Spark plug

A spark plug including a center electrode, an insulator, a metal shell, a first ground electrode, and a second ground electrode. The center electrode extends in an axial direction. The insulator has an axial hole extending in the axial direction. The center electrode is to be inserted into the axial hole. The metal shell is arranged at an outer periphery of the insulator. The first ground electrode has electrical continuity with the metal shell, and forms a first gap with a front end surface of the center electrode. The second ground electrode has electrical continuity with the metal shell, is sealed to metal shell, extends from the metal shell to a position facing a side surface of the center electrode, and forms an annular second gap between the side surface of the center electrode and an inner peripheral surface of the second ground electrode.

RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP14/63470 filed May 21, 2014, which claims the benefit of Japanese Patent Application No. 2013-109156, filed May 23, 2013.

FIELD OF THE INVENTION

The present invention relates to a spark plug.

BACKGROUND OF THE INVENTION

Conventionally, a spark plug is used in an internal combustion engine. The configuration of the spark plug generally includes a center electrode and a ground electrode. The center electrode and the ground electrode form the gap for causing a spark.

Improving the durability of the spark plug suppresses various malfunctions thereby reducing maintenance of the internal combustion engine. In this respect, the durability of the spark plug can be affected by various factors. For example, during the operation of the internal combustion engine, an increase in temperature of the electrode might cause electrode wear. The advance of the electrode wear might not allow the spark plug to provide the intended performance (for example, causes an ignition failure).

An advantage of the present invention is a new technique that improves the durability of the spark plug.

SUMMARY OF THE INVENTION

The present invention has been conceived to solve the above-mentioned problems, and can be realized as the following application examples.

Application Example 1

In accordance with a first aspect of the present invention, there is provided a spark plug having a center electrode, an insulator, a metal shell, a first ground electrode, and a second ground electrode. The center electrode extends in an axial direction. The insulator has an axial hole extending in the axial direction. The center electrode is to be inserted into the axial hole. The metal shell is arranged at an outer periphery of the insulator. The first ground electrode has electrical continuity with the metal shell. The first ground electrode forms a first gap with a front end surface of the center electrode. The second ground electrode has electrical continuity with the metal shell. The second ground electrode is sealed to metal shell. The second ground electrode extends from the metal shell to a position facing a side surface of the center electrode. The second ground electrode forms an annular second gap between the side surface of the center electrode and an inner peripheral surface of the second ground electrode. A proportion of a size of the first gap to a size of the second gap is equal to or more than 0.80 and equal to or less than 1.25.

With this configuration, both the first ground electrode and the second ground electrode are used for discharge. This allows improving the durability of the spark plug.

Application Example 2

In accordance with a second aspect of the present invention, there is provided a spark plug according to the application example 1, wherein the first ground electrode includes a first nickel portion that is a portion formed by nickel or a nickel alloy. The first nickel portion has a nickel content of 90 weight % or more. The second ground electrode includes a second nickel portion that is a portion formed by nickel or a nickel alloy. The second nickel portion has a nickel content of 90 weight % or more.

With this configuration, respective thermal conductivities of the first ground electrode and the second ground electrode are improved. This allows suppressing the wear of the first ground electrode and the second ground electrode due to high temperature.

Application Example 3

In accordance with a third aspect of the present invention, there is provide a spark plug according to the application example 1 or 2, wherein at least one of the first ground electrode and the second ground electrode includes: a surface layer that forms a surface thereof; and a core portion that is formed inside of the surface layer and has a larger thermal conductivity than a thermal conductivity of the surface layer.

With this configuration, the thermal conductivity is improved by the core portion. This allows suppressing the wear of the ground electrode due to high temperature.

Application Example 4

In accordance with a fourth aspect of the present invention, there is provided a spark plug according to the application example 3, wherein the first ground electrode is sealed to the second ground electrode.

With this configuration, the temperature of the first ground electrode is likely to increase compared with the case where the first ground electrode is sealed directly to the metal shell. However, the thermal conductivity is improved by the core portion. This allows suppressing the wear of the ground electrode due to high temperature.

Application Example 5

In accordance with a fifth aspect of the present invention, there is provided a spark plug according to any one of the application examples 1 to 4, wherein a shortest distance between a surface of the second ground electrode and a surface of the insulator is twice or more as large as a maximum value between the size of the first gap and the size of the second gap.

This configuration allows suppressing occurrence of discharge along the surface of the insulator even in the case where the first gap and the second gap are large due to the wear of the ground electrode. Accordingly, the durability of the spark plug can be improved.

Application Example 6

In accordance with a sixth aspect of the present invention, there is provided a spark plug according to any one of the application examples 1 to 5, wherein the first ground electrode includes a first noble metal portion that is formed by a noble metal or a noble metal alloy in a position forming the first gap. The second ground electrode includes a second noble metal portion that is formed by a noble metal or a noble metal alloy in a position forming the second gap. In the center electrode, at least a first portion and a second portion are formed by a noble metal or a noble metal alloy. The first portion forms the first gap with the first noble metal portion. The second portion forms the second gap with the second noble metal portion.

This configuration allows suppressing the wear of each of the center electrode, the first ground electrode, and the second ground electrode.

Application Example 7

In accordance with a seventh aspect of the present invention, there is provided a spark plug according to the application example 6, wherein the noble metal or the noble metal alloy is iridium or an iridium alloy.

This configuration allows appropriately suppressing the wear of each of the center electrode, the first ground electrode, and the second ground electrode.

Here, the present invention can be realized by various forms, for example, can be realized in a form of a spark plug, an internal combustion engine on which the spark plug is mounted or similar form.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A. First Embodiment

A1. Configuration of Spark Plug

FIG. 1is a sectional view of a spark plug100of a first embodiment. The line CL shown in the drawing denotes the central axis of the spark plug100. Hereinafter, the central axis CL is also referred to as an “axial line CL” and the direction parallel to the central axis CL is also referred to as an “axial direction.” The radial direction of the circle around the central axis CL is also referred to simply as a “radial direction” and the direction of the circumference of the circle around the central axis CL is also referred to as a “circumferential direction.” In the drawings, the first direction D1and the second direction D2are parallel to the axial line CL. The second direction D2is the direction opposite to the first direction D1. As describe later, a center electrode20, a first ground electrode30, and a second ground electrode90, which form a spark gap (also referred to simply as a “gap”), form the end portion on the first direction D1side of the spark plug100. Hereinafter, the first direction D1side is also referred to as a “front end side,” and the second direction D2side is also referred to as a “rear end side.”

The spark plug100includes a ceramic insulator10, the center electrode20, the first ground electrode30, the second ground electrode90, a terminal metal fitting40, a metal shell50, a conductive seal60, a resistor element70, a conductive seal80, a front-end-side packing8, a talc9as one example of a buffer, a first rear-end-side packing6, and a second rear-end-side packing7. The right side in the drawing shows an expansion (i.e., enlarged) figure of the cross section of the portions forming gaps g1and g2described later in the center electrode20, the first ground electrode30, and the second ground electrode90viewed from another direction.

The ceramic insulator10is an approximately cylindrically-shaped member with a through hole12(an axial hole). The through hole12extends along the central axis CL so as to pass through the ceramic insulator10. The ceramic insulator10is formed by sintering alumina (another insulating material can also be adopted). The ceramic insulator10includes a nose portion13, a first outer-diameter contracted portion15, a front-end-side trunk portion17, a flange portion19, a second outer-diameter contracted portion11, and a rear-end-side trunk portion18that are arranged from the front end side toward the rear end side in this order.

The flange portion19is the portion positioned approximately in the center of the axial direction of the ceramic insulator10, and is the maximum outer diameter portion of the ceramic insulator10. On the front end side of the flange portion19, the front-end-side trunk portion17is disposed. On the front end side of the front-end-side trunk portion17, the first outer-diameter contracted portion15is disposed. The outer diameter of the first outer-diameter contracted portion15gradually decreases from the rear end side toward the front end side. On the front end side of the first outer-diameter contracted portion15, the nose portion13is disposed. In the state where the spark plug100is installed on an internal combustion engine (not shown), the nose portion13is exposed to a combustion chamber.

On the rear end side of the flange portion19, the second outer-diameter contracted portion11is disposed. The outer diameter of the second outer-diameter contracted portion11gradually decreases from the front end side toward the rear end side. On the rear end side of the second outer-diameter contracted portion11, the rear-end-side trunk portion18is disposed.

