Spark plug

Abstract

To provide a spark plug capable of controlling a form and a length of a discharge path.SOLUTION: A spark plug comprises: a center electrode insulated and held by a main metal fitting; and a grounding electrode connected to the main metal fitting. The other end part of the grounding electrode is opposite to an apical surface of the center electrode with an interval. In a cross section obtained by cutting the other end part in a flat surface vertical to a drawing direction of the grounding electrode, when dividing an outer form line into two of a first line and a second line by one end of the longest line segment of line segments connecting two points on an outer form line of the other end part and the other end, a distance between the outer form line in a direction vertical to the line segment and the line segment in at least one of the first line and the second line becomes gradually longer until a specific point on the outer form line as directing to the other end from the one end of the line segment gradually shorter from a boundary of the specific point. A cross point of a vertical line drawn from the specific point to the line segment and the line segment is existed at a position other than, a middle point of the line segment.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a spark plug.

Background Art

[0002] A spark plug including a ground electrode connected to a main metal fitting and a center electrode insulated and held by the main metal fitting ignites a combustible mixture (fluid) by spark discharge between the center electrode and the ground electrode and grows a flame kernel. The spark discharge continues until the energy of the coil of the ignition device is consumed. The discharge path connecting the center electrode and the ground electrode is extended downstream along the flow of the fluid. When the discharge path extends, the voltage between the electrodes increases, and a short circuit or re-discharge of the discharge path occurs. Discharge behaviors such as the extension of the discharge path, the short circuit of the discharge path, and re-discharge affect combustion. Patent Document 1 and Patent Document 2 disclose prior arts in which the flow is changed by a ground electrode and the discharge path is changed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, there is room for improvement in the prior art.

[0005] The present invention has been made to meet this requirement, and an object thereof is to provide a spark plug capable of controlling the shape and length of a discharge path extending downstream.

Means for Solving the Problems

[0006] To achieve this object, the spark plug of the present invention includes a cylindrical main fitting extending along the axis from the tip side to the rear end side, a center electrode insulated and held by the main fitting, and a ground electrode extending from one end connected to the main fitting toward the other end. The other end faces the tip surface of the center electrode with a gap therebetween. In a cross-section obtained by cutting the other end in a plane perpendicular to the extending direction of the ground electrode, when the outer contour line of the other end is divided into two lines, a first line and a second line, by the two ends of the longest line segment connecting two points on the outer contour line, at least one of the first line and the second line has a distance between the outer contour line and the line segment in a direction perpendicular to the line segment that gradually increases from one end of the line segment to a specific point on the outer contour line and then gradually decreases with the specific point as a boundary. The intersection of the perpendicular line dropped from the specific point to the line segment and the line segment exists at a point other than the midpoint of the line segment.

Advantages of the Invention

[0007] According to the first aspect, the fluid flowing toward one end on the outer contour line of the other end of the ground electrode decreases in velocity and increases in pressure as it approaches the one end. The flow is accelerated by the portion between the one end and the specific point on the outer contour line, and the pressure gradually decreases. The flow after passing the specific point decelerates as it approaches the other end on the outer contour line, and the pressure gradually increases. Since the other end of the ground electrode can control the velocity and pressure distribution of the flow around the other end, the shape and length of the discharge path extending downstream can be controlled.

[0008] According to the second aspect, in the first aspect, the longest line segment connecting two points on the outer contour line of the other end intersects a plane perpendicular to the axis. This can increase the influence on the flow around the other end, so that the discharge path can be controlled more precisely.

[0009] According to the third aspect, in the first or second aspect, the distance between a specific point on the outer contour line of the other end portion and the tip surface of the center electrode is shorter than the distance between a point on the outer contour line other than the specific point and the tip surface. Therefore, it is easier for the discharge point (the starting point of the discharge path) of the ground electrode to occur around the specific point, and the discharge voltage at which a spark occurs between the tip surface of the center electrode and the other end portion of the ground electrode can be prevented from increasing. Since the discharge path starting from around the specific point of the first line can be controlled by the plane including the first line, the discharge path can be easily controlled.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0011] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a side sectional view of a spark plug 10 in the first embodiment with respect to the axis O as a boundary. In FIG. 1, the lower side of the paper surface is the tip side of the spark plug 10, and the upper side of the paper surface is the rear end side of the spark plug 10 (the same applies to FIGS. 2 to 5). As shown in FIG. 1, the spark plug 10 includes a center electrode 13 and a ground electrode 18. The center electrode 13 is disposed in the insulator 11, and the ground electrode 18 is connected to a main fitting 16 disposed in the insulator 11.

