Internal combustion engine spark plug and method of manufacturing the same

A spark plug includes a plug cover provided at a tip end portion of a housing to cover an auxiliary combustion chamber where an electrical discharge gap is located. The plug cover has a curved surface portion. The curved surface portion includes an outer curved surface convexly curved outward. The curved surface portion has an inclined injection hole formed therein to allow communication between the auxiliary combustion chamber and an outside. The inclined injection hole is opened to be inclined, relative to a plug axial direction, outward in a plug radial direction toward a tip end side. In a cross section including a central axis of the inclined injection hole and along the plug axial direction, a tangent line of a portion of the outer curved surface that overlaps an extended line of the central axis of the inclined injection hole is orthogonal to the extended line.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2021-073795 filed on Apr. 26, 2021, the description of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Technical Field

The present disclosure relates to an internal combustion engine spark plug and a method of manufacturing the same.

Related Art

An internal combustion engine spark plug that includes an auxiliary combustion chamber at its tip end is known.

SUMMARY

An aspect of the present disclosure provides an internal combustion engine spark plug including: a cylindrical insulator; a center electrode that is held on an inner circumferential side of the insulator and protrudes from the insulator toward a tip end side; a cylindrical housing that holds the insulator on its inner circumferential side; a ground electrode that forms an electrical discharge gap between itself and the center electrode; and a plug cover provided at a tip end portion of the housing to cover an auxiliary combustion chamber where the electrical discharge gap is located. The plug cover has a curved surface portion that includes an outer curved surface convexly curved outward. The curved surface portion has an inclined injection hole formed therein to allow communication between the auxiliary combustion chamber and an outside, the inclined injection hole being opened to be inclined, relative to a plug axial direction, outward in a plug radial direction toward the tip end side. In a cross section including a central axis of the inclined injection hole and along the plug axial direction, a tangent line of a portion of the outer curved surface that overlaps an extended line of the central axis of the inclined injection hole is orthogonal to the extended line.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As disclosed in Japanese Unexamined Patent Application Publication No. 2020-009747, for example, an internal combustion engine spark plug that includes an auxiliary combustion chamber at its tip end is known. In the spark plug, the auxiliary combustion chamber is covered by a plug cover, whose tip end portion is decreased in thickness. This intends to prevent the tip end portion from reaching high temperatures and prevent ignition of a gas mixture prior to electrical discharge generated by the spark plug (i.e., pre-ignition).

The spark plug described in Japanese Unexamined Patent Application Publication No. 2020-009747 gives consideration to heat received from combustion in a main combustion chamber, but not to heat received from flames injected from the auxiliary combustion chamber to the main combustion chamber via an injection hole formed in the plug cover. To be more specific, no consideration is given to preventing an inner circumferential surface of the injection hole and its surrounding regions from reaching high temperatures owing to the flames. It can therefore be said that there still remains a concern about pre-ignition originated from the inner circumferential surface of the injection hole and its surrounding regions, leaving room for further improvement.

The present disclosure has been made in view of such problems, and intends to provide an internal combustion engine spark plug that can prevent pre-ignition, and a method of manufacturing the same.

First Embodiment

With reference toFIGS.1-6, an embodiment relating to an internal combustion engine spark plug and a method of manufacturing the same will be described.

As shown inFIGS.1and2, an internal combustion engine spark plug1in the present embodiment has a cylindrical insulator3, a center electrode4, a cylindrical housing2, a ground electrode6, and a plug cover5. The center electrode4is held on an inner circumferential side of the insulator3and protrudes from the insulator3toward a tip end side. The housing2holds the insulator3on its inner circumferential side. The ground electrode6forms an electrical discharge gap G between itself and the center electrode4. The plug cover5is provided at a tip end portion of the housing2to cover an auxiliary combustion chamber50where the electrical discharge gap G is located.

The plug cover5has a curved surface portion52. The curved surface portion52includes an outer curved surface521convexly curved outward. The curved surface portion52has an inclined injection hole51formed therein to allow communication between the auxiliary combustion chamber50and an outside. The inclined injection hole51is opened to be inclined, relative to a plug axial direction Z, outward in a plug radial direction toward the tip end side. As shown inFIG.1, in a cross section including a central axis of the inclined injection hole51and along the plug axial direction Z, a tangent line521L of a portion of the outer curved surface521that overlaps an extended line51L of the central axis of the inclined injection hole51is orthogonal to the extended line51L. Here, the term “orthogonal to” does not mean “being strictly orthogonal to”, but includes, for example, a variation range within ±5° from “being strictly orthogonal to”.

