Spark plug

A spark plug includes a plug cap having a plurality of through holes. In a cross section including an axial line, the plug cap includes an enlarging portion in a portion of a region between a first imaginary straight line and a second imaginary straight line, the first imaginary straight line extending through a front end of the end portion of a ground electrode and being perpendicular to the axial line, the second imaginary straight line extending through a back end of one of inner open ends of the through holes that is closest to a front end of the spark plug and being perpendicular to the axial line. The enlarging portion being a portion in which a minimum distance from an outer surface to an inner surface of the plug cap increases with increasing distance in a direction from back to front.

FIELD OF THE INVENTION

The present invention relates to a spark plug including a pre-chamber for a combustion chamber of an engine.

BACKGROUND OF THE INVENTION

A spark plug including a pre-chamber for a combustion chamber of an engine is known. For example, see Japanese Unexamined Patent Application Publication No. 2017-103179 (“PTL 1”). This type of spark plug includes a plug cap that has through holes and that is connected to a front end portion of a metal shell. The spark plug ignites combustible air-fuel mixture that has flowed into the plug cap from the combustion chamber through the through holes. The air-fuel mixture is combusted to generate an expansion pressure that causes a gas flow including flame to be injected into the combustion chamber through the through holes, so that the combustible air-fuel mixture in the combustion chamber is rapidly combusted due to the injected jet flow.

According to the technology described in PTL 1, the plug cap has a substantially constant wall thickness from the back end to the front end thereof. Therefore, a front end portion of the plug cap is easily cooled, and the thermal energy of the gas flow including flame is easily transferred to the plug cap. This causes a reduction in the energy of the jet flow injected into the combustion chamber through the through holes, resulting in a reduction in the rate of combustion in the combustion chamber or misfiring. Thus, combustion stability is reduced.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above-described problem, and an object of the present invention is to provide a spark plug with which stable combustion can be achieved.

To achieve the above-described object, a spark plug according to the present invention includes a metal shell having a tubular shape and extending along an axial line in a direction from front to back; a center electrode retained in the metal shell in an insulated manner; a ground electrode electrically connected to the metal shell and disposed such that a spark gap is formed between the center electrode and an end portion of the ground electrode; and a plug cap connected to a front end portion of the metal shell, the plug cap covering the center electrode and the end portion of the ground electrode from the front and having a plurality of through holes in a region in front of the ground electrode. In a cross section including the axial line, the plug cap includes an enlarging portion in at least a portion of a region between a first imaginary straight line and a second imaginary straight line, the first imaginary straight line extending through a front end of the end portion of the ground electrode and being perpendicular to the axial line, the second imaginary straight line extending through a back end of one of inner open ends of the through holes that is closest to a front end of the spark plug and being perpendicular to the axial line, the enlarging portion being a portion in which a minimum distance from an outer surface to an inner surface of the plug cap increases with increasing distance in a direction from back to front. A cross-sectional area of a region surrounded by the enlarging portion along a plane perpendicular to the axial line decreases with increasing distance in the direction from back to front.

According to a spark plug of a first aspect, the enlarging portion is provided between the front end of the end portion of the ground electrode and the back end of one of the inner open ends of the through holes in the plug cap that is closest to the front end of the spark plug, the enlarging portion being a portion in which the minimum distance from the outer surface to the inner surface of the plug cap increases with increasing distance in the direction from back to front. Heat conduction through the enlarging portion toward the back becomes more difficult with increasing distance in the direction from front to back, and heat capacity of the enlarging portion increases with increasing distance in the direction from back to front. Therefore, the temperature in a front region of the enlarging portion is not excessively reduced. As a result, the jet flow injected into the combustion chamber has sufficient thermal energy.

