spark plug

Abstract

Spark plug (100) comprising a tubular insulator (10) having an axial hole (12) extending in the direction of an axial line (CL), a center electrode (20) comprising a part (20t) arranged in the axial hole (12), and a tubular metal shell (50) fixed to an outer circumference of the insulator (10), wherein the metal shell (50) comprises a curved part (58) which is curved in such a way that it widens radially outwards, wherein the curved part (58) of the metal shell (50) has a Vickers hardness of 350 HV to 450 HV, and wherein in a section containing the axial line (CL) a comparison expression DA / DD ≥ 15.7 is satisfied, where DA is the distance in the direction of the axial line (CL) between a front end (Pf) and a rear end (Pr) of an outer circumferential surface (58o) of the curved part (58) and DD is a maximum distance between the outer circumferential surface (58o) of the curved part (58) and an imaginary straight line (DDL) connecting the front end (Pf) and the rear end (Pr) of the outer circumferential surface (58o) of the curved part (58), measured in a direction perpendicular to the imaginary straight line (DDL).

Description

TECHNICAL AREA

[0001] The present invention relates to a spark plug. STATE OF THE ART

[0002] A conventional spark plug consists of an insulator with an axial hole, a center electrode with a portion positioned within the insulator's axial hole, and a tubular metal shell attached to the outer circumference of the insulator. To improve the gas tightness between the metal shell and the insulator, the metal shell is fixed to the insulator, for example, by partially deforming it.

[0003] Further relevant prior art is disclosed in the following documents: EP 2 770 593 A2 and EP 2 541 703 A1.

[0004] Furthermore, the subsequently published DE 10 2018 111 204 A1 describes another relevant technical background. State-of-the-art patent document

[0005] Patent document 1: Published Japanese patent application (kokai) JP H09 - 219 273 A SUMMARY OF THE INVENTION PROBLEM STATEMENT OF THE INVENTION

[0006] In recent years, there has been an increasing demand for a reduction in the diameter of spark plugs. However, reducing the diameter of the metal shell can sometimes lead to deformation of the shell into an undesirable shape, as this compromises its strength. Increasing the hardness of a portion of the metal shell can counteract this deformation. However, under certain circumstances, the hardened portion may develop cracks or splits during prolonged use.

[0007] The present invention provides a technique for suppressing a deformation of the metal shell to an undesired shape and for suppressing an impairment of the durability of a deformed part of the metal shell. PROBLEM SOLVING

[0008] The following describes various application examples of the invention.

[0009] To solve the problem described above, a spark plug with the feature of claim 1 is specified. Further advantages of the embodiment are defined in the dependent claims. [Application example 1]

[0010] A spark plug comprises a tubular insulator having an axial hole extending in the direction of an axial line, a center electrode comprising a portion arranged in the axial hole, and a tubular metal shell fixed to an outer circumference of the insulator, the metal shell comprising a curved portion curved such that it widens radially outwards; wherein the curved portion of the metal shell has a Vickers hardness of 350 HV to 450 HV;and wherein in a section containing the axial line a comparison expression DA / DD ≥ 15.7 is satisfied, where DA is the distance in the direction of the axial line between a front end and a rear end of an outer circumferential surface of the curved part and DD is a maximum distance between the outer circumferential surface of the curved part and an imaginary straight line connecting the front end and the rear end of the outer circumferential surface of the curved part, measured in a direction perpendicular to the imaginary straight line.

[0011] Because, according to this configuration, the curved part has a Vickers hardness of 350 HV to 450 HV, deformation of the curved part into an undesirable shape can be suppressed. And because the ratio of the distance DA to the maximum distance DD (the DA / DD ratio) is 15.7 or greater, large bending of the curved part is suppressed; thus, a reduction in the durability of the curved part can be prevented. [Application example 2]

[0012] A spark plug according to application example 1, wherein in the section containing the axial line the comparison expression 4.8 ≤ DA / DB is satisfied, where DB is the average wall thickness of the curved part measured in a direction perpendicular to the outer circumferential surface of the curved part.

[0013] Because this configuration prevents the average wall thickness DB from becoming excessively large, the curved part can simply be formed with a corresponding curvature. [Application example 3]

[0014] A spark plug according to application example 1 or 2, wherein in the section containing the axial line a comparison expression DA / DB ≤ 6.3 is satisfied, where DB is the average wall thickness of the curved part measured in a direction perpendicular to the outer circumferential surface of the curved part.

[0015] Because this configuration prevents the average wall thickness DB from becoming excessively small, excessive deformation of the curved part can be suppressed.

[0016] The technology of the present invention can be implemented in various ways: for example, as a spark plug, as a method for manufacturing the spark plug, as an ignition device that uses the spark plug, as an internal combustion engine that contains the spark plug, and as an internal combustion engine that contains the ignition device with the spark plug used therein. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a sectional view of a spark plug 100 according to an embodiment of the present invention. Fig. Figure 2 is an explanatory view that illustrates a distance DA and a maximum distance DD of a metal shell 50. Fig. Figure 3 is an explanatory view illustrating a curved part 58 and an average wall thickness DB of the metal shell 50. Fig. Section 4 contains a table that specifies relationships between structural parameters of samples of spark plug 100 and the results of a first evaluation test, and a curve diagram that shows the relationship between a DA / DD ratio, a shelf life Tm and evaluation results RA. Fig. Section 5 contains a table that specifies relationships between structural parameters of spark plug 100 samples and the results of a second evaluation test, and a curve diagram that shows the relationship between the distance DA, a DA / DB ratio and evaluation results RB. EXECUTIONS OF THE INVENTION A. Implementation: A-1. Spark plug configuration:

