Patent Publication Number: US-11050221-B2

Title: Spark plug with anti-loosening feature

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2019-069848 filed on Apr. 1, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a spark plug. 
     BACKGROUND 
     A spark plug for ignition is attached to an internal combustion engine, such as a gasoline engine, by engaging a screw portion formed on the outer surface of a metallic shell of the spark plug with an internal thread provided in an engine head. In general, the screw portion has a uniform pitch diameter (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2015-225774). 
     In general, as the length of the screw portion increases, the degree of freedom in determining the location of a water jacket of an engine advantageously increases. However, in the case where the pitch diameter of the screw portion is uniform, an increase in the length of the screw portion causes an increase in the area of contact between the screw portion and the engine head, and, as a result, the surface pressure per unit area decreases. Therefore, there has been a problem that the longer the screw portion, the more easily the spark plug loosens upon receipt of vibration generated in the combustion cycle of the engine. 
     The present invention has been accomplished in order to solve the above-mentioned problem, and can be realized as the following modes. 
     SUMMARY 
     (1) According to one mode of the present invention, a spark plug is provided. The spark plug includes a tubular metallic shell having an attachment screw portion having an external thread, an insulator disposed inside the metallic shell and having an axial hole, and a center electrode disposed in the axial hole. The attachment screw portion has a pitch diameter local maximum portion at which the external thread has a locally maximum pitch diameter. The pitch diameter local maximum portion is located on a forward end side of a rear end of the center electrode in a direction along an axial line of the metallic shell. In the spark plug of this mode, upon contact with the engine head, the surface pressure increases at the pitch diameter local maximum portion. Therefore, wobbling of the spark plug can be prevented, whereby loosening of the spark plug can be prevented. 
     (2) In the spark plug of the above-described mode, the pitch diameter of the external thread may become the maximum at the pitch diameter local maximum portion. In the spark plug of this mode, since heat from the interior of an engine is efficiently released from the metallic shell through the pitch diameter local maximum portion, heat dissipation is enhanced. 
     (3) The spark plug of the above-described mode may be configured such that the insulator has an insulator step portion protruding outward, the metallic shell has a metallic shell internal step portion protruding inward, the insulator step portion is in contact with the metallic shell internal step portion via a packing, and the pitch diameter local maximum portion is located on a forward end side of a rear end of the metallic shell internal step portion in the direction along the axial line. In the spark plug of this mode, since heat from the interior of an engine is efficiently released from the metallic shell through the pitch diameter local maximum portion, heat dissipation is enhanced. 
     (4) In the spark plug of the above-described mode, the pitch diameter local maximum portion may be located at a position of the metallic shell internal step portion in the direction along the axial line. In the spark plug of this mode, since heat from the interior of an engine is efficiently released from the metallic shell through the pitch diameter local maximum portion, heat dissipation is enhanced. 
     (5) In the spark plug of the above-described mode, the attachment screw portion may have a length of 26.5 mm or more in the direction along the axial line. The spark plug of this mode can be preferably applied to a spark plug which has a long attachment screw portion and generally tends to loosen easily. 
     Notably, the present invention can be embodied in various forms. For example, the present invention can be embodied as an engine head or the like to which a spark plug is attached. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanatory view showing a partially sectioned spark plug; 
         FIG. 2  is a graph showing a change in the pitch diameter of an external thread of an attachment screw portion; 
         FIG. 3  is a view showing the positional relation of a pitch diameter local maximum portion in the spark plug; 
         FIG. 4  is a graph showing a change in the pitch diameter of the external thread in a second embodiment; 
         FIG. 5  is a graph showing a change in the pitch diameter of the external thread in a third embodiment; and 
         FIG. 6  is a graph showing a change in the pitch diameter of the external thread in a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A. First Embodiment 
       FIG. 1  is an explanatory view showing a partially sectioned spark plug  100 . In  FIG. 1 , an external shape of the spark plug  100  is shown on the right side of an axial line CA, which is the center axis of the spark plug  100 , and a cross-sectional shape of the spark plug  100  is shown on the left side of the axial line CA. In the description of the present embodiment, the lower side of  FIG. 1  will be referred to as the “forward end side” of the spark plug  100 , and the upper side of  FIG. 1  will be referred to as the “rear end side” of the spark plug  100 . 
     The spark plug  100  includes an insulator  10  having an axial hole  12  extending along the axial line CA, a center electrode  20  disposed in the axial hole  12 , a tubular metallic shell  50  disposed around 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 member disposed inside the metallic shell  50 . Specifically, the insulator  10  is a tubular member having the axial hole  12  at its center. A portion of the center electrode  20  is accommodated in a forward end portion of the axial hole  12 , and a portion of a metal terminal  40  is accommodated in a rear end portion of the axial hole  12 . A central trunk portion  19  having a large 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  whose outer diameter decreases toward the center electrode  20  is formed on the forward end side of the forward trunk portion  17 . 
