Spark plug

Abstract

To provide a spark plug that can promote flame propagation.SOLUTION: A spark plug comprises: a columnar center electrode that extends along an axis; a subject fitting that insulates and holds the center electrode; and a ground electrode that is electrically connected with the subject fitting. A spark gap is formed between a side face of the center electrode and an end face of the ground electrode, and the subject fitting includes a cylindrical leading end having the spark gap located therein. On a cross section that is perpendicular to the axis, and that includes the center of gravity of the end face, the distance between a center line of the ground electrode including the center of gravity and a contact on the tangent line of the side face of the center electrode is larger than 0 and shorter than the distance between the center of gravity and an edge of the end face of the ground electrode.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a spark plug in which a ground electrode is electrically connected to a main fitting.

Background Art

[0002] In a spark plug including a center electrode, a main fitting that insulates and holds the center electrode, and a ground electrode electrically connected to the main fitting, a prior art in which a spark gap formed between a side surface of the center electrode and an end surface of the ground electrode is provided inside a cylindrical tip portion of the main fitting is disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the prior art, a technique for promoting flame propagation is required.

[0005] The present invention has been made to meet this requirement, and an object thereof is to provide a spark plug capable of promoting flame propagation.

Means for Solving the Problems

[0006] A first aspect for achieving this object includes a cylindrical center electrode extending along an axis, a main fitting that insulates and holds the center electrode, and a ground electrode electrically connected to the main fitting. A spark gap is formed between a side surface of the center electrode and an end surface of the ground electrode, and the main fitting includes a cylindrical tip portion in which the spark gap is located inside. In a cross section perpendicular to the axis and including the center of gravity of the end surface, the distance between the center line of the ground electrode including the center of gravity and the contact point on the tangent line of the side surface of the center electrode is greater than 0 and shorter than the distance between the edge of the end surface of the ground electrode and the center of gravity.

[0007] In the second embodiment, a groove extending in the circumferential direction is provided on the inner circumference of the tip portion, as in the first embodiment.

[0008] A third embodiment is that, in the first or second embodiment, the shape of the cross-section perpendicular to the center line of the portion of the ground electrode located inside the tip is a polygon with rounded corners, a circle, or an ellipse. [Effects of the Invention]

[0009] According to the present invention, in a cross-section perpendicular to the axis of the spark plug and including the centroid of the end face of the ground electrode, the distance between the center line of the ground electrode including the centroid and the point of contact at the tangent to the side surface of the central electrode is greater than 0 and shorter than the distance between the edge of the end face of the ground electrode and the centroid. Compared to the case where the distance between the center line of the ground electrode and the point of contact is longer than the distance between the edge of the end face of the ground electrode and the centroid, the discharge points generated on the ground electrode and the central electrode can be dispersed, thereby reducing spark wear on the ground electrode and the central electrode. Furthermore, since a difference in heat loss is created in the circumferential direction of the central electrode, flame propagation can be promoted towards the direction of smaller heat loss. [Brief explanation of the drawing]

[0010] [Figure 1] This is a partial cross-sectional view of a spark plug in one embodiment. [Figure 2] This is a cross-sectional view of a spark plug, enlarged from the portion shown as II in Figure 1. [Figure 3] This is a cross-sectional view of the spark plug along line III-III in Figure 2. [Figure 4] This is a magnified cross-sectional view of the spark plug, specifically the portion indicated by IV in Figure 2. [Figure 5] (a) is a cross-sectional view of the ground electrode along the Va-Va line in Figure 3, (b) is a cross-sectional view of the ground electrode in a modified example, and (c) is a cross-sectional view of the ground electrode in another modified example. [Modes for carrying out the invention]

[0011] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Figure 1 is a partial cross-sectional view of a spark plug 10 in one embodiment. Figure 1 shows a cross-section including the axis O of the tip portion of the spark plug 10. In Figure 1, the lower side of the paper is referred to as the tip side of the spark plug 10, and the upper side of the paper is referred to as the rear end side of the spark plug 10 (the same applies to Figures 2 and 3). As shown in Figure 1, the spark plug 10 comprises an insulator 11, a center electrode 15, a main body fitting 20, and a ground electrode 25.

