Spark plug

Abstract

To provide a technique that can suppress the occurrence of irregular discharges at random positions.SOLUTION: A spark plug includes a center electrode extending along an axis, a cylindrical insulator that holds the center electrode on its inner circumference, a cylindrical main body metal fitting that holds the insulator on its inner circumference, and a ground electrode joined to a tip of the main body metal fitting. The spark plug has a protrusion that protrudes circumferentially and toward the tip from a joint between the ground electrode and the main body metal fitting, and the protrusion is more distant from the main body metal fitting the more distant as it is away from the ground electrode in the circumferential direction, the length of the protrusion in the circumferential direction is 0.3 mm or more and 1.3 mm or less, and the length in the axial direction is 0.3 mm or more and 1.3 mm or less.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to spark plugs. [Background technology]

[0002] As an ignition spark plug used in an internal combustion engine, there is known a spark plug in which a voltage is applied between a ground electrode connected to a tip of a metallic shell and a center electrode, thereby generating a spark discharge in a gap formed between the tip of the center electrode and the tip of the ground electrode (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-036492 A Summary of the Invention [Problem to be solved by the invention]

[0004] In order to improve thermal efficiency, the compression ratio is increased, and high boost and lean burn are promoted, which may cause discharges between gaps other than the intended gap. When such irregular discharges occur, the frequency of regular discharges in the intended gap decreases, and combustion stability deteriorates. In particular, when irregular discharges occur at random positions, there is a problem that combustion stability deteriorates further. Therefore, there has been a demand for technology that can suppress the occurrence of irregular discharges at random positions. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms.

[0006] (1) According to one embodiment of the present disclosure, there is provided a spark plug. The spark plug includes a center electrode extending along an axis, a cylindrical insulator that holds the center electrode on its inner periphery, a cylindrical metal shell that holds the insulator on its inner periphery, and a ground electrode joined to a tip of the metal shell, and has a protruding portion that protrudes from a joint between the ground electrode and the metal shell in a circumferential direction toward the tip side, the protruding portion is more distant from the metal shell as it is farther from the ground electrode in the circumferential direction, the length in the circumferential direction is 0.3 mm to 1.3 mm, and the length in the axial direction is 0.3 mm to 1.3 mm. According to this embodiment of the spark plug, since the protruding portion protrudes from the joint between the ground electrode and the metal shell in a circumferential direction toward the tip side, the electric field intensity can be locally increased at the protruding portion. As a result, irregular discharges occurring in the spark plug can be converged to the protruding portion, and irregular discharges occurring at random positions can be suppressed.

[0007] (2) In the spark plug described in (1) above, the insulator may have a locking portion whose outer diameter decreases toward the tip side along the axial direction, the metal shell may have a shelf portion whose inner diameter decreases toward the tip side along the axial direction, the shelf portion may hold the insulator in a state in which the locking portion is locked via a packing, and the inner diameter of the metal shell in at least a portion on the tip side of the shelf portion may be 6.5 mm or less. According to this form of spark plug, even in a configuration in which the inner diameter of at least a portion on the tip side of the metal shell is small and irregular discharge is likely to occur between the center electrode and the metal shell, it is possible to suppress irregular discharge from occurring at random positions.

[0008] (3) In the spark plug according to (1) or (2), the inner diameter of the tip of the metallic shell may be 6.5 mm or less. With this spark plug, even in a configuration in which the inner diameter of the tip of the metallic shell is small and irregular discharge is likely to occur between the center electrode and the metallic shell, it is possible to suppress irregular discharge from occurring at random positions.

[0009] The present disclosure can be realized in various forms, for example, in the form of a manufacturing method for a spark plug, an engine head having a spark plug attached thereto, and the like. [Brief description of the drawings]