     The metallic shell  50  is a 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 an attachment screw portion  52 , which are disposed in this order from the rear end side. A tool for attaching the spark plug  100  to an engine head  90  is engaged with the tool engagement portion  51 . The attachment screw portion  52  is a portion where an external thread is formed over the entire circumference along the outer periphery of the metallic shell  50  and which is screwed into an attachment screw hole  93  of the engine head  90 . The seal portion  54  is formed in a flange shape at the proximal end of the attachment screw portion  52 . An annular gasket  65  formed by bending a plate is inserted between the seal portion  54  and the engine head  90 . The metallic shell  50  has an annular forward-end-side end surface  57  which defines a center opening through which the forward end of the leg portion  13  of the insulator  10  and the forward end of the center electrode  20  project. 
     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 . 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 the outer circumferential surface of the rear trunk portion  18  of the insulator  10  and a portion of the inner circumferential surface of the metallic shell  50 , which portion extends from the tool engagement portion  51  to the crimp portion  53 . 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, and as a result, the compression deformation portion  58  is deformed by compression. As a result of the compressive deformation of the compression deformation portion  58 , 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 CA, thereby increasing gastightness in the metallic shell  50 . 
     The metallic shell  50  has a metallic shell internal step portion  56  protruding inward. The position of the rear end of the metallic shell internal step portion  56  in the direction along the axial line CA is denoted by P 2 . The insulator  10  has an insulator step portion  15  located at the rear end of the leg portion  13  and protruding outward. The metallic shell internal step portion  56  of the metallic shell  50  is in contact with the insulator step portion  15  via an annular sheet packing  68 . This sheet packing  68  is a member that maintains gastightness between the metallic shell  50  and the insulator  10 , and prevents leakage of combustion gas. In the present embodiment, the packing  68  is a sheet packing. 
     The center electrode  20  is a rod shaped member composed of an electrode member  21  and a core material  22  embedded in the electrode member  21  and having a thermal conductivity higher than that of the electrode member  21 . The electrode member  21  is made of a nickel alloy whose main component is nickel, and the core material  22  is made of copper or an alloy whose main component is copper. A noble metal tip made of, for example, an iridium alloy may be joined to an end portion of the center electrode  20  on the forward end side. 
     The center electrode  20  has a flange  23  formed near an end portion thereof on the rear end side and bulging outward. The flange  23  comes into contact, from the rear end side, with an axial hole internal step portion  14  protruding inward within the axial hole  12  of the insulator  10 , whereby the center electrode  20  is positioned within the insulator  10 . On the rear end side of the center electrode  20 , the center electrode  20  is electrically connected to the metal terminal  40  via a seal  64  and a ceramic resistor  63 . Notably, the position of the rear end of the center electrode  20  in the direction along the axial line CA is denoted by P 1 . 
     The ground electrode  30  is made of an alloy whose main component is nickel. 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 from the proximal end  32  along the axial line CA toward the forward end side, and is bent 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 . A noble metal tip  31  is disposed on the surface of the distal end portion  33  of the ground electrode  30  which faces the center electrode  20 . A gap for spark discharge is formed between the noble metal tip  31  and the center electrode  20 . The noble metal tip  31  is made of, for example, platinum, iridium, ruthenium, rhodium, or an alloy thereof. 
       FIG. 2  is a graph showing a change in the pitch diameter of the external thread of the attachment screw portion  52 . In  FIG. 2 , a change in the pitch diameter is shown by a broken line, and the shape of the external thread is shown by a continuous line for reference. In order to facilitate understanding of the contents, the change in the pitch diameter shown by the broken line is rendered greater than the actual change amount, and the change shown by the broken line does not mean the absolute value of the actual change. In  FIG. 2 , the vertical axis represents the pitch diameter of the external thread, and the horizontal axis represents the position in the direction along the axial line CA. In the present specification, the “pitch diameter of the external thread” shows the value prescribed in JIS B 0205. 
     As shown in  FIG. 2 , in the spark plug  100  of the present embodiment, the attachment screw portion  52  has a pitch diameter local maximum portion PA at which the pitch diameter of the external thread becomes the maximum. The “pitch diameter local maximum portion” means a certain portion which is larger in pitch diameter than a portion on the forward end side of the certain portion and a portion on the rear end side of the certain portion. In the present embodiment, the pitch diameter at the pitch diameter local maximum portion PA is 11.100 mm. 