The spark plug1in the present embodiment can be used, for example, as ignition means in an internal combustion engine of an automobile and the like. As shown inFIG.4, the spark plug1is attached to an internal combustion engine10by allowing a threaded portion21formed on an outer circumferential surface of the housing2to be screwed into an internally-threaded portion of a plug hole711in a cylinder head71. With the spark plug1attached to the internal combustion engine10, the housing2is in thermal contact with the cylinder head71of the internal combustion engine via the threaded portion21.

The internal combustion engine10includes a piston74that reciprocates in a cylinder70. Reciprocation of the piston74changes the volume of a main combustion chamber101. The internal combustion engine10also has an intake port721and an exhaust port731both formed therein, which are respectively equipped with an intake valve72and an exhaust valve73.

The spark plug1then has one end in its axial direction Z arranged in the main combustion chamber101of the internal combustion engine10. With respect to the spark plug1in its axial direction Z, a side where the spark plug1is exposed to the main combustion chamber101shall be termed a tip end side, and a side opposite thereto shall be termed a base end side. The axial direction Z of the spark plug1shall be termed a plug axial direction Z or simply termed a Z direction, as appropriate. Note that a plug central axis PC shall mean a central axis PC of the spark plug1. As shown inFIGS.1and2, the plug central axis PC is also a central axis of the center electrode4in the present embodiment. The plug radial direction means a radial direction of a circle which is on a plane orthogonal to the plug central axis PC and whose center is on the plug central axis PC.

As shown inFIG.4, with the spark plug1attached to the internal combustion engine10, the plug cover5provides a partition between the auxiliary combustion chamber50and the main combustion chamber101. The inclined injection hole51allows communication between the auxiliary combustion chamber50and the main combustion chamber101. The plug cover5is constituted of a material, an example of which includes iron, nickel, alloyed iron or nickel, stainless steel, and the like.

As shown inFIG.1, the plug cover5is joined to the tip end portion of the housing2by welding and the like such that its central axis CC overlaps the plug central axis PC. The plug cover5and the housing2are in thermal contact with each other.

In the present embodiment, the plug cover5has a circumferential wall portion53and a bottom wall portion54. The circumferential wall portion53is an approximately cylindrically-shaped portion that covers a part of an outer circumferential side of the auxiliary combustion chamber50. The bottom wall portion54is a portion that covers a tip end side of the auxiliary combustion chamber50. The curved surface portion52links a tip end of the circumferential wall portion53and an outer circumference of the bottom wall portion54in a curved surface-like manner.

In the present embodiment, the curved surface portion52has a uniform thickness. Here, the term “uniform” shall permit a case where, for example, a thickness difference between the thickest portion and the thinnest portion of the curved surface portion52falls within 5% from the average thickness of the curved surface portion52.

Furthermore, the curved surface portion52includes an inner curved surface522convexly curved outward. In the cross section including the central axis of the inclined injection hole51and along the plug axial direction Z, a tangent line522L of a portion of the inner curved surface522that overlaps an extended line51L of the central axis of the inclined injection hole51is orthogonal to the extended line51L.

The curved surface portion52is formed on the plug cover5entirely in a plug circumferential direction. Each of the outer curved surface521and the inner curved surface522is formed on the curved surface portion52also entirely in the plug circumferential direction. Note that the plug circumferential direction shall refer to a circumferential direction of a circle which is on a plane orthogonal to the plug central axis PC and whose center is on the plug central axis PC.

As shown inFIG.3, in a cross section including the central axis CC of the plug cover5, the outer curved surface521and the inner curved surface522are respectively formed to extend along virtual circles VC1, VC2. Furthermore, the virtual circle VC1along which the outer curved surface521extends and the virtual circle VC2along which the inner curved surface522extends are circles concentric with each other.