[0017] Fig. Figure 1 is a sectional view showing a spark plug 100 according to one embodiment. The drawing shows a central axis CL (also referred to as the "axial line CL") of the spark plug 100 and a flat section of the spark plug 100 containing the central axis CL. Hereinafter, the direction parallel to the central axis CL is referred to as the "direction of the axial line CL" or simply as the "axial direction" or "forward-backward direction". The radial direction of a circle centered on the axial line CL can also be referred to as the "radial direction". The radial direction is a direction orthogonal to the axial line CL. The circumferential direction of the circle centered on the axial line CL is also referred to as the "circumferential direction". As for the direction parallel to the central axis CL, the one shown in Figure 1 is the "radial direction". Fig. The downward direction is referred to as the front-end direction Df or forward direction Df, and the upward direction is referred to as the rear-end direction Dfr or reverse direction Dfr. The front-end direction Df extends from a metal terminal member 40 to a center electrode 20, these members being described further below. The side of the front-end direction Df in Fig. 1 is designated as a front end side of the spark plug 100, and the rear end side Dfr in Fig. 1 is referred to as a rear end of the spark plug 100.

[0018] The spark plug 100 comprises a tubular insulator 10 with a through-hole 12 (also referred to as axial hole 12) extending along the axial line CL, the center electrode 20 held in the through-hole 12 at the front end of the through-hole 12, the metal terminal 40 held in the through-hole 12 at the rear end of the through-hole 12, a resistor 73 held in the through-hole 12 between the center electrode 20 and the metal terminal 40, an electrically conductive first seal 72 in contact with the center electrode 20 and the resistor 73 to electrically connect these elements 20 and 73, an electrically conductive second seal 74 in contact with the resistor 73 and the metal terminal 40 to electrically connect these elements 73 and 40, and a tubular metal shell 50. fixed to the outer circumference of the insulator 10, and an earth electrode 30,one end of which is connected to an annular front end surface 55 of the metal shell 50 and the other end of which faces the central electrode 20 with a distance g between them.

[0019] The insulator 10 comprises a large-diameter section 14, which has the largest outer diameter and is formed approximately in the middle in the axial direction. The insulator 10 comprises a rear-end stem section 13, which is formed at the rear end of the large-diameter section 14. The insulator 10 comprises a front-end stem section 15, which is formed at the front end of the large-diameter section 14 and has an outer diameter that is smaller than that of the rear-end stem section 13. The insulator 10 comprises an outer diameter reduction section 16 and a leg section 19, which are formed at the front end of the front-end stem section 15 in that order towards the front end. The outer diameter of the outer diameter reduction section 16 decreases gradually in the forward direction Df.The insulator 10 comprises an inner diameter reduction part 11, which is adjacent to the outer diameter reduction part 16 (in the example of . Fig. 1 in the front-end stem section 15) and whose inner diameter gradually decreases in the forward direction Df. The insulator 10 is preferably designed for mechanical strength, thermal strength and electrical strength, for example by burning aluminum oxide (although other electrically insulating materials can also be used).

[0020] The center electrode 20 is a metal element and is located in the through-hole 12 of the insulator 10 at an end portion of the through-hole 12 on the forward-direction Df side. The center electrode 20 comprises a generally circular, columnar rod section 28 and a first tip 29 connected (for example, by laser welding) to the front end of the rod section 28. The rod section 28 comprises a head section 24 located on the backward-direction Dfr side and a shaft section 27 connected to the forward-direction Df side of the head section 24. The shaft section 27 extends in the forward-direction Df parallel to the axial line CL. A portion of the head section 24 on the forward-direction Df side is a sleeve section 23, the outer diameter of which is larger than that of the shaft section 27. A surface of the cuff part 23 on the forward direction Df side is held by the inner diameter reduction part 11 of the insulator 10.The shaft section 27 is connected to the forward-direction Df side of the cuff section 23. The first point 29 is connected to the front end of the shaft section 27. The rod section 28 is an example of a base section to which the first point 29 is connected.

[0021] The rod section 28 comprises an outer layer 21 and a core 22, which is arranged on the inner circumferential side of the outer layer 21. The outer layer 21 is made of a material (e.g., an alloy containing nickel as a major component) that has greater oxidation resistance than the core 22. The major component is the component with the largest proportion (in weight percent). The core 22 is made of a material (e.g., pure copper or an alloy containing copper as a major component) that has greater thermal conductivity than the outer layer 21. The first tip 29 is made of a material (e.g., a precious metal such as iridium (Ir) or platinum (Pt)) that has greater resistance to discharge.A portion of the center electrode 20, located on the forward-direction Df side and comprising the first tip 29, projects from the axial hole 12 of the insulator 10 in the forward direction Df. A portion 20t of the center electrode 20 on the reverse-direction Dfr side is located in the axial hole 12. In this way, the center electrode 20 is positioned in the axial hole 12 of the insulator 10 such that it encompasses a portion (at least a portion of portion 20t) located in a front end section 10t of the insulator 10. The front end section 10t of the insulator 10 comprises the front end of the insulator 10. The first tip 29 can also be omitted. Furthermore, the core 22 can be omitted.

