial hole  12  extending along the axial line O, a center electrode  20  disposed in the axial hole  12 , a tubular metallic shell  50  which surrounds the insulator  10 , and a ground electrode  30  whose proximal end  32  is fixed to the metallic shell  50 . 
     The insulator  10  is a ceramic insulator formed by firing a ceramic material such as alumina. The insulator  10  is a tubular member having the axial hole  12  at its center. The forward end side of the axial hole  12  accommodates a portion of the center electrode  20  and the rear end side of the axial hole  12  accommodates a portion of a metal terminal  40 . A central trunk portion  19  having an increased outer diameter is formed on the insulator  10  at the center in the axis direction. A rear trunk portion  18  having an outer diameter smaller than that of the central trunk portion  19  is formed on the rear end side of the central trunk portion  19 . A forward trunk portion  17  having an outer diameter smaller than that of the rear trunk portion  18  is formed on the forward end side of the central trunk portion  19 . A leg portion  13  having an outer diameter smaller than that of the forward trunk portion  17  and decreasing toward the center electrode  20  is formed on the forward end side of the forward trunk portion  17 . 
     The metallic shell  50  is a cylindrical tubular metal member which surrounds and holds a portion of the insulator  10 , from a portion of the rear trunk portion  18  to the leg portion  13 . The metallic shell  50  is made of, for example, low-carbon steel, and entirely plated with nickel, zinc or the like. The metallic shell  50  includes a tool engagement portion  51 , a seal portion  54  and a mounting screw portion  52 , which are disposed in this order from the rear end side. A tool for mounting the spark plug  100  on an engine head is fitted on the tool engagement portion  51 . The mounting screw portion  52  has threads which are threaded into a mounting screw hole of the engine head. The seal portion  54  is formed in a flange shape at the proximal end of the mounting screw portion  52 . An annular gasket  65  formed by bending a plate is inserted between the seal portion  54  and the engine head. The metallic shell  50  has, at its forward end, an annular end surface  57  which surrounds a center opening of the metallic shell  50 . The forward end of the leg portion  13  of the insulator  10  and the forward end of the center electrode  20  project from the center opening. 
     A crimp portion  53  having a reduced thickness is provided on the rear end side of the tool engagement portion  51  of the metallic shell  50 . Further, a compression deformation portion  58  having a reduced thickness like the crimp portion  53  is provided between the seal portion  54  and the tool engagement portion  51 . Annular ring members  66  and  67  are disposed between a portion of the inner circumferential surface of the metallic shell  50  extending from the tool engagement portion  51  to the crimp portion  53  and the outer circumferential surface of the rear trunk portion  18  of the insulator  10 . Powder of talc  69  is charged between the two ring members  66  and  67 . At the time of manufacture of the spark plug  100 , the crimp portion  53  is pressed forward so that the crimp portion  53  is bent inward, thereby allowing the compression deformation portion  58  to be deformed by compression. As the compression deformation portion  58  is deformed by compression, the insulator  10  is pressed forward in the metallic shell  50  via the ring members  66 ,  67  and the talc  69 . As a result of this pressing, the talc  69  is compressed in the direction of the axial line O, thereby increasing airtightness in the metallic shell  50 . 
     The insulator  10  is disposed in the metallic shell  50 . Specifically, an insulator step portion  15  of the insulator  10  located at the rear end of the leg portion  13  thereof is pressed against a metallic shell step portion  56  formed on the inner circumference of the mounting screw portion  52  via an annular sheet packing  68 . This sheet packing  68  is a member that maintains airtightness between the metallic shell  50  and the insulator  10 , and prevents leakage of combustion gas. 
     The center electrode  20  is a rod shaped member formed by embedding a core material  22  in an electrode member  21  with the core material  22  having a thermal conductivity higher than that of the electrode member  21 . The electrode member  21  is made of a nickel alloy which contains nickel as a main component, and the core material  22  is made of copper or an alloy which contains copper as a main component. The center electrode  20  has a diameter of, for example, 0.9 mm at its forward end. 