In addition, the cross-sectional area of the region surrounded by the enlarging portion along a plane perpendicular to the axial line decreases with increasing distance in the direction from back to front. Therefore, the speed of the gas flow in a front region of the enlarging portion can be increased. Accordingly, the jet flow injected into the combustion chamber has sufficient kinetic energy. Since the jet flow has sufficient thermal energy and sufficient kinetic energy, stable combustion of combustible air-fuel mixture can be achieved in the combustion chamber.

According to a spark plug of a second aspect, in the cross section including the axial line, a line showing an inner surface of the enlarging portion has a substantially constant radius of curvature. Accordingly, protrusions are not easily formed on the inner surface of the enlarging portion. As a result, overheating of the inner surface of the enlarging portion at the protrusions can be prevented. Thus, not only can the effects of the first aspect be obtained, but combustible air-fuel mixture that has flowed into the plug cap from the combustion chamber through the through holes can be prevented from undergoing pre-ignition with protrusions on the inner surface of the enlarging portion serving as ignition sources.

According to a spark plug of a third aspect, in the cross section including the axial line, the minimum distance from the outer surface to the inner surface of the plug cap is smallest at a front end of the inner surface of the plug cap. In this case, the heat capacity of a portion including the front end of the inner surface of the plug cap is less than the heat capacity of a portion including the front end of the enlarging portion. Accordingly, the temperature of the portion including the front end of the inner surface of the plug cap can be easily reduced. Thus, not only can the effects of the first and second aspects be obtained, but the combustible air-fuel mixture that has flowed into the plug cap from the combustion chamber through the through holes can be prevented from undergoing pre-ignition with the front end of the inner surface of the plug cap serving as an ignition source.

According to a spark plug of a fourth aspect, in the cross section including the axial line, the minimum distance from the outer surface to the inner surface of the plug cap is smallest at a front end of the outer surface of the plug cap. In this case, the heat capacity of the portion including the front end of the inner surface of the plug cap can be reduced in a region including the front end of the outer surface. Accordingly, the temperature of the portion including the front end of the outer surface of the plug cap can be easily reduced. Thus, not only can the effects of the third aspect be obtained, but the combustible air-fuel mixture in the combustion chamber can be prevented from undergoing pre-ignition with the front end of the outer surface of the plug cap serving as an ignition source.

According to a spark plug of a fifth aspect, a front end of the outer surface of the plug cap is a flat surface. In this case, overheating of the front end of the outer surface of the plug cap can be reduced. Therefore, not only can the effects of the first to fourth aspects be obtained, but the combustible air-fuel mixture in the combustion chamber can be prevented from undergoing pre-ignition with the front end of the outer surface of the plug cap serving as an ignition source.

According to a spark plug of a sixth aspect, the plug cap includes a straight portion formed between the enlarging portion and back ends of the plurality of through holes. A cross-sectional area of a region surrounded by the straight portion along a plane perpendicular to the axial line is constant over entire length of the straight portion in a direction of the axial line. Accordingly, the gas flow can be accelerated with less loss in the gas flow in the straight portion. Since the jet flow injected into the combustion chamber has sufficient kinetic energy, not only can the effects of the first to fifth aspects be obtained, but stable combustion of the combustible air-fuel mixture can be achieved in the combustion chamber.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings.FIG. 1is a partially sectioned view of a spark plug10according to a first embodiment. The bottom ofFIG. 1is defined as the front of the spark plug10, and the top ofFIG. 1is defined as the back of the spark plug10. This also applies toFIGS. 2 to 5.FIG. 1shows a cross section of a front end portion of the spark plug10including an axial line O. As illustrated inFIG. 1, the spark plug10includes an insulator11, a center electrode13, a metal shell20, a ground electrode30, and a plug cap40.

The insulator11is a substantially cylindrical member having an axial hole12that extends along the axial line O, and is made of a ceramic, such as alumina, having good mechanical characteristics and high insulation properties at high temperatures. The center electrode13is disposed in a front region of the axial hole12in the insulator11. The center electrode13is electrically connected to a metal terminal14in the axial hole12. The metal terminal14is a rod-shaped member to which a high-voltage cable (not shown) is connected, and is made of a conductive metal material (for example, low-carbon steel). The metal terminal14is fixed to the back end of the insulator11.