[0022] The metal connecting element 40 is a rod-like link extending parallel to the axial line CL. The metal connecting element 40 is made of an electrically conductive material (e.g., a metal containing iron as a major component). The metal connecting element 40 has a cap fastening part 49, a sleeve part 48, and a shaft part 41, which are arranged sequentially in the forward direction Df. The shaft part 41 is inserted into a portion of the axial hole 12 of the insulator 10 on the reverse-direction Dfr side. The cap fastening part 49 projects from the axial hole 12 on the rear end face of the insulator 10.

[0023] The resistor 73 is positioned in the axial hole 12 of the insulator 10 between the metal terminal 40 and the center electrode 20 to suppress electrical noise. The resistor 73 is made of an electrically conductive material (e.g., a mixture of glass, carbon particles, and ceramic particles). The first seal 72 is positioned between the resistor 73 and the center electrode 20, and the second seal 74 is positioned between the resistor 73 and the metal terminal 40. These seals 72 and 74 are made of an electrically conductive material (e.g., a mixture of metal particles and glass similar to the material of the resistor 73). The center electrode 20 is electrically connected to the metal terminal 40 via the first seal 72, the resistor 73, and the second seal 74.

[0024] The metal shell 50 is a tubular element having a through-hole 59 extending along the axial line CL. In this embodiment, the central axis of the metal shell 50 coincides with the axial line CL. The insulator 10 is inserted into the through-hole 59 of the metal shell 50, and the metal shell 50 is fixed to the outer circumference of the insulator 10. The metal shell 50 is made of an electrically conductive material (e.g., a metal such as carbon steel containing iron as a major component). A portion on the forward-direction Df side of the insulator 10 projects outward from the through-hole 59. A portion on the reverse-direction Dfr side of the insulator 10 also projects outward from the through-hole 59.

[0025] The metal shell 50 comprises a tool gripping portion 51 and a front-end stem portion 52. A spark plug wrench (not shown) can engage in the tool gripping portion 51. The front-end stem portion 52 comprises a front end surface 55 of the metal shell 50. The front-end stem portion 52 has a threaded portion 57 formed on its outer circumferential surface, which can be screwed into a mounting hole of an internal combustion engine (not shown). The threaded portion 57 has an external thread extending in the direction of the axial line CL.

[0026] The metal shell 50 includes a flange-like intermediate stem section 54, which projects radially outward from its outer circumferential surface between the tool engagement section 51 and the front-end stem section 52. The outer diameter of the intermediate stem section 54 is larger than the maximum outer diameter of the threaded section 57 (i.e., the outer diameter of the thread tooth). A surface 54f of the intermediate stem section 54 on the forward-direction Df side is a seat surface (referred to as seat surface 54f) and provides a seal in conjunction with a mounting part (e.g., a cylinder head) of the internal combustion engine in which the mounting hole is drilled.

[0027] An annular seal 9 is arranged between the threaded portion 57 of the front-end stem section 52 and the seating surface 54f of the intermediate stem section 54. When the spark plug 100 is mounted on the internal combustion engine, the seal 9 is compressed and deformed to seal the gap between the seating surface 54f of the metal shell 50 and the mounting part (e.g., the cylinder head) of the internal combustion engine (not shown). However, the seal 9 can also be omitted. In this case, the seating surface 54f of the metal shell 50 comes into direct contact with the mounting part of the internal combustion engine to seal the gap between the seating surface 54f and the mounting part of the internal combustion engine.

[0028] The front-end stem portion 52 of the metal shell 50 has a projection 56 that extends radially inwards. The projection 56 has a smaller inner diameter than at least one portion on the reverse-direction Dfr side. In this embodiment, the inner diameter of a surface 56r (also referred to as the rear surface 56r) on the reverse-direction Dfr side gradually decreases in the forward direction Df. A front-end seal 8 is held between the rear surface 56r of the projection 56 and the outer diameter reduction portion 16 of the insulator 10. In this embodiment, the front-end seal 8 is, for example, a plate-like ring made of iron (although other materials, such as metallic materials like copper, etc., can also be used).The projection 56 (in particular, a portion of the projection 56 forming a rear surface 56r thereof) indirectly holds the outer diameter reduction section 16 of the insulator 10 from the forward direction Df-side via the seal 8. The seal 8 can also be omitted. In this case, the projection 56 (in particular, a portion of the projection 56 forming the rear surface 56r) can be in contact with the outer diameter reduction section 16 of the insulator 10. That is, the projection 56 can directly hold the insulator 10. In this way, the projection 56 corresponds to a holding element for directly or indirectly holding the outer diameter reduction section 16 of the insulator 10.

[0029] The metal shell 50 comprises a rear end section 53 on the rear end face of the tool gripping part 51. The rear end section 53 forms the rear end of the metal shell 50 and has a smaller wall thickness than the tool gripping part 51. The metal shell 50 further comprises a connecting part 58, which is formed between the intermediate stem section 54 and the tool gripping part 51 for connecting the parts 54 and 51. The connecting part 58 has a smaller wall thickness than the intermediate stem section 54 and the tool gripping part 51. Annular ring links 61 and 62 are inserted between an inner circumferential surface of the metal shell 50, which extends from the tool gripping part 51 to the rear end section 53, and an outer circumferential surface of the rear end stem section 13 of the insulator 10. Furthermore, a talc powder 70 is loaded between these ring links 61 and 62.When, during the manufacturing process of the spark plug 100, the rear end section 53 is bent radially inwards for crimping, the connecting section 58 is deformed outwards under the application of force. This fixes the metal shell 50 and the insulator 10 together. In this embodiment, the connecting section 58 is curved such that it widens radially outwards (hereinafter, the connecting section 58 is also referred to as the curved section 58). During this crimping step, the talc is compressed, thereby improving the airtight seal between the metal shell 50 and the insulator 10. The seal 8 is pressed between the outer diameter reduction section 16 of the insulator 10 and the projection 56 of the metal shell 50 to provide a seal between the metal shell 50 and the insulator 10.