[0012] The insulator 11 is a substantially cylindrical member having an axial hole 12 extending along the axis O. The insulator 11 is made of a ceramic such as alumina, which has excellent mechanical properties and insulation properties at high temperatures. The insulator 11 includes a locking portion 13 and a tip portion 14 adjacent to the tip side of the locking portion 13. The outer diameter of the tip portion 14 is smaller than the outer diameter of the locking portion 13.

[0013] A central electrode 15 is positioned within the axial hole 12 along the axis O, at least from the locking portion 13 to the tip portion 14 of the insulator 11. The central electrode 15 includes a rod-shaped base material 16 mainly composed of Ni, a tip 17 positioned at the tip of the base material 16, mainly composed of one or more noble metals such as Pt, Rh, Ru, and Ir, and a molten portion 18 that joins the tip 17 and the base material 16. The tip 17 and the molten portion 18 can be omitted.

[0014] The tip of the central electrode 15 protrudes from the insulator 11 towards the tip. The central electrode 15 is electrically connected to the terminal fitting 19 within the axial hole 12. The terminal fitting 19 is a rod-shaped member to which an ignition device (not shown) is connected, and is made of a conductive metal material (e.g., low-carbon steel). The terminal fitting 19 is fixed to the rear end of the insulator 11.

[0015] The main fitting 20 is a substantially cylindrical member formed from a conductive metallic material (e.g., copper, copper alloy, low-carbon steel, etc.). The main fitting 20 is positioned on the outer circumference of the insulator 11. The main fitting 20 includes a cylindrical tip portion 21 located on the outer circumference side of at least the locking portion 13 and tip portion 14 of the insulator 11. The tip portion 21 has a male thread 22 on its outer circumference and a shelf portion 23 on its inner circumference. The male thread 22 fits into a female thread provided in the spark plug hole of an engine (not shown). The shelf portion 23 is located on the tip side of the locking portion 13 of the insulator 11 and locks the locking portion 13. The tip portion 21 extends further forward than the tip portion 14 of the insulator 11.

[0016] A hole 24 is provided in the tip portion 21, closer to the tip than the shelf portion 23, with a recessed portion of the inner circumference of the tip portion 21. In this embodiment, the hole 24 penetrates the tip portion 21 radially, has a circular cross-section, and a portion of the male thread 22 is missing due to the hole 24. A portion of the ground electrode 25 is positioned inside the hole 24. The ground electrode 25, with a portion positioned inside the hole 24, protrudes from the tip portion 21 toward the central electrode 15.

[0017] A cap 26 closes the tip end of the main fitting 20, and a space 29 is provided inside the cap 26. The material of the cap 26 is exemplified by a metallic material mainly composed of one or more elements such as Fe, Ni, and Cu. In this embodiment, the cap 26 is connected to the tip end 21 via a molten portion 28. A through hole 27 is provided in the cap 26 that connects the inside and outside of the space 29.

[0018] Figure 2 is a cross-sectional view of the spark plug 10 with the portion shown by II in Figure 1 enlarged. The ground electrode 25 has a columnar shape and includes one end portion 31 that faces the side surface 30 of the center electrode 15 to form a spark gap G, and the other end portion 32 disposed in the hole 24. The one end portion 31 is located inside the inner circumference 33 of the tip portion 21. The one end portion 31 is cylindrical and the cross-section of the one end portion 31 is circular. The other end portion 32 is inside the outer circumference 34 of the tip portion 21 and is located inside the bottom valley 22a of the male thread 22. The other end portion 32 is cylindrical and the cross-section of the other end portion 32 is circular and fits into the hole 24. The hole 24 passing through the tip portion 21 is blocked by the other end portion 32. The end surface 35 of the one end portion 31 of the ground electrode 25 faces the side surface 30 of the center electrode 15, and the end surface 36 of the other end portion 32 is located in the hole 24.

[0019] The ground electrode 25 includes a base material 37 mainly composed of, for example, Ni, a chip 38 mainly composed of one or more of noble metals such as Pt, Rh, Ru, and Ir, and a melting portion 39 that joins the chip 38 and the base material 37. One end portion 31 of the ground electrode 25 includes a part of the chip 38, and the other end portion 32 includes a part of the base material 37. The chip 38 and the melting portion 39 can be omitted.