The metal shell20is a substantially cylindrical member made of a conductive metal material (for example, low-carbon steel). The metal shell20includes a front end portion22having an external thread21formed on an outer peripheral surface thereof, a seating portion23that is adjacent to the back end of the front end portion22, and a tool engagement portion24provided behind the seating portion23. The external thread21is screwed into a threaded hole2in an engine1. The seating portion23is a portion that seals a clearance between the threaded hole2in the engine1and the external thread21, and has an outer diameter greater than the outer diameter of the external thread21. The tool engagement portion24engages with a tool, such as a wrench, used to screw the external thread21into the threaded hole2in the engine1.

The ground electrode30is a rod-shaped member made of a metal material containing, for example, Ni as a main component. In the present embodiment, the ground electrode30is disposed at a position where the external thread21is provided, and extends through the front end portion22to project into the inside of the front end portion22. The ground electrode30includes an end portion31that faces the center electrode13. The plug cap40is connected to the front end portion22of the metal shell20.

The plug cap40is a portion that covers the center electrode13and the end portion31of the ground electrode30from the front. The plug cap40is made of a metal material containing, for example, Ni as a main component. The plug cap40has a plurality of through holes41in a region in front of the ground electrode30. When the spark plug10is installed by screwing the external thread21into the threaded hole2in the engine1, the plug cap40is exposed in the combustion chamber3of the engine1. The through holes41connect a pre-chamber42in the plug cap40to the combustion chamber3.

FIG. 2is an enlarged sectional view of part II of the spark plug10shown inFIG. 1including the axial line O. The front end portion22of the metal shell20has a recess25that is recessed radially inward in a region where the external thread21is provided. The front end portion22also has a hole26, which is thinner than the recess25, in a region radially inside the recess25. The hole26extends through the front end portion22in a radial direction. The ground electrode30is inserted through the hole26and joined to the front end portion22by a melted portion27. A spark gap33is formed between the end portion31of the ground electrode30and the center electrode13. Since the ground electrode30is joined to the metal shell20in the region where the external thread21is provided, heat is transferred from the ground electrode30to the engine1through the external thread21.

The plug cap40has an outer surface43that is sphere-cap-shaped and an inner surface44that is cone-shaped. The through holes41have outer open ends45in the outer surface43of the plug cap40and inner open ends46in the inner surface44of the plug cap40. Each through hole41is inclined toward the front in the direction from the inner open end46to the outer open end45thereof. In the present embodiment, back ends47of the inner open ends46of the through holes41are all positioned on a plane perpendicular to the axial line O. The plug cap40is joined to the front end portion22of the metal shell20by a melted portion48.

In a cross section including the axial line O, the plug cap40includes a first enlarging portion53and a second enlarging portion55in a region51between a first imaginary straight line49and a second imaginary straight line50. The first imaginary straight line49extends through a front end32of the end portion31of the ground electrode30and is perpendicular to the axial line O. The second imaginary straight line50extends through the back end47of one of the inner open ends46that is closest to the front end of the spark plug10and is perpendicular to the axial line O. The second enlarging portion55is located in front of the first enlarging portion53. The first enlarging portion53and the second enlarging portion55are each a portion in which the minimum distance from the outer surface43to the inner surface44of the plug cap40increases with increasing distance in the direction from back to front. The minimum distance from the outer surface43to the inner surface44of the plug cap40is the length of the shortest line segment between a point on the outer surface43and a point on the inner surface44in the region51.

The plug cap40includes a first straight portion52, a second straight portion54, and a third straight portion56in addition to the first enlarging portion53and the second enlarging portion55in the region51. The first straight portion52, the first enlarging portion53, the second straight portion54, the second enlarging portion55, and the third straight portion56are arranged in that order in the direction from back to front. The front end of the melted portion48is in contact with the first straight portion52.