[0012] The insulator 11 is a substantially cylindrical member having a shaft hole 12 along the axis O. The insulator 11 is formed of ceramics such as alumina that is excellent in mechanical properties and insulation properties at high temperatures. The center electrode 13 is disposed in the shaft hole 12. The center electrode 13 is a rod-shaped metallic member having conductivity. The center electrode 13, for example, has a bottomed cylindrical base material mainly composed of Ni covering a core material mainly composed of copper. It is possible to omit the core material. The tip surface 14 of the center electrode 13 is disposed outside the shaft hole 12.

[0013] The center electrode 13 is electrically connected to the terminal fitting 15 within the shaft hole 12. The terminal fitting 15 is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is formed of a metallic material having conductivity (for example, low-carbon steel or the like). The terminal fitting 15 is fixed to the rear end of the insulator 11 with the tip side inserted into the shaft hole 12.

[0014] The main body fitting 16 is a substantially cylindrical member formed of a metallic material having conductivity (for example, low-carbon steel or the like). The main body fitting 16 surrounds the tip side of the insulator 11 and holds the insulator 11 inside. A male screw 17 is formed on the outer peripheral surface of the main body fitting 16. The male screw 17 is a portion that screws into a screw hole of an engine (not shown).

[0015] The ground electrode 18 is a rod-shaped member formed of a metallic material having conductivity (for example, an Ni-based alloy or the like). The ground electrode 18 extends from one end portion 19 connected to the main body fitting 16 toward the other end portion 20. The ground electrode 18 includes a bent portion 21 between the one end portion 19 and the other end portion 20. The ground electrode 18 extends along the axis O from the one end portion 19 toward the bent portion 21, and extends substantially perpendicular to the axis O from the bent portion 21 toward the other end portion 20. The other end portion 20 of the ground electrode 18 intersects the axis O. The other end portion 20 faces the tip surface 14 of the center electrode 13 with a gap therebetween.

[0016] FIG. 2 is a cross-sectional view including the axis O of the spark plug 10 taken along line II-II in FIG. 1. In FIG. 2, illustrations of the spark plug 10 other than the center electrode 13 and the ground electrode 18 are omitted. The cross-sectional view of FIG. 2 shows a portion including the outer contour line 22 of the other end portion 20 that appears when the other end portion 20 is cut by a plane perpendicular to the extending direction of the other end portion 20 of the ground electrode 18. The shape of the outer contour line 22 is a so-called airfoil shape, specifically, a flat-bottom airfoil.

[0017] The line segment 23 is the longest line segment among the line segments connecting two points on the outer contour line 22. The outer contour line 22 is divided into two, a first line 26 and a second line 27, by one end 24 and the other end 25 of the line segment 23. The first line 26 is a convex curve toward the tip surface 14 of the center electrode 13. The second line 27 is a curve located on the tip side of the line segment 23, and a curve having one end 24 as an end point and a straight line having the other end 25 as an end point are connected. The shape of the outer contour line 22 can ensure the distance in the direction perpendicular to the line segment 23 between the first line 26 and the second line 27, so that the mechanical strength of the other end portion 20 can be increased.

[0018] The curvature of the outer contour line 22 at the other end 25 is larger than the curvature of the outer contour line 22 at one end 24. From the viewpoint of ensuring the heat resistance of the ground electrode 18, it is preferable that the other end 25 has a thickness of 0.4 mm or more in the direction perpendicular to the line segment 23.

[0019] The distance between the center line 28 and the first line 26 in the direction perpendicular to the line segment 23 is equal to the distance between the center line 28 and the second line 27 in the direction perpendicular to the line segment 23. The center line 28 is a convex curve located between the line segment 23 and the first line 26. Both ends of the center line 28 coincide with one end 24 and the other end 25 of the line segment 23.

[0020] The first line 26 is formed such that the distance between the line segment 23 and the first line 26 in the direction perpendicular to the line segment 23 gradually increases from one end 24 to a specific point 29 on the first line 26 as it goes from one end 24 to the other end 25 of the line segment 23, and gradually decreases after passing the specific point 29. The intersection point 31 of the perpendicular line 30 dropped from the specific point 29 to the line segment 23 and the line segment 23 exists at a position other than the midpoint 32 of the line segment 23.

