spark plug

Abstract

A spark plug (100) comprising: a tubular metal casing (50) with a fastening screw section (52) having an external thread; an insulator (10) arranged within the metal casing (50) and having an axial opening (12); and a center electrode (20) arranged in the axial opening (12), wherein the fastening screw section (52) has a section with a local maximum pitch diameter (PA) at which the external thread has a locally maximum pitch diameter, wherein the section with the local maximum pitch diameter (PA) is located on a front end face of a rear end (P1) of the center electrode (20) in a direction along an axial line (CA) of the metal casing (50), wherein the insulator (10) has an outwardly projecting insulator step section (15), and the metal casing (50) has an inwardly projecting inner metal casing step section (56).the insulator stage section (15) is in contact with the inner metal shell stage section (56) via a seal (68), and the section with the local maximum of the pitch diameter (PA) is arranged at a position of the inner metal shell stage section (56) in the direction along the axial line (CA).

Description

TECHNICAL AREA 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 section formed on the outer surface of a metal housing of the spark plug with an internal thread provided in an engine head. In general, the screw section has a uniform pitch diameter (see, for example, the disclosed Japanese patent application (kokai) JP 2015 - 225 774 A). Generally, increasing the length of the screw section advantageously increases the degree of freedom in determining the position of the engine's water jacket. However, if the pitch diameter of the screw section is uniform, increasing the length of the screw section increases the contact area between the screw section and the engine head, and consequently, the surface pressure per unit area decreases. Therefore, the longer the screw section, the more easily the spark plug loosens when absorbing the vibrations generated during the engine's combustion cycle. Further relevant prior art is disclosed in JP 2006 - 236 769 A. This document relates to a spark plug for an internal combustion engine with an external thread on a main metal fitting, wherein the effective diameter of the external thread is not constant along the axial direction, but rather a region with maximum effective diameter and a region with a smaller effective diameter are provided to prevent the spark plug from loosening under thermal and mechanical stress. The present invention was carried out to solve the above-mentioned problem and can be implemented in the following modes. SUMMARY To solve the problem described above, a spark plug with the features of claim 1 is specified. Advantageous embodiments are defined in the dependent claims. In particular, the present invention can be embodied in various forms. For example, the present invention can be implemented in the form of a cylinder head or similar assembly to which a spark plug is attached. Fig. 1 is an explanatory view showing a partially sectioned spark plug; Fig. 2 is a diagram showing a change in the pitch diameter of an external thread of a fastening screw section; Fig. 3 is a view showing the positional relationship of the section with the local maximum of the pitch diameter in the spark plug; Fig. 4 is a diagram showing a change in the pitch diameter of the external thread in a second embodiment; Fig. 5 is a diagram showing a change in the pitch diameter of the external thread in a third embodiment; and Fig. 6 is a diagram showing a change in the pitch diameter of the external thread in a fourth embodiment. DETAILED DESCRIPTION OF PREFERRED EXECUTION FORMS A. First embodiment: Fig. 1 is an explanatory view showing a partially sectioned spark plug 100. In Fig. 1, an outer shape of the spark plug 100 is shown on the right side of an axial line CA, which is the central 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 is referred to as the "front end" of the spark plug 100 and the upper side of Fig. 1 as the "rear end" of the spark plug 100. The spark plug 100 comprises an insulator 10 with an axial opening 12 extending along the axial line CA, a center electrode 20 arranged in the axial opening 12, a tubular metal shell 50 arranged around the insulator 10, and a ground electrode 30, the proximal end 32 of which is attached to the metal shell 50. The insulator 10 is a ceramic insulator produced by firing a ceramic material such as aluminum oxide. The insulator 10 is an element arranged within the metal shell 50. Specifically, the insulator 10 is a tubular element with an axial opening 12 in its center. Part of the central electrode 20 is located in a front end section of the axial opening 12, and part of a metal terminal 40 is located in a rear end section of the axial opening 12. A central body section 19 with a large outer diameter is formed axially in the center of the insulator 10. A rear body section 18 with an outer diameter smaller than that of the central body section 19 is formed on the rear end of the central body section 19.A front body section 17 with an outer diameter smaller than that of the rear body section 18 is formed at the front end of the central body section 19. A leg section 13, whose outer diameter decreases towards the central electrode 20, is formed at the front end of the front body section 17. The metal sleeve 50 is a tubular metal element that surrounds and holds a section of the insulator 10, extending from part of the rear body section 18 to the leg section 13. The metal sleeve 50 is made, for example, of low-carbon steel and is completely coated with nickel, zinc, or a similar alloy. The metal sleeve 50 includes a tool engagement section 51, a sealing section 54, and a fastening screw section 52, arranged in that order from the rear end. A tool for fastening the spark plug 100 to a motor head 90 engages with the tool engagement section 51. The fastening screw section 52 is a section in which an external thread is formed around the entire