The first enlarging portion53is adjacent to the front end of the first straight portion52, and the second straight portion54is adjacent to the front end of the first enlarging portion53. The second enlarging portion55is adjacent to the front end of the second straight portion54, and the third straight portion56is adjacent to the front end of the second enlarging portion55. The back ends47of the inner open ends46are in contact with the third straight portion56.

The cross-sectional area of a first inner region57, which is a region surrounded by the first straight portion52, along a plane perpendicular to the axial line O is constant over the entire length of the first straight portion52in an axial line direction. The cross-sectional area of the first inner region57along a plane perpendicular to the axial line O is equal to the cross-sectional area of the pre-chamber42along a plane perpendicular to the axial line O at the front end32of the end portion31of the ground electrode30.

The cross-sectional area of a second inner region58, which is a region surrounded by the first enlarging portion53, along a plane perpendicular to the axial line O decreases with increasing distance in the direction from back to front. The cross-sectional area of a third inner region59, which is a region surrounded by the second straight portion54, along a plane perpendicular to the axial line O is constant over the entire length of the second straight portion54in the axial line direction. The cross-sectional area of a fourth inner region60, which is a region surrounded by the second enlarging portion55, along a plane perpendicular to the axial line O decreases with increasing distance in the direction from back to front. The cross-sectional area of a fifth inner region61, which is a region surrounded by the third straight portion56, along a plane perpendicular to the axial line O is constant over the entire length of the third straight portion56in the axial line direction.

A front end region62, which is a region in front of the fifth inner region61in the pre-chamber42, includes a front end63of the inner surface44of the plug cap40. The cross-sectional area of the front end region62along a plane perpendicular to the axial line O at the back end of the front end region62is smaller than the cross-sectional area of the first enlarging portion53along a plane perpendicular to the axial line O at the back end of the first enlarging portion53. The front end63of the inner surface44is a portion of a sphere-cap-shaped curved surface. The front end63of the inner surface44is spaced from the inner open ends46of the through holes41. The front end63of the inner surface44is located on the axial line O. The minimum distance from the outer surface43to the inner surface44of the plug cap40is smallest at the front end63of the inner surface44. In particular, in the present embodiment, the minimum distance from the outer surface43to the inner surface44of the plug cap40is smallest between a front end64of the outer surface43of the plug cap40and the front end63of the inner surface44of the plug cap40. The front end64of the outer surface43is a flat surface that is perpendicular to the axial line O.

The spark plug10generates a discharge in the spark gap33to ignite combustible air-fuel mixture that has flowed into the plug cap40from the combustion chamber3through the through holes41. The air-fuel mixture is combusted to generate an expansion pressure that causes a gas flow including flame to be injected into the combustion chamber3through the through holes41, so that the combustible air-fuel mixture in the combustion chamber3is combusted due to the injected jet flow.

The spark plug10is configured such that the first enlarging portion53and the second enlarging portion55are provided in the region51between the front end32of the end portion31of the ground electrode30and the back end47of one of the inner open ends46of the through holes41in the plug cap40that is closest to the front end of the spark plug10. The first enlarging portion53and the second enlarging portion55are each a portion in which the minimum distance from the outer surface43to the inner surface44of the plug cap40in a cross section including the axial line O increases with increasing distance in the direction from back to front. Heat conduction through the first enlarging portion53and the second enlarging portion55toward the back becomes more difficult with increasing distance in the direction from front to back, and the heat capacities of the first enlarging portion53and the second enlarging portion55increase with increasing distance in the direction from back to front. Therefore, the temperature in a front region of the second enlarging portion55is not excessively reduced when, for example, load on the engine1is low. Accordingly, the jet flow injected into the combustion chamber3has sufficient thermal energy.