[0030] The earth electrode 30 is a metal element comprising a rod-like body section 37. An end section 33 (also referred to as the near end section 33) of the body section 37 is connected to the front end face 55 of the metal shell 50 (e.g., by resistance welding). The body section 37 extends in the front end direction Df from the near end section 33 connected to the metal shell 50, is bent towards the central axis CL, extends in a direction crossing the axial line CL, and reaches a far end section 34. The far end section 34 of the earth electrode 30 and the first tip 29 of the central electrode 20 form an intermediate space g between them. That is, the far end section 34 of the earth electrode 30 is located on the forward-direction Df side of the first tip 29 of the central electrode 20 and faces the first tip 29 with the intermediate space g between them. A second tip similar to the first tip 29 can be connected to the distant end part 34 of the body part 37.In this way, the first tip 29 and the second tip in between can form the discharge gap g.

[0031] Body part 37 has an outer layer 31 and an inner layer 32, which is arranged on the inner circumferential side of the outer layer 31. The outer layer 31 is made of a material (e.g., an alloy containing nickel as a major component) that has greater oxidation resistance than the inner layer 32. The inner layer 32 is made of a material (e.g., pure copper or an alloy containing copper as a major component) that has higher heat resistance than the outer layer 31. However, the inner layer 32 can also be omitted. A-2. Construction of the curved part 58

[0032] Fig. Figure 2 is an explanatory view that illustrates the curved part 58 of the metal shell 50. Fig. Figure 2 shows a section of a portion of the metal shell 50 containing the curved part 58. The section is a shallow section containing the axial line CL. For clarity, the section of the metal shell 50 is not hatched. The intermediate stem part 54 is connected to the forward-direction Df side of the curved part 58, while the tool engagement part 51 is connected to the reverse-direction Dfr side of the curved part 58. An outer circumferential surface 58o and an inner circumferential surface 58i of the curved part 58 are curved such that they widen radially outwards. Fig. 2 is Do, the distance between the axial line CL and the outer circumferential surface 50° of the metal shell 50, measured in a direction perpendicular to the axial line CL. The distance Do changes with the position in the direction parallel to the axial line CL.

[0033] In Fig. 2. Pf is the front end of the outer circumferential surface 58° in the section, and Pr is the rear end of the outer circumferential surface 58° in the section. DA is the distance between the front end Pf and the rear end Pr, measured in the direction parallel to the axial line CL. The distance DA thus indicates the length of the curved part 58° in the direction parallel to the axial line CL. In the section of Fig. 2. On a portion 58o1 of the outer circumferential surface 58o on the forward direction Df-side, the distance Do gradually decreases in the forward direction Df. The distance Do reaches a minimum value Do1 at the front end Pf. As the outer circumferential surface 50o extends from the front end Pf to the forward direction Df-side, the distance Do becomes greater than the minimum value Do1. Similarly, on a portion 58o2 of the outer circumferential surface 58o on the backward direction Dfr-side, the distance Do gradually decreases in the backward direction Dfr. At the rear end Pr, the distance Do reaches a minimum value Do2. As the outer circumferential surface 50o extends from the rear end Pr to the backward direction Dfr-side, the distance Do becomes greater than the minimum value Do2.Therefore, parts of the outer circumferential surface 58o where the distance Do between the axial line CL and the outer circumferential surface 58o is minimized can be used as the front end Pf and the rear end Pr of the outer circumferential surface 58o. The distance Do1 at the front end Pf can be different from the distance Do2 at the rear end Pr.

[0034] An imaginary straight line DDL in Fig. 2 connects the front end Pf and the back end Pr in the section of Fig. 2. DD is the maximum distance between the imaginary straight line DDL and the outer circumferential surface 58o of the curved part 58, measured in a direction perpendicular to the imaginary straight line DDL. The imaginary straight line DDL can be a straight line parallel to the axial line CL or it can be non-parallel to the axial line CL, i.e., the imaginary straight line DDL can be inclined with respect to the axial line CL.

[0035] As mentioned above, during the manufacturing of the spark plug 100, the curved part 58 of the metal shell 50 is deformed. For example, before deformation, the curved part 58 is a cylindrical section whose center coincides with the axial line CL. By applying force to the cylindrical section, it is deformed in such a way that it expands radially outwards, thus exhibiting a curved shape. If the degree of deformation of the curved part 58 is large, its durability may be reduced compared to a small degree of deformation. For example, if the spark plug 100 is used for a long period of time, cracks may form in the curved part 58. The smaller the degree of deformation of the curved part 58 during manufacturing, the greater the ratio of the distance Da to the maximum distance DD, i.e., the ratio DA / DD.The greater the DA / DD ratio, the greater the durability of the curved part 58.