[0021] In this embodiment, the line segment 23 intersects the plane 33 perpendicular to the axis O. That is, the angle θ formed between the plane 33 and the line segment 23 satisfies θ≠0°. Also, the distance between the specific point 29 on the first line 26 and the tip surface 14 of the center electrode 13 is shorter than the distances between the tip surface 14 and the points on the outer contour line 22 other than the specific point 29.

[0022] When the spark plug 10 is disposed in an engine (not shown), the tip surface 14 of the center electrode 13 and the ground electrode 18 are exposed in the combustion chamber of the engine. The spark plug 10 is disposed in the engine with one end 24 of the ground electrode 18 facing the upstream side of the flow in the combustion chamber and the other end 25 of the ground electrode 18 facing the downstream side (exhaust valve side).

[0023] When the secondary voltage of the ignition coil of an ignition device (not shown) rises and the insulation between the center electrode 13 and the ground electrode 18 breaks down, first, a spark (hereinafter referred to as a "capacitance spark") is generated between the center electrode 13 and the ground electrode 18 by the electrical energy stored in the secondary circuit. Next, a spark with a long duration (hereinafter referred to as an "induced spark") is generated by the electromagnetic energy of the ignition coil. The generation of the induced spark continues until the energy of the ignition coil is consumed. The discharge path connecting the center electrode 13 and the ground electrode 18 extends along the flow of the fluid (flammable mixture). When the discharge path extends, the voltage between the center electrode 13 and the ground electrode 18 rises, and a phenomenon called short-circuiting or re-discharge of the discharge path occurs.

[0024] The flame kernel generated by the spark discharge grows from between the tip surface 14 of the center electrode 13 and the other end portion 20 of the ground electrode 18, or grows from the downstream between the electrodes due to the extension of the discharge path. By controlling the flow around the other end portion 20 of the ground electrode 18 and controlling the shape and length of the discharge path that extends along the flow, the position where the flame kernel grows downstream between the electrodes can be controlled, leading to an improvement in ignitability.

[0025] Inside the engine (not shown), the flow towards one end 24 on the outer contour line 22 of the other end 20 of the ground electrode 18 decreases in velocity and increases in pressure as it approaches the one end 24. The fluid whose pressure has increased at the one end 24 is accelerated from the one end 24 towards a specific point 29 on the first line 26 by that pressure, and the pressure gradually decreases. In this portion, a negative pressure gradient occurs along the first line 26. The negative pressure gradient helps the flow.

[0026] The fluid that has passed the specific point 29 on the first line 26 decelerates and the pressure gradually increases as it moves towards the other end 25. In this portion, a positive pressure gradient occurs along the first line 26. The positive pressure gradient hinders the flow. In this way, a pressure gradient is generated around the other end 20, and since the velocity and pressure distribution of the flow around the other end 20 can be controlled, the shape and length of the discharge path stretched along the flow can be controlled. Therefore, the position where the flame kernel grows downstream of the other end 20 can be controlled.

[0027] A spark discharge is likely to occur between the portion of the ground electrode 18 close to the center electrode 13 or the protruding portion and the center electrode 13. Since the distance between the specific point 29 on the first line 26 of the other end 20 and the tip surface 14 of the center electrode 13 is shorter than the distance between the tip surface 14 and a point on the outer contour line 22 other than the specific point 29, the discharge point (the starting point of the discharge path) of the ground electrode 18 is likely to occur around the specific point 29. Therefore, the discharge voltage at which a capacitive spark occurs between the tip surface 14 of the center electrode 13 and the other end 20 of the ground electrode 18 can be prevented from becoming high. Furthermore, since the discharge path starting around the specific point 29 of the other end 20 can be controlled by the convex surface including the first line 26 of the other end 20, the discharge path can be easily controlled.

[0028] Since the center line 28 is located between the line segment 23 and the first line 26 on the outer contour line 22, it becomes easier to change the direction of the flow at the other end 25 compared to the case where the center line 28 coincides with the line segment 23 (when the center line is a straight line) like a symmetric wing. Therefore, the discharge path can be controlled at a higher level.

[0029] Since the angle θ formed between the line segment 23 connecting two points on the outer shape line 22 and the plane 33 is θ ≠ 0°, it becomes easier to change the flow direction at the other end 25 compared to the case where the line segment 23 and the plane 33 coincide. Therefore, the discharge path can be controlled at a higher level. Since the flow at the other end 25 can be significantly changed by the angle θ, when the discharge path is flowed downstream along the tumble flow in the combustion chamber, the discharge path can be extended toward the center of the combustion chamber. Since a flame kernel can be grown at a position close to the center of the combustion chamber, the ignitability can be improved.