[0010] [Figure 1] 1 is a partial cross-sectional view showing a schematic configuration of a spark plug. [Diagram 2] FIG. 2 is a view taken along the arrow A in FIG. [Diagram 3] 10 is an explanatory diagram showing a configuration of a main part of a spark plug according to a modified example. FIG. [Figure 4] 10 is an explanatory diagram showing a configuration of a main part of a spark plug according to a modified example. FIG. [Diagram 5] 10 is an explanatory diagram showing a configuration of a main part of a spark plug according to a modified example. FIG. [Figure 6] 10 is an explanatory diagram showing a configuration of a main part of a spark plug according to a modified example. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] A. Embodiment FIG. 1 is a partial cross-sectional view showing a schematic configuration of a spark plug 100 according to an embodiment of the present disclosure. In FIG. 1, the outer shape of the spark plug 100 is shown on the right side of the drawing, and the cross-sectional shape of the spark plug 100 is shown on the left side of the drawing, with the axis CA being the axis of the spark plug 100 as the boundary. In the following description, the lower side of FIG. 1 along the axis CA (the side where a ground electrode 40, which will be described later, is arranged) is called the leading end side, and the upper side of FIG. 1 (the side where a terminal fitting 50, which will be described later, is arranged) is called the rear end side. For convenience of description, an engine head 90 to which the spark plug 100 is attached is shown by a broken line in FIG. 1. The spark plug 100 is attached to the engine head 90 so that its leading end is exposed in a combustion chamber 95.

[0012] The spark plug 100 includes an insulator 10, a center electrode 20, a metal shell 30, a ground electrode 40, and a terminal fitting 50. An axis CA of the spark plug 100 coincides with the axes of the insulator 10, the center electrode 20, the metal shell 30, and the terminal fitting 50.

[0013] The insulator 10 has a generally cylindrical external shape with a through hole 11 formed along an axis CA. The through hole 11 accommodates a part of the center electrode 20 at the front end and a part of the terminal fitting 50 at the rear end. Thus, the insulator 10 holds the center electrode 20 on its inner periphery. A front end portion of the insulator 10 is accommodated in an axial hole 38 of a metallic shell 30, which will be described later, and a rear end portion of the insulator 10 is exposed from the axial hole 38. The insulator 10 is made of a porcelain insulator formed by firing a ceramic material such as alumina.

[0014] The insulator 10 has a large diameter portion 14, a locking portion 15, a step portion 17, and a small diameter portion 16. The large diameter portion 14 is located on the rear end side of the insulator 10 in the direction along the axis CA. The diameter of the through hole 11 in the large diameter portion 14 is formed to be approximately constant. The locking portion 15 is formed to have an outer diameter that is smaller toward the tip side in the direction along the axis CA at the tip side of the large diameter portion 14. The step portion 17 is configured by forming the diameter of the through hole 11 to be smaller toward the tip side in the direction along the axis CA. In other words, the step portion 17 is formed to protrude radially inward in the through hole 11. The step portion 17 supports a flange portion 22 of the center electrode 20. The small diameter portion 16 is connected to the tip side of the step portion 17, and the diameter of the through hole 11 is formed to be smaller than that of the step portion 17. A part of the leg portion 21 of the center electrode 20 described later is accommodated in the through hole 11 of the small diameter portion 16. A front end of the insulator 10 is provided on the rear end side of a front end 37 of the metallic shell 30 in the direction along the axis CA.

[0015] The center electrode 20 is a rod-shaped electrode extending along the axis CA. The center electrode 20 is held in the through hole 11 of the insulator 10. The center electrode 20 has a leg 21, a flange 22, and a head 23. The leg 21 is formed extending in a direction along the axis CA, and a part of the tip side is exposed from the through hole 11. A precious metal tip formed of, for example, a platinum or iridium alloy may be joined to the tip end of the leg 21. The flange 22 is formed to be continuous with the rear end side of the leg 21 and to protrude toward the outer periphery. The flange 22 abuts against the step portion 17 of the insulator 10 from the rear end side, thereby positioning the center electrode 20 in the through hole 11 of the insulator 10. The head 23 is formed to be continuous with the rear end side of the flange 22 and to be extended in a direction along the axis CA. The rear end side of the center electrode 20 is electrically connected to the terminal fitting 50 through the through hole 11 of the insulator 10 via a front end side seal material 61, a resistor 62, and a rear end side seal material 63. The center electrode 20 of this embodiment is formed of a nickel alloy containing nickel as a main component. In this disclosure, the term "main component" means the component that is contained in the largest amount.

[0016] The resistor 62 is formed from ceramic powder, a conductive material, glass, and an adhesive. The resistor 62 functions as an electrical resistor between the terminal fitting 50 and the center electrode 20, thereby suppressing the generation of noise when spark discharge occurs. The leading end sealing material 61 and the trailing end sealing material 63 each contain a conductive glass powder as a material. In this embodiment, the leading end sealing material 61 and the trailing end sealing material 63 contain a powder obtained by mixing copper powder and calcium borosilicate glass powder as a material.

