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] Known spark plugs for ignition used in internal combustion engines are those that generate spark discharge in a gap formed between the tip of the center electrode and the tip of the ground electrode when a voltage is applied between the center electrode and a ground electrode connected to the tip of a metallic shell (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-036492 Summary of the Invention [Problem to be solved by the invention]

[0004] In order to improve thermal efficiency, an increase in the compression ratio, a high boost rate, and lean burn are being promoted, which can cause discharges at locations other than the intended gap. When such irregular discharges occur, the frequency of regular discharges at the intended gap decreases, thereby deteriorating combustion stability. In particular, when irregular discharges occur at random locations, there is a problem that combustion stability is further deteriorated. Therefore, there has been a need for technology that can suppress the occurrence of irregular discharges at random locations. [Means for solving the problem]

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

[0006] (1) According to one aspect of the present disclosure, there is provided a spark plug. The spark plug includes a center electrode extending along an axis, a cylindrical insulator holding the center electrode on its inner periphery, a cylindrical metal shell holding the insulator on its inner periphery, and a ground electrode joined to the tip of the metal shell. The spark plug has a protruding portion that protrudes circumferentially and toward the tip from a joint between the ground electrode and the metal shell, the protruding portion being increasingly separated from the metal shell in the circumferential direction, the length of the protruding portion in the circumferential direction being 0.3 mm to 1.3 mm, and the length of the protruding portion in the axial direction being 0.3 mm to 1.3 mm. According to this aspect of the spark plug, the protruding portion that protrudes circumferentially and toward the tip from the joint between the ground electrode and the metal shell can locally increase electric field strength at the protruding portion. As a result, irregular discharges occurring in the spark plug can be concentrated at the protruding portion, thereby suppressing irregular discharges from occurring at random positions.

[0007] (2) In the spark plug described in (1) above, the insulator may have a locking portion whose outer diameter decreases toward the tip along the axial direction, the metallic shell may have a shelf portion whose inner diameter decreases toward the tip 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 at least a portion of the metallic shell further toward the tip than 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 of the metallic shell toward the tip side is small and irregular discharge is likely to occur between the center electrode and the metallic shell, irregular discharge can be suppressed from occurring at random positions.

[0008] (3) In the spark plug described in (1) or (2), the inner diameter of the front end of the metallic shell may be 6.5 mm or less. According to this spark plug, even in a configuration in which the inner diameter of the front end of the metallic shell is small and irregular discharge is likely to occur between the center electrode and the metallic shell, irregular discharge can be suppressed from occurring at random positions.

[0009] The present disclosure can be realized in various forms, for example, a method for manufacturing a spark plug, an engine head to which a spark plug is attached, and the like. [Brief explanation of the drawings]

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

[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, which is the axial center 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, described later, is disposed) will be referred to as the leading end side, and the upper side of FIG. 1 (the side where a terminal fitting 50, described later, is disposed) will be referred to as the trailing end side. For ease of explanation, an engine head 90 to which the spark plug 100 is attached is indicated by a dashed line in FIG. 1. The spark plug 100 is attached to the engine head 90 so that its leading end is exposed within 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 portion of the center electrode 20 at the front end and a portion of the metal terminal 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 metal shell 30 (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 at the rear end 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 generally constant. The locking portion 15 is formed so that its outer diameter decreases toward the tip end along the axis CA, located distally of the large diameter portion 14. The step portion 17 is configured so that the diameter of the through hole 11 decreases toward the tip end along the axis CA. In other words, the step portion 17 protrudes radially inward from 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 distal end of the step portion 17, and the diameter of the through hole 11 is smaller than that of the step portion 17. The through hole 11 in the small diameter portion 16 accommodates a portion of a leg portion 21 of the center electrode 20, which will be described later. In the direction along the axis CA, the front end of the insulator 10 is located rearward of the front end 37 of the metallic shell 30.

[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 portion 21, a flange portion 22, and a head portion 23. The leg portion 21 is formed to extend in a direction along the axis CA, and a portion of its tip end is exposed from the through-hole 11. A noble metal tip made of, for example, a platinum or iridium alloy may be joined to the tip end of the leg portion 21. The flange portion 22 is continuous with the rear end of the leg portion 21 and extends outward. The flange portion 22 abuts against the step portion 17 of the insulator 10 from its rear end, thereby positioning the center electrode 20 within the through-hole 11 of the insulator 10. The head portion 23 is continuous with the rear end of the flange portion 22 and extends in a direction along the axis CA. The rear end side of the center electrode 20 is electrically connected to the terminal fitting 50 within the through hole 11 of the insulator 10 via a front end seal material 61, a resistor 62, and a rear end seal material 63. The center electrode 20 of this embodiment is formed from a nickel alloy containing nickel as its main component. In this disclosure, the term "main component" refers to the component that is contained in the largest amount.