In the present embodiment, as shown inFIG.2, the curved surface portion52of the plug cover5has four inclined injection holes51formed therein. Each of the inclined injection holes51is formed to have an approximately circular cross-sectional shape orthogonal to its central axis. Each inclined injection hole51is formed to allow the extended line51L of its central axis to substantially pass through the plug central axis PC. In the present embodiment, one of the inclined injection holes51overlaps the ground electrode6when seen in the Z direction.

As shown inFIGS.1and2, the ground electrode6protrudes from its fixed end portion61, which is fixed to the housing2, into the auxiliary combustion chamber50.

Next, description will be given a method of manufacturing the spark plug1in the present embodiment.

The method of manufacturing the spark plug1in the present embodiment includes a cover forming step and an injection hole forming step. As shown inFIG.5, the cover forming step includes forming the plug cover5in a state where the inclined injection hole51is yet to be opened, such that a tangent line523L of a portion of the outer curved surface521that overlaps the extended line51L of the central axis of the inclined injection hole51to be opened is orthogonal to the extended line51L.

As shown inFIGS.5and6, the injection hole forming step includes, after the cover forming step, opening the inclined injection hole51at the curved surface portion52such that the tangent line523L of the portion of the outer curved surface521that overlaps the extended line51L of the central axis of the inclined injection hole51to be opened is orthogonal to the extended line51L.

In the present embodiment, the injection hole forming step is performed on the plug cover5that is yet to be fixed to the housing2. The injection hole forming step includes opening the inclined injection hole51by subjecting the curved surface portion52of the plug cover5to punch-stamping or drill-cutting. When stamping or cutting is performed, the inclined injection hole51is opened from the outer curved surface521toward the inner curved surface522at the curved surface portion52. Furthermore, the inclined injection hole51is opened along a direction of a normal to the portion of the outer curved surface521that overlaps the extended line51L of the central axis of the inclined injection hole51to be opened.

Furthermore, as shown inFIG.5, the cover forming step includes forming the plug cover5in the state where the inclined injection hole51is yet to be opened, such that a tangent line524L of a portion of the inner curved surface522that overlaps the extended line51L of the central axis of the inclined injection hole51to be opened is orthogonal to the extended line51L.

As shown inFIGS.5and6, the injection hole forming step includes opening the inclined injection hole51at the curved surface portion52such that the tangent line524L of the portion of the inner curved surface522that overlaps the extended line51L of the central axis of the inclined injection hole51to be opened is orthogonal to the extended line51L.

Furthermore, the cover forming step includes forming the plug cover5in the state where the inclined injection hole51is yet to be opened, by plastically working a plate-like member having a uniform thickness. As shown inFIG.5, the cover forming step also includes forming the curved surface portion52such that, in the cross section including the central axis CC of the plug cover5, the outer curved surface521and the inner curved surface522respectively extend along the virtual circles VC1, VC2. The cover forming step also includes forming the curved surface portion52such that the virtual circle VC1along which the outer curved surface521extends and the virtual circle VC2along which the inner curved surface522extends are circles concentric with each other.

With the cover forming step and the injection hole forming step described above, the plug cover5having the inclined injection hole51formed therein can be obtained. The plug cover5having the inclined injection hole51formed therein is then fixed to the housing2by welding or the like. As shown inFIGS.1and2, the spark plug1in the present embodiment can thereby be obtained.

Next, actions and effects of the present embodiment will be described.

In the above-described spark plug1, in the cross section including the central axis of the inclined injection hole51and along the plug axial direction Z, the tangent line521L of the portion of the outer curved surface521that overlaps the extended line51L of the central axis of the inclined injection hole51is orthogonal to the extended line51L of the central axis of the inclined injection hole51. The inclined injection hole51thus tends to have an inner circumferential surface511decreased in area. The amount of heat received by the inner circumferential surface511of the inclined injection hole51, owing to flames injected from the auxiliary combustion chamber50to the main combustion chamber via the inclined injection hole51, can thus be reduced. The inner circumferential surface511of the inclined injection hole51and its surrounding regions can thus be prevented from reaching high temperatures. Consequently, pre-ignition can be prevented.