       FIG. 3  is a view showing the positional relation of the pitch diameter local maximum portion PA in the spark plug  100 . In the present embodiment, the pitch diameter local maximum portion PA is located on the forward end side of the rear end P 1  (see also  FIG. 1 ) of the center electrode  20  in the direction along the axial line CA. 
     The spark plug  100  of the present embodiment is less likely to loosen after having been fixed to the engine head  90 . The mechanism for preventing loosening of the spark plug  100  will now be described. 
     In general, a forward-end-side portion of the spark plug  100  fixed to the engine head  90  receives a larger amount of heat from the interior of the engine as compared with a rear-end-side portion of the spark plug  100 . Specifically, whereas the temperature of a forward-end-side portion of the metallic shell  50  of the spark plug  100  increases to about 600° C., the temperature of a rear-end-side portion of the metallic shell  50  increases only to about 100° C. As a result, the forward-end-side portion of the metallic shell  50  thermally expands in a greater amount as compared with the rear-end-side portion of the metallic shell  50 . 
     In the spark plug  100  of the present embodiment, since the pitch diameter local maximum portion PA is located on the forward end side of the rear end P 1  of the center electrode  20 , the amount of thermal expansion becomes larger as compared with the case where the pitch diameter local maximum portion PA is located on the rear end side of the rear end P 1  of the center electrode  20 . As a result, upon contact with the engine head  90 , the surface pressure increases at the pitch diameter local maximum portion PA. Therefore, wobbling of the spark plug  100  can be prevented, whereby loosening of the spark plug  100  can be prevented. 
     In the spark plug  100  of the present embodiment, the pitch diameter local maximum portion PA is larger in pitch diameter as compared with a portion on the forward end side of the pitch diameter local maximum portion PA. Therefore, it is possible to prevent damage of the attachment screw hole  93  of the engine head  90 , which damage would otherwise occur due to excessive thermal expansion of the portion on the forward end side of the pitch diameter local maximum portion PA. 
     In the spark plug  100  of the present embodiment, the pitch diameter of the external thread becomes the maximum at the pitch diameter local maximum portion PA. When the metallic shell  50  receives heat from the engine, the heat is conducted to the engine head  90  through the pitch diameter local maximum portion PA in contact with the engine head  90 , whereby the heat is released from the metallic shell  50 . In the present embodiment, since the pitch diameter local maximum portion PA is located on the forward end side of the rear end P 1  of the center electrode  20 , the heat is efficiently released from the metallic shell  50  through the pitch diameter local maximum portion PA as compared with the case where the pitch diameter local maximum portion PA is located on the rear end side of the rear end P 1  of the center electrode  20 . Therefore, the spark plug  100  of the present embodiment is excellent in heat dissipation. 
     In the spark plug  100  of the present embodiment, the pitch diameter of the external thread becomes the maximum at the pitch diameter local maximum portion PA. When the spark plug  100  of the present embodiment is fixed to the engine head  90 , the spark plug  100  comes into contact with the engine head  90  at the pitch diameter local maximum portion PA, so that the surface pressure concentrates at the pitch diameter local maximum portion PA. By virtue of this, loosening of the spark plug  100  can be prevented more effectively as compared with the case where the pitch diameter is uniform. 
     As shown in  FIG. 3 , in the present embodiment, the pitch diameter local maximum portion PA is located at the same position as the rear end P 2  of the metallic shell internal step portion  56  in the direction along the axial line CA. Namely, in the present embodiment, in the direction along the axial line CA, the pitch diameter local maximum portion PA is located at the position of the metallic shell internal step portion  56 . When the insulator  10  and the center electrode  20  receive heat from the interior of the engine, the heat is conducted to the metallic shell  50  through the packing  68  and the metallic shell internal step portion  56 , and is conducted to the engine head  90  through the pitch diameter local maximum portion PA. In the present embodiment, since the pitch diameter local maximum portion PA is located at the position of the metallic shell internal step portion  56  in the direction along the axial line CA, the heat is efficiently released from the metallic shell  50  through the pitch diameter local maximum portion PA. Therefore, the spark plug  100  of the present embodiment is excellent in heat dissipation. 
     In the present embodiment, the length of the attachment screw portion  52  in the direction along the axial line CA is 26.5 mm or more. The “length of the attachment screw portion  52 ” refers to the length from a forward most portion of the thread to the forward-end-side surface of the seal portion  54 . In general, the longer the length of the attachment screw portion  52 , the larger the area through which the external thread of the attachment screw portion  52  comes into contact with the internal thread of the engine head  90 , and the greater the likeliness of occurrence of loosening. However, the spark plug of the present embodiment can prevent loosening effectively. Notably, the lower limit of the length of the attachment screw portion  52  in the direction along the axial line CA is not limited to 26.5 mm, and the length may be, for example, 18 mm or more. Although no limitation is imposed the upper limit of the length of the attachment screw portion  52  in the direction along the axial line CA, from the viewpoint of easiness of manufacture of the spark plug  100 , the length is preferably 70 mm or less, more preferably, 50 mm or less. 