[0016] The resistor 62 is made of 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 noise generated when spark discharge occurs. The leading-end seal material 61 and the trailing-end seal material 63 each contain conductive glass powder as their material. In this embodiment, the leading-end seal material 61 and the trailing-end seal material 63 contain a powder mixture of copper powder and borosilicate calcium glass powder as their 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 formed, for example, from 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 engaging portion 31 engages with a tool (not shown) when attaching the spark plug 100 to the engine head 90. The male thread portion 32 has a thread formed on the outer peripheral surface at the tip end of the metallic shell 30, and is screwed into a 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 fitted 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 portion closer to the rear end than the tool engagement portion 31. The compressive 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, protruding radially inward. The inner diameter of the shelf portion 34 decreases toward the tip 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. In this embodiment, the packing 68 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 still more preferably 6.2 mm or less. Furthermore, from the viewpoint of ensuring the thickness of the insulator 10 and preventing 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 increases or decreases in a portion 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 portion 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. Furthermore, from the viewpoint of ensuring the thickness of the insulator 10 and suppressing deterioration in voltage resistance, the inner diameter of the metal shell 30 in at least a portion 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.

[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 metallic 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 portion where one end of the ground electrode 40 and the tip 37 of the metallic shell 30 are joined is also referred to as a "joint 80." If the ground electrode 40 and the metallic shell 30 are joined by welding, the joint 80 corresponds to the welded surface. Like the center electrode 20, the ground electrode 40 is formed of a nickel alloy containing nickel as a primary component. In this embodiment, an electrode tip 42 is provided on the ground electrode 40 at a portion 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 referred to as a discharge gap or a spark gap.

[0024] The terminal fitting 50 is provided at the rear end of the spark plug 100. The front end of the terminal fitting 50 is housed in the through hole 11 of the insulator 10, and the rear end of the terminal fitting 50 is exposed through the through hole 11. A high-voltage cable (not shown) is connected to the terminal fitting 50, and high voltage is applied to it. 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 is a schematic view as seen from the side where the ground electrode 40 is disposed in the circumferential direction. The spark plug 100 of this embodiment has a protruding portion 70. The protruding portion 70 is formed to protrude from a joint 80 between the ground electrode 40 and the metallic shell 30. The protruding portion 70 protrudes from the joint 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 generally plate-like external shape. The protruding portion 70 is formed so that the farther the protruding portion 70 is from the metallic shell 30 in the circumferential direction, the farther it is from the ground electrode 40. 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 each be 0.3 mm or more, the electric field strength can be effectively increased. Furthermore, by having the lengths in the circumferential direction and the direction of the axis CA each be 1.3 mm or less, deformation and breakage of the protrusion 70 can be suppressed, thereby suppressing defects caused by the protrusion 70. Here, the length of the protrusion 70 can be measured by projecting it using a projector. Note that the circumferential length of the protrusion 70 is also relatively small, so it can approximate 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 be within the above numerical range. 2, X denotes the length (dimension) of each protrusion 70 in the circumferential direction, and Y denotes the length (dimension) of each protrusion 70 in the direction of the axis CA.

[0027] The method for forming the protrusion 70 is not particularly limited, and it may be formed as follows. For example, when joining the metallic shell 30 and the ground electrode 40 by welding, the shape and dimensions of a jig used during welding may be adjusted to control the formation of weld burrs of a desired size, thereby forming the protrusion 70. Alternatively, for example, a plate-like member or the like of a desired size may be welded to the joint 80 between the metallic shell 30 and the ground electrode 40, and the protrusion 70 may be formed by grinding the welded plate-like member or the like to the desired size.