The spark plug1generates electrical discharge at the electrical discharge gap G to thereby ignite a gas mixture in the auxiliary combustion chamber50and form flames. The flames generated in the auxiliary combustion chamber50are then injected to the main combustion chamber via the inclined injection hole51. The inner circumferential surface511of the inclined injection hole51therefore receives heat owing to the injected flames. The inner circumferential surface511of the inclined injection hole51thus tends to receive a larger amount of heat and reach higher temperatures as it has a larger area. Here, in the spark plug1in the present embodiment, in the cross section including the central axis of the inclined injection hole51and along the Z direction, the tangent line521L is orthogonal to the extended line51L of the central axis of the inclined injection hole51. As shown inFIG.6, the inclined injection hole51thus tends to be decreased in length D in its opening direction. The inner circumferential surface511of the inclined injection hole51thus tends to be decreased in area. The amount of heat received by the inner circumferential surface511of the inclined injection hole51, owing to flames injected to the main combustion chamber, can thus be reduced. The inner circumferential surface511of the inclined injection hole51and its surrounding regions can thus be prevented from reaching high temperatures. An internal combustion engine of an automobile and the like, having the spark plug1installed therein, can thus prevent pre-ignition even during high-load operation and the like. Consequently, the internal combustion engine can improve fuel efficiency and output.

In the cross section including the central axis of the inclined injection hole51and along the plug axial direction Z, the tangent line522L (seeFIG.1) is orthogonal to the extended line51L. The inclined injection hole51thus tends to be further decreased in length in its opening direction. The inner circumferential surface511of the inclined injection hole51thus tends to be further decreased in area. Consequently, pre-ignition can further be prevented.

In the cross section including the central axis CC of the plug cover5, the outer curved surface521and the inner curved surface522are respectively formed to extend along the virtual circles VC1, VC2. Furthermore, the virtual circle VC1along which the outer curved surface521extends and the virtual circle VC2along which the inner curved surface522extends are circles concentric with each other. The inclined injection hole51thus tends to be much further decreased in length in its opening direction. The inner circumferential surface511of the inclined injection hole51thus tends to be much further decreased in area. Consequently, pre-ignition can much further be prevented.

The above-described method of manufacturing the spark plug1includes opening the inclined injection hole51at the curved surface portion52such that the tangent line523L of the portion of the outer curved surface521that overlaps the extended line51L of the central axis of the inclined injection hole51to be opened is orthogonal to the extended line51L of the central axis of the inclined injection hole51. The inclined injection hole51can thus be formed such that the inner circumferential surface511of the inclined injection hole51is decreased in area. Consequently, the spark plug1that can prevent pre-ignition can be manufactured.

Furthermore, the cover forming step includes forming the plug cover5in the state where the inclined injection hole51is yet to be opened, such that the tangent line523L of the portion of the outer curved surface521that overlaps the extended line51L of the central axis of the inclined injection hole51to be opened is orthogonal to the extended line51L. This facilitates forming the inclined injection hole51such that the inner circumferential surface511of the inclined injection hole51is decreased in area. Consequently, the spark plug1that can prevent pre-ignition can be manufactured efficiently.

As shown inFIGS.7and8, assume a case where the inclined injection hole51is opened such that the tangent line523L is not orthogonal to the extended line51L. If a plug cover8in this comparative embodiment is manufactured as shown inFIG.8, the inclined injection hole51tends to be increased in length D in its opening direction, in comparison with the case, as shown inFIG.6, where the inclined injection hole51is opened such that the tangent line523L is orthogonal to the extended line51L. The inner circumferential surface511of the inclined injection hole51therefore tends to be increased in area. In the case of the present embodiment, on the contrary, the inclined injection hole51is opened such that the tangent line523L is orthogonal to the extended line51L, as described above. As shown inFIG.6, the inclined injection hole51thus tends to be decreased in length D in its opening direction. The inner circumferential surface511of the inclined injection hole51therefore tends to be decreased in area. Consequently, the spark plug1that can prevent pre-ignition can be manufactured.

Furthermore, in the present embodiment, the inclined injection hole51is opened along the direction of the normal to the portion of the outer curved surface521that overlaps the extended line51L of the central axis of the inclined injection hole51to be opened. When the inclined injection hole51is opened from the outer curved surface521toward the inner curved surface522by punch-stamping or drill-cutting, a tip end of the punch or drill is thus less likely to slip. This facilitates opening the inclined injection hole51at the curved surface portion52by stamping or cutting. In other words, the inclined injection hole51can be opened efficiently without laser processing, electrical discharge machining, or the like. Consequently, the spark plug1that can prevent pre-ignition can be manufactured efficiently.