The cross-sectional area of each of the second inner region58surrounded by the first enlarging portion53and the fourth inner region60surrounded by the second enlarging portion55along a plane perpendicular to the axial line O decreases with increasing distance in the direction from back to front. Therefore, the speed of the gas flow including flame in the front region of the second enlarging portion55can be increased. Accordingly, the jet flow injected into the combustion chamber3has sufficient kinetic energy. As a result, owing to the high-energy jet flow, reduction in the rate of combustion in the combustion chamber3and misfiring can be prevented, and the combustible air-fuel mixture in the combustion chamber3can be rapidly combusted.

Since the second straight portion54is provided between the first enlarging portion53and the second enlarging portion55, the inclination of the inner surface44of the plug cap40with respect to the axial line O varies in a cross section including the axial line O. Therefore, a turbulent flow can be easily generated in the pre-chamber42. Accordingly, the rate of combustion in the pre-chamber42can be increased.

The spark plug10is configured such that, in a cross section including the axial line O, the minimum distance from the outer surface43to the inner surface44of the plug cap40is smallest at the front end63of the inner surface44of the plug cap40. Therefore, the heat capacity of a portion including the front end63of the inner surface44of the plug cap40is less than the heat capacities of portions including the front ends of the first enlarging portion53and the second enlarging portion55. Accordingly, the temperature of the portion including the front end63of the inner surface44of the plug cap40can be easily reduced by, for example, radiation. Therefore, when, for example, load on the engine1is high, the combustible air-fuel mixture that has flowed into the plug cap40from the combustion chamber3through the through holes41can be prevented from undergoing pre-ignition with the front end63of the inner surface44of the plug cap40serving as an ignition source.

The spark plug10is configured such that, in a cross section including the axial line O, the minimum distance from the outer surface43to the inner surface44of the plug cap40is smallest at the front end64of the outer surface43of the plug cap40. Therefore, the heat capacity of the portion including the front end63of the inner surface44of the plug cap40can be reduced in a region including the front end64of the outer surface43. Accordingly, the temperature of the portion including the front end64of the outer surface43of the plug cap40can be easily reduced by, for example, radiation. Therefore, when, for example, load on the engine1is high, the combustible air-fuel mixture in the combustion chamber3can be prevented from undergoing pre-ignition with the front end64of the outer surface43of the plug cap40serving as an ignition source.

The spark plug10is configured such that the front end64of the outer surface43of the plug cap40is a flat surface. Accordingly, compared to when the outer surface43has a thinned front end, such as a sphere-cap-shaped front end, overheating of the front end64of the outer surface43of the plug cap40can be reduced. Therefore, when, for example, load on the engine1is high, the combustible air-fuel mixture in the combustion chamber3can be prevented from undergoing pre-ignition with the front end64of the outer surface43of the plug cap40serving as an ignition source.

The plug cap40includes the third straight portion56formed between the second enlarging portion55and the back ends47of the through holes41. Since the cross-sectional area of the fifth inner region61surrounded by the third straight portion56along a plane perpendicular to the axial line O is constant over the entire length of the third straight portion56in the axial line direction, the gas flow can be accelerated with less loss in the gas flow in the third straight portion56. As a result, the jet flow injected into the combustion chamber3has sufficient kinetic energy, so that stable combustion of the combustible air-fuel mixture can be achieved in the combustion chamber3.

A second embodiment will be described with reference toFIG. 3. In the first embodiment, the plug cap40includes a plurality of enlarging portions (first enlarging portion53and second enlarging portion55). In contrast, in the second embodiment, a plug cap71includes one enlarging portion73. Components that are the same as those described in the first embodiment are denoted by the same reference signs, and description thereof is omitted.FIG. 3is a sectional view of a spark plug70according to the second embodiment including the axial line O. Similar to the first embodiment,FIG. 3is an enlarged view of part II (seeFIG. 1) of the spark plug70. This also applies toFIGS. 4 and 5.