circumference of the outer perimeter of the metal sleeve 50 and which screws into a fastening screw hole 93 of the motor head 90.The sealing section 54 is flange-shaped at the proximal end of the mounting screw section 52. A ring seal 65, formed by bending a plate, is inserted between the sealing section 54 and the motor head 90. The metal casing 50 has an annular front end surface 57 that defines a central opening through which the front end of the leg section 13 of the insulator 10 and the front end of the center electrode 20 protrude. A crimp section 53 with a reduced thickness is provided on the rear end of the tool engagement section 51 of the metal casing 50. A compression section 58 with a reduced thickness, similar to the crimp section 53, is provided between the sealing section 54 and the tool engagement section 51. Annular ring elements 66 and 67 are arranged between the outer circumferential surface of the rear body section 18 of the insulator 10 and a section of the inner circumferential surface of the metal casing 50, this section extending from the tool engagement section 51 to the crimp section 53. Talc powder 69 is loaded between the two ring elements 66 and 67. During the manufacture of the spark plug 100, the crimp section 53 is pressed forward, causing it to bend inwards and, as a result, the compression section 58 is deformed by compression.As a result of the compression deformation of the compression deformation section 58, the insulator 10 in the metal shell 50 is pushed forward over the ring elements 66, 67 and the talc 69. This compression causes the talc 69 to be compressed in the direction of the axial line CA, thereby increasing the gas tightness in the metal shell 50. The metal shell 50 has an inwardly projecting inner metal shell step section 56. The position of the rear end of the inner metal shell step section 56 in the direction of the axial line CA is designated P2. The insulator 10 has an insulator step section 15, which is located at the rear end of the leg section 13 and projects outwards. The inner metal shell step section 56 of the metal shell 50 is in contact with the insulator step section 15 via an annular sheet metal seal 68. This sheet metal seal 68 is an element that maintains the gas tightness between the metal shell 50 and the insulator 10 and prevents the escape of combustion gas. In the present embodiment, the seal 68 is a sheet metal packing. The central electrode 20 is a rod-shaped element consisting of an electrode element 21 and a core material 22 embedded in the electrode element 21. The core material 22 has a higher thermal conductivity than the electrode element 21. The electrode element 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 precious metal tip, e.g., made of an iridium alloy, can be connected to an end section of the central electrode 20 at its front end. The central electrode 20 has a flange 23 formed near one end section of its rear end face, which bulges outwards. The flange 23 comes into contact with an inner axial opening step section 14 at the rear end face, which projects inwards within the axial opening 12 of the insulator 10, with the central electrode 20 positioned inside the insulator 10. At the rear end face of the central electrode 20, the electrode 20 is electrically connected to the metal terminal 40 via a seal 64 and a ceramic resistor 63. Specifically, the position of the rear end of the central electrode 20 in the direction along the axial line CA is designated P1. The ground electrode 30 is made of an alloy whose main component is nickel. The proximal end 32 of the ground electrode 30 is attached to the end face 57 of the metal sheath 50. The ground electrode 30 extends from the proximal end 32 along the axial line CA towards the front end and is bent at an intermediate section such that a side face of a distal end section 33 faces the front end face of the center electrode 20. A precious metal tip 31 is arranged on the surface of the distal end section 33 of the ground electrode 30 facing the center electrode 20. A gap for the spark discharge is formed between the precious metal tip 31 and the center electrode 20. The precious metal tip 31 is made, for example, of 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 fastening screw section 52. In Fig. 2, a change in the pitch diameter is represented by a dashed line, and the shape of the external thread by a solid reference line. To facilitate understanding, the change in pitch diameter represented by the dashed line is shown as larger than the actual amount of change, and the change represented by the dashed line does not represent 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 along the axial line CA. In this description, the "pitch diameter of the external thread" refers to the value specified in JIS B 0205. As shown in Fig. 2, in the present embodiment of the spark plug 100, the fastening screw section 52 has a section with a local maximum pitch diameter PA, where the pitch diameter of the external thread is at its maximum. The “section with a local maximum pitch diameter PA” means a specific section whose pitch diameter is larger than that of a section at the front end and a section at the rear end of the specific section. In the present embodiment, the flank diameter at the section with the local maximum pitch diameter PA is 11.100 mm. Fig. 3 is a view showing the positional relationship of the section with the local maximum of the pitch diameter PA in the spark plug 100. In the present embodiment, the section with the local maximum of the pitch diameter PA is located on the front end of the rear end P1 (see also Fig. 1) of the center electrode 20 in the direction of the axial line CA. The spark plug 100 of the current embodiment is less likely to come loose after being attached to the engine head 90. The mechanism for preventing the spark plug 100 from coming loose will now be described. In general, the front end of the spark plug 100, attached to the cylinder head 90, receives a greater amount of