The cover forming step includes forming the plug cover5in the state where the inclined injection hole51is yet to be opened, such that the tangent line524L (seeFIG.5) is orthogonal to the extended line51L. The injection hole forming step includes opening the inclined injection hole51at the curved surface portion52such that the tangent line524L is orthogonal to the extended line51L. The inclined injection hole51can thus be formed such that the inclined injection hole51is further decreased in length in its opening direction. The inclined injection hole51can thus be formed such that the inner circumferential surface511of the inclined injection hole51is further decreased in area. Consequently, the spark plug1that can further prevent pre-ignition can be manufactured.

Furthermore, the curved surface portion52has a uniform thickness. This facilitates opening the inclined injection hole51such that both of the tangent line523L and the tangent line524L are orthogonal to the extended line51L. The inclined injection hole51thus tends to be further decreased in length in its opening direction. The inner circumferential surface511of the inclined injection hole51thus tends to be further decreased in area. Consequently, pre-ignition can further be prevented.

Furthermore, in the present embodiment, the cover forming step includes forming the plug cover5in the state where the inclined injection hole51is yet to be opened, by plastically working a plate-like member having a uniform thickness. The cover forming step also includes forming the curved surface portion52such that, in the cross section including the central axis CC of the plug cover5, the outer curved surface521and the inner curved surface522respectively extend along the virtual circles VC1, VC2. The cover forming step also includes forming the curved surface portion52such that the virtual circle VC1along which the outer curved surface521extends and the virtual circle VC2along which the inner curved surface522extends are circles concentric with each other. The inclined injection hole51can thus be formed such that the inclined injection hole51is much further decreased in length in its opening direction. The inclined injection hole51can thus be formed such that the inner circumferential surface511of the inclined injection hole51is much further decreased in area. Consequently, the spark plug1that can much further prevent pre-ignition can be manufactured.

As described above, in the spark plug1in the present embodiment, the inner circumferential surface511of the inclined injection hole51tends to be decreased in area. A flame jet injected to the main combustion chamber via the inclined injection hole51is therefore less likely to be cooled by the inner circumferential surface511of the inclined injection hole51. The flame jet can thus be intensified. Consequently, ignition properties in the main combustion chamber can be improved.

In the cover forming step, the plug cover5can be formed to suit the opening direction of the inclined injection hole51to be opened. This facilitates forming the inclined injection hole51such that the inclined injection hole51is opened in a desired direction with its inner circumferential surface511decreased in area. In other words, the inclined injection hole51can be formed such that the inner circumferential surface511is decreased in area, while the flame jet can be prevented from approaching the piston and the like in consideration of an inclination angle of a base end surface of the main combustion chamber, a cylinder diameter, the shape of a base end surface of the piston, and the like. The inclined injection hole51can thus be formed such that cooling loss of the flame jet can be reduced while the heat received by the inner circumferential surface511of the inclined injection hole51is reduced. Consequently, the spark plug1that can prevent pre-ignition and improve ignition properties can be manufactured.

In the spark plug1in the present embodiment, the inclined injection hole51tends to be decreased in length in its opening direction. The flame jet thus tends to be injected toward the main combustion chamber in a spreading manner and its injecting speed tends to be decreased. The flame jet can thus further be prevented from approaching the piston and the like. Cooling loss of the flame jet can further be reduced. Consequently, ignition properties can further be improved. Furthermore, the decrease in flame jet injecting speed can prospectively reduce pseudo knocking sounds caused by the flame jet being injected.

The tangent line522L is orthogonal to the extended line51L, so that the direction of gas expansion caused by combustion in the auxiliary combustion chamber50tends to coincide with the opening direction of the inclined injection hole51. This facilitates intensifying the flame jet. Consequently, ignition properties can be improved.

According to the present embodiment, as described above, the internal combustion engine spark plug1that can prevent pre-ignition and the method of manufacturing the same can be provided.

Second Embodiment

As shown inFIG.9, the present embodiment is obtained by modifying the first embodiment in position to form the inclined injection hole51.