     The center electrode  20  has a flange  23  formed near the rear end thereof and bulging outward. The flange  23  comes into contact with an axial hole step portion  14  formed in the axial hole  12  from the rear end side to thereby position the center electrode  20  in the insulator  10 . The rear end of the center electrode  20  is electrically connected to the metal terminal  40  via a seal  64  and a ceramic resistor  63 . 
     The ground electrode  30  is made of an alloy which contains nickel as a main component. The proximal end  32  of the ground electrode  30  is fixed to the end surface  57  of the metallic shell  50 . The ground electrode  30  extends forward from the proximal end  32  along the axial line O, and is curved at an intermediate portion such that one side surface of a distal end portion  33  faces the forward end surface of the center electrode  20 . In the present embodiment, a noble metal tip  31  in a cylindrical shape is disposed on a surface of the distal end portion  33  of the ground electrode  30  which faces the center electrode  20 . The diameter of the noble metal tip  31  is, for example, 1.0 mm. The noble metal tip  31  forms a gap between the noble metal tip  31  and the center electrode  20  for spark discharge. The gap is, for example, 0.8 mm. The noble metal tip  31  is made of, for example, platinum (Pt), iridium (Ir), ruthenium (Ru), rhodium (Rh), or an alloy thereof. 
       FIG. 2  is an enlarged view of the noble metal tip  31  and the center electrode  20 . The noble metal tip  31  is fixed to the ground electrode  30  via an intermediate tip  70 . In the present embodiment, the surface of a gap-side end  36  of the noble metal tip  31  and the surface of a gap-side end  26  of the center electrode  20  are approximately parallel to each other. Further, the center axis of the noble metal tip  31  extends through the surface of the gap-side end  26  of the center electrode  20 , and the center axis of the center electrode  20  extends through the surface of the gap-side end  36  of the noble metal tip  31 . In the present embodiment, the center axis of the noble metal tip  31  and the center axis of the center electrode  20  coincide with each other and with the axial line O. Further, in another embodiment, the center axis of the noble metal tip  31  and the center axis of the center electrode  20  may be offset from each other. Moreover, the center axis of the noble metal tip  31  and the center axis of the center electrode  20  may intersect with each other or may be in a skew relation. 
     The intermediate tip  70  is made of the same material as that of the ground electrode  30 . The intermediate tip  70  has a cylindrical straight portion  71  and a flange  72 . The diameter of the straight portion  71  is, for example, 1.1 mm. The flange  72  is disposed at the forward end of the straight portion  71 . The noble metal tip  31  is laser-welded to the straight portion  71  of the intermediate tip  70 . The intermediate tip  70  with the noble metal tip  31  joined thereto is joined to the ground electrode  30  by resistance-welding the flange  72  to the ground electrode  30  while pressing the flange  72  for contact with the ground electrode  30 . A weld portion  34  is present between the noble metal tip  31  and the intermediate tip  70 . The weld portion  34  is formed by materials of the noble metal tip  31  and the intermediate tip  70  which are melted during laser welding and then solidified. The maximum diameter R 1  of the weld portion  34  is, for example, 1.3 mm. The flange  72  may not be necessarily provided. 
     In the present embodiment, the weld portion  34  bulges outward. This bulge is formed, for example, by using a centrifugal force by rotating the noble metal tip  31  and the intermediate tip  70  while continuously irradiating the outer peripheries of the noble metal tip  31  and the intermediate tip  70  with a laser beam. Alternatively, the bulge can be formed by applying a load to the noble metal tip  31  and the intermediate tip  70  such that the noble metal tip  31  and the intermediate tip  70  are compressed during joining of the noble metal tip  31  and the intermediate tip  70 . 