[0036] Fig. Figure 3 is an explanatory view that illustrates the average wall thickness DB of the curved part 58 of the metal shell 50. Similar to Fig. 2 shows Fig. Figure 3 shows a section of a portion of the metal shell 50 containing the curved part 58. On the outer circumferential surface 58o of the curved part 58, n measuring positions P1 to Pn (n is an integer equal to or greater than 2) are marked. These measuring positions P1 to Pn are arranged at equal intervals dx in the forward direction Df from the rear end Pf of the outer circumferential surface 58o. The interval dx is the distance in the direction parallel to the axial line CL. A plurality of measuring positions P1 to Pn are arranged throughout the region from the rear end Pr to the front end Pf. The distance de between the front end Pf and the measuring position Pn furthest along the forward direction Df side is equal to or less than the interval dx. Similar to the interval dx, the distance de is a distance in the direction parallel to the axial line CL.

[0037] Fig. Figure 3 shows the wall thicknesses T1 to Tn at measuring positions P1 to Pn. In the section of Fig. 3. The wall thicknesses T1 to Tn extend in corresponding directions perpendicular to the outer circumferential surface 58o. In particular, the wall thicknesses T1 to Tn extend in directions perpendicular to tangent lines tangential to the outer circumferential surface 58o at the measuring positions P1 to Pn. Fig. Figure 3 is an explanatory view showing the thickness T1 at the i-th measurement position Pi (i is an integer greater than 1 and equal to or less than n). The tangent line LTi in Fig. Line 3 is tangential to a line representing the outer circumferential surface 58o at measurement position Pi. A perpendicular line LPi is a line extending through measurement position Pi and is perpendicular to the tangent line LTi. The wall thickness Ti is the distance between the outer circumferential surface 58o and the inner circumferential surface 58i, measured along the perpendicular line LPi. The wall thicknesses T1 to Tn at measurement positions P1 to Pn are measured in the same way as the wall thickness T1 at measurement position Pi. The average wall thickness DB is the average of n wall thicknesses T1 to Tn.

[0038] If the ratio of the distance DA to the average wall thickness DB, i.e., the ratio DA / DB, is small because the average wall thickness DB is large relative to the distance DA corresponding to the length of the curved part 58, a difficulty arises in forming the curved part 58 due to deformation. Therefore, during the manufacture of the spark plug 100, the curved part 58 may not exhibit adequate deformation. If the curved part 58 does not exhibit adequate deformation, the force received by the curved part 58 during the manufacture of the spark plug 100 is likely to cause cracking in the curved part 58. If the ratio DA / DB is larger because the average wall thickness DB is small relative to the distance DA corresponding to the length of the curved part 58, deformation simply develops during the formation of the curved part 58.Therefore, during the manufacture of the spark plug 100, the curved part 58 can easily develop excessive deformation (such excessive deformation is referred to as a bulge).

[0039] As mentioned above, when fixing the metal shell 50 to the insulator 10 ( Fig. 1) The curved part 58 is formed by deformation. By forming the curved part 58 by deformation, the gas tightness between the insulator 10 and the metal shell 50 is improved. By increasing the force exerted on the seal 8 between the projection 56 of the metal shell 50 and the outer diameter reduction part 16 of the insulator 10, the gas tightness provided by the packing 8 is improved.

[0040] To improve the degree of freedom in the design of an internal combustion engine, a thin spark plug can be used. For example, the nominal size of the threaded part 57 is set to less than M10. Such a thin spark plug can utilize the metal shell 50 with a small wall thickness. With a metal shell 50 having a small wall thickness, the strength of the metal shell 50 may be reduced. For example, the curved part 58 may assume an undesirable shape during the deformation process of the spark plug 100. A reduction in the strength of the metal shell 50 can be mitigated by increasing its hardness. However, if the hardness of the metal shell 50 is too high, a defect may occur in a part subjected to deformation during manufacturing, such as the curved part 58. For example, the curved part 58 may become brittle.Embrittlement of the curved part 58 can lead to cracking in the curved part 58 if the spark plug 100 is used for a long period of time.

[0041] In the first and second evaluation tests described below, samples of spark plug 100, which have a thin metal shell 50 with high hardness, were examined for the appropriate design of the curved part 58. In the samples, the threaded parts 57 of the metal shells 50 had a nominal size of M8. In the evaluation tests, the metal shells 50 with high hardness were used instead of the metal shells of ordinary spark plugs. The curved parts 58 of the spark plug 100 samples used exhibited a higher Vickers hardness than the curved parts of ordinary spark plugs. In particular, the curved parts 58 exhibited a Vickers hardness of 350 HV to 450 HV (the Vickers hardness of the curved part 58 is measured on the outer circumferential surface of the curved part 58).By using metal shells 50, which provide the curved parts 58 with a hardness of such a degree, it is possible to prevent the curved parts 58 from assuming an undesirable shape during deformation during manufacturing. Specifically, the material for the metal shells 50 of the samples is carbon steel. The hardness is increased by increasing the carbon content (e.g., in weight percent) of the carbon steel.

[0042] The Vickers hardness of the curved part 58 was measured using the method specified in JIS Z2244. The load applied to an indenter for the measurement was 1.96 N. A portion of the curved part 58 against which the indenter was pressed for a measurement of the Vickers hardness of the curved part 58 can be a portion of a section of the metal shell 50 containing the central axis CL, wherein the portion corresponds to a section of the curved part 58 and not to the outer circumferential surface of the curved part 58.