[0030] As the angle θ formed between the line segment 23 and the plane 33 increases, the flow is more likely to separate, and the flow velocity at the other end 25 tends to decrease. When controlling the discharge path to extend significantly downstream, although it also depends on the degree of curvature of the center line 28, for example, θ ≤ 20° is preferable.

[0031] The second embodiment will be described with reference to FIG. 3. In the second embodiment, the shape of the outer shape line 42 of the other end portion 41 of the ground electrode 40 facing the center electrode 13 will be described in the case of a so-called symmetric wing. In the second embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description will be omitted.

[0032] FIG. 3 is a cross-sectional view including the axis O of the spark plug in the third embodiment. In FIG. 3, illustrations of parts of the spark plug other than the center electrode 13 and the ground electrode 40 are omitted. The ground electrode 40 is connected to the main body fitting 16 instead of the ground electrode 18 of the spark plug 10 in the first embodiment. The cross-sectional view of FIG. 3 shows a portion including the outer shape line 42 of the other end portion 41 that appears when the other end portion 41 is cut by a plane perpendicular to the extending direction of the other end portion 41 of the ground electrode 40.

[0033] The line segment 43 is the longest line segment among the line segments connecting two points on the outer contour line 42. The outer contour line 42 is divided into two lines, a first line 46 and a second line 47, by one end 44 and the other end 45 of the line segment 43. The first line 46 is a convex curve that faces the tip surface 14 of the center electrode 13. The second line 47 is a convex curve that bulges toward the tip side. The outer contour line 42 is symmetric with respect to the line segment 43. The curvature of the outer contour line 42 at the other end 45 is larger than the curvature of the outer contour line 42 at the one end 44.

[0034] The first line 46 is formed such that the distance between the line segment 43 and the first line 46 in the direction perpendicular to the line segment 43 gradually increases from the one end 44 to a specific point 48 on the first line 46 as it goes from the one end 44 to the other end 45 of the line segment 43, and gradually decreases with the specific point 48 as the boundary. The intersection point 50 of the perpendicular line 49 dropped from the specific point 48 to the line segment 43 and the line segment 43 exists at a position other than the midpoint 51 of the line segment 43. In the present embodiment, the line segment 43 intersects the plane 33 perpendicular to the axis O. That is, the angle θ formed by the plane 33 and the line segment 43 is θ≠0°.

[0035] The spark plug in the second embodiment is arranged in the engine with one end 44 of the ground electrode 40 facing the upstream side of the flow in the combustion chamber and the other end 45 of the ground electrode 40 facing the downstream side. The spark plug in the second embodiment can achieve the same operational effects as the first embodiment, except for the function of the center line 28 located between the first line 26 and the second line 27 in the first embodiment.

[0036] The third embodiment will be described with reference to FIG. 4. In the third embodiment, a case where the other end 65 of the outer contour line 62 of the other end portion 61 of the ground electrode 60 facing the center electrode 13 is warped will be described. In the third embodiment, the same parts as those described in the first embodiment are denoted by the same reference numerals, and the following description thereof is omitted.

[0037] FIG. 4 is a cross-sectional view including the axis O of the spark plug in the third embodiment. In FIG. 4, illustration of parts of the spark plug other than the center electrode 13 and the ground electrode 60 is omitted. The ground electrode 60 is connected to the main body fitting 16 instead of the ground electrode 18 of the spark plug 10 in the first embodiment. The cross-sectional view of FIG. 4 shows a part including the outer contour line 62 of the other end 61 when the other end 61 is cut by a plane perpendicular to the extending direction of the other end 61 of the ground electrode 60. The shape of the outer contour line 62 is a so-called airfoil shape.

[0038] The line segment 63 is the longest line segment among the line segments connecting two points on the outer contour line 62. The outer contour line 62 is divided into two, a first line 66 and a second line 67, by one end 64 and the other end 65 of the line segment 63. The first line 66 is a convex curve toward the tip surface 14 of the center electrode 13. The second line 67 is a curve whose concavity and convexity change with a point on the second line 67 as a boundary. The second line 67 intersects the line segment 63. The curvature of the outer contour line 62 at the other end 65 is larger than the curvature of the outer contour line 62 at the one end 64.