[0020] The ground electrode 25 is connected to the tip portion 21 through a melting portion 40 in the hole 24. The melting portion 40 is formed by irradiating a laser beam onto the end surface 36 of the ground electrode 25 disposed in the hole 24. The melting portion 40 is formed by melting a part including a part of the end surface 36 of the ground electrode 25 and a part including a part of the tip portion 21. In this embodiment, the melting portion 40 is continuous over the entire circumference of the end surface 36 of the ground electrode 25.

[0021] Figure 3 is a cross-sectional view of the spark plug 10 taken along the line III-III in Figure 2. The line III-III in Figure 2 is a straight line perpendicular to the axis O (see Figure 1) of the spark plug 10, and the line III-III passes through the centroid 41 (see Figure 3) of the end surface 35 of the ground electrode 25. Figure 3 is a cross-section perpendicular to the axis O and including the centroid 41 of the end surface 35 of the ground electrode 25. The centroid 41 of the end surface 35 is the geometric center calculated by known means when the end surface 35 is regarded as a planar figure. <OOOO096> The center line C of the ground electrode 25, which passes through the centroid 41 of the end face 35, is a straight line passing through the center of the cross-section of the ground electrode 25 as shown in Figure 3. In this embodiment, the center line C coincides with the III-III line (see Figure 2). The tangent line 44 is a tangent line of the side surface 30 of the center electrode 15 that is perpendicular to the center line C. The distance D2 between the contact point 45 of the side surface 30 at the tangent line 44 and the center line C is greater than 0 and shorter than the distance D1 between the edge 42 of the end face 35 and the centroid 41. The distance D2 does not need to include 0, but for example, it should be 0.02 mm or more.

[0023] The space 29 created by the spark plug 10 (see Figure 1) attached to the engine (not shown) is a sub-chamber provided in the engine's combustion chamber. Due to the engine's valve operation, fuel flows from the combustion chamber into the space 29 (sub-chamber) through the through-hole 27. The spark plug 10 ignites the fuel by a discharge between the center electrode 15 and the ground electrode 25. Since the contact point 45 on the side surface 30 of the center electrode 15 is closer to one of the two edges 42, 43 of the end face 35 of the ground electrode 25, the discharge starting point (discharge point) of the ground electrode 25 is likely to occur near the edge 42 on the end face 35 where the electric field is concentrated. After ignition, the flame kernel that has grown through initial flame formation propagates, and combustion spreads into the space 29.

[0024] On the other hand, the flame kernel generated by the discharge suffers heat loss due to the central electrode 15 and the ground electrode 25, and thermal energy is removed. Near the end face 35 of the ground electrode 25, the heat loss by the central electrode 15 and the ground electrode 25 is less the greater the distance from the central electrode 15. Therefore, the heat loss is smaller near the edge 43 of the end face 35 of the ground electrode 25 compared to the edge 42 of the end face 35. Because a difference in heat loss occurs in the direction of the edges 42 and 43 of the end face 35, i.e., in the circumferential direction of the central electrode 15, flame propagation can be promoted towards the direction of less heat loss (counterclockwise in Figure 3) with the central electrode 15 as the center.

[0025] Because the combustion spreads differently in the circumferential direction of the central electrode 15 compared to the end face 35 of the ground electrode 25, a swirl flow (lateral swirling flow) centered on the central electrode 15 is generated in space 29 (see Figure 1). The expansion pressure generated by the combustion in space 29 causes a gas flow containing flames to be injected from the through hole 27 into the combustion chamber. The fuel in the combustion chamber is burned by this flame jet. Because the swirl flow generated in space 29 increases the combustion speed of the fuel in space 29, the ignition delay time of the fuel in the combustion chamber caused by the flames ejected from space 29 into the combustion chamber can be shortened.

[0026] Furthermore, due to the swirl flow generated within the space 29, the flame propagates to the upper part of the space 29 (near the insulator 11), where the distance between the end face 35 of the ground electrode 25 and the through hole 27 is longer than the distance between the end face 35 and the through hole 27. This makes it easier for the unburned gas in the upper part of the space 29 to burn. Compared to the case where unburned gas remains in the upper part of the space 29, the expansion pressure generated by combustion within the space 29 can be increased, thus increasing the speed of the flame ejected from the space 29 into the combustion chamber. Consequently, ignition performance can be improved.