[0043] In the evaluation tests, the intervals dx were used for measuring the average wall thickness DB ( Fig. 3) set to 0.1 mm. During the production of the spark plug 100 samples prior to crimping, the curved parts 58 had a cylindrical shape with a fixed wall thickness. By applying force during crimping, the curved parts 58 were deformed in such a way that they were curved radially outwards. A-3. Test results

[0044] Fig. Figure 4(A) is a table showing the relationships between the structural parameters of spark plug 100 samples and the results of the first evaluation test. This table shows the relationships between the sample number, the distance DA (unit: mm), the maximum distance DD (unit: mm), the DA / DD ratio, the shelf life Tm (unit: hours), and the evaluation result RA. Ten types of samples A1 to A10, each with a different configuration of the curved part 58 (specifically, a combination of the distance DA and the maximum distance DD), were evaluated. The samples have the same structural parameters except for the distance DA and the maximum distance DD. For example, the average wall thickness DB was 0.6 mm.

[0045] The durability time Tm indicates the resistance to corrosion of the curved part 58 and was measured by the following evaluation test. Samples A1 to A10, each consisting of ten pieces, were prepared. A corrosive liquid containing calcium nitrate was prepared. All samples were immersed in the corrosive liquid. The corrosive liquid was maintained at a temperature of 130°C. After the start of the test, the ten types of samples were removed from the corrosive liquid at two-hour intervals, such that one piece of each sample type was removed from the corrosive liquid. The removed samples were visually inspected for damage, such as a crack or defect, in the curved part 58 of the metal shells 50. The durability time Tm indicates the time elapsed until damage to the curved part 58 was detected.

[0046] The RA rating indicates the result of the shelf life Tm assessment. An RA rating of A indicates that the shelf life Tm is 10 hours or more. An RA rating of B indicates that the shelf life Tm is less than 10 hours.

[0047] Fig. Figure 4(B) is a curve diagram showing the relationship between the DA / DD ratio, the shelf life Tm, and the assessment results RA. The horizontal axis represents the DA / DD ratio, and the vertical axis represents the shelf life Tm. A circular mark indicates a sample with an assessment result RA of A, and a triangular mark indicates a sample with an assessment result RA of B.

[0048] As in Fig. 4(A) and Fig. As shown in Figure 4(B), samples A1 to A10 exhibited DA / DD ratios of 9.3, 10.9, 14.2, 15.1, 15.7, 15.8, 16.9, 18.3, 19.3, and 21.3, respectively. The RA evaluation results of four sample types (A1 to A4) with a DA / DD ratio of 15.1 or less were equal to B. The RA evaluation results of six sample types (A5 to A10) with a DA / DD ratio of 15.7 or more were equal to A.

[0049] Therefore, if the DA / DD ratio was high, the RA rating result was good. As mentioned above with reference to Fig. As described in section 2, the following reason is assumed: because with a larger DA / DD ratio the deformation of the curved part 58 during the manufacture of the spark plug 100 is smaller, the durability of the curved part 58 can be improved.

[0050] Therefore, to improve the durability of the curved part 58, the DA / DD ratio is preferably large. The samples whose evaluation results RA were equal to A exhibited D / A / DD ratios of 15.7, 15.8, 16.9, 18.3, 19.3, and 21.3, respectively. A preferred range of the DA / DD ratio can be determined using these six values. In particular, any one of the six values ​​can be used as the lower limit of a preferred range of the DA / DD ratio. For example, the DA / DD ratio can be 15.7 or greater. Any one of these values ​​equal to or greater than the lower limit can be used as the upper limit of the DA / DD ratio. For example, the DA / DD ratio can be 21.3 or less.

[0051] The larger the DA / DD ratio, the smaller the deformation of the curved part 58. Therefore, the larger the DA / DD ratio, the greater the durability of the curved part 58. The DA / DD ratio can thus exceed 21.3. However, if the DA / DD ratio is large, the deformation of the curved part 58 during manufacturing is small. Consequently, the gas tightness of the insulator 10 and the metal shell 50 tends to deteriorate. Preferably, the DA / DD ratio is set such that an adequate gas tightness between the insulator 10 and the metal shell 50 is achieved.

[0052] The durability of the curved part 58 is influenced by the degree of deformation of the curved part 58. The degree of deformation of the curved part 58 is primarily specified by the DA / DD ratio. Even if a combination of the distance DA and the maximum distance DD of a particular curved part 58 differs from the combination of the distance DA and the maximum distance DD of the curved part 58 of a sample, if the curved part 58 has the same DA / DD ratio, the curved parts 58 are likely to exhibit similar durability. Therefore, it is assumed that the aforementioned preferred range of the DA / DD ratio can be applied to various combinations of the distance DA and the maximum distance DD.

[0053] Fig. Figure 5(A) is a table that specifies relationships between structural parameters of spark plug 100 samples and the results of the second evaluation test. The table shows the relationships between the sample number, the distance DA (unit: mm), the average wall thickness DB (unit: mm), the DA / DB ratio, the evaluation result RB, the maximum distance DD (unit: mm), and the DA / DD ratio. Twenty-six types of samples, B1 to B26, each with a different design of the curved part 58 (specifically, a combination of the distance DA, the average wall thickness DB, and the maximum distance DD), were evaluated. The samples have the same structural parameters except for the distance DA, the average wall thickness DB, and the maximum distance DD.