heat from the engine interior than the rear end. Specifically, the temperature of the front end of the spark plug's metal casing 50 rises to approximately 600°C, while the temperature of the rear end only rises to approximately 100°C. Consequently, the front part of the metal casing 50 expands thermally to a greater extent than the rear part. Since, in the present embodiment of the spark plug 100, the section with the local maximum of the pitch diameter PA is located on the front end of the rear end P1 of the center electrode 20, the amount of thermal expansion is greater than in the case where the section with the local maximum of the pitch diameter PA is located on the rear end of the rear end P1 of the center electrode 20. Consequently, upon contact with the cylinder head 90, the surface pressure on the section with the local maximum of the pitch diameter PA increases. Therefore, wobbling of the spark plug 100 can be prevented, thus preventing the spark plug 100 from coming loose. In the spark plug 100 of the present embodiment, the section with the local maximum of the pitch diameter PA is larger than that of a section on the front end side of the section with the local maximum of the pitch diameter PA. Therefore, it is possible to prevent damage to the mounting screw hole 93 of the engine head 90, which would otherwise occur due to excessive thermal expansion of the section on the front end side of the section with the local maximum of the pitch diameter PA. In the spark plug 100 of the present embodiment, the pitch diameter of the external thread reaches its maximum at the section with the local maximum pitch diameter PA. When the metal casing 50 receives heat from the engine, the heat is conducted to the engine head 90 via the section with the local maximum pitch diameter PA, thereby dissipating the heat from the metal casing 50. Since, in the present embodiment, the section with the local maximum pitch diameter PA is located at the front end of the rear end P1 of the center electrode 20, the heat is dissipated more efficiently from the metal casing 50 through the section with the local maximum pitch diameter PA compared to the case where the section with the local maximum pitch diameter PA is located at the rear end P1 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 reaches its maximum at the section with the local maximum of the pitch diameter PA. When the spark plug 100 of the present embodiment is attached to the engine head 90, the spark plug 100 comes into contact with the engine head 90 at the section with the local maximum of the pitch diameter PA, so that the surface pressure is concentrated at this section. This more effectively prevents the spark plug 100 from loosening compared to the case of a uniform pitch diameter. As shown in Fig. 3, in the present embodiment the section with the local maximum of the pitch diameter PA is located at the same position as the rear end P2 of the inner metal casing stage section 56 in the direction of the axial line CA. Specifically, in the present embodiment, the section with a local maximum of the pitch diameter PA is located at the position of the inner metal casing stage section 56 in the direction along the axial line CA. When the insulator 10 and the center electrode 20 receive heat from the interior of the motor, the heat is conducted to the metal casing 50 through the seal 68 and the inner metal casing stage section 56, and to the motor head 90 through the section with the local maximum of the pitch diameter PA.Since, in the present embodiment, the section with a local maximum of the pitch diameter PA is located at the position of the inner metal shell step section 56 in the direction of the axial line CA, the heat is efficiently dissipated from the metal shell 50 via the section with a local maximum of the pitch diameter PA. Therefore, the spark plug 100 of the present embodiment has excellent heat dissipation properties. In the present embodiment, the length of the fastening screw section 52 in the direction of the axial line CA is 26.5 mm or more. The "length of the fastening screw section 52" refers to the length from the foremost part of the thread to the front end face of the sealing section 54. In general, the longer the length of the fastening screw section 52, the larger the area over which the external thread of the fastening screw section 52 comes into contact with the internal thread of the engine head 90, and the greater the likelihood of loosening. However, the spark plug in the present embodiment can effectively prevent loosening. It is noteworthy that the lower limit of the length of the fastening screw section 52 in the direction of the axial line CA is not limited to 26.5 mm and the length can be, for example, 18 mm or more.Although the upper limit of the length of the fastening screw section 52 in the direction of the axial line CA is not limited, from the point of view of the ease 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 graphic showing a change in the pitch diameter of the external thread in a second embodiment. In Fig. 4, a change in the pitch diameter is also represented by a dashed line, and the shape of the external thread is shown by a solid reference line, as in Fig. 2. To facilitate understanding, the change in pitch diameter indicated by the dashed line is shown as larger than the actual change, and the change indicated by the dashed line does not represent 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 along the axial line CA. The above also applies to Figs. 5 and 6, which will be discussed later. The second embodiment is structurally identical to the first embodiment, except for the way in which the pitch diameter of the external thread changes. In the second embodiment, the section with a local maximum of the pitch diameter PA is located at the front end of the rear end P2 of the inner metal casing stage section 56. Due to this configuration, when the metal casing 50 receives heat from the motor, the heat is dissipated more efficiently