[0028] The spark plug 100 of this embodiment described above has a protruding portion 70 that protrudes circumferentially from the joint 80 between the metallic shell 30 and the ground electrode 40 toward the tip end. The protruding portion 70 is formed so that the greater the distance from the metallic shell 30 in the circumferential direction, the farther the protruding portion 70 is from the ground electrode 40. This allows the electric field strength to be locally increased at the protruding portion 70. As a result, the location at which an irregular discharge occurs can be controlled so that the irregular discharge occurs at the protruding portion 70 where the electric field strength is high. This prevents the irregular discharge from occurring at random locations, thereby preventing deterioration of combustion stability. Furthermore, 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. This effectively increases the electric field strength at the protruding portion 70 and also prevents deformation of the protruding portion 70.

[0029] Furthermore, according to the spark plug 100 of this embodiment, irregular discharges occurring in the spark plug 100 can be converged to the protruding portion 70, thereby suppressing the occurrence of irregular discharges in areas other than the protruding portion 70. More specifically, it is possible to suppress the occurrence of irregular discharges, known as deep sparks, such as discharges along the outer surface of the insulator 10 toward 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 or along the leading end surface (electrical surface portion) of the metal shell around the joint between the ground electrode and the metal shell, the effect of locally increasing the electric field strength is low, and irregular discharge cannot be converged to that position. However, in the spark plug 100 of this embodiment, the protrusion 70 is formed so that the farther it is from the metal shell 30 in the circumferential direction, the farther it is from the ground electrode 40. Therefore, the electric field strength can be locally increased at the relatively sharp protrusion 70, and irregular discharge can be converged to the protrusion 70. As a result, combustion stability can be improved.

[0031] Furthermore, unlike the present application, in a configuration in which the electric field strength in the electric surface portion is increased by providing an auxiliary gap at an arbitrary position, the cost required to process the auxiliary gap increases, which causes a problem of losing marketability. However, with the spark plug 100 of this embodiment, the electric field strength can be increased by the protrusion 70 that protrudes circumferentially from the joint portion 80 toward the tip side, so that the increase in the cost required for processing can be suppressed.

[0032] Generally, due to factors such as the impact of engine downsizing resulting from vehicle weight reduction, there is a demand for smaller diameter spark plugs to be installed in engines. However, as the diameter of a spark plug is made smaller, the inner diameter of the metallic shell tends to be made smaller, which makes it more likely that irregular discharge, known as side sparks, will occur between the center electrode at the front end of the spark plug and the metallic shell. Furthermore, when the inner diameter of the metallic shell is made smaller, irregular discharge, known as back sparks, which discharges along the outer surface of the insulator toward the rear end of the metallic shell, will also be more likely to occur.

[0033] However, according to the spark plug 100 of this embodiment, even in such a reduced-diameter configuration, it is possible to locally increase the electric field strength. Therefore, even if an irregular discharge occurs between the center electrode 20 and the metallic shell 30 at the front end portion of the spark plug 100, the location of the irregular discharge can be converged to the protruding portion 70. Furthermore, because the irregular discharge occurring in the spark plug 100 can be converged to the protruding portion 70, it is possible to suppress the occurrence of irregular discharge in areas other than the protruding portion 70, such as an irregular discharge that discharges along the outer surface of the insulator 10 toward the rear end portion of the metallic shell 30. Therefore, the spark plug 100 of this embodiment is particularly suitable for use in a reduced-diameter configuration, such as a configuration in which the inner diameter of the metallic shell 30 in at least a portion of the metallic shell 30 distal to the shelf portion 34 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 in which two protrusions were formed, protruding 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 of 3 mm from the end of the metal shell to the center electrode, 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 occurred was 0.8 mm, and in Table 2, a spark plug was used in which the discharge gap at which regular discharge occurred 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, while in Table 2, air at 1.1 MPa was used, assuming a NA (normal aspiration) engine. Of 100 irregular lateral discharges, known as lateral sparks, the percentage of irregular discharges occurring in the circumferential direction, up to the tips of the ground electrode and the protrusions formed on both ends of the ground electrode, was determined. A percentage of 80% or more was evaluated as A, and a percentage of less than 80% was evaluated as B.

[0037] An engine durability test was also conducted. The engine used was an in-line 4-cylinder, 1.3L, naturally aspirated engine. The test was conducted under the condition of 100 hours of WOT (wide open throttle) durability, and an evaluation was conducted to see if any breakage or deformation of the protrusions was observed. Those in which breakage or deformation of the protrusions was observed were rated 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, for samples with protrusions measuring 0.3 mm to 1.3 mm in circumferential length and 0.3 mm to 1.3 mm in axial length, the rate of side sparks occurring in the circumferential direction from the ground electrode to the tips of the protrusions formed at both ends of the ground electrode was 80% or more. This indicates that the proportion of irregular discharges in the direction in which the protrusions are formed is high, and irregular discharges occurring at random locations can be suppressed. Furthermore, for all samples in which at least one of the circumferential and axial lengths of the protrusions was 1.4 mm, breakage or deformation of the protrusions was observed during the engine durability test, which was undesirable. For samples in which the circumferential and axial lengths of the protrusions were 1.3 mm or less, no breakage or deformation of the protrusions was observed during the engine durability test, which was favorable.