Into the front end side of the through hole12of the ceramic insulator10, the center electrode20is inserted. The center electrode20is a rod-shaped member that extends along the central axis CL. The center electrode20includes an electrode base material21, a core material22, and a column-shaped tip28. The core material22is buried inside of the electrode base material21. The tip28is sealed to the front end side of the electrode base material21, and has the center on the central axis CL. The rear end portion of the core material22is exposed from the electrode base material21so as to form the rear end portion of the center electrode20. The other portion of the core material22is coated with the electrode base material21. However, the entire core material22may be covered with the electrode base material21. The electrode base material21is formed by using, for example, an alloy containing nickel. The core material22is formed of, for example, an alloy containing copper. The tip28is formed of an alloy containing iridium (however, another conductive material (for example, a metallic material) can also be adopted). The tip28is sealed to the electrode base material21by, for example, laser beam welding. A part of the rear end side of the center electrode20is arranged within the through hole12of the ceramic insulator10. A part of the front end side of the center electrode20is exposed on the front end side of the ceramic insulator10.

Into the rear end side of the through hole12of the ceramic insulator10, the terminal metal fitting40is inserted. The terminal metal fitting40is a rod-shaped member that extends along the central axis CL. The terminal metal fitting40is formed using low-carbon steel (however, another conductive material (for example, a metallic material) can also be adopted). The terminal metal fitting40includes a flange portion42, a plug cap installation portion41, and a nose portion43. The plug cap installation portion41forms the portion on the rear end side with respect to the flange portion42. The nose portion43forms the portion on the front end side with respect to the flange portion42. The plug cap installation portion41is exposed on the rear end side of the ceramic insulator10. The nose portion43is inserted into the through hole12of the ceramic insulator10.

In the through hole12of the ceramic insulator10, the resistor element70is arranged between the terminal metal fitting40and the center electrode20. The resistor element70reduces the radio wave noise during the occurrence of the spark. The resistor element70is formed by the composition containing glass particles such as B2O3—SiO2-based glass particles, ceramic particles such as ZrO2ceramic particles, and a conductive material such as carbon particles and metal.

In the through hole12, the clearance between the resistor element70and the center electrode20is filled with the conductive seal60. The clearance between the resistor element70and the terminal metal fitting40is filled with the conductive seal80. As a result, the center electrode20and the terminal metal fitting40electrically connect to each other via the resistor element70and the conductive seals60and80. The conductive seal is formed using, for example, various glass particles described above and metal particles (such as Cu and Fe).

The metal shell50is a cylindrically-shaped metal shell for securing the spark plug100to an engine head (not shown) of the internal combustion engine. The metal shell50is formed using a low-carbon steel material (or another conductive material (for example, a metallic material) can also be adopted). In the metal shell50, a through hole59is formed. The through hole59passes through along the central axis CL. The ceramic insulator10is inserted into the through hole59of the metal shell50. The metal shell50is secured to the outer periphery of the ceramic insulator10. The front end of the ceramic insulator10is exposed from the front end of the metal shell50. The rear end of the ceramic insulator10is exposed from the rear end of the metal shell50.

The metal shell50includes a body55, a seal portion54, a deformed portion58, a tool engagement portion51, and a crimp portion53that are arranged from the front end side toward the rear end side in this order. The shape of the seal portion54is approximately cylindrically shaped. On the front end side of the seal portion54, the body55is disposed. The outer diameter of the body55is smaller than the outer diameter of the seal portion54. On the outer peripheral surface of the body55, a screw portion52is formed to be threadably mounted on the mounting hole of the internal combustion engine. Between the seal portion54and the screw portion52, an annular gasket5is fitted by insertion. The gasket5is formed by folding a metal plate.

The body55of the metal shell50includes an inner-diameter contracted portion56. The inner-diameter contracted portion56is arranged on the front end side with respect to the flange portion19of the ceramic insulator10. The internal diameter of the inner-diameter contracted portion56gradually decreases from the rear end side toward the front end side. Between the inner-diameter contracted portion56of the metal shell50and the first outer-diameter contracted portion15of the ceramic insulator10, the front-end-side packing8is sandwiched. The front-end-side packing8is made of steel, and is an O-shaped ring. Here, another material (for example, a metallic material such as copper) can also be adopted.

On the rear end side of the seal portion54, the deformed portion58is disposed. The deformed portion58has a wall thickness thinner than that of the seal portion54. The deformed portion58is deformed such that the center portion projects toward the outside in the radial direction (the direction away from the central axis CL). On the rear end side of the deformed portion58, the tool engagement portion51is disposed. The shape of the tool engagement portion51is a shape (for example, a hexagonal prism) with which a spark plug wrench is engaged. On the rear end side of the tool engagement portion51, the crimp portion53is disposed. The crimp portion53has a wall thickness thinner than that of the tool engagement portion51. The crimp portion53is arranged on the rear end side with respect to the second outer-diameter contracted portion11of the ceramic insulator10so as to form the rear end of the metal shell50. The crimp portion53is flexed to radially inside.

Between the inner peripheral surface of the portion on the rear end side of the metal shell50and the outer peripheral surface of the ceramic insulator10, an annular space SP is formed. This space SP is a space formed by the inner peripheral surface of the metal shell50and the outer peripheral surface of the ceramic insulator10at a position between the crimp portion53and the second outer-diameter contracted portion11. On the rear end side within this space SP, the first rear-end-side packing6is arranged. On the front end side within this space SP, the second rear-end-side packing7is arranged. In this embodiment, these rear-end-side packings6and7are C-shaped rings made of steel (another material can also be adopted). Between the two rear-end-side packings6and7within the space SP, the powders of the talc9are filled up.

The crimp portion53is crimped so as to be folded to the inside. Accordingly, the ceramic insulator10is pressed to the front end side within the metal shell50via the packings6and7and the talc9. Thus, the front-end-side packing8is pressed between the first outer-diameter contracted portion15and inner-diameter contracted portion56. The front-end-side packing8seals between the metal shell50and the ceramic insulator10. The above-described configuration suppresses the gas inside of the combustion chamber of the internal combustion engine to leak to the outside through between the metal shell50and the ceramic insulator10.

The first ground electrode30includes a base material32and a tip38. The base material32is sealed to the front end of the metal shell50. The tip38is sealed to a front end portion31of the base material32. The base material32extends from the end sealed to the metal shell50toward the first direction D1, folded by approximately 90 degrees toward the central axis CL. The front end portion31is arranged on the front end side of the center electrode20. The X direction Dx in the drawings is the direction vertical to the central axis CL from the sealed portion between the metal shell50and the base material32toward the central axis CL. The partial expansion figure inFIG. 1shows the cross section that includes the central axis CL and is vertical to the X direction Dx. The tip38is sealed by, for example, laser beam welding on the base material32in the position facing the front end surface of the tip28of the center electrode20, specifically, on the surface on the second direction D2side of the front end portion31. The shape of the tip38is a circular plate shape having the center on the central axis CL. The base material32is formed using a nickel alloy containing nickel of 90 weight % or more. The tip38is formed using an alloy containing iridium. The surface on the second direction D2side of the tip38of the first ground electrode30and the surface (front end surface) on the first direction D1side of the tip28of the center electrode20form a first gap g1.

The second ground electrode90includes a supporting portion92and a cylindrically-shaped tip98(also referred to as the “cylindrical tip98”). The supporting portion92includes a hole forming portion91that forms a column-shaped through hole having the center on the central axis CL, and is sealed to the front end portion of the metal shell50. The tip98is sealed to the inner peripheral surface of the hole forming portion91, and has the center on the central axis CL. The cylindrical tip98is sealed to the inner peripheral surface of the hole forming portion91by, for example, brazing. The supporting portion92is sealed to the inner peripheral surface of the front end portion of the metal shell50(details will be described later). The supporting portion92is formed using a nickel alloy that contains nickel of 90 weight % or more. The cylindrical tip98is formed using an alloy that contains iridium. The inner peripheral surface of the cylindrical tip98of the second ground electrode90and the outer peripheral surface of the tip28of the center electrode20form an annular second gap g2.