[0054] The evaluation result RB indicates whether or not a defect is present in the curved part 58. In this evaluation test, the outer circumferential surface of the curved part 58 of each spark plug 100 sample was visually inspected to determine whether or not a defect was present in the curved part 58. Specifically, the curved part 58 was inspected for defects such as cracking, fine defects indicative of cracking, and excessive deformation. An evaluation result RB of A indicates the absence of a defect, and an evaluation result RB of B indicates the presence of a defect.

[0055] Fig. Figure 5(B) is a curve diagram showing the relationship between the distance DA, the ratio DA / DB, and the rating scores RB. The horizontal axis represents the distance DA, and the vertical axis represents the ratio DA / DB. A circular marker represents a sample with a rating score RB of A, and a triangular marker represents a sample with a rating score RB of B.

[0056] As in Fig. 5(A) and Fig. As shown in Figure 5(B), the distance DA is distributed over a range from 2.85 mm to 4.01 mm. The ratio DA / DB is distributed over a range from 4.1 to 7.3.

[0057] As in Fig. As shown in 5(A), the assessment results RB of samples B1 to B17 were equal to A. As in Fig. 5(A) and Fig. As shown in Figure 5(B), the distances DA of samples (B1 to B17) with ratings of A are distributed over a wide range from 2.92 mm to 3.97 mm. Furthermore, the DA / DB ratios of samples with ratings of A are distributed over a range from 4.8 to 6.3. In particular, the DA / DD ratios of samples with ratings of A are distributed over a range from 15.8 to 25.3.

[0058] For the samples (especially the six types of samples B18 to B23) with a DA / DB ratio of less than 4.8, the assessment results RB were equal to B. These samples developed cracking or a fine defect indicative of cracking. When the DA / DB ratio is small, the curved part 58 develops cracking or an indication of cracking from the area in relation to Fig. The reason described in section 3 is particularly relevant when the DA / DB ratio is small because the average wall thickness B is large relative to the distance DA corresponding to the length of the curved part 58. In such cases, a difficulty arises in forming the curved part 58 by means of deformation. Therefore, during the manufacture of the spark plug 100, the curved part 58 is not formed with sufficient deformation, resulting in cracking or indications of cracking in the curved part 58.

[0059] For the samples (especially the three types of samples B24 to B26) with a DA / DB ratio greater than 6.3, the evaluation results RB were equal to B. These samples exhibited excessive deformation of the curved part 58. When the DA / DB ratio is large, the curved part 58 develops excessive deformation due to the Fig. The reason described in section 3 is particularly relevant when the DA / DB ratio is large, because the average wall thickness DB is small relative to the distance DA corresponding to the length of the curved part 58. In such cases, deformation simply develops during the formation of the curved part 58. Therefore, during the manufacture of the spark plug 100, the curved part 58 may develop excessive deformation.

[0060] The samples with the evaluation results RB of A exhibited DA / DB ratios of 4.8, 4.9, 5.0, 5.2, 5.4, 5.6, 5.7, 5.9, 6.0, 6.1, and 6.3, respectively. A preferred range of the DA / DB ratio can be determined using these 11 values. In particular, any one of the 11 values ​​can be used as the lower limit of a preferred range of the DA / DB ratio.

[0061] For example, the DA / DB ratio can be 4.8 or higher. Furthermore, any value equal to or greater than the lower limit can be used as the upper limit of the DA / DB ratio. For example, the DA / DB ratio can be 6.3 or lower.

[0062] It is assumed that a defect in the curved part 58 is influenced by the ease of deformation of the curved part 58 during the manufacture of the spark plug 100. The ease of deformation of the curved part 58 is mainly determined by the DA / DB ratio. Even if a combination of the distance DA and the average wall thickness DB of a particular curved part 58 differs from the combination of the distance DA and the average wall thickness DB of the curved part 58 of a sample, it is assumed that if the curved parts 58 have the same DA / DB ratio, they will have the same probability of a defect occurring. Therefore, it is assumed that the aforementioned preferred range of the DA / DB ratio can be applied to various combinations of the distance DA and the average wall thickness DB.