[0017] The metal shell 30 has a generally cylindrical external shape with an axial hole 38 formed in the direction along the axis CA, and holds the insulator 10 within the axial hole 38. More specifically, the metal shell 30 surrounds and holds a portion of the insulator 10 ranging from a part of the large diameter portion 14 to the small diameter portion 16. The metal shell 30 is made of, for example, low carbon steel, and is entirely plated with nickel plating, zinc plating, or the like. The metal shell 30 includes a tool engagement portion 31, a male thread portion 32, a seat portion 33, a crimped portion 35, a compressive deformation portion 36, and a shelf portion 34.

[0018] The tool engagement portion 31 engages with a tool (not shown) when the spark plug 100 is attached to the engine head 90. The male thread portion 32 has a thread formed on the outer circumferential surface at the tip portion of the metallic shell 30, and is screwed into the female thread portion 93 of the engine head 90. The seat portion 33 is continuous with the rear end side of the male thread portion 32 and is formed in a flange shape. An annular gasket 65 formed by bending a plate is inserted between the seat portion 33 and the engine head 90.

[0019] The crimped portion 35 is formed to have a thinner wall thickness at a rear end side than the tool engagement portion 31. The compressed deformation portion 36 is formed to have a thinner wall thickness between the tool engagement portion 31 and the seat portion 33. Annular ring members 66, 67 are interposed between the axial hole 38 of the metal shell 30 and the outer circumferential surface of the large diameter portion 14 of the insulator 10 from the tool engagement portion 31 to the crimped portion 35 in the direction along the axis CA, and talc 69 powder is filled between the ring members 66, 67. As will be described later, the metal shell 30 is assembled to the insulator 10 by being crimped at the crimped portion 35.

[0020] The shelf portion 34 is formed on the inner peripheral surface of the male thread portion 32 so as to protrude radially inward. The shelf portion 34 has an inner diameter that decreases toward the tip side along the direction of the axis CA. An annular packing 68 is provided between the locking portion 15 and the shelf portion 34. The metal shell 30 holds the insulator 10 with the locking portion 15 locked by the shelf portion 34 via the packing 68. The packing 68 is a member that maintains airtightness between the metal shell 30 and the insulator 10, and prevents the outflow of combustion gas. The packing 68 in this embodiment is formed of a plate packing.

[0021] In this embodiment, the inner diameter of the metal shell 30 on the tip side of the shelf portion 34 is formed to be approximately constant. In other words, the diameter of the axial hole 38 on the tip side of the shelf portion 34 of the metal shell 30 is formed to be approximately constant. The inner diameter of the metal shell 30 at the tip 37 is preferably 7.5 mm or less, more preferably 7.0 mm or less, even more preferably 6.5 mm or less, and even more preferably 6.2 mm or less. From the viewpoint of ensuring the thickness of the insulator 10 and suppressing deterioration of the voltage resistance, the inner diameter of the metal shell 30 at the tip 37 is preferably 5.0 mm or more, more preferably 5.2 mm or more, and even more preferably 5.5 mm or more.

[0022] The inner diameter of the metal shell 30 on the tip side of the shelf portion 34 does not have to be approximately constant. For example, the metal shell 30 may have a region where the inner diameter is enlarged or reduced in a part on the tip side of the shelf portion 34. In such a configuration, the inner diameter of the metal shell 30 in at least a part on the tip side of the shelf portion 34 is preferably 7.5 mm or less, more preferably 7.0 mm or less, even more preferably 6.5 mm or less, and even more preferably 6.2 mm or less. Moreover, the inner diameter of the metal shell 30 in at least a part on the tip side of the shelf portion 34 is preferably 5.0 mm or more, more preferably 5.2 mm or more, and even more preferably 5.5 mm or more, from the viewpoint of ensuring the thickness of the insulator 10 and suppressing deterioration of the voltage resistance.

[0023] The ground electrode 40 is formed of a bent rod-shaped metal member. One end of the ground electrode 40 is joined to the tip 37 of the metal shell 30, and the other end of the ground electrode 40 is bent so as to face the tip of the center electrode 20. In the following description, the part where one end of the ground electrode 40 and the tip 37 of the metal shell 30 are joined is also called a "joint 80". When the ground electrode 40 and the metal shell 30 are joined by welding, the joint 80 corresponds to the welded surface. The ground electrode 40 is formed of a nickel alloy containing nickel as a main component, similar to the center electrode 20. In this embodiment, an electrode tip 42 is provided at a part of the ground electrode 40 facing the tip of the center electrode 20. A gap G for spark discharge is formed between the electrode tip 42 and the tip of the center electrode 20. The gap G is also called a discharge gap or a spark gap.