A2. Configuration of Electrode

FIGS. 2A to 2Dare schematic diagrams showing the configurations of the electrodes20,30, and90of the spark plug100.FIG. 2Ashows a partial sectional view (a sectional view including the central axis CL) parallel to the X direction Dx on the first direction D1side of the spark plug100.FIG. 2Bshows a sectional view (a sectional view including the central axis CL) of the same portion vertical to the X direction Dx.FIG. 2Cshows a schematic diagram of the spark plug100observed from the first direction D1side toward the second direction D2.FIG. 2Dshows a schematic diagram of the remaining portion after the first ground electrode30is deleted from the schematic diagram ofFIG. 2C. In the drawings, the two directions Dx and Dy perpendicular to the central axis CL are shown in addition to the first direction D1and the second direction D2. The Y direction Dy is a direction perpendicular to the X direction Dx.FIG. 2Ais the cross section taken along the line A-A ofFIG. 2C, and is the cross section that divides the base material32of the first ground electrode30in half.FIG. 2Bis the cross section taken along the line B-B ofFIG. 2C.

Here,FIG. 2AandFIG. 2Bshow the appearances of the ceramic insulator10observed facing the direction vertical to the cross section. Here, the right side inFIG. 2Ashows an expansion figure of the portion including the tip28. InFIG. 2C, the first ground electrode30is hatched. InFIG. 2D, the tip28and the second ground electrode90are hatched.

As shown inFIG. 2AandFIG. 2D, the cylindrical tip98of the second ground electrode90surrounds the peripheral area of the tip28of the center electrode20on the outside in the radial direction over the whole circumference. The annular second gap g2is formed by an inner peripheral surface98s(the surface on the inside of the radial direction inFIG. 2A) of the cylindrical tip98and an outer peripheral surface28s2(the surface on the outside in the radial direction) of the tip28of the center electrode20.

As shown inFIG. 2BandFIG. 2D, the supporting portion92of the second ground electrode90is a plate-shaped member that extends from the −Dy direction side to the +Dy direction side of the central axis CL along the Y direction Dy. Here, the +Dy direction denotes the Y direction Dy, and the −Dy direction denotes the direction opposite to the Y direction Dy. In the drawings, two connecting portions92sand92tforming the supporting portion92are shown. The first connecting portion92sis the portion on the −Dy direction side with respect to the central axis CL in the supporting portion92. On the outside in the radial direction in the first connecting portion92s, an end portion921is sealed to the metal shell50on the −Dy direction side with respect to the central axis CL. The second connecting portion92tis the portion on the +Dy direction side with respect to the central axis CL in the supporting portion92. On the outside in the radial direction in the second connecting portion92t, an end portion921is sealed to the metal shell50on the +Dy direction side with respect to the central axis CL. The respective shapes of the first connecting portion92sand the second connecting portion92tare mutually the same.

As shown inFIG. 2B, the supporting portion92(specifically, the connecting portions92sand92t) extends from the connecting portion (that is, the hole forming portion91) with the cylindrical tip98toward the outside in the radial direction, is bent toward the second direction D2, extends toward the second direction D2side, and reaches the end portion921. The outer peripheral surface of the end portion921is sealed to the inner peripheral surface of the metal shell50by welding. For example, a boundary portion W95between the end portion921of the supporting portion92and the metal shell50is welded by laser beam welding from the first direction D1side. Accordingly, the second ground electrode90has electrical continuity with the metal shell50.

As shown inFIG. 2A, in the end portion on the first direction D1side, the metal shell50(specifically, the body55), a large internal diameter portion501is formed. The large internal diameter portion501has a relatively large internal diameter. On the second direction D2side of the large internal diameter portion501, a small internal diameter portion502is formed. The small internal diameter portion502has an internal diameter smaller than that of the large internal diameter portion501. In the boundary portion between the large internal diameter portion501and the small internal diameter portion502, a level difference (i.e., annular surface) is formed. At the level difference, the internal diameter changes in a stepped pattern. The second ground electrode90is fitted to this large internal diameter portion501from the first direction D1side toward the second direction D2.

As shown inFIG. 2BandFIG. 2D, the second ground electrode90is constituted such that the two end portions921of the supporting portion92are brought into contact with the inner peripheral surface of the large internal diameter portion501of the metal shell50. Specifically, in the case of observation facing the direction parallel to the central axis CL as shown inFIG. 2D, the shapes of the edges on the outer periphery side of the two end portions921are arc shapes having diameters that is larger than the internal diameter of the small internal diameter portion502and is slightly smaller than the internal diameter of the large internal diameter portion501. Accordingly, in the case where the second ground electrode90is fitted to the large internal diameter portion501, the surfaces on the second direction D2side of the two end portions921of the supporting portion92are brought into contact with the level difference (annular surface) between the large internal diameter portion501and the small internal diameter portion502. Accordingly, this inhibits the second ground electrode90from getting into the small internal diameter portion502, thus suppressing the displacement of the second ground electrode90in the first direction D1with respect to the metal shell50. Additionally, the two end portions921of the supporting portion92are brought into contact with the inner peripheral surface of the large internal diameter portion501. This suppresses the displacement (the displacement of the second ground electrode90with respect to the metal shell50) in the direction perpendicular to the central axis CL. As a result, a size dg2(also referred to as the “second gap size dg2”) of the second gap g2is approximately constant over the whole circumference on the outer peripheral surface28s2of the tip28of the center electrode20.

As shown inFIG. 2A, the first ground electrode30is welded to a front end surface501sof the metal shell50(for example, by laser beam welding). Accordingly, the first ground electrode30has electrical continuity with the metal shell50. As shown inFIG. 2C, the first ground electrode30is arranged to extend in the X direction Dx vertical to the direction (that is, the Y direction Dy) extending the supporting portion92of the second ground electrode90. As shown in the expansion figure inFIG. 2A, a front end surface28s1of the tip28of the center electrode20is a planar surface perpendicular to the central axis CL. Additionally, a surface38son the second direction D2side of the tip38of the first ground electrode30is a planar surface perpendicular to the central axis CL. These surfaces28s1and38sform the first gap g1. In the first gap g1, a size dg1(also referred to as the “first gap size dg1”), that is, the distance between the two surfaces28s1and38sis approximately constant irrespective of the position in the first gap g1. During manufacturing of the spark plug100, the degree of bending of the first ground electrode30is adjusted such that the first gap size dg1becomes a predetermined size.

As described above, the first ground electrode30has the tip38formed of the noble metal alloy (specifically, the alloy containing iridium) in the position forming the first gap g1. The second ground electrode90has the cylindrical tip98formed of the noble metal alloy (specifically, the alloy containing iridium) in the position forming the second gap g2. In the center electrode20, at least the portion forming the first gap g1with the tip38(that is, the front end surface28s1of the tip28) and the portion forming the second gap g2with the cylindrical tip98(that is, the outer peripheral surface28s2of the tip28) are formed of noble metal alloys (specifically, alloys containing iridium). Accordingly, this allows suppressing the wear of each of the center electrode20, the first ground electrode30, and the second ground electrode90.

A3. First Evaluation Test

The following describes the first evaluation test using samples of the spark plug. In the first evaluation test, the relationship between: the ratio of the first gap size dg1to the second gap size dg2, and the bias eccentricity of the number of discharges between the first gap g1and the second gap g2was evaluated. To evaluate this relationship, the first evaluation test employed test samples of the spark plug that includes a center electrode with the tip28, a first ground electrode with the tip38, and a second ground electrode with the cylindrical tip98(not shown). The configurations of the center electrode and the first ground electrode of the test samples are similar to the configurations of the center electrode20and the first ground electrode30inFIG. 1andFIG. 2AtoFIG. 2D. For the second ground electrode, the shape of a supporting portion is not same as the shape of the supporting portion92inFIG. 1andFIG. 2AtoFIG. 2D. However, the supporting portion for the test samples includes a hole forming portion that allows insertion of the cylindrical tip98similarly to the hole forming portion91described inFIG. 2AtoFIG. 2D. The cylindrical tip98is sealed to the inner peripheral surface of the hole forming portion. The supporting portion for the test samples is sealed to a front end portion of a metal shell. To appropriately perform the above-described evaluation, the respective three tips28,38, and98for the test samples are the same as the three tips28,38, and98described inFIG. 2AtoFIG. 2D. The configuration of the sample is otherwise similar to the configuration of the spark plug100inFIG. 1. In the first evaluation test, samples of four spark plugs with mutually different ratios dg1/dg2(hereinafter referred to as “gap ratios”) of the first gap size dg1to the second gap size dg2(inFIG. 2A) were used to measure the rate (hereinafter referred to as a “second discharge rate”) of the number of discharges that occurred between the center electrode and the second ground electrode to the number (here, 100) of all discharges that occurred in the sample of the spark plug. Here, a discharge occurs between the center electrode and the first ground electrode or between the center electrode and the second ground electrode. Table 1 below shows the measurement result.