The plug cap71of the spark plug70is joined to the front end portion22of the metal shell20by the melted portion48. The plug cap71has the outer open ends45of the through holes41in the outer surface43thereof and the inner open ends46of the through holes41in an inner surface72thereof.

The plug cap71includes the enlarging portion73in the region51. The enlarging portion73is formed such that the minimum distance from the outer surface43to the inner surface72of the plug cap71in a cross section including the axial line O increases with increasing distance in the direction from back to front. The front end of the melted portion48is in contact with the enlarging portion73. The back ends47of the inner open ends46are in contact with the enlarging portion73. The cross-sectional area of an inner region74, which is a region surrounded by the enlarging portion73, along a plane perpendicular to the axial line O decreases with increasing distance in the direction from back to front. A front end region75, which is a region in front of the inner region74in the pre-chamber42, includes the front end63of the inner surface72of the plug cap71.

In a cross section including the axial line O, a line showing the inner surface72of the enlarging portion73has a substantially constant radius of curvature. Since the inner surface72of the enlarging portion73has no significant inflection points, protrusions are not easily formed on the inner surface72of the enlarging portion73. As a result, overheating of the inner surface72of the enlarging portion73at protrusions can be prevented, so that the combustible air-fuel mixture that has flowed into the plug cap71from the combustion chamber3through the through holes41can be prevented from undergoing pre-ignition with protrusions on the inner surface72of the enlarging portion73serving as ignition sources. Preferably, the line showing the inner surface72of the enlarging portion73in a cross section including the axial line O has an exactly constant radius of curvature over the entire length of the enlarging portion73in the axial line direction.

A third embodiment will be described with reference toFIG. 4. In the first embodiment, the plug cap40includes the second straight portion54between the first enlarging portion53and the second enlarging portion55. In contrast, in the third embodiment, a plug cap81includes an enlarging portion84having a perpendicular surface83that is perpendicular to the axial line O. Components that are the same as those described in the first embodiment are denoted by the same reference signs, and description thereof is omitted.FIG. 4is a sectional view of a spark plug80according to the third embodiment including the axial line O.

The plug cap81of the spark plug80is joined to the front end portion22of the metal shell20by the melted portion48. The plug cap81has an inner surface82including the perpendicular surface83, which has an annular shape and is perpendicular to the axial line O. The plug cap81has the outer open ends45of the through holes41in the outer surface43thereof and the inner open ends46of the through holes41in the inner surface82thereof.

The plug cap81includes the enlarging portion84in the region51. The front end of the melted portion48is in contact with the enlarging portion84. The back ends47of the inner open ends46are in contact with the enlarging portion84. The cross-sectional area of an inner region85, which is a region surrounded by the enlarging portion84, along a plane perpendicular to the axial line O decreases with increasing distance in the direction from back to front. A front end region86, which is a region in front of the inner region85in the pre-chamber42, includes the front end63of the inner surface82of the plug cap81.

The enlarging portion84has the perpendicular surface83. In a cross section including the axial line O, the minimum distance from an intersection point43abetween a straight line83aincluding the perpendicular surface83and the outer surface43to a point on the perpendicular surface83(portion of the inner surface82) increases with increasing distance in a radially inward direction of the perpendicular surface83. The enlarging portion84is formed such that the minimum distance between a point on the outer surface43in a region in front of the intersection point43aand the inner surface82is greater than the minimum distance between a point on the outer surface43in a region behind the intersection point43aand the inner surface82. Thus, the enlarging portion84is formed such that the minimum distance from the outer surface43to the inner surface82of the plug cap81increases with increasing distance in the direction from back to front.

Since the spark plug80according to the third embodiment includes the enlarging portion84, effects similar to those of the spark plug10according to the first embodiment, which includes the first enlarging portion53and the second enlarging portion55, can be obtained. In addition, since the enlarging portion84has the perpendicular surface83, a turbulent flow can be easily generated in the pre-chamber42. As a result, the rate of combustion in the pre-chamber42can be increased.