In the present embodiment, as shown inFIG.9, the inclined injection hole51is formed so as not to overlap the ground electrode6when seen in the Z direction.

The other features are similar to those of the first embodiment. Note that unless otherwise specified, a sign used in the second and following embodiments and identical to a sign used in the embodiment(s) that has/have already been discussed represents a component or the like similar to that in the embodiment(s) that has/have already been discussed.

The present embodiment also has actions and effects similar to those of the first embodiment.

Third Embodiment

As shown inFIG.10, the present embodiment is obtained by modifying the first embodiment in shape of the plug cover5.

In the present embodiment, as shown inFIG.10, the virtual circle VC1along which the outer curved surface521extends and the virtual circle VC2along which the inner curved surface522extends have approximately the same diameters. Furthermore, in the cross section including the central axis CC of the plug cover5, the virtual circle VC1along which the outer curved surface521extends and the virtual circle VC2along which the inner curved surface522extends have centers different from each other.

In the present embodiment, the bottom wall portion54of the plug cover5further has an axial-direction injection hole55formed therein. The axial-direction injection hole55is formed such that an extended line55L of its central axis extends along the plug axial direction Z. Furthermore, the axial-direction injection hole55is formed at a position closer to the plug central axis PC than the inclined injection hole51is.

The other configurations and the actions and effects are similar to those of the first embodiment.

Fourth Embodiment

As shown inFIG.11, the present embodiment is obtained by modifying the third embodiment in shape of the plug cover5.

In the present embodiment, as shown inFIG.11, the virtual circle VC2along which the inner curved surface522extends has a diameter equal to or smaller than a radius of the virtual circle VC1along which the outer curved surface521extends.

Furthermore, the curved surface portion52has a smaller thickness further from the circumferential wall portion53and further from the bottom wall portion54.

Except for this, the present embodiment is similar to the third embodiment.

The curved surface portion52has a smaller thickness further from the circumferential wall portion53and further from the bottom wall portion54. The inner circumferential surface511of the inclined injection hole51thus tends to be further decreased in area. Consequently, pre-ignition can further be prevented.

Except for this, the present embodiment has actions and effects similar to those of the third embodiment.

Fifth Embodiment

As shown inFIG.12, the present embodiment is obtained by modifying the first embodiment in shape of the plug cover5.

In the present embodiment, as shown inFIG.12, the curved surface portion52of the plug cover5assumes the shape of a surface curved convexly on the tip end side. Furthermore, the curved surface portion52has a uniform thickness.

In the cross section including the central axis CC of the plug cover5, the outer curved surface521and the inner curved surface522are respectively formed to extend along virtual ellipses VC3, VC4.

In the cross section including the central axis of the inclined injection hole51and along the Z direction, the tangent line521L and the tangent line522L are orthogonal to the extended line51L.

The other configurations and the actions and effects are similar to those of the first embodiment.

Sixth Embodiment

As shown inFIG.13, the present embodiment is obtained by modifying the fifth embodiment in shape of the plug cover5.

In the present embodiment, as shown inFIG.13, the curved surface portion52has a larger thickness toward the base end side. In other words, the curved surface portion52has a smaller thickness as it is closer to the plug central axis PC.

Furthermore, in the cross section including the central axis of the inclined injection hole51and along the Z direction, the tangent line522L is inclined relative to the extended line51L.

Except for this, the present embodiment is similar to the fifth embodiment.

The curved surface portion52has a larger thickness toward the base end side. Heat of a tip end portion of the curved surface portion52thus tends to be transferred to the base end side. The tip end portion of the curved surface portion52can thus be prevented from reaching high temperatures. Consequently, pre-ignition can be prevented.

In other words, the tip end portion of the curved surface portion52is at a longer distance from the housing2than a base end portion of the curved surface portion52is. The tip end portion of the curved surface portion52is therefore less likely to dissipate heat via the housing2to the outside than the base end portion of the curved surface portion52is. The tip end portion of the curved surface portion52therefore tends to reach higher temperatures than the base end portion of the curved surface portion52does. Here, the spark plug1in the present embodiment has the curved surface portion52whose thickness is larger toward the base end side. Heat of the tip end portion of the curved surface portion52thus tends to be transferred to the base end side of the curved surface portion52. Heat of the tip end portion of the curved surface portion52thus tends to be dissipated via the housing2to the outside. Consequently, pre-ignition can be prevented.