The dimensions in common between the four samples used for the evaluation test are as follows.1) Outer Diameter of Tip28of Center Electrode: 2.2 mm2) Internal Diameter of Cylindrical Tip98: 2.8 mm3) Second Gap Size dg2: 0.3 mm

The four samples are different in the first gap size dg1from one another. The bent state of the first ground electrode (for example, a bend radius or similar state) is adjusted so as to adjust the first gap size dg1.

The testing method is as follows. The sample of the spark plug is arranged in a container for experiment filled with air. The internal pressure of the container is raised to 1 MPa. This pressure is determined assuming the pressure during ignition in the combustion chamber of the internal combustion engine. In this state, a voltage is applied to the sample of the spark plug to conduct a discharge. Every time a discharge is conducted, it is confirmed that the ground electrode that has caused a discharge is the first ground electrode or the second ground electrode by visual check. Hereinafter, the ground electrode that has caused the discharge is referred to as a “discharge ground electrode.” The discharge is repeatedly conducted so as to calculate the second discharge rate, that is, the rate of the number of discharges that have occurred between the center electrode and the second ground electrode to the number of all discharges.

As shown in table 1, the second discharge rate becomes higher as the gap ratio becomes larger. As the reason for this result, it is estimated that this is because a discharge is less likely to occur in the first gap g1in the case where the gap ratio is large since the first gap size dg1is larger than the second gap size dg2compared with the case where the gap ratio is small. Specifically, as shown in Table 1, in the case where the gap ratio is 0.70, the second discharge rate is 30%. That is, the discharge ground electrode is biased to the first ground electrode. In the case where the gap ratio is 1.30, the second discharge rate is 70%. That is, the discharge ground electrode is biased to the second ground electrode. In the case where the gap ratio is 0.80, the second discharge rate is 45%. In the case where the gap ratio is 1.25, the second discharge rate is 55%. In these two cases, discharge occurs approximately equally between the first ground electrode and the second ground electrode.

Setting the gap ratio within the range of 0.80 or more and 1.25 or less allows approximately equally using both the first ground electrode and the second ground electrode for discharge. This consequently allows suppressing significant wear of one ground electrode compared with the other ground electrode, thus improving the durability of the spark plug. For example, stable discharges can be achieved over a long period of time.

Here, the test sample has the three tips28,38, and98that form the first gap g1and the second gap g2similarly to the spark plug100shown inFIGS. 2A to 2D. Accordingly, the above-described preferred range of the gap ratio is applicable to the spark plug100inFIGS. 2A to 2D, and thus spark plugs in various configurations with the three tips28,38, and98.

Here, the distance between the two discharging surfaces (here, the outer peripheral surface28s2of the tip28and the inner peripheral surface98sof the cylindrical tip98) that form the second gap g2might change corresponding to the position on the discharging surface. For example, the displacement (particularly, the displacement in the direction perpendicular to the central axis CL) of the center electrode20might be larger than zero. Alternatively, the displacement of the second ground electrode90might be larger than zero. In the case where this displacement occurs, the distance between the two discharging surfaces28s2and98smight change corresponding to the position on the discharging surface28s2. In this case, it is only necessary to adopt the shortest distance between the two discharging surfaces (here, the two discharging surfaces28s2and98s) that form the second gap g2as the second gap size dg2. Similarly, the distance between the two discharging surfaces (here, the front end surface28s1of the tip28and the surface38sof the tip38) that form the first gap g1might change corresponding to the position on the discharging surface. In this case, it is only necessary to adopt the shortest distance between the two discharging surfaces (here, the two discharging surfaces28s1and38s) that form the first gap g1as the first gap size dg1. The first gap size dg1and the second gap size dg2thus obtained are used to calculate a gap ratio (dg1/dg2). This gap ratio (dg1/dg2) is preferred to be within the range of 0.80 or more and 1.25 or less. This allows approximately equally using both the first ground electrode30and the second ground electrode90for discharge.

Here, the difference in likelihood of discharge between the first gap g1and the second gap g2is estimated to be caused mainly by the difference between the first gap size dg1and the second gap size dg2. Accordingly, the above-described preferred range of the gap ratio is estimated to be applicable irrespective of the configuration other than the gap sizes dg1and dg2. For example, the above-described preferred range is estimated to be applicable irrespective of the material (here, the material of the tip28and the material of the tip38) of the portion that forms the first gap g1in the electrode, the material (here, the material of the tip28and the material of the cylindrical tip98) of the portion that forms the second gap g2in the electrode, and the area of the portions that form the gaps g1and g2on the surfaces of the electrodes20,30, and90.

A4. Second Evaluation Test

The following describes the second evaluation test using samples of the spark plug. In the second evaluation test, the rate of occurrence of a creeping discharge in the spark plug (referred to as a “used spark plug”) after the operation of the internal combustion engine mounted with the sample of the spark plug for 1000 hours was measured.

FIGS. 3A and 3Bare explanatory views of creeping discharges. The following describes the creeping discharge using the spark plug100shown inFIG. 1andFIGS. 2A to 2D. The drawings show the expansion figures of the portions including the gaps g1and g2in the sectional views shown inFIG. 1andFIG. 2B.FIG. 3Ashows a schematic diagram of the spark plug100before being used.FIG. 3Bshows a schematic diagram of the spark plug100(the spark plug100after the operation for 1000 hours) after being used. InFIG. 3A, bold lines p1and p2show examples of discharge paths. The first discharge path p1is an exemplary path of a discharge that might occur in the first gap g1, and is a path from the front end surface28s1of the tip28to the surface38sof the tip38. The second discharge path p2is an exemplary path of a discharge that occurs in the second gap g2, and is a path from the outer peripheral surface28s2of the tip28to the inner peripheral surface98sof the cylindrical tip98.

FIG. 3Ashows a distance h that denotes the shortest distance between the surface of the ceramic insulator10and the surface of the second ground electrode90. In this embodiment, the shortest distance h is the same as the distance (the distance measured in parallel to the central axis CL) between a surface10s(referred to as the “front end surface10s”) on the first direction D1side of the ceramic insulator10and a surface92uson the second direction D2side of the supporting portion92in the second ground electrode90. Before the spark plug100is used, the shortest distance h>the first gap size dg1is satisfied and the shortest distance h>the second gap size dg2is satisfied. The first gap size dg1is the same as the second gap size dg2.

The electrodes20,30, and90might wear by the operation for 1000 hours. Particularly, wear is likely to occur in the portion that causes a discharge, that is, the front end surface28s1of the tip28, the outer peripheral surface28s2of the tip28, the surface38sof the tip38, and the inner peripheral surface98sof the tip98.FIG. 3Bshows a schematic diagram after use for 1000 hours. In the drawing, respective surfaces28s1e,28s2e,38se, and98seare surfaces obtained by wear of the respective original surfaces28s1,28s2,38s, and98s. In the first gap g1after use, a first gap size dg1eis larger than the first gap size dg1before use (inFIG. 3A). In the second gap g2after use, a second gap size dg2eis larger than the second gap size dg2before use. Hereinafter, the first gap size dg1before use is also referred to as the “first initial gap size dg1.” The second gap size dg2before use is also referred to as a “second initial gap size dg2.” Here, the electrode wear might progress non-uniformly. In this case, the shortest distance between the front end surface28s1eand the surface38secorresponds to the first gap size dg1eafter use. The shortest distance between the outer peripheral surface28s2eand the inner peripheral surface98secorresponds to the second gap size dg2eafter use.

InFIG. 3B, a bold line px denotes an exemplary path of a creeping discharge. This creeping discharge path px goes from the surface92usof the supporting portion92of the second ground electrode90to the front end surface10sof the ceramic insulator10, goes toward the center electrode20along this front end surface10s, and reaches the outer peripheral surface of the center electrode20(here, the outer peripheral surface of the electrode base material21). The creeping discharge that creeps on the front end surface10sof the ceramic insulator10in this method might occur in the case where the discharges in the gaps g1and g2are less likely to occur. For example, as the gap sizes dg1eand dg2eare larger with respect to the shortest distance h, in other words, as the shortest distance h is smaller with respect to the gap sizes dg1eand dg2e, the creeping discharge is more likely to occur. When this creeping discharge occurs, the ceramic insulator10might be damaged. Accordingly, the rate of occurrence of an unintended creeping discharge is preferred to be small.