[0027] Due to the discharge between the center electrode 15 and the ground electrode 25, the discharge point on the ground electrode 25 tends to occur near the edge 42 of the end face 35. If the discharge points on the ground electrode 25 and the center electrode 15 are concentrated in a narrow area, spark wear on the ground electrode 25 and the center electrode 15 will be significantly localized. However, since the distance D2 between the contact point 45 on the side surface 30 of the center electrode 15 and the center line C is shorter than the distance D1 between the edge 42 of the end face 35 of the ground electrode 25 and the center of gravity 41, the discharge points on the ground electrode 25 and the center electrode 15 can be dispersed compared to the case where the distance D2 is longer than the distance D1. This reduces localized spark wear on the ground electrode 25 and the center electrode 15.

[0028] Figure 4 is a cross-sectional view of the inner circumference 33 of the tip portion 21 of the spark plug 10, enlarged from the portion indicated by IV in Figure 2. A groove 46 extending in the circumferential direction is provided on the inner circumference 33 of the tip portion 21. Examples of grooves 46 include grinding marks after polishing the inner circumference 33 of the tip portion 21 with a grinding wheel, and cutting marks after cutting with a cutting tool. Cutting marks are created around the axis of rotation of the spindle when the workpiece is rotated with the spindle of a lathe or the like, and a cutting tool on the carriage is applied to the workpiece to cut the inner circumference 33 of the tip portion 21. Alternatively, grooves 46 can also be created by laser processing, in which an assist gas is blown onto the inner circumference 33 of the tip portion 21 while a laser beam is irradiated, and the molten material is removed.

[0029] The spark plug 10 has grooves 46 that extend circumferentially on the inner circumference 33 of its tip 21, which reduces the resistance of the swirl flow that circumferentially rotates inside the tip 21. This allows for a faster combustion speed of the fuel in the space 29 compared to a case without circumferential grooves 46.

[0030] The grooves 46 are provided at least one of the forward and rear ends of the ground electrode 25, relative to the position of the other end 32. The grooves 46 provided on the forward side of the other end 32 can reduce the resistance of the swirl flow between the spark gap G and the through hole 27. The grooves 46 provided on the rear end of the other end 32 can reduce the resistance of the swirl flow in the upper part of the space 29 (near the insulator 11).

[0031] A groove 46 extending in the circumferential direction may be provided on the inner surface of the cap 26. The groove 46 on the inner surface of the cap 26 can reduce the resistance of the swirl flow near the through hole 27. The groove 46 on the inner surface of the cap 26 can be made by polishing, cutting, laser processing, etc., similar to the groove 46 on the inner circumference 33 of the tip portion 21. It is also possible to manufacture the cap 26 with the groove 46 by powder metallurgy.

[0032] Figure 5(a) is a cross-sectional view of the ground electrode 25 along the Va-Va line in Figure 3. The cross-sectional shape of the portion of the ground electrode 25 located inside the tip 21, perpendicular to the center line C, is circular. Because the cross-section of the ground electrode 25 is circular, it is less likely to obstruct the swirl flowing around the center electrode 15, thereby reducing the resistance of the swirl flow caused by the ground electrode 25. If one end 31 (see Figure 2) of the ground electrode 25 is made thinner than the other end 32, the resistance of the swirl flowing near the end 31 of the ground electrode 25 can be further reduced compared to the case where the ground electrode 25 has roughly the same thickness from one end 31 to the other end 32.

[0033] Figure 5(b) is a cross-sectional view of the ground electrode 47 in a modified example. The ground electrode 47 is positioned at the tip 21 of the spark plug 10 in place of the ground electrode 25. The cross-sectional shape of the portion of the ground electrode 47 located inside the tip 21, perpendicular to the center line C, is elliptical. Because the cross-section of the ground electrode 47 is elliptical, the ground electrode 47 is less likely to obstruct the swirl flowing around the center electrode 15, thereby reducing the resistance of the swirl flow caused by the ground electrode 47.