[0024] The terminal fitting 50 is provided at an end portion on the rear end side of the spark plug 100. The front end side of the terminal fitting 50 is received in the through hole 11 of the insulator 10, and the rear end side of the terminal fitting 50 is exposed from the through hole 11. A high voltage cable (not shown) is connected to the terminal fitting 50, and high voltage is applied thereto. This application causes a spark discharge in the gap G. The spark generated in the gap G ignites the air-fuel mixture in the combustion chamber 95.

[0025] FIG. 2 is a view taken along the arrow A in FIG. 1. FIG. 2 shows a schematic view seen from the side where the ground electrode 40 is arranged in the circumferential direction. The spark plug 100 of this embodiment has a protruding portion 70. The protruding portion 70 is formed protruding from a joint portion 80 between the ground electrode 40 and the metal shell 30. The protruding portion 70 protrudes from the joint portion 80 toward the tip side in the circumferential direction of the spark plug 100 and along the axis CA. The protruding portion 70 has a substantially plate-like external shape. The protruding portion 70 is formed so as to be farther away from the metal shell 30 as it moves away from the ground electrode 40 in the circumferential direction. In this embodiment, the two protruding portions 70 protrude in directions that move away from each other in the circumferential direction.

[0026] The length of the protrusion 70 in the circumferential direction is 0.3 mm or more and 1.3 mm or less, more preferably 0.4 mm or more and 1.3 mm or less. The length of the protrusion 70 in the direction of the axis CA is 0.3 mm or more and 1.3 mm or less, more preferably 0.4 mm or more and 1.3 mm or less. By having the lengths in the circumferential direction and the direction of the axis CA of 0.3 mm or more, respectively, the electric field intensity can be effectively increased. In addition, by having the lengths in the circumferential direction and the direction of the axis CA of 1.3 mm or less, respectively, the deformation and breakage of the protrusion 70 can be suppressed, and therefore the occurrence of defects due to the protrusion 70 can be suppressed. Here, the length of the protrusion 70 can be measured by projecting it using a projector. In addition, since the dimension of the circumferential length of the protrusion 70 is also relatively small, it can be approximated to the value measured by projecting it using a projector. In an embodiment having two protrusions 70 as in this embodiment, it is preferable that the dimension of at least one of the protrusions 70 is within the above numerical range. In FIG. 2, the circumferential length (dimension) of each protrusion 70 is designated as X, and the length (dimension) of each protrusion 70 in the direction of the axis CA is designated as Y.

[0027] The method for forming the protrusion 70 is not particularly limited, but may be as follows. For example, when the metal shell 30 and the ground electrode 40 are joined by welding, the shape and dimensions of a jig used during welding may be adjusted to control the generation of weld burrs of a desired dimension to form the protrusion 70. Also, for example, a plate member or the like of a desired dimension may be welded to the joint 80 between the metal shell 30 and the ground electrode 40, and the protrusion 70 may be formed by cutting the plate member or the like after welding to the desired dimension.

[0028] The spark plug 100 of the present embodiment described above has the protruding portion 70 protruding from the joint 80 between the metal shell 30 and the ground electrode 40 in the circumferential direction toward the tip side, and the protruding portion 70 is formed so that the further it is from the metal shell 30 in the circumferential direction, the further it is from the ground electrode 40. This makes it possible to locally increase the electric field strength at the protruding portion 70. As a result, it is possible to control the occurrence position of the irregular discharge so that the irregular discharge occurs at the protruding portion 70 where the electric field strength is high, so that it is possible to suppress the occurrence of the irregular discharge at a random position and to suppress the deterioration of the combustion stability. In addition, since the length of the protruding portion 70 in the circumferential direction is 0.3 mm or more and 1.3 mm or less, and the length in the direction of the axis CA is 0.3 mm or more and 1.3 mm or less, it is possible to effectively increase the electric field strength at the protruding portion 70 and to suppress the deformation of the protruding portion 70.