The creeping discharge that might occur in the spark plug100inFIGS. 2A to 2Dhas been described above. The sample of the spark plug used in the second evaluation test is the same as the sample used for the first evaluation test. The supporting portion of the sample includes the surface92us, which realizes the shortest distance h between the surface of the ceramic insulator10and the surface of the second ground electrode, similarly to the supporting portion92inFIG. 3AandFIG. 3B. Accordingly, in the test sample, in the case where the tips28,38, and98wear due to discharge, the creeping discharge might occur similarly to the spark plug100shown inFIG. 3B.

In the second evaluation test, samples of four spark plugs with different shortest distances h were used to measure the rate of occurrence of the creeping discharge after the operation for 1000 hours. Table 2 below shows the measurement result.

TABLE 2Initial Distance Ratio (h/dg)1.81.92.02.1Occurrence Rate of301000Creeping Dischargeafter Use for 1000 Hours

In Table 2, an initial distance ratio (h/dg) is the ratio of the shortest distance h to the initial gap sizes dg1and dg2of the sample of the spark plug before use. The occurrence rate of the creeping discharge after use for 1000 hours is the rate of the number of creeping discharges with respect to the number of all discharges in the case where the sample of the spark plug after use for 1000 hours is used and discharge is repeated under the same condition as that of the first evaluation test. Whether or not the discharge was the creeping discharge was confirmed by visual check.

The dimensions in common between the four samples used for the evaluation test are as follows.1) Outer Shape of Tip28of Center Electrode: 2.2 mm2) Internal Diameter of Cylindrical Tip98: 2.8 mm3) First Initial Gap Size dg1: 0.3 mm4) Second Initial Gap Size dg2: 0.3 mm

The four samples are different in the shortest distance h from one another. The length along the central axis CL of the nose portion13of the ceramic insulator10is adjusted so as to adjust the shortest distance h.

As shown in Table 2, as the initial distance ratio becomes larger, the rate of the creeping discharge becomes smaller. The reason for this result is estimated as follows. As described above, the gap sizes dg1eand dg2emight become larger than the initial gap sizes dg1and dg2due to the operation for 1000 hours. Here, in the case where the initial distance ratio is large, the proportion of the gap sizes dg1eand dg2eafter use to the shortest distance h is small compared with the case where the initial distance ratio is small. That is, in the case where the initial distance ratio is large, the discharge is likely to occur in the gaps g1and g2compared with the case where the initial distance ratio is small. Accordingly, in the case where the operating period is the same, that is, in the case where the electrode wear occurs approximately equally, the rate of the creeping discharge becomes smaller as the initial distance ratio becomes larger.

Specifically, as shown in Table 2, in the case where the initial distance ratio is equal to or more than 2.0, more specifically, in the case where the initial distance ratio is 2.0 or 2.1, the occurrence rate of the creeping discharge is zero percent. In the case where the initial distance ratio is 1.9, the occurrence rate of the creeping discharge is 10%. In the case where the initial distance ratio is 1.8, the occurrence rate of the creeping discharge is 30%. Setting the initial distance ratio to be equal to or more than 2 in this method allows suppressing the creeping discharge. This consequently allows improving the durability of the spark plug.

Here, the first initial gap size dg1may be different from the second initial gap size dg2. In this case, the shortest distance h is preferred to be twice or more as large as the maximum value among the first initial gap size dg1and the second initial gap size dg2. This configuration allows suppressing the creeping discharge even in the case where any of the first ground electrode30and the second ground electrode90wears.

In each case, various values can be adopted as the upper limit of the initial distance ratio. For example, the initial distance ratio may be set to be equal to or less than “2.1” that is the evaluated value in the second evaluation test. As the upper limit of the initial distance ratio, the value larger than 2.1 (for example, any value selected from 3, 3.5, and 4) may be adopted (the initial distance ratio is equal to or less than the upper limit). In the case where the first initial gap size dg1is different from the second initial gap size dg2, the ratio of the shortest distance h to the maximum value between the first initial gap size dg1and the second initial gap size dg2can be adopted as the initial distance ratio. Here, in the case where the shortest distance h is large, the portion (referred to as the outside portion) positioned on the outside of the through hole12of the ceramic insulator10in the center electrode20is often large. In the case where the outside portion of the center electrode20is long, the durability of the center electrode20is likely to be low. Accordingly, the shortest distance h, and thus the initial distance ratio is preferred to be small.

As described above, in the test sample, in the case where the tips28,38, and98wear due to discharge, the creeping discharge might occur similarly to the spark plug100shown inFIG. 3B. Accordingly, the above-described preferred range of the initial distance ratio is applicable to the spark plug100inFIGS. 2A to 2D, and thus spark plugs in various configurations with the three tips28,38, and98and the supporting portion that realizes the shortest distance h.

Here, the rate of electrode wear (for example, an increased amount of the gap sizes dg1and dg2per unit of operating period) might change corresponding to the materials of the tips28,38, and98, the presence of the tips28,38, and98, the area of the portions that form the gaps g1and g2on the surfaces of the electrodes20,30, and90, and similar parameter. In each case, when the shortest distance h is twice or more as large as the maximum value among the first initial gap size dg1and the second initial gap size dg2, the shortest distance h larger than the gap sizes dg1and dg2can be maintained until the gap sizes dg1and dg2increases double. This allows suppressing the creeping discharge over a long period of time compared with the case where the shortest distance h is less than twice as large as the above-described maximum value. In this method, the durability of the spark plug can be improved. However, the shortest distance h may be less than twice as large as the maximum value between the first initial gap size dg1and the second initial gap size dg2.

Here, in the embodiment inFIGS. 3A and 3B, the shortest distance h is the distance measured in parallel to the first direction D1. The arrangement of the point on the ceramic insulator and the point on the second ground electrode to specify the shortest distance h can be various arrangements corresponding to the shape of the ceramic insulator10and the shape of the second ground electrode. For example, the distance measured along the oblique direction intersecting with the first direction D1between the ceramic insulator and the second ground electrode may be the shortest distance.

B. Second Embodiment

FIGS. 4A to 4Dare schematic diagrams showing a second embodiment of the spark plug.FIG. 4Ashows a sectional view similar to that ofFIG. 2A.FIG. 4Bshows a sectional view similar to that ofFIG. 2B.FIG. 4Cshows a schematic diagram similar to that ofFIG. 2C.FIG. 4Dshows a schematic diagram similar to that ofFIG. 2D. There are two differences from the spark plug100of the first embodiment. The first difference is that the base material32of the first ground electrode30of the first embodiment is replaced by a surface layer36, which forms the surface, and a core portion37, which is formed inside of the surface layer36. The second difference is that the supporting portion92of the first embodiment is replaced by a surface layer96, which forms the surface, and a core portion97, which is formed inside of the surface layer96. The other configuration of a spark plug100aof the second embodiment is the same as the configuration of the spark plug100of the first embodiment (in the drawings, like reference signs designate corresponding or identical configurations, and therefore such configurations will not be further elaborated here). For example, the arrangement of the tips28,38, and98forming the gaps g1and g2is the same as the arrangement in the embodiment shown inFIGS. 2A to 2D. Here, inFIG. 4C, the core portion37is hatched. InFIG. 4D, the core portion97is hatched.

In the second embodiment, a first ground electrode30aincludes the surface layer36, the core portion37, which is disposed inside of the surface layer36, and the tip38, which is sealed to a front end portion31aof the first ground electrode30a. The outer shape of the surface layer36is the same as the outer shape of the base material32of the first embodiment. As shown inFIG. 4A, the core portion37extends from the sealed portion with the metal shell50and extends to the middle of the first ground electrode30athat reaches the front end portion31a. The front end portion31ais the portion corresponding to the front end portion31(inFIG. 2A) of the first embodiment.

The core portion37is formed using a material with a higher thermal conductivity than that of the surface layer36. Accordingly, the heat transfer by the first ground electrode30acan be promoted compared with the case where the core portion37is omitted. As a result, this simply allows transferring heat from the first ground electrode30ato the metal shell50during the operation of the internal combustion engine. Accordingly, this allows suppressing the state where the temperature of the first ground electrode30abecomes high and the long-continued state where the temperature of the first ground electrode30ais high. As a result, this allows suppressing the wear of the first ground electrode30a(for example, oxidation of the surface of the first ground electrode30a).