[0039] The distance between the center line 68 and the first line 66 in the direction perpendicular to the line segment 63 is equal to the distance between the center line 68 and the second line 67 in the direction perpendicular to the line segment 63. The center line 68 is a convex curve located between the line segment 63 and the first line 66. Both ends of the center line 68 coincide with one end 64 and the other end 65 of the line segment 63.

[0040] The first line 66 is formed such that the distance between the line segment 63 and the first line 66 in the direction perpendicular to the line segment 63 gradually increases from one end 64 of the line segment 63 to a specific point 69 on the first line 66 as it goes from one end 64 to the other end 65, and gradually decreases with the specific point 69 as a boundary. The intersection 71 of the perpendicular 70 dropped from the specific point 69 to the line segment 63 and the line segment 63 exists at a position other than the midpoint 72 of the line segment 63.

[0041] In the present embodiment, the line segment 63 intersects the plane 33 perpendicular to the axis O. That is, the angle θ formed between the plane 33 and the line segment 63 is such that θ≠0°. Also, the distance between the specific point 69 on the first line 66 and the tip surface 14 of the center electrode 13 is shorter than the distance between the tip surface 14 and a point on the outer contour line 62 other than the specific point 69.

[0042] The spark plug in the third embodiment is arranged in the engine with one end 64 of the ground electrode 60 facing the upstream side of the flow in the combustion chamber and the other end 65 of the ground electrode 60 facing the downstream side. Thereby, the same operational effects as those in the first embodiment can be achieved.

[0043] The fourth embodiment will be described with reference to FIG. 5. In the third embodiment, the case where the first line 66 of the other end portion 61 of the ground electrode 60 faces the tip surface 14 of the center electrode 13 has been described. In the fourth embodiment, the case where the second line 67 faces the tip surface 14 of the center electrode 13 will be described. In the fourth embodiment, parts that are the same as those described in the first embodiment or the third embodiment are denoted by the same reference numerals, and the following description thereof is omitted.

[0044] FIG. 5 is a cross-sectional view including the axis O of the spark plug in the fourth embodiment. In FIG. 5, illustrations of parts of the spark plug other than the center electrode 13 and the ground electrode 80 are omitted. The ground electrode 80 is connected to the main body fitting 16 instead of the ground electrode 18 of the spark plug 10 in the first embodiment. The cross-sectional view of FIG. 5 shows a portion including the outer contour line 62 of the other end portion 81 when the other end portion 81 is cut by a plane perpendicular to the extending direction of the other end portion 81 of the ground electrode 80.

[0045] The first line 66 of the outer contour line 62 is a convex curve toward the tip side. The second line 67 of the outer contour line 62 faces the tip surface 14 of the center electrode 13. Spark discharge mainly occurs between the surface including the second line 67 of the other end portion 81 and the tip surface 14 of the center electrode 13.

[0046] In this embodiment, the line segment 63 intersects the plane 33 perpendicular to the axis O. That is, the angle θ formed by the plane 33 and the line segment 63 satisfies θ≠0°. The spark plug in the fourth embodiment is arranged in the engine with one end 64 of the ground electrode 80 facing the upstream side of the flow in the combustion chamber and the other end 65 of the ground electrode 80 facing the downstream side.

[0047] The flow toward one end 64 on the outer contour line 62 of the other end portion 81 of the ground electrode 80 decreases in velocity and increases in pressure as it approaches the one end 64. The fluid with increased pressure at the one end 64 is accelerated from the one end 64 toward a specific point 69 on the first line 66 by that pressure, and the pressure gradually decreases. The fluid passing the specific point 69 on the first line 66 decelerates and the pressure gradually increases as it approaches the other end 65. A pressure gradient is generated around the other end portion 81, and the velocity and pressure distribution of the flow around the other end portion 81 can be controlled. The discharge path connecting the surface including the second line 67 of the other end portion 81 and the tip surface 14 of the center electrode 13 changes in shape and length riding on the flow around the other end portion 81, so the shape and length of the discharge path can be controlled.

[0048] Depending on the position where the spark plug is arranged in the engine (not shown) and the position of the injector that supplies fuel into the engine, etc., it may be possible to improve the ignitability more by facing the surface including the first line 66 of the other end portion 81 toward the tip side than by facing the surface including the second line 67 toward the tip side. The spark plug in the fourth embodiment is advantageous in such a case.