[0029] Furthermore, according to the spark plug 100 of the present embodiment, irregular discharges occurring in the spark plug 100 can be converged to the protruding portion 70, so that it is possible to suppress the occurrence of irregular discharges in regions other than the protruding portion 70. More specifically, it is possible to suppress the occurrence of irregular discharges, so-called deep sparks, such as discharges along the outer surface of the insulator 10 to the rear end portion of the metallic shell 30. As a result, it is possible to further suppress the deterioration of combustion stability.

[0030] Unlike the present application, in a configuration in which burrs or the like are formed along the side surface of the ground electrode around the joint between the ground electrode and the metal shell, or in a configuration in which burrs or the like are formed along the tip surface (electric surface portion) of the metal shell, the effect of locally increasing the electric field strength is low, and irregular discharge cannot be converged to that position. However, according to the spark plug 100 of the present embodiment, the protrusion 70 is formed so that the further it is from the metal shell 30 in the circumferential direction, the further it is from the ground electrode 40. Therefore, the electric field strength can be locally increased at the protrusion 70 formed relatively sharply, and irregular discharge can be converged to the protrusion 70. As a result, combustion stability can be improved.

[0031] In addition, unlike the present application, in a configuration in which an auxiliary gap is provided at an arbitrary position to increase the electric field strength in the electric surface portion, the cost required for processing the auxiliary gap increases, resulting in a problem of loss of marketability. However, according to the spark plug 100 of this embodiment, the electric field strength can be increased by the protruding portion 70 that protrudes from the joint portion 80 in the circumferential direction and toward the tip side, so that an increase in the cost required for processing can be suppressed.

[0032] Generally, due to the influence of engine size reduction caused by weight reduction of vehicles, there is a demand for a smaller diameter of the spark plug to be installed in the engine. However, as the diameter of the spark plug is made smaller, the inner diameter of the metal shell also tends to be made smaller, so that irregular discharge, so-called side sparks, is more likely to occur between the center electrode at the tip of the spark plug and the metal shell. In addition, when the inner diameter of the metal shell is made small, irregular discharge, so-called deep sparks, which discharges along the outer surface of the insulator to the rear end of the metal shell, is also more likely to occur.

[0033] However, according to the spark plug 100 of the present embodiment, even in such a reduced-diameter configuration, the electric field strength can be locally increased. Therefore, even if an irregular discharge occurs between the center electrode 20 and the metallic shell 30 at the front end of the spark plug 100, the position where the irregular discharge occurs can be converged to the protruding portion 70. In addition, since the irregular discharge occurring in the spark plug 100 can be converged to the protruding portion 70, the occurrence of an irregular discharge in an area other than the protruding portion 70, such as an irregular discharge that is discharged along the outer surface of the insulator 10 to the rear end of the metallic shell 30, can be suppressed. Therefore, the spark plug 100 of the present embodiment can be particularly suitably used in a reduced-diameter configuration, such as a configuration in which the inner diameter of the metallic shell 30 in at least a part on the front end side of the shelf portion 34 of the metallic shell 30 is 6.5 mm or less, or a configuration in which the inner diameter at the front end 37 of the metallic shell 30 is 6.5 mm or less.

[0034] The effects obtained by the spark plug 100 of this embodiment will be described below with reference to test results. Tests were conducted using a spark plug having two protrusions that protrude in directions away from each other in the circumferential direction from the joint between the ground electrode and the metallic shell.

[0035] (1) Test 1 <Sample> Spark plugs were used in which the circumferential length and axial length of the protrusions were different, as shown in Tables 1 and 2 below. The spark plugs used had a nominal diameter of M12, iridium precious metal tips at the ends of the ground electrode and center electrode, a distance from the end of the metal shell to the center electrode of 3 mm, and an inner diameter of the end of the metal shell of 7 mm. In Table 1, a spark plug was used in which the discharge gap at which regular discharge occurs was 0.8 mm, and in Table 2, a spark plug was used in which the discharge gap at which regular discharge occurs was 1.1 mm.

[0036] <Test conditions and evaluation methods> In Table 1, air at 1.4 MPa was used as the pressurized gas, assuming a turbo engine, and in Table 2, air at 1.1 MPa was used, assuming a NA (normal aspiration) engine, and discharges were performed for each. Of 100 irregular discharges in the lateral direction, known as lateral sparks, the percentage of irregular discharges that occurred in the circumferential direction, within the range of the ground electrode and the tips of the protrusions formed on both ends of the ground electrode, was calculated. A percentage of 80% or more was rated as A, and a percentage of less than 80% was rated as B.