Here, as the material of the surface layer36, various materials can be adopted. For example, an alloy containing nickel can be adopted similarly to the base material32of the first embodiment. As the material of the core portion37, various materials with higher thermal conductivities than that of the surface layer36can be adopted. For example, copper or an alloy containing copper can be adopted.

In the second embodiment, a second ground electrode90aincludes the surface layer96, the core portion97, which is disposed inside of the surface layer96, and the cylindrical tip98, which is sealed to the inner peripheral surface of the surface layer96. The outer shape of the surface layer96is the same as the outer shape of the supporting portion92of the first embodiment. Hereinafter, the whole of the surface layer96and the core portion97is referred to as a “supporting portion92a.” Reference sign obtained by adding the character “a” to the tail end of reference sign of the element corresponding to the supporting portion92inFIGS. 2A to 2Dis given to the element of the supporting portion92a. For example, a first connecting portion92sadenotes the same portion as the first connecting portion92sinFIG. 2D. Additionally, an end portion921adenotes the same portion as the end portion921inFIG. 2B. As shown inFIG. 4BandFIG. 4D, the core portion97extends from the proximity of the end on the −Dy direction side of the supporting portion92ato the proximity of the end on the +Dy direction side within the supporting portion92aalong the Y direction Dy. Additionally, the core portion97is formed in a ring shape to bypass the through hole and a hole forming portion91a.

The core portion97is formed using the material with the higher thermal conductivity than that of the surface layer96. Accordingly, the heat transfer by the second ground electrode90acan be promoted compared with the case where the core portion97is omitted. As a result, this simply allows transferring heat from the second ground electrode90ato the metal shell50during the operation of the internal combustion engine. Accordingly, this allows suppressing the state where the temperature of the second ground electrode90abecomes high and the long-continued state where the temperature of the second ground electrode90ais high. As a result, this allows suppressing the wear of the second ground electrode90a(for example, oxidation of the surface of the second ground electrode90a).

Here, as the material of the surface layer96, various materials can be adopted. For example, an alloy containing nickel can be adopted similarly to the supporting portion92of the first embodiment. As the material of the core portion97, various materials with higher thermal conductivities than that of the surface layer96can be adopted. For example, copper or an alloy containing copper can be adopted.

The configuration of the portion other than the above-described two differences of the spark plug100aof the second embodiment is the same as the configuration of the spark plug100of the first embodiment. Accordingly, the spark plug100aof the second embodiment can achieve the same advantage as that of the spark plug100of the first embodiment. For example, the proportion of the first gap size dg1to the second gap size dg2is set to be equal to or more than 0.80 and equal to or less than 1.25. This allows approximately equally using both the first ground electrode30aand the second ground electrode90afor discharge. This consequently allows suppressing significant wear of one ground electrode compared with the other ground electrode, thus improving the durability of the spark plug100a. Additionally, similarly to the first embodiment described inFIGS. 3A and 3B, setting the shortest distance h to be twice or more as large as the maximum value between the first initial gap size dg1and the second initial gap size dg2allows suppressing the creeping discharge. As a result, the durability of the spark plug100can be improved. Additionally, the first gap g1is formed by the noble metal alloy (specifically, the tip28and the tip38). This allows suppressing the wear of each of the center electrode20and the first ground electrode30a. Additionally, the second gap g2is formed by the noble metal alloy (specifically, the tip28and the cylindrical tip98). This allows suppressing the wear of each of the center electrode20and the second ground electrode90a. Additionally, as the noble metal, iridium is used. This allows appropriately suppressing the wear of the electrodes20,30a, and90a.

FIGS. 5A to 5Dare schematic diagrams showing a third embodiment of the spark plug.FIG. 5Ashows a sectional view similar to that ofFIG. 4A.FIG. 5Bshows a sectional view similar to that ofFIG. 4B.FIG. 5Cshows a schematic diagram similar to that ofFIG. 4C.FIG. 5Dshows a schematic diagram similar to that ofFIG. 4D. There are three differences from the spark plug100aof the second embodiment as follows.

1) The first difference is that the large internal diameter portion501of the metal shell50is omitted.

2) The second difference is that a supporting portion92b(here, a surface layer96b) of a second ground electrode90bextends toward the outside in the radial direction up to the position of the outer peripheral surface of a front end portion501bof a metal shell50b.

3) The third difference is that a first ground electrode30bis sealed to a surface92bson the first direction D1side of the supporting portion92bof the second ground electrode90b. As shown inFIG. 5BandFIG. 5C, in the case of the observation facing the direction in parallel to the central axis CL, the direction in which the first ground electrode30bextends from the sealed portion with the metal shell50btoward the central axis CL is parallel to the direction (here, the Y direction Dy) in which the second ground electrode90bextends.

The other configuration of a spark plug100bof the third embodiment is the same as the configuration of the spark plug100aof the second embodiment (in the drawings, like reference signs designate corresponding or identical configurations, and therefore such configurations will not be further elaborated here). For example, the configuration of the metal shell50bof the third embodiment is the same as the configurations of the metal shells50of the first and second embodiments except that the portion that forms the large internal diameter portion501is omitted. The arrangement of the tips28,38, and98forming the gaps g1and g2is the same as the arrangements in the embodiments shown inFIGS. 2A to 2DandFIGS. 4A to 4D.

As shown inFIG. 5BandFIG. 5D, the second ground electrode90bincludes the supporting portion92band the cylindrical tip98. The supporting portion92bincludes the hole forming portion91asame as that of the embodiment ofFIG. 4B. The cylindrical tip98is sealed to the inner peripheral surface of this hole forming portion91a. As shown inFIG. 5BandFIG. 5D, the core portion97is disposed inside of the supporting portion92bsimilarly to the embodiment inFIG. 4BandFIG. 4D. The remaining portion other than the core portion97in the supporting portion92bis the surface layer96b. The surface layer96bis formed using a nickel alloy.

As shown inFIG. 5B, an end portion921bof the supporting portion92bis the end portion921bon the outside in the radial direction and on the second direction D2side. In this end portion921b, an end face92s2on the second direction D2side is sealed to the end face (referred to as a “front end surface501sb”) on the first direction D1side of the metal shell50b. For example, a boundary portion W95bbetween the supporting portion92band the metal shell50bis welded by laser beam welding from outside in the radial direction. These surfaces92s2and501sbare each a planar surface perpendicular to the central axis CL.FIG. 5BandFIG. 5Dshow two connecting portions92sband92tb. The first connecting portion92sbis the portion on the −Dy direction side with respect to the central axis CL in the supporting portion92b. The second connecting portion92tbis the portion on the +Dy direction side with respect to the central axis CL in the supporting portion92b. The end portion921bof the first connecting portion92sbis sealed to the metal shell50bon the −Dy direction side with respect to the central axis CL. The end portion921bof the second connecting portion92tbis sealed to the metal shell50bon the +Dy direction side with respect to the central axis CL.

In this embodiment, as shown inFIG. 5D, the shapes of edges92soon the outer periphery side of the two end faces92s2in the supporting portion92bare the same as a part of the circle (that is, the arc) having approximately the same diameter as the outer diameter of the front end surface501sbof the metal shell50b. As shown inFIG. 5D, the shapes of edges92sion the inner peripheral side of the two end faces92s2in the supporting portion92bare the same as a part of the circle (that is, the arc) having a slightly smaller diameter than the internal diameter of the front end surface501sbof the metal shell50b. Accordingly, the front end surface501sbof the metal shell50bcan be simply sealed to the two end faces92s2of the supporting portion92b. This allows enhancing the sealing strength. Additionally, the edges92soon the outer periphery side of the two end faces92s2in the supporting portion92bis arranged on the edge on the outer periphery side of the front end surface501sbof the metal shell50b. This allows suppressing the displacement (the displacement in the direction perpendicular to the central axis CL) of the second ground electrode90bwith respect to the metal shell50b. As a result, the second gap size dg2is approximately constant over the whole circumference on the outer peripheral surface28s2of the tip28of the center electrode20.

As shown inFIG. 5B, the first ground electrode30bis sealed to the surface92bson the first direction D1side of the supporting portion92bof the second ground electrode90b(for example, by laser beam welding). The configuration of the first ground electrode30bis the same as the configuration obtained by omitting the portion overlapping with the second ground electrode90binFIG. 5Bin the first ground electrode30ain the case where the first ground electrode30ainFIG. 4Ais superimposed onFIG. 5Bsuch that the tips38overlap with each other. Similarly to the first ground electrode30ainFIG. 4A, the first ground electrode30bincludes a surface layer36b, a core portion37b, which is formed inside of the surface layer36b, and the tip38.