A fourth embodiment will be described with reference toFIG. 5. In the first to third embodiments, the through holes41formed in the plug caps40,71, and81are at the same position in the axial line direction. In contrast, in the fourth embodiment, a plurality of through holes93,97, and99are provided at different positions in the axial line direction. Components that are the same as those described in the first embodiment are denoted by the same reference signs, and description thereof is omitted.FIG. 5is a sectional view of a spark plug90according to the fourth embodiment.

The spark plug90includes a plug cap91joined to the front end portion22of the metal shell20by the melted portion48. The plug cap91has the plurality of through holes93,97,99. The through hole93has an outer open end94in the outer surface43of the plug cap91and an inner open end95in an inner surface92of the plug cap91. The through hole97has an inner open end98in the inner surface92. The through hole99has an outer open end100in the outer surface43and an inner open end101in the inner surface92. The inner open end95is closest to the front end of the spark plug90, and the inner open end101is closest to the back end of the spark plug90. The through holes93,97, and99are inclined toward the front in the direction from the inner surface92to the outer surface43of the plug cap91.

In a cross section including the axial line O, the plug cap91includes an enlarging portion105in a region104between a first imaginary straight line49and a second imaginary straight line103. The first imaginary straight line49extends through the front end32of the end portion31of the ground electrode30and is perpendicular to the axial line O. The second imaginary straight line103extends through a back end96of the inner open end95, which is closest to the front end of the spark plug90, and is perpendicular to the axial line O. The enlarging portion105is formed such that the minimum distance from the outer surface43to the inner surface92of the plug cap91increases with increasing distance in the direction from back to front. The cross-sectional area of a first inner region107, which is a region surrounded by the enlarging portion105, along a plane perpendicular to the axial line O decreases with increasing distance in the direction from back to front.

A straight portion106is provided adjacent to the front end of the enlarging portion105. The straight portion106is in contact with a back end102of the inner open end101of the through hole99, which is one of the through holes93,97, and99that is closest to the back end of the spark plug90. The cross-sectional area of a second inner region108, which is a region surrounded by the straight portion106, along a plane perpendicular to the axial line O is constant over the entire length of the straight portion106in the axial line direction. A front end region109, which is a region in front of the second inner region108in the pre-chamber42, includes the front end63of the inner surface92of the plug cap91.

Since the spark plug90according to the fourth embodiment includes the enlarging portion105and the straight portion106, effects similar to those of the spark plug10according to the first embodiment, which includes the second enlarging portion55and the third straight portion56, can be obtained. In addition, since the inner surface92of the enlarging portion105includes radially inwardly convex inflection points near the straight portion106in a cross section including the axial line O, a turbulent flow can be easily generated in the pre-chamber42. As a result, the rate of combustion in the pre-chamber42can be increased.

Although the present invention has been described based on embodiments, the present invention is not limited to the above-described embodiments in any way, and it can be easily understood that various improvements and modifications are possible within the spirit of the present invention. For example, the shapes of the plug caps40,71,81, and91and the number, shapes, sizes, etc., of the through holes41,93,97, and99may be set as appropriate.

Although the plug caps40,71,81, and91are each welded to the metal shell20in the embodiments, the plug caps are not necessarily limited to this. For example, a plug cap may, of course, be a front end portion of a tubular member having a closed front end and connected to the front end portion22of the metal shell20. The tubular member is disposed to surround the outer periphery of the front end portion22of the metal shell20. An external thread formed on the outer peripheral surface of the tubular member is screwed into the threaded hole2in the engine1.

The tubular member (plug cap) may be connected to the front end portion22of the metal shell20by, for example, forming an internal thread on an inner peripheral surface of the tubular member and screwing the internal thread onto the external thread21formed on the front end portion22. Alternatively, a back end portion of the tubular member and the seating portion23of the metal shell20may be joined together by, for example, welding. Alternatively, a flange may be formed on the back end portion of the tubular member, and the seating portion23of the metal shell20and the flange may be joined together by, for example, welding. The tubular member may be made of, for example, a metal material such as a nickel-based alloy or a ceramic such as silicon nitride.