     In the present embodiment, over the entire circumference of the weld portion  34 , the diameter R 1  of the weld portion  34  is larger than the diameter of a portion of the intermediate tip  70  which is adjacent to the weld portion  34 , that is, the diameter R 2  of the straight portion  71 . Further, in the present embodiment, the noble metal tip  31  is entirely located inside a virtual tapered cylindrical surface  80  which extends from the peripheral edge  25  of the forward end of the center electrode  20  so as to come into contact with the outer circumferential edge  35  of the weld portion  34 . The virtual tapered cylindrical surface  80  is a virtual surface formed by connecting the peripheral edge  25  of the forward end of the center electrode  20  and the outer circumferential edge  35  of the weld portion  34  by straight lines over the entire circumferences thereof. 
     Further, in the present embodiment, the distance L 1  between the weld portion  34  and a surface  73  of the flange  72  which faces toward the noble metal tip  31 , that is, the distance L 1  of the straight portion  71 , is 10% or more of the distance L 2  between the surface  73  of the flange  72  and the gap-side end  36  of the noble metal tip  31 . 
     In the aforementioned spark plug  100  of the present embodiment, the diameter R 1  of the weld portion  34  is larger than the diameter R 2  of the straight portion  71 . Accordingly, when sparks flow due to, for example, a high flow velocity of an air-fuel mixture in a cylinder of the internal combustion engine for which the spark plug is provided, sparks are more likely to fly to the weld portion  34  rather than to the electrode components such as the intermediate tip  70  and the ground electrode  30  compared with the case where the diameter R 1  of the weld portion  34  is similar to the diameter R 2  of the straight portion  71  (the case where the weld portion  34  does not bulge outward). Therefore, sparks are less likely to fly to the electrode components such as the intermediate tip  70  and the ground electrode  30 , which are spaced from the noble metal tip  31  by a greater distance, compared with the weld portion  34 . Thus, consumption (erosion) of the electrode components can be reduced. In addition, since the weld portion  34  contains the component of the noble metal tip  31 , the weld portion  34  is more durable than the electrode components. Accordingly, even if sparks fly to the weld portion  34 , the amount of consumption can be decreased compared with the case where sparks fly to the electrode components. 
     In the present embodiment, the noble metal tip  31  is located inside the virtual tapered cylindrical surface  80  which extends from the peripheral edge  25  of the forward end of the center electrode  20  so as to come into contact with the outer circumferential edge  35  of the weld portion  34 . Accordingly, when sparks flow due to, for example, a high flow velocity of an air-fuel mixture in a cylinder of the internal combustion engine for which the spark plug is provided, sparks are more likely to fly to the weld portion  34  compared with the case where the noble metal tip  31  is partially present outside the virtual tapered cylindrical surface  80 . Therefore, flying of sparks directly onto the electrode material is more effectively reduced. 
     In the present embodiment, over the entire circumference of the weld portion  34 , the diameter R 1  of the weld portion  34  is larger than the diameter R 2  of the straight portion  71 . Accordingly, flying of sparks directly to the electrode component is more effectively reduced. 
     In the present embodiment, since the length L 1  of the straight portion  71  is 10% or more of the distance L 2  between the surface  73  of the flange  72  and the gap-side end  36  of the noble metal tip  31 , the straight portion  71  can be made sufficiently long. Accordingly, the intermediate tip  70  can be easily resistance-welded to the ground electrode  30  by using the straight portion  71  and the surface  73  of the flange  72 . 
     In the above embodiment, over the entire circumference of the weld portion  34 , the diameter R 1  of the weld portion  34  is larger than the diameter R 2  of the straight portion  71 . However, the diameter R 1  of the weld portion  34  may be larger than the diameter R 2  of the straight portion  71  only over a portion of the circumference of the weld portion  34 . When the diameter R 1  of the weld portion  34  is larger than the diameter R 2  of the straight portion  71  only over a portion of the circumference of the weld portion  34 , flying of sparks directly to the electrode component can prevented in a region corresponding to that portion of the circumference of the weld portion  34 . 
     In the above embodiment, the length L 1  of the straight portion  71  is 10% or more of the distance L 2  between the surface  73  of the flange  72  and the gap-side end  36  of the noble metal tip  31 . However, the length L 1  of the straight portion  71  may be less than 10% of the distance L 2  as far as the intermediate tip  70  can be joined to the ground electrode  30 . 