[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 tips of the ground electrode and center electrode, a distance from the tip of the metal shell to the center electrode of 3 mm, and an inner diameter of the tip 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 occurred was 0.8 mm, and in Table 4, a spark plug was used in which the discharge gap at which regular discharge occurred 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, while in Table 4, air at 1.1 MPa was used, assuming a naturally aspirated engine. Of the 100 irregular lateral discharges, known as lateral sparks, the percentage of irregular discharges occurring in the circumferential direction, up to the tips of the protrusions formed on both ends of the ground electrode, was determined. 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. Engine durability tests were also conducted in the same manner as in Test 1, and those with breakage or deformation of the protrusions 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, similar results to those of Test 1 were obtained in Test 2, which used spark plugs with a relatively small inner diameter at the tip of the metallic shell. More specifically, in samples formed with protruding portions having 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 circumferential direction between the ground electrode and the tips of the protruding portions formed at both ends of the ground electrode was 70% or more, resulting in favorable results. Furthermore, in samples formed with protruding portions having a circumferential length of 0.4 mm to 1.3 mm and an axial length of 0.4 mm to 1.3 mm, the rate of side sparks occurring in the circumferential direction between the ground electrode and the tips of the protruding portions formed at both ends of the ground electrode was 80% or more, resulting in even better results. In other words, it was shown that the rate of irregular discharges in the direction in which the protruding portions were formed was high, and that irregular discharges occurring at random positions could be suppressed. 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. Variations 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 in the circumferential direction from the side where the ground electrode 40 is located, but the present disclosure is not limited to this. The thickness of the protruding portion 70 does not have to be substantially constant; for example, the protruding portion 70 may be formed so that its thickness becomes thinner or thicker from its base end toward its tip end.

[0049] FIGS. 3 to 6 are explanatory diagrams showing the configuration of essential parts of a spark plug 100a, 100b, 100c, or 100d according to a modification. Similar to FIG. 2, FIGS. 3 to 6 are schematic views of the front end portion of the spark plug 100a, 100b, 100c, or 100d, viewed in the circumferential direction from the side where the ground electrode 40 is located. 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 refers to 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. Furthermore, as shown in FIGS. 4 and 5, the protrusions 70b and 70c of the spark plugs 100b and 100c may have at least a partially bent shape. More specifically, as in a spark plug 100b shown in FIG. 4, the protruding portion 70b may be bent toward the axis CA of the spark plug 100b. Alternatively, as in a spark plug 100c shown in FIG. 5, the protruding portion 70c may be bent away from the axis CA of the spark plug 100c. Also, for example, as shown in FIG. 6, the number of protruding portions 70d may be one. Furthermore, such configurations may be combined as appropriate. More specifically, for example, two protruding portions 70a formed at different angles may be provided, two protruding portions 70b and 70c may be provided that are bent in different directions, or one protruding portion 70d may be provided that is bent. Even with such a configuration, the electric field strength can be increased in the protruding portions 70a, 70b, 70c, and 70d, thereby generating irregular discharges in the protruding portions 70a, 70b, 70c, and 70d.

[0050] The present invention is not limited to the above-described embodiments 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 section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described 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...engaging 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...ledge portion, 35...crimped portion, 36...compression deformation portion, 37...tip, 38...axial hole, 40...ground electrode, 42...electrode tip, 50...terminal metal fitting, 61... Front end sealing material, 62...resistor, 63...rear 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 portion, 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 its inner periphery; a cylindrical metallic shell that holds the insulator on its inner periphery; a ground electrode joined to a tip of the metallic 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 characterized by:

2. 2. The spark plug according to claim 1, the insulator has a locking portion whose outer diameter decreases toward a tip end along the axial direction, the metallic shell has a shelf portion whose inner diameter decreases toward a tip end 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 part on the tip side beyond the shelf portion is 6.5 mm or less.

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

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