The first ground electrode30bis sealed to the metal shell50bvia the second ground electrode90b. In this case, the heat transfer from the first ground electrode30bto the metal shell50bis suppressed compared with the case where the first ground electrode30bis sealed directly to the metal shell50b. Accordingly, the temperature of the first ground electrode30bis likely to increase. However, the core portion37bis buried in the first ground electrode30b. Accordingly, this allows suppressing the state where the temperature of the first ground electrode30bbecomes high and the long-continued state where the temperature of the first ground electrode30bis high. As a result, this allows suppressing the wear of the first ground electrode30b(for example, oxidation of the surface of the first ground electrode30b).

The configuration of the portion other than the above-described differences of the spark plug100bof the third embodiment is the same as the configuration of the spark plug100aof the second embodiment. Accordingly, the spark plug100bof the third embodiment can achieve the same advantage as that of the spark plug100aof the second embodiment. For example, the proportion of the first gap size dg1to the second gap size dg2is set to be equal to or more than 0.80 and equal to or less than 1.25. This allows approximately equally using both the first ground electrode30band the second ground electrode90bfor discharge. This consequently allows suppressing significant wear of one ground electrode compared with the other ground electrode, thus improving the durability of the spark plug100b. Similarly to the first embodiment described inFIGS. 3A and 3B, setting the shortest distance h to be twice or more as large as the maximum value between the first initial gap size dg1and the second initial gap size dg2allows suppressing the creeping discharge. As a result, the durability of the spark plug100bcan be improved. Additionally, the first gap g1is formed by the noble metal alloy (specifically, the tip28and the tip38). This allows suppressing the wear of each of the center electrode20and the first ground electrode30b. Additionally, the second gap g2is formed by the noble metal alloy (specifically, the tip28and the cylindrical tip98). This allows suppressing the wear of each of the center electrode20and the second ground electrode90b. Additionally, as the noble metal, iridium is used. This allows appropriately suppressing the wear of the electrodes20,30b, and90b. Additionally, the core portion37bwith the higher thermal conductivity than that of the surface layer36bis buried inside of the first ground electrode30b. Accordingly, this allows suppressing the state where the temperature of the first ground electrode30bbecomes high and the long-continued state where the temperature of the first ground electrode30bis high during the operation of the internal combustion engine. As a result, this allows suppressing the wear of the first ground electrode30b(for example, oxidation of the surface of the first ground electrode30b). Additionally, the core portion97with the higher thermal conductivity than that of the surface layer96bis buried inside of the second ground electrode90b. Accordingly, this allows suppressing the state where the temperature of the second ground electrode90bbecomes high and the long-continued state where the temperature of the second ground electrode90bis high during the operation of the internal combustion engine. As a result, this allows suppressing the wear of the second ground electrode90b(for example, oxidation of the surface of the second ground electrode90b).

FIGS. 6A to 6Dare schematic diagrams showing a fourth embodiment of the spark plug.FIG. 6Ashows a sectional view similar to that ofFIG. 5A.FIG. 6Bshows a sectional view similar to that ofFIG. 5B.FIG. 6Cshows a schematic diagram similar to that ofFIG. 5C.FIG. 6Dshows a schematic diagram similar to that ofFIG. 5D. There is a difference from the spark plug100bof the third embodiment only in that the sealed surface between a metal shell50cand a supporting portion92cchanges in a stepped shape. The other configuration of a spark plug100cis the same as the configuration of the spark plug100bof the third embodiment (in the drawings, like reference signs designate corresponding or identical configurations, and therefore such configurations will not be further elaborated here). For example, the configuration of the metal shell50cof the fourth embodiment is the same as the configurations of the metal shells50of the first and second embodiments except that the shape of a front end portion501cis different. Additionally, the configuration of a second ground electrode90cof the fourth embodiment is the same as the configuration of the second ground electrode90binFIG. 5Aexcept that the shape (the shape of the portion to be sealed to the metal shell50c) of an end portion921cof the supporting portion92cis different from the shape (the shape of the portion to be sealed to the metal shell50b) of the end portion921bof the supporting portion92binFIG. 5B. The arrangement of the tips28,38, and98that form the gaps g1and g2is the same as the arrangement of the embodiments inFIGS. 2A to 2D,FIGS. 4A to 4D, andFIGS. 5A to 5D. Here, the right side ofFIG. 6Bshows an expansion figure of the sealed portion between the metal shell50cand the second ground electrode90c.

As shown inFIG. 6BandFIG. 6D, the second ground electrode90cincludes the supporting portion92cand the cylindrical tip98. The configuration other than the shape of the sealed surface with the metal shell50cin the configuration of the supporting portion92cis the same as the configuration of the supporting portion92binFIG. 5BandFIG. 5D. The cylindrical tip98is sealed to the inner peripheral surface of the hole forming portion91aof the supporting portion92c. The same core portion97as that of the third embodiment is disposed inside of the supporting portion92c. The remaining portion other than the core portion97in the supporting portion92cis a surface layer96c. In the drawings, a first connecting portion92scis the portion on the −Dy direction side with respect to the central axis CL in the supporting portion92c, and a second connecting portion92tcis the portion on the +Dy direction side with respect to the central axis CL in the supporting portion92c. As shown inFIG. 6B, the end portion921cof the first connecting portion92scis sealed to the metal shell50con the −Dy direction side with respect to the central axis CL. The end portion921cof the second connecting portion92tcis sealed to the metal shell50con the +Dy direction side with respect to the central axis CL.

As shown in the expansion figure inFIG. 6B, the end portion921cof the supporting portion92cincludes an inside portion941d, which is the portion on the inner peripheral side, and an outside portion941e, which is the portion on the outside in the radial direction of the inside portion941d. As shown inFIG. 6B, a surface941dson the second direction D2side of the inside portion941dand a surface941eson the second direction D2side of the outside portion941eare both planar surfaces perpendicular to the central axis CL. However, the surface941esof the outside portion941eis positioned on the first direction D1side with respect to the surface941dsof the inside portion941d. In the boundary portion between the inside portion941dand the outside portion941e, an outer peripheral surface941fs(also referred to as the partial cylindrical surface941fs) is formed. The outer peripheral surface941fshas the same shape as that of a part of a cylinder having the center on the central axis CL.

As shown inFIG. 6B, the front end portion501cof the metal shell50cincludes an inside portion501dand an outside portion501e, which is the portion on the outside in the radial direction of the inside portion501d. A surface501dson the first direction D1side of the inside portion501dand a surface501eson the first direction D1side of the outside portion501eare each a planar surface perpendicular to the central axis CL. However, the surface501esof the outside portion501eis positioned on the first direction D1side with respect to the surface501dsof the inside portion501d. In the boundary portion between the inside portion501dand the outside portion501e, an inner peripheral surface501fs(also referred to as the partial cylindrical surface501fs) is formed. The inner peripheral surface501fshas the same shape as that of a part of a cylinder having the center on the central axis CL.

As shown inFIG. 6B, the second ground electrode90cis fitted to the front end portion501cof the metal shell50cfrom the first direction D1side toward the second direction D2. The surface941esof the outside portion941eof the supporting portion92cis brought into contact with the surface501esof the outside portion501eof the metal shell50c. The surface941dsof the inside portion941dof the supporting portion92cis brought into contact with the surface501dsof the inside portion501dof the metal shell50c. A boundary portion W95cbetween the supporting portion92cand the metal shell50cis welded by laser beam welding from outside in the radial direction.

The partial cylindrical surface941fsof the supporting portion92cis brought into contact with the partial cylindrical surface501fsof the metal shell50c. Accordingly, this allows suppressing the displacement (the displacement in the direction perpendicular to the central axis CL) of the second ground electrode90cwith respect to the metal shell50c. As a result, the second gap size dg2is approximately constant over the whole circumference on the outer peripheral surface of the tip28of the center electrode20.

As shown inFIG. 6B, the first ground electrode30bis sealed to the surface92bson the first direction D1side of the supporting portion92cof the second ground electrode90c(for example, by laser beam welding). Here, a depressed portion or a cutout may be disposed on the surface92bson the first direction D1side of the supporting portion92cof the second ground electrode90c, and one end portion of the first ground electrode30bmay be arranged to be sealed to the depressed portion or the cutout.