     B. Second Embodiment 
       FIG. 3  is an enlarged view of the center electrode  20  in a spark plug according to a second embodiment of the present invention. The spark plug of the second embodiment has the same structure as that of the spark plug of the first embodiment except for the structures of the center electrode and the ground electrode. 
     In the spark plug of the second embodiment, a noble metal tip  27  in a cylindrical shape is fixed to the forward end of the center electrode  20 . The noble metal tip  27  forms a gap between the noble metal tip  27  and the ground electrode  30  for spark discharge. The diameter of the noble metal tip  27  is, for example, 0.7 mm. The center electrode  20  and the noble metal tip  27  are laser-welded. Accordingly, a weld portion  28  is present between the center electrode  20  and the noble metal tip  27 . The maximum diameter R 3  of the weld portion  28  is, for example, 1.2 mm. The diameter R 4  of the portion of the center electrode  20  which is adjacent to the weld portion  28  is, for example, 1.0 mm. 
     In the present embodiment, a noble metal tip  31   a  in a cylindrical shape is directly joined to the ground electrode  30 . The diameter of the noble metal tip  31   a  is, for example, 1.0 mm. The gap between the noble metal tip  31   a  and the noble metal tip  27  on the center electrode  20  is, for example, 0.8 mm. In the present embodiment, the noble metal tip  31   a  is resistance-welded to the ground electrode  30 . The noble metal tip  31   a  may be fixed to the ground electrode  30  via an intermediate tip as in the case of the first embodiment. 
     In the present embodiment, the surface of a gap-side end  36   a  of the noble metal tip  31   a  and the surface of a gap-side end  26   a  of the noble metal tip  27  of the center electrode  20  are approximately parallel to each other. Further, the center axis of the noble metal tip  31   a  on the ground electrode  30  extends through the surface of the gap-side end  26   a  of the noble metal tip  27  on the center electrode  20 , and the center axis of the noble metal tip  27  on the center electrode  20  extends through the surface of the gap-side end  36   a  of the noble metal tip  31   a  on the ground electrode  30 . In the present embodiment, the center axis of the noble metal tip  31   a  on the ground electrode  30  and the center axis of the noble metal tip  27  on the center electrode  20  coincide with each other and with the axial line O. Further, in another embodiment, the center axis of the noble metal tip  31   a  on the ground electrode  30  and the center axis of the noble metal tip  27  on the center electrode  20  may be offset from each other. Moreover, the center axis of the noble metal tip  31   a  on the ground electrode  30  and the center axis of the noble metal tip  27  on the center electrode  20  may intersect with each other or may be in a skew relation. 
     In the present embodiment, the weld portion  28  bulges outward. Specifically, over the entire circumference of the weld portion  28 , the diameter R 3  of the weld portion  28  is larger than the diameter R 4  of the portion of the center electrode  20  which is adjacent to the weld portion  28 . Further, the noble metal tip  27  on the center electrode  20  is entirely located inside a virtual tapered cylindrical surface  80   a  which extends from the peripheral edge  37  of the gap-side end of the noble metal tip  31   a  on the ground electrode  30  so as to come into contact with the outer circumferential edge  29  of the weld portion  28 . 
     In the aforementioned second embodiment, the diameter R 3  of the weld portion  28  is larger than the diameter R 4  of the portion of the center electrode  20  which is adjacent to the weld portion  28 . Accordingly, when sparks flow due to, for example, a high flow velocity of an air-fuel mixture in the cylinder of the internal combustion engine, sparks are more likely to fly to the weld portion  29  rather than to the electrode component (center electrode  20 ) compared with the case where the diameter R 3  of the weld portion  28  is similar to the diameter R 4  of the portion adjacent to the weld portion  28  (the case where the weld portion  28  does not bulge outward). Therefore, sparks are less likely to fly to the electrode component which is spaced from the noble metal tip  31   a  by a greater distance compared with the weld portion  29 . Thus, consumption of the center electrode  20 , which is an electrode component, can be reduced. 