[0034] The ground electrode 47 has an elliptical cross-section, with the major axis aligned along the circumferential direction of the tip portion 21 and the minor axis aligned along the axial direction of the tip portion 21. This makes it less likely for the ground electrode 47 to obstruct the swirl flowing around the central electrode 15 compared to when the major axis of the ellipse is aligned along the axial direction of the tip portion 21, thereby reducing the resistance of the swirl flow caused by the ground electrode 47.

[0035] Figure 5(c) is a cross-sectional view of the ground electrode 48 in another modified example. The ground electrode 48 is positioned at the tip 21 of the spark plug 10 in place of the ground electrode 25. The cross-sectional shape of the portion of the ground electrode 48 located inside the tip 21, perpendicular to the center line C, is a polygon with rounded corners. In this embodiment, the cross-section of the ground electrode 48 is a quadrilateral with four rounded corners, but it is not limited to this. The polygon of the cross-section can be appropriately selected from a triangle, pentagon, hexagon, etc. Because the cross-section of the ground electrode 48 is a polygon with rounded corners, it is less likely to obstruct the swirl flowing around the center electrode 15 compared to a ground electrode with a polygonal cross-section with sharp corners. Therefore, the resistance of the swirl flow by the ground electrode 48 can be reduced.

[0036] Although the present invention has been described above based on embodiments, it can be easily inferred that the present invention is not limited in any way to the above embodiments, and that various improvements and modifications are possible without departing from the spirit of the present invention.

[0037] In the embodiment, the case where the center line C of the ground electrode 25 coincides with a straight line (III-III line) perpendicular to the axis O of the spark plug 10 has been described, but it is not necessarily limited to this. It is certainly possible to position the ground electrode 25 such that the center line C of the ground electrode 25 and the axis O of the spark plug 10 intersect at an angle.

[0038] In the embodiment, the case where the grounding electrode 25 is columnar (straight) has been described, but it is not necessarily limited to this. It is certainly possible to prepare a bent grounding electrode 25, connect the end of the grounding electrode 25 to the main fitting 20 or cap 26, and position the end face 35 of the grounding electrode 25 opposite the side surface 30 of the central electrode 15.

[0039] In the embodiment described, the other end 32 of the grounding electrode 25 is placed in a hole 24 provided in the tip portion 21, but this is not necessarily the only option. It is certainly possible to join the other end 32 of the grounding electrode 25 to the tip portion 21 or the cap 26 without providing a hole 24.

[0040] In the embodiment, the case where the hole 24 is circular has been described, but it is not necessarily limited to this. Other shapes of the hole 24 include ellipses, semicircles, polygons such as triangles, squares, and hexagons, and polygons with rounded corners. The cross-sectional shape of the ground electrode 25 placed in the hole 24 is appropriately set to fit into the hole 24, depending on the shape of the hole 24.

[0041] In the embodiment described, a case was described in which the molten portion 40 is continuously provided around the center line of the hole 24, but it is not necessarily limited to this. It is certainly possible to provide the molten portion 40 intermittently around the center line of the hole 24, or to fix the ground electrode 25 into the hole 24 by press-fitting it instead of welding.

[0042] In the embodiment, a case was described in which a portion of the end face 36 of the ground electrode 25 remains unmelted, but this is not necessarily the only case. It is also possible that the entire end face 36 of the ground electrode 25 melts into the molten portion 40 and the end face 36 disappears.

[0043] In the embodiment described, the size of the hole 24 is constant in the radial direction of the tip portion 21, but this is not necessarily the only case. For example, it is certainly possible to provide a so-called tapered hole in the tip portion 21, where the size of the hole decreases from the outer side to the inside in the radial direction of the tip portion 21, and to place the ground electrode 25 in that hole, or to place the ground electrode 25 in a hole 24 with a counterbore on the outer circumference 34 of the tip portion 21.

[0044] In the embodiment described, a case was described in which a hole 24 is provided in the male thread portion 22 of the tip portion 21, but it is not necessarily limited to this. For example, it is certainly possible to provide a cylindrical portion without the male thread 22 in the tip portion and to provide a ground electrode 25 by drilling a hole in the cylindrical portion.