[0037] In addition, an engine durability test was conducted. The engine used was an in-line 4-cylinder, 1.3L, naturally aspirated engine, and the test was conducted under the condition of 100 hours of WOT (wide open throttle) durability, and an evaluation was conducted on whether or not breakage or deformation of the protruding parts was observed. Those that found breakage or deformation of the protruding parts were rated as C.

[0038] <Evaluation Results> The evaluation results in Test 1 are shown in Tables 1 and 2 below.

[0039] [Table 1]

[0040] [Table 2]

[0041] As shown in Tables 1 and 2, in the samples in which the protrusions were formed with a circumferential length of 0.3 mm to 1.3 mm and an axial length of 0.3 mm to 1.3 mm, the rate of side sparks occurring in the range from the ground electrode and the tips of the protrusions formed at both ends of the ground electrode in the circumferential direction was 80% or more. In other words, it was shown that the rate of irregular discharge in the direction in which the protrusions were formed was high, and it was possible to suppress the occurrence of irregular discharges at random positions. In addition, in all samples in which at least one of the circumferential length and the axial length of the protrusions was 1.4 mm, breakage or deformation of the protrusions was observed in the engine durability test, which was undesirable. In the samples in which the circumferential length and the axial length of the protrusions were 1.3 mm or less, no breakage or deformation of the protrusions was observed in the engine durability test, which was good.

[0042] (2) Test 2 <Sample> Spark plugs were used in which the circumferential length and axial length of the protrusions were different, as shown in Tables 3 and 4 below. The spark plugs used had a nominal diameter of M12, iridium precious metal tips at the ends of the ground electrode and center electrode, a distance from the end of the metal shell to the center electrode of 3 mm, and an inner diameter of the end of the metal shell of 6.5 mm. In Table 3, a spark plug was used in which the discharge gap at which regular discharge occurs was 0.8 mm, and in Table 4, a spark plug was used in which the discharge gap at which regular discharge occurs was 1.1 mm.

[0043] <Test conditions and evaluation methods> In Table 3, air at 1.4 MPa was used as the pressurized gas assuming a turbo engine, and in Table 4, air at 1.1 MPa was used assuming a naturally aspirated engine, and discharges were performed for each. Of 100 irregular discharges in the lateral direction, known as lateral sparks, the percentage of irregular discharges occurring in the circumferential direction in the range from the ground electrode to the tips of the protrusions formed on both ends of the ground electrode was calculated. Those with this percentage of 80% or more were rated AA, those with a percentage of 70% or more but less than 80% were rated A, and those with a percentage of less than 70% were rated B. In addition, an engine durability test was performed in the same manner as in Test 1, and those in which breakage or deformation of the protrusions was observed were rated C.

[0044] <Evaluation Results> The evaluation results in Test 2 are shown in Tables 3 and 4 below.

[0045] [Table 3]

[0046] [Table 4]

[0047] As shown in Tables 3 and 4, in Test 2 using a spark plug with a relatively small inner diameter at the tip of the metal shell, the same results as in Test 1 were obtained. More specifically, in the sample in which a protruding portion having a length of 0.3 mm to 1.3 mm in the circumferential direction and a length of 0.3 mm to 1.3 mm in the axial direction was formed, the rate of side sparks occurring in the range from the ground electrode to the tip of the protruding portion formed at both ends of the ground electrode in the circumferential direction was 70% or more, and good results were obtained. In addition, in the sample in which a protruding portion having a length of 0.4 mm to 1.3 mm in the circumferential direction and a length of 0.4 mm to 1.3 mm in the axial direction was formed, the rate of side sparks occurring in the range from the ground electrode to the tip of the protruding portion formed at both ends of the ground electrode in the circumferential direction was 80% or more, and even better results were obtained. In other words, it was shown that the rate of irregular discharge in the direction in which the protruding portion was formed was high, and it was possible to suppress the occurrence of irregular discharge at random positions. In addition, in all samples in which at least one of the circumferential length and axial length of the protrusion was 1.4 mm, breakage or deformation of the protrusion was observed in the engine durability test, which was undesirable. In samples in which the circumferential length and axial length of the protrusion was 1.3 mm or less, no breakage or deformation of the protrusion was observed in the engine durability test, which was good.