     In the present embodiment, the noble metal tip  27  on the center electrode  20  is located inside the virtual tapered cylindrical surface  80   a  which extends from the peripheral edge  37  of the gap-side end of the noble metal tip  31   a  on the ground electrode  30  so as to come into contact with the outer circumferential edge  29  of the weld portion  28 . Accordingly, when sparks flow due to, for example, a high flow velocity of an air-fuel mixture in the cylinder of the internal combustion engine, sparks are more likely to fly to the weld portion  28  compared with the case where the noble metal tip  27  on the center electrode  20  is partially present outside the virtual tapered cylindrical surface  80   a . Therefore, flying of sparks directly to the electrode component is more effectively reduced. In the present embodiment, over the entire circumference of the weld portion  28 , the diameter R 3  of the weld portion  28  is larger than the diameter R 4  of the portion of the center electrode  20  which is adjacent to the weld portion  28 . Accordingly, flying of sparks directly to the center electrode  20  is effectively reduced. 
     In the second embodiment, over the entire circumference of the weld portion  28 , the diameter R 3  of the weld portion  28  is larger than the diameter R 4  of the portion of the center electrode  20  which is adjacent to the weld portion  28 . However, the diameter of the weld portion  28  may be larger, only over a portion of the circumference of the weld portion  28 , than the diameter of the portion of the center electrode  20  which is adjacent to the weld portion  28 . When the diameter of the weld portion  28  is larger, only over a portion of the circumference of the weld portion  28 , than the diameter of the portion of the center electrode  20  which is adjacent to the weld portion  28 , flying of sparks directly to the center electrode  20  can be restrained in a region corresponding to that portion of the circumference of the weld portion  28 . 
     The present invention is not limited to the above-described embodiments and may be embodied in various other forms without departing from the scope of the invention. For example, the technical features in the embodiments corresponding to the technical features in the modes described in “Summary of the Invention” can be appropriately replaced or combined in order to solve some of or all the foregoing problems or to achieve some of or all the foregoing effects. A technical feature which is not described as an essential feature in the present specification may be appropriately deleted. 
     DESCRIPTION OF SYMBOLS 
     
         
         
           
               10  . . . insulator 
               12  . . . axial hole 
               13  . . . leg portion 
               14  . . . axial hole step portion 
               15  . . . insulator step portion 
               17  . . . forward trunk portion 
               18  . . . rear trunk portion 
               19  . . . central trunk portion 
               20  . . . center electrode 
               21  . . . electrode member 
               22  . . . core material 
               23  . . . flange 
               25  . . . peripheral edge of forward end 
               26 ,  26   a  . . . gap-side end 
               27  . . . (first) noble metal tip 
               28  . . . weld portion 
               29  . . . outer circumferential edge 
               30  . . . ground electrode 
               31 ,  31   a  . . . noble metal tip (second noble metal tip) 
               32  . . . proximal end 
               33  . . . distal end portion 
               34  . . . weld portion 
               35  . . . outer circumferential edge 
               36 ,  36   a  . . . gap-side end 
               37  . . . peripheral edge of gap-side end 
               40  . . . metal terminal 
               50  . . . metallic shell 
               51  . . . tool engagement portion 
               52  . . . mounting screw portion 
               53  . . . crimp portion 
               54  . . . seal portion 
               56  . . . metallic shell step portion 
               57  . . . end surface 
               58  . . . compression deformation portion 
               63  . . . ceramic resistor 
               64  . . . seal 
               65  . . . gasket 
               66 ,  67  . . . ring member 
               68  . . . sheet packing 
               69  . . . talc 
               70  . . . intermediate tip 
               71  . . . straight portion 
               72  . . . flange 
               73  . . . surface 
               80 ,  80   a  . . . virtual tapered cylindrical surface 
               100  . . . spark plug