[0045] In the embodiment, the case in which the hole 24 penetrates the tip portion 21 has been described, but it is not necessarily limited to this. Even if the hole does not penetrate the tip portion 21, if a part of the inner circumference 33 of the tip portion 21 is recessed, the other end 32 of the ground electrode 25 can be placed inside the hole.

[0046] In the embodiment described, the case in which the other end 32 of the grounding electrode 25 is approximately the same thickness as the one end 31 of the grounding electrode 25 was described, but it is not necessarily limited to this. It is certainly possible to make the thickness of the one end 31 and the other end 32 of the grounding electrode 25 different.

[0047] In this embodiment, the case where the cross-sectional shape of one end 31 and the cross-sectional shape of the other end 32 of the ground electrode 25 are the same has been described, but it is not necessarily limited to this. It is certainly possible to make the cross-sectional shapes of one end 31 and the other end 32 different.

[0048] In the embodiment, a case in which a cap 26 is placed on the tip side of the tip portion 21 of the main fitting 20 has been described, but it is not necessarily limited to this. It is of course possible to omit the cap 26. This is because even without the cap 26, if one end 31 of the ground electrode 25 is surrounded by the tip portion 21 of the main fitting 20, it is possible to generate a flow circulating around the tip portion 21.

[0049] In the embodiment described, a case in which a hemispherical cap 26 is placed on the main fitting 20 has been explained, but the invention is not necessarily limited to this. The shape of the cap 26 can be set as appropriate. Other examples of cap shapes for the cap 26 include a bottomed cylindrical shape and a disc shape.

[0050] In this embodiment, the case in which a cap 26 is welded to the main fitting 20 has been described, but it is not necessarily limited to this. It is certainly possible to prepare a cylindrical member with a cap at its tip and connect it to the main fitting 20 to form a space 29. The cylindrical member is a cylindrical member with its tip closed by a cap, and has a female thread on its inner surface that connects to the male thread 22 of the main fitting 20. The outer surface of the cylindrical member has a male thread that connects to the female thread of the engine's spark plug hole. By connecting the female thread of the cylindrical member to the male thread 22 of the main fitting 20, a cap is positioned on the tip side of the main fitting 20. A through hole 27 is provided in this cap.

[0051] The means of connecting the cylindrical member to the main fitting 20 and arranging the cap on the tip side of the main fitting 20 is not limited to connecting the female thread on the inner circumference of the cylindrical member to the male thread 22 of the main fitting 20. It is naturally possible to connect the cylindrical member to the main fitting by other means. Other means include, for example, joining the cylindrical member and the main fitting by welding. Examples of materials for the cylindrical member include metallic materials such as Ni-based alloys and stainless steel, and ceramics such as silicon nitride. [Explanation of Symbols]

[0052] 10 Spark plugs 15 Center electrode 20 Main fittings 21 Tip 25,47,48 Ground electrode 30 Side view of the central electrode 33 Inner circumference of the tip 35 End face of ground electrode 41 Center of gravity of the end face 42 Edge of end face 44 Tangent line 45 contacts 46 Groove C center line D1,D2 distance G Spark Gap O axis

Claims

1. A cylindrical central electrode extending along the axis, The main metal fitting that insulates and holds the central electrode, The main body fitting comprises a grounding electrode electrically connected to the main body fitting, A spark gap is formed between the side surface of the central electrode and the end surface of the ground electrode. The main fitting is a spark plug that includes a cylindrical tip portion in which the spark gap is located on the inside, In a cross-section perpendicular to the aforementioned axis and including the centroid of the end face, A spark plug in which the distance between the center line of the ground electrode including the center of gravity and the point of contact of the tangent to the side surface of the center electrode that is perpendicular to the center line is greater than 0 and shorter than the distance between the edge of the end face and the center of gravity.

2. The spark plug according to claim 1, wherein a groove extending in the circumferential direction is provided on the inner circumference of the tip portion.

3. The spark plug according to claim 1 or 2, wherein the shape of the cross-section perpendicular to the center line of the portion of the ground electrode located inside the tip is a polygon with rounded corners, a circle, or an ellipse.

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