[0048] B. Modifications The configuration of the spark plug 100 of the above embodiment is merely an example and can be modified in various ways. For example, the protruding portion 70 of the above embodiment has a substantially constant thickness when viewed from the side where the ground electrode 40 is disposed in the circumferential direction, but the present disclosure is not limited to this. The thickness of the protruding portion 70 does not have to be substantially constant, and for example, the protruding portion 70 may be formed so that the thickness becomes thinner or thicker from the base end side to the tip end side.

[0049] 3 to 6 are explanatory diagrams showing the configuration of the main part of the spark plug 100a, 100b, 100c, or 100d in the modified example. As in FIG. 2, FIG. 3 to FIG. 6 are schematic diagrams showing the front end of the spark plug 100a, 100b, 100c, or 100d as viewed from the side where the ground electrode 40 is disposed in the circumferential direction. For example, as shown in FIG. 3, the protrusion 70a of the spark plug 100a may have any angle. Here, the angle of the protrusion 70a means the angle between the surface along the front end 37 of the metal shell 30 and the protrusion 70a, or the angle between the side surface of the ground electrode 40 and the protrusion 70a. Also, for example, as shown in FIG. 4 and FIG. 5, the protrusions 70b and 70c of the spark plugs 100b and 100c may have a shape having at least a bent portion. More specifically, as in the spark plug 100b shown in FIG. 4, the protruding portion 70b may be bent in a direction approaching the axis CA of the spark plug 100b, and as in the spark plug 100c shown in FIG. 5, the protruding portion 70c may be bent in a direction away from the axis CA of the spark plug 100c. Also, for example, as shown in FIG. 6, the number of the protruding portion 70d may be one. Also, such configurations may be appropriately combined. More specifically, for example, the protruding portion 70a may be formed at different angles, the protruding portions 70b and 70c may be bent in different directions, or one protruding portion 70d may be bent. With such a configuration, the electric field intensity can be increased in the protruding portions 70a, 70b, 70c, and 70d, so that irregular discharge can be generated in the protruding portions 70a, 70b, 70c, and 70d.

[0050] The present invention is not limited to the above-mentioned embodiment, and can be realized in various configurations without departing from the spirit of the present invention. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention column can be appropriately replaced or combined to solve some or all of the above-mentioned problems or to achieve some or all of the above-mentioned effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0051] 10...insulator, 11...through hole, 14...large diameter portion, 15...engagement portion, 16...small diameter portion, 17...step portion, 20...center electrode, 21...leg portion, 22...flange portion, 23...head portion, 30...metal shell, 31...tool engagement portion, 32...male thread portion, 33...seat portion, 34...shelf portion, 35...swaged portion, 36...compression deformation portion, 37...tip, 38...shaft hole, 40...ground electrode, 42...electrode tip, 50...terminal metal fitting, 61... Leading end sealing material, 62... resistor, 63... trailing end sealing material, 65... gasket, 66, 67... ring member, 68... packing, 69... talc, 70, 70a, 70b, 70c, 70d... protrusion, 80... joint, 90... engine head, 93... female thread, 95... combustion chamber, 100, 100a, 100b, 100c, 100d... spark plug, CA... axis, G... gap

Claims

1. A center electrode extending along an axis; a cylindrical insulator that holds the center electrode on an inner circumferential side; a cylindrical metal shell that holds the insulator on an inner periphery thereof; a ground electrode joined to a tip end of the metal shell, a protrusion protruding from a joint between the ground electrode and the metallic shell in a circumferential direction toward a tip end side, The protrusion is the further away from the ground electrode in the circumferential direction, the further away from the metallic shell; The length in the circumferential direction is 0.3 mm or more and 1.3 mm or less, The length in the direction of the axis is 0.3 mm or more and 1.3 mm or less. A spark plug comprising:

2. 2. The spark plug according to claim 1, The insulator has a locking portion whose outer diameter decreases toward a tip side along the axial direction, the metallic shell has a shelf portion whose inner diameter decreases toward a tip side along the axial direction, and the shelf portion holds the insulator in a state in which the locking portion is locked via a packing, the inner diameter of the metallic shell at least in a portion on a tip side beyond the shelf portion is 6.5 mm or less.

3. The spark plug according to claim 1 or 2, The inside diameter of the tip of the metallic shell is 6.5 mm or less.

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