Here, the configuration of the portion other than the above-described difference of the spark plug100cof the fourth embodiment is the same as the configuration of the spark plug100bof the third embodiment. Accordingly, the spark plug100cof the fourth embodiment can achieve various advantages similar to those of the spark plug100bof the third embodiment. For example, the proportion of the first gap size dg1to the second gap size dg2is set to be equal to or more than 0.80 and equal to or less than 1.25. This allows approximately equally using both the first ground electrode30band the second ground electrode90cfor discharge. This consequently allows suppressing significant wear of one ground electrode compared with the other ground electrode, thus improving the durability of the spark plug100c. Similarly to the first embodiment described inFIGS. 3A and 3B, setting the shortest distance to be twice or more as large as the maximum value between the first initial gap size dg1and the second initial gap size dg2allows suppressing the creeping discharge. As a result, the durability of the spark plug100ccan be improved. Additionally, the first gap g1is formed by the noble metal alloy (specifically, the tip28and the tip38). This allows suppressing the wear of each of the center electrode20and the first ground electrode30b. Additionally, the second gap g2is formed by the noble metal alloy (specifically, the tip28and the cylindrical tip98). This allows suppressing the wear of each of the center electrode20and the second ground electrode90c. Additionally, as the noble metal, iridium is used. This allows appropriately suppressing the wear of the electrodes20,30b, and90c. Additionally, the core portion37bwith the higher thermal conductivity than that of the surface layer36bis buried inside of the first ground electrode30b. Accordingly, this allows suppressing the wear of the first ground electrode30b. Additionally, the core portion97with the higher thermal conductivity than that of the surface layer96cis buried inside of the second ground electrode90c. Accordingly, this allows suppressing the wear of the second ground electrode90c.

(1) In the above-described respective embodiments, the first ground electrode is preferred to include a first nickel portion that is the portion formed by nickel or a nickel alloy, and the nickel content of the first nickel portion is preferred to be equal to or more than 90 weight %. For example, in the above-described embodiments, the base material32inFIG. 2Aand the surface layers36and36binFIG. 4A,FIG. 5B, andFIG. 6Beach correspond to the first nickel portion. An increase in nickel content allows improving the thermal conductivity of the first ground electrode. Accordingly, this simply allows transferring heat from the first ground electrode to the metal shell during the operation of the internal combustion engine. Thus, this allows suppressing the state where the temperature of the first ground electrode becomes high and the long-continued state where the temperature of the first ground electrode is high. As a result, this allows suppressing the wear of the first ground electrode (for example, oxidation of the surface of the first ground electrode). However, the nickel content of the first nickel portion of the first ground electrode may be less than 90 weight %.

Similarly, the second ground electrode is preferred to include a second nickel portion that is the portion formed by nickel or a nickel alloy, and the nickel content of the second nickel portion is preferred to be equal to or more than 90 weight %. For example, in the above-described embodiments, the entire supporting portion92inFIG. 2Aand the surface layers96,96b, and96cinFIG. 4B,FIG. 5B, andFIG. 6Beach corresponds to the second nickel portion. In the case where the nickel content of this second nickel portion is equal to or more than 90 weight %, this simply allows transferring heat from the second ground electrode to the metal shell during the operation of the internal combustion engine. Thus, this allows suppressing the state where the temperature of the second ground electrode becomes high and the long-continued state where the temperature of the second ground electrode is high. As a result, this allows suppressing the wear of the second ground electrode (for example, oxidation of the surface of the second ground electrode). However, the nickel content of the second nickel portion of the second ground electrode may be less than 90 weight %.

However, the first ground electrode may be formed using a conductive material other than nickel without containing nickel. Similarly, the second ground electrode may be formed using a conductive material other than nickel without containing nickel.

(2) In the above-described embodiments that include the core portions37and37bof the first ground electrodes, the core portions37and37bmay be omitted. Additionally, in the embodiment without the core portion, the core portion (for example, the core portions37and37b) may be added. Additionally, in the embodiment that includes the core portion97of the second ground electrode, the core portion97may be omitted. In the embodiment without the core portion97, the core portion97may be added. In this method, the core portion may be disposed only in any one of the first ground electrode and the second ground electrode. The core portion may be omitted from both the first ground electrode and the second ground electrode. The core portion may be disposed in both the first ground electrode and the second ground electrode.

As the material of the core portion, various materials with larger thermal conductivities than that of the surface layer disposed in the peripheral area of the core portion can be adopted. For example, a conductive material such as copper, an alloy containing copper, and silver can be adopted.

(3) In the above-described respective embodiments, respective noble metal tips apart from one another may be disposed in the portion that forms the first gap g1and the portion that forms the second gap g2in the center electrode20. Additionally, the above-described respective embodiments, at least one of the noble metal tips38and98disposed in the ground electrode may be omitted. In the above-described respective embodiments, the noble metal tips of one or more portions optionally selected from the portion that forms the first gap g1of the center electrode20, the portion that generates the second gap g2of the center electrode20, the portion that forms the first gap g1of the first ground electrode, and the portion that forms the second gap g2of the second ground electrode may be omitted.

The material of the noble metal tip is not limited to iridium or an alloy containing iridium, and other various materials can be adopted. For example, platinum or an alloy containing platinum may be adopted. Generally, a noble metal or a noble metal alloy can be adopted. Additionally, the respective materials of the noble metal tips in the portion that forms the first gap g1of the center electrode20, the portion that generates the second gap g2of the center electrode20, the portion that forms the first gap g1of the first ground electrode, and the portion that forms the second gap g2of the second ground electrode may be selected independently from one another. For example, the tip28may be formed using the noble metal (for example, iridium). The noble metal tip38and the cylindrical tip98may be formed using the noble metal alloy (for example, an iridium alloy).

(4) The area of the discharging surface (in the above-described respective embodiments, the area of the inner peripheral surface98sof the cylindrical tip98) that forms the second gap g2of the second ground electrode is preferred to be twice or more as large as the area of the discharging surface (in the above-described respective embodiments, the area of the surface38sof the tip38) that forms the first gap g1of the first ground electrode. This configuration achieves the area of the discharging surface three times as large as the area in the case where the second ground electrode is omitted, thus improving the durability of the spark plug. For example, a stable discharge can be achieved over a long period of time.

(5) To suppress the displacement (particularly, the displacement in the direction intersecting with the central axis CL) of the second ground electrode with respect to the metal shell, the second ground electrode is preferred to be the surface in contact with the metal shell and to have the surface (referred to as a “position specifying surface”) specified by the normal line intersecting with the first direction D1. For example, in the above-described embodiments, the surfaces on the outside in the radial direction of the two end portions921and921aof the supporting portions92and92ainFIGS. 2A to 2DandFIGS. 4A to 4Dand the surfaces (the partial cylindrical surface941fs) on the outside in the radial direction of the inside portion941dof the two end portions921cin the supporting portion92cinFIGS. 6A to 6Drespectively correspond to the position specifying surface. In these embodiments, the normal direction of the position specifying surface is the same as the radial direction in the position specifying surface. Generally, the second ground electrode is preferred to have two or more position specifying surfaces that are arranged in mutually different directions observed from the central axis CL and have mutually different normal directions. This configuration allows appropriately suppressing the displacement (the displacement in the direction intersecting with the central axis CL) of the second ground electrode with respect to the metal shell. For example, the configuration where the depressed portion or the convex portion of the second ground electrode is fitted to the convex portion or the depressed portion of the metal shell can be adopted. Here, the normal direction of the position specifying surface may be the direction obliquely inclined with respect to the planar surface perpendicular to the central axis CL. However, to suppress the displacement in the first direction D1of the second ground electrode, the normal direction of the position specifying surface is preferred to be the same as the radial direction in the position specifying surface.

Here, the configurations of the center electrode, the first ground electrode, and the second ground electrode are not limited to the above-described configurations. Other various configurations can be adopted.

The present invention has been described above based on the embodiment and the modifications. The above-described embodiments of the invention are for ease of understanding of the present invention and do not limit the present invention. The present invention may be modified or improved without departing from the gist and the claims of the present invention, and includes the equivalents.

INDUSTRIAL APPLICABILITY

The present invention is preferably applicable to a spark plug that includes a center electrode, a first ground electrode that forms a first gap with a front end surface of the center electrode, and a second ground electrode that forms an annular second gap between the side surface of the center electrode and the inner peripheral surface of the second ground electrode.

DESCRIPTION OF REFERENCE SIGNS