[0049] As described above, the present invention has been described based on the embodiments. However, it can be easily inferred that the present invention is not limited to the above embodiments at all, and various improvements and modifications are possible without departing from the spirit of the present invention. For example, the shapes of the outer contour lines 22, 42, 62 of the other end portions 20, 41, 61, 81 of the ground electrodes 18, 40, 60, 80 are examples and are set as appropriate.

[0050] In the embodiments, the outer contour lines 22, 42, 62 that appear when the other ends 20, 41, 61, 81 of the ground electrodes 18, 40, 60, 80 are cut in a plane including the axis O have been described, but the present invention is not necessarily limited thereto. The other ends of the ground electrodes and the axis O do not necessarily intersect perpendicularly, and they may intersect obliquely, or the axis O and the other ends of the ground electrodes may not intersect. When the axis O and the other ends of the ground electrodes do not intersect, the outer contour line that appears when the other ends are cut in a plane perpendicular to the extending direction of the ground electrodes (particularly the other ends) is specified.

[0051] In the first, third, and fourth embodiments, the case where the line segments 23, 63 intersect the plane 33 perpendicular to the axis O has been described, but the present invention is not necessarily limited thereto. It is of course possible to provide the ground electrodes 18, 60, 80 such that the line segments 23, 63 are included in the plane 33. Since the center lines 28, 68 of the ground electrodes 18, 60, 80 are bent, even when the line segments 23, 63 are included in the plane 33, the flow around the other ends 20, 61, 81 can be controlled by the bent other ends 20, 61, 81.

[0052] In the first embodiment, the case where the surface including the first line 26 of the other end 20 is directed toward the tip surface 14 of the center electrode 13 has been described, but the present invention is not necessarily limited thereto. Depending on the position where the spark plug 10 is disposed in an engine (not shown) and the position of an injector that supplies fuel into the engine, etc., it is of course possible to direct the surface including the second line 27 of the other end 20 toward the tip surface 14 of the center electrode 13 so as to improve the ignitability.

[0053] Although not described in the embodiments, in order to improve the wear resistance, it is of course possible to provide a discharge member made of a noble metal such as Pt, Ir, Ru, Rh, W, or an alloy mainly composed of a noble metal or W at a portion where a discharge point can be formed among the other ends 20, 41, 61, 81 of the ground electrodes 18, 40, 60, 80. In order to reduce the influence on the shape of the outer contour lines 22, 42, 62 of the other ends 20, 41, 61, 81, the discharge member is preferably, for example, a film covering the base material or embedded in the base material. The entire other ends 20, 41, 61, 81 may be replaced with the discharge member.

Description of Symbols

[0054] 10 Spark plug 13 Central electrode 14 Tip surface 16 Body fitting 18, 40, 60, 80 Ground electrode 19 One end 20, 41, 61, 81 Other end 22, 42, 62 Outer contour line 23, 43, 63 Line segment 24, 44, 64 One end 25, 45, 65 Other end 26, 46, 66 First line 27, 47, 67 Second line 29, 48, 69 Specific point 30, 49, 70 Perpendicular line 31, 50, 71 Intersection point 32, 51, 72 Midpoint 33 Plane O Axis

Claims

1. A cylindrical main fitting extending along an axis from a front end side to a rear end side, A central electrode insulated and held by the main fitting, A ground electrode connected to the main fitting and extending from one end to the other end, and comprising: The other end is a spark plug facing the front end surface of the central electrode with a gap therebetween, In a cross section obtained by cutting the other end with a plane perpendicular to the extending direction of the ground electrode, When the outer contour line of the other end is divided into two lines, a first line and a second line, by one end and the other end of the longest line segment among the line segments connecting two points on the outer contour line, At least one of the first line and the second line is a convex curve in which the distance between the outer contour line and the line segment in a direction perpendicular to the line segment gradually increases from the one end to the other end of the line segment up to a specific point on the outer contour line and gradually decreases with the specific point as a boundary, The intersection point of the perpendicular line dropped from the specific point to the line segment and the line segment exists other than the midpoint of the line segment, and the angle θ formed by a plane perpendicular to the axis and the line segment is 0° ≤ θ ≤ 20°. A spark plug.

2. The spark plug according to claim 1, wherein the line segment intersects a plane perpendicular to the axis.

3. In a cross section obtained by cutting the other end with a plane perpendicular to the extending direction of the ground electrode, The spark plug according to claim 1 or 2, wherein the distance between the specific point and the front end surface is shorter than the distance between a point on the outer contour line other than the specific point and the front end surface.

Images