     B. Second Embodiment 
       FIG. 4  is a graph showing a change in the pitch diameter of the external thread in a second embodiment. In  FIG. 4  as well, a change in the pitch diameter is shown by a broken line, and the shape of the external thread is shown by a continuous line for reference as in the case of  FIG. 2 . In order to facilitate understanding of the contents, the change in the pitch diameter shown by the broken line is rendered greater than the actual change amount, and the change shown by the broken line does not mean the absolute value of the actual change. In  FIG. 4 , the vertical axis represents the pitch diameter of the external thread, and the horizontal axis represents the position in the direction along the axial line CA. The above also applies to  FIGS. 5 and 6  which will be referred to later. 
     The second embodiment is identical in structure with the first embodiment except the manner in which the pitch diameter of the external thread changes. In the second embodiment, the pitch diameter local maximum portion PA is located on the forward end side of the rear end P 2  of the metallic shell internal step portion  56 . By virtue of this configuration, when the metallic shell  50  receives heat from the engine, the heat is efficiently released from the metallic shell  50  through the pitch diameter local maximum portion PA as compared with the case where the pitch diameter local maximum portion PA is located on the rear end side of the rear end P 2  of the metallic shell internal step portion  56 . Therefore, in the present embodiment, the heat dissipation of the metallic shell  50  is enhanced. 
     C. Third Embodiment 
       FIG. 5  is a graph showing a change in the pitch diameter of the external thread in a third embodiment. The third embodiment is identical in structure with the first embodiment except the manner in which the pitch diameter of the external thread changes. 
     In the third embodiment, the pitch diameter of the external thread is constant on the rear end side of a position near the rear end P 1  of the center electrode  20 . The pitch diameter of the external thread increases gradually from the position near the rear end P 1  of the center electrode  20  until reaching the pitch diameter local maximum portion PA, and decreases gradually from the pitch diameter local maximum portion PA until reaching the forward end. This configuration can also prevent loosening of the spark plug  100 . 
     D. Fourth Embodiment 
       FIG. 6  is a graph showing a change in the pitch diameter of the external thread in a fourth embodiment. The fourth embodiment is identical in structure with the first embodiment except the manner in which the pitch diameter of the external thread changes. 
     In the fourth embodiment, the pitch diameter of the external thread is constant on the rear end side of a position near the rear end P 1  of the center electrode  20 . The pitch diameter of the external thread increases gradually from the position near the rear end P 1  of the center electrode  20  until reaching a position near the rear end P 2  of the metallic shell internal step portion  56 , and becomes the maximum at the position near the rear end P 2 . The external thread has a pitch diameter local maximum portion PA which extends from the position near the rear end P 2  and has a predetermined width. This configuration can also prevent loosening of the spark plug  100 . 
     E. Other Embodiments 
     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 the “SUMMARY” section 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. 
     In the above-described embodiments, the pitch diameter becomes the maximum at the pitch diameter local maximum portion PA. The position at which the pitch diameter becomes the maximum is not limited to the pitch diameter local maximum portion PA. The pitch diameter may become maximum at a portion other than the pitch diameter local maximum portion PA. 
     In the above-described embodiments, in the direction along the axial line CA, the pitch diameter local maximum portion PA is located at the same position as the rear end P 2  of the metallic shell internal step portion  56  or located on the forward end side of the rear end P 2  of the metallic shell internal step portion  56 . However, the position of the pitch diameter local maximum portion PA is not limited to these positions. The pitch diameter local maximum portion PA may be located on the rear end side of the rear end P 2  of the metallic shell internal step portion  56  in the direction along the axial line CA. 
     In the above-described embodiments, as shown in  FIG. 3 , the metallic shell internal step portion  56  protrudes inward in relation to portions located on the forward end side and rear end side, respectively, of the metallic shell internal step portion  56 . However, this is not a limitation. It is sufficient that the metallic shell internal step portion  56  protrudes inward in relation to a portion located on the rear end side of the metallic shell internal step portion  56 . 
     In the above-described embodiment, the insulator step portion  15  is located on the forward end side of the axial hole internal step portion  14  in the direction along the axial line CA. However, this is not a limitation. In the direction along the axial line CA, the insulator step portion  15  may be located at the same position as the axial hole internal step portion  14  or located on the rear end side of the axial hole internal step portion  14 . Notably, it is preferred from the viewpoint of manufacture of the spark plug  100  that the insulator step portion  15  is located on the forward end side of the axial hole internal step portion  14  in the direction along the axial line CA.