Except for this, the present embodiment has actions and effects similar to those of the fifth embodiment.

The first to sixth embodiments, described above, each have four inclined injection holes51formed in the curved surface portion52. It is noted that the curved surface portion can have less than or more than four inclined injection holes formed therein.

The present disclosure is not limited to each of the above-described embodiments, and is applicable to various embodiments without departing from the gist thereof.

An aspect of the present disclosure provides an internal combustion engine spark plug (1) including:

a cylindrical insulator (3);

a center electrode (4) that is held on an inner circumferential side of the insulator and protrudes from the insulator toward a tip end side;

a cylindrical housing (2) that holds the insulator on its inner circumferential side;

a ground electrode (6) that forms an electrical discharge gap (G) between itself and the center electrode; and

a plug cover (5) provided at a tip end portion of the housing to cover an auxiliary combustion chamber (50) where the electrical discharge gap is located,

the plug cover having a curved surface portion (52) that includes an outer curved surface (521) convexly curved outward,

the curved surface portion having an inclined injection hole (51) formed therein to allow communication between the auxiliary combustion chamber and an outside, the inclined injection hole (51) being opened to be inclined, relative to a plug axial direction (Z), outward in a plug radial direction toward the tip end side, and

in a cross section including a central axis of the inclined injection hole and along the plug axial direction, a tangent line (521L) of a portion of the outer curved surface that overlaps an extended line (51L) of the central axis of the inclined injection hole being orthogonal to the extended line.

Another aspect of the present disclosure provides a method of manufacturing an internal combustion engine spark plug (1) including

a cylindrical insulator (3),

a center electrode (4) that is held on an inner circumferential side of the insulator and protrudes from the insulator toward a tip end side,

a cylindrical housing (2) that holds the insulator on its inner circumferential side,

a ground electrode (6) that forms an electrical discharge gap (G) between itself and the center electrode, and

a plug cover (5) provided at a tip end portion of the housing to cover an auxiliary combustion chamber (50) where the electrical discharge gap is located,

the plug cover having a curved surface portion (52) that includes an outer curved surface (521) convexly curved outward, and

the curved surface portion having an inclined injection hole (51) formed therein to allow communication between the auxiliary combustion chamber and an outside, the inclined injection hole (51) being opened to be inclined, relative to a plug axial direction (Z), outward in a plug radial direction toward the tip end side,

the method including:

a cover forming step of forming the plug cover in a state where the inclined injection hole is yet to be opened, such that a tangent line (523L) of a portion of the outer curved surface that overlaps an extended line (51L) of a central axis of the inclined injection hole to be opened is orthogonal to the extended line; and

an injection hole forming step of, after the cover forming step, opening the inclined injection hole at the curved surface portion such that the tangent line of the portion of the outer curved surface that overlaps the extended line of the central axis of the inclined injection hole to be opened is orthogonal to the extended line.

In the above-described spark plug, in the cross section including the central axis of the inclined injection hole and along the plug axial direction, the tangent line of the portion of the outer curved surface that overlaps the extended line of the central axis of the inclined injection hole is orthogonal to the extended line of the central axis of the inclined injection hole. The inner circumferential surface of the inclined injection hole thus tends to be decreased in area. The amount of heat received by the inner circumferential surface of the inclined injection hole, owing to flames injected from the auxiliary combustion chamber to the main combustion chamber via the inclined injection hole, can thus be reduced. The inner circumferential surface of the inclined injection hole and its surrounding regions can thus be prevented from reaching high temperatures. Consequently, pre-ignition can be prevented.

In the above-described method of manufacturing the spark plug, the inclined injection hole is opened at the curved surface portion such that the tangent line of the portion of the outer curved surface that overlaps the extended line of the central axis of the inclined injection hole to be opened is orthogonal to the extended line of the central axis of the inclined injection hole. The inclined injection hole can thus be formed such that the inner circumferential surface of the inclined injection hole is decreased in area. Consequently, a spark plug that can prevent pre-ignition can be manufactured.

As described above, according to the above-described aspects, an internal combustion engine spark plug that can prevent pre-ignition and the method of manufacturing the same can be provided.