[0063] The samples with an assessment result RB of A in Fig. 5(A) exhibited DA / DD ratios of 15.8, 15.9, 16.0, 16.2, 17.1, 17.4, 17.7, 18.0, 18.3, 18.9, 19.4, 19.9, 20.1, 24.3, and 25.3, respectively. A preferred range of DA / DD ratios can be determined using 21 values ​​derived from these 15 values ​​and six DA / DD ratio values ​​from six samples with RA scores of A in Fig. 4(A) shall be determined. In particular, any of the 21 values ​​may be used as the lower bound of a preferred range of the DA / DD ratio. For example, the DA / DD ratio may be 15.7 or greater. Furthermore, any of these values ​​equal to or greater than the lower bound may be used as the upper bound of the DA / DD ratio. For example, the DA / DD ratio may be 25.3 or less. B. Modified embodiments (1) The curved part 58 of the metal shell 50 may also have various other configurations than those described above. For example, the DA / DB ratio may be less than 4.8. The DA / DB ratio may be greater than 6.3. In both cases, if the Vickers hardness of the curved part 58 of the finished spark plug 100 falls within the range of 350 HV to 450 HV, insufficient strength of the curved part 58 can be avoided, even when using a thin metal shell 50 whose threaded part 57 has a nominal size of less than M10. And if the DA / DB ratio is 15.7 or greater, the durability of the curved part 58 can be improved because excessive deformation of the curved part 58 during the manufacture of the spark plug 100 is suppressed. (2) Instead of carbon steel, various other electrically conductive materials can also be used for forming the metal shell 50. For example, a metal material containing iron, carbon, and some other components (such as chromium) can be used. As a method for adjusting the Vickers hardness of the curved part 58, various other methods can also be used instead of the method for adjusting the carbon content of the material for the metal shell 50. For example, the content of other components contained in the material for the metal shell 50 can be adjusted. (3) As regards the construction of spark plug 100, the following may be used instead of the one in Fig. In addition to the setup shown in Figure 1, various other configurations can also be used. For example, the front-end seal 8 can be omitted. In this case, the projection 56 of the metal shell 50 directly holds the outer diameter reduction section 16 of the insulator 10. Furthermore, the resistor 73 can be omitted. A magnetic element can be arranged in the axial hole 12 of the insulator 10 between the center electrode 20 and the metal terminal element 40. Additionally, the first tip 29 can be located away from the center electrode 20. Finally, a discharge gap can be located between the ground electrode and a side surface (a surface located away from the axial line CL in the direction perpendicular to the axial line CL) of the center electrode instead of at the front end surface (e.g., the surface of the first tip 29 on the forward-direction Df side). Fig.1) the center electrode. The total number of discharge gaps can be two or more. Furthermore, the ground electrode 30 can be omitted. In this case, a discharge can be generated between the center electrode 20 of the spark plug and another element arranged in a combustion chamber.

[0064] In any case, the nominal size of the threaded part 57 of the metal shell 50 can be M10 or larger or smaller than M10. As mentioned above, when manufacturing the spark plug with the metal shell 50, which has a thin threaded part 57 of less than M10 nominal size and whose curved part 58 has a high Vickers hardness of 350 HV to 450 HV, deformation of the curved part 58 into an undesirable shape can be suppressed. And if the DA / DD ratio of the curved part 58 is 15.7 or greater, the durability of the curved part 58 can be improved.

[0065] The invention has been explained with reference to the embodiment and modified embodiments described above. The embodiment and the modified embodiments are intended to clarify the invention but do not limit it. The embodiments described here can be modified or improved in various ways without departing from the scope of the invention as defined by the claims and their equivalents. INDUSTRIAL APPLICABILITY

[0066] The present invention can advantageously be applied to spark plugs. LIST OF REFERENCE MARKS

[0067] 8: Front end seal; 9: Seal; 10: Insulator; 10t: Front end section; 11: Inner diameter reducing section; 12: Through hole (axial hole); 13: Rear end stem section; 14: Large diameter section; 15: Front end stem section; 16: Outer diameter reducing section; 19: Leg section; 20: Center electrode; 20t: Section; 21: Outer layer; 22: Core; 23: Sleeve section; 24: Head section; 27: Shaft section; 28: Rod section; 29: First tip; 30: Ground electrode; 31: Outer layer; 32: Inner layer; 33: Near end section; 34: Far end section; 37: Body section; 40: Metal connecting element; 41: Shaft section; 48: Sleeve section; 49: Cap fastening part; 50: Metal shell; 50o: Outer circumferential surface; 51: Tool gripping part; 52: Front end stem part; 53: Rear end part; 54: Intermediate stem part; 54f: Seat surface; 55: Front end surface; 56: Projection; 56r: Rear surface; 57: Threaded part; 58: Curved part (connecting part); 58i: Inner circumferential surface; 58o: Outer circumferential surface; 58o1: Part;58o2: Part; 59: Through hole, 61: Ring link, 70: Talc, 72: First seal; 73: Resistor; 74: Second seal; 100: Spark plug; g: Discharge gap; CL: Center axis (axial line); Df: Front end direction (forward direction); and Dfr: Rear end direction (reverse direction).;

Claims

[1] Spark plug (100) comprising a tubular insulator (10) having an axial hole (12) extending in the direction of an axial line (CL), a center electrode (20) comprising a part (20t) arranged in the axial hole (12), and a tubular metal shell (50) fixed to an outer circumference of the insulator (10), wherein the metal shell (50) comprises a curved part (58) which is curved in such a way that it widens radially outwards, wherein the curved part (58) of the metal shell (50) has a Vickers hardness of 350 HV to 450 HV, and wherein in a section containing the axial line (CL) a comparison expression DA / DD ≥ 15.7 is satisfied, where DA is the distance in the direction of the axial line (CL) between a front end (Pf) and a rear end (Pr) of an outer circumferential surface (58o) of the curved part (58) and DD is a maximum distance between the outer circumferential surface (58o) of the curved part (58) and an imaginary straight line (DDL) connecting the front end (Pf) and the rear end (Pr) of the outer circumferential surface (58o) of the curved part (58), measured in a direction perpendicular to the imaginary straight line (DDL). [2] Spark plug (100) according to claim 1, wherein in the section containing the axial line (CL) the comparison expression 4.8 ≤ DA / DB is satisfied, where DB is the average wall thickness of the curved part (58) measured in a direction perpendicular to the outer circumferential surface (58o) of the curved part (58). [3] Spark plug (100) according to claim 1 or 2, wherein in the section containing the axial line (CL) a comparison expression DA / DB ≤ 6.3 is satisfied, where DB is the average wall thickness of the curved part (58) measured in a direction perpendicular to the outer circumferential surface (58o) of the curved part (58).

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