from the metal casing 50 through the section with a local maximum of the pitch diameter PA, compared to the case where the section with a local maximum of the pitch diameter PA is located at the rear end of the rear end P2 of the inner metal casing stage section 56. Therefore, the heat dissipation of the metal casing 50 is improved in the present embodiment. C. Third embodiment Fig. 5 is a diagram showing a change in the pitch diameter of the external thread in a third embodiment. The third embodiment is structurally identical to the first embodiment, except for the way in which the pitch diameter of the external thread changes. In the third embodiment, the pitch diameter of the external thread is constant at the rear end near the rear end P1 of the center electrode 20. The pitch diameter of the external thread gradually increases from the position near the rear end P1 of the center electrode 20 until the section with a local maximum of the pitch diameter PA is reached, and gradually decreases from the section with a local maximum of the pitch diameter PA until the front end is reached. This configuration can also prevent the spark plug 100 from loosening. D. Fourth embodiment Fig. 6 is a diagram showing a change in the pitch diameter of the external thread in a fourth embodiment. The fourth embodiment is structurally identical to the first embodiment, except for the way in which the pitch diameter of the external thread changes. In the fourth embodiment, the pitch diameter of the external thread is constant at the rear end near the rear end P1 of the center electrode 20. The pitch diameter of the external thread gradually increases from this position until it reaches a position near the rear end P2 of the inner metal sheath step section 56, where it reaches its maximum. The external thread has a section with a local maximum pitch diameter PA extending from the position near the rear end P2 and having a predetermined width. This configuration can also prevent the spark plug 100 from loosening. E. Other embodiments: The present invention is not limited to the embodiments described above and can be embodied in various other forms without deviating from the scope of the invention. For example, the technical features in the embodiments described in the "SUMMARY" section can be combined to solve some or all of the aforementioned problems or to achieve some or all of the aforementioned effects. The scope of protection of the application is determined by the claims. In the embodiments described above, the pitch diameter reaches its maximum at the section with a local maximum of the pitch diameter PA. The position at which the pitch diameter reaches its maximum is not limited to the section with a local maximum of the pitch diameter PA. The pitch diameter can reach its maximum in a different region than the section with a local maximum of the pitch diameter PA. In the embodiments described above, the section with a local maximum of the pitch diameter PA is located in the direction along the axial line CA at the same position as the rear end P2 of the inner metal shell step section 56 or at the front end of the rear end P2 of the inner metal shell step section 56. However, the position of the section with a local maximum of the pitch diameter PA is not limited to these positions. The section with a local maximum of the pitch diameter PA can be located at the rear end of the rear end P2 of the inner metal shell step section 56 in the direction of the axial line CA. In the embodiments described above, as shown in Fig. 3, the inner metal shell step section 56 projects inwards with respect to sections located on the front end and rear end of the inner metal shell step section 56, respectively. However, this is not a limitation. It is sufficient that the inner metal shell step section 56 projects inwards with respect to a section located on the rear end of the inner metal shell step section 56. In the embodiment described above, the insulator stage section 15 is located at the front end of the inner axial opening stage section 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 stage section 15 can be located in the same position as the inner axial opening stage section 14 or at the rear end of the inner axial opening stage section 14. In particular, from the point of view of the manufacture of the spark plug 100, it is preferable that the insulator stage section 15 be located at the front end of the inner axial opening stage section 14 in the direction of the axial line CA.

Claims

A spark plug (100) comprising: a tubular metal casing (50) with a fastening screw section (52) having an external thread; an insulator (10) arranged within the metal casing (50) and having an axial opening (12); and a center electrode (20) arranged in the axial opening (12), wherein the fastening screw section (52) has a section with a local maximum pitch diameter (PA) at which the external thread has a locally maximum pitch diameter, wherein the section with the local maximum pitch diameter (PA) is located on a front end face of a rear end (P1) of the center electrode (20) in a direction along an axial line (CA) of the metal casing (50), wherein the insulator (10) has an outwardly projecting insulator step section (15), and the metal casing (50) has an inwardly projecting inner metal casing step section (56).the insulator stage section (15) is in contact with the inner metal shell stage section (56) via a seal (68), and the section with the local maximum of the pitch diameter (PA) is arranged at a position of the inner metal shell stage section (56) in the direction along the axial line (CA). Spark plug (100) according to claim 1, wherein the pitch diameter of the external thread reaches its maximum at the section with the local maximum of the pitch diameter (PA). Spark plug (100) according to one of claims 1 or 2, wherein the fastening screw section (52) has a length of 26.5 mm or more in the direction along the axial line (CA).

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