Although the ground electrode30that extends through the front end portion22of the metal shell20is disposed at a position where the external thread21is provided in the embodiments, the position of the ground electrode is not necessarily limited to this. For example, the plug cap may be disposed such that the front end surface of the front end portion22of the metal shell20is exposed, and the ground electrode may, of course, be connected to the front end surface of the front end portion22. The ground electrode may have either a straight shape or a bent shape. The ground electrode may be joined to the plug cap.

Although the inner open ends46,95,98, and101of the through holes41,93,97, and99appear in cross sections of the plug caps40,71,81, and91along a plane including the axial line O in the embodiments, the through holes are not necessarily limited to this. The through holes may, of course, be formed in the plug caps40,71,81, and91such that positions of the inner open ends thereof relative to the axial line O are shifted so that the inner open ends do not appear in cross sections along a plane including the axial line O. In such a case, the positions of the inner open ends of the through holes can be determined based on the inner open ends that appear in cross sections of the plug caps40,71,81, and91along a plane parallel to the axial line O. The regions51and104in cross sections along a plane including the axial line O are determined based on the determined positions of the inner open ends of the through holes. The minimum distances from the outer surfaces43to the inner surfaces44,72,82, and92of the plug caps40,71,81, and91are measured in the cross sections along a plane including the axial line O.

Although the front end64of the outer surface43of each of the plug caps40,71,81, and91is a flat surface in the embodiments, the front end64is not necessarily limited to this. The front end64of the outer surface43of each of the plug caps40,71,81, and91may, of course, instead be sphere-cap-shaped or cone-shaped.

Although the front end63of each of the inner surfaces44,72,82, and92of the plug caps40,71,81, and91is a portion of a curved surface in the embodiments, the front end63is not necessarily limited to this. The front end63of each of the inner surfaces44,72,82, and92of the plug caps40,71,81, and91may, of course, instead be a flat surface.

Although the front ends63of the inner surfaces44,72,82, and92of the plug caps40,71,81, and91are spaced from the front ends of the inner open ends46,95,98, and101of the through holes41,93,97, and99in the embodiments, the plug caps are not necessarily limited to this. The front ends of the inner open ends46,95,98, and101of the through holes41,93,97, and99may instead be in contact with the front ends63of the inner surfaces44,72,82, and92.

Although the front ends of the inner open ends46and95of the through holes41and93formed in the plug caps40,71,81, and91are positioned behind the front ends63of the inner surfaces44,72,82, and92of the plug caps40,71,81, and91in the embodiments, the positions of the front ends of the inner open ends46and95are not necessarily limited to this. The positions of the front ends of the inner open ends46and95may, of course, instead be the same as the positions of the front ends63of the inner surfaces44,72,82, and92in the axial line direction. In this case, the gas flow in the pre-chamber42is smoothly introduced into the through holes41and93.

In the fourth embodiment, the lines showing the inner surface92of the enlarging portion105in a cross section including the axial line O include radially inwardly convex inflection points. However, the enlarging portion105is not necessarily limited to this. For example, the lines showing the inner surface92of the enlarging portion105may, of course, have radially outwardly convex inflection points at positions behind the radially inwardly convex inflection points. Also in other embodiments, the inner surfaces of the enlarging portions may, of course, have radially outwardly convex inflection points.

Each embodiment may be modified by providing one or more portions of the structures of other embodiments in addition to the structure thereof or in place of one or more portions of the structure thereof or by omitting a portion of the structure thereof.

For example, in the first embodiment, the first straight portion52may, of course, be omitted and the first enlarging portion53may be extended toward the back so that the first enlarging portion53is in contact with the melted portion48. Also, in the first embodiment, the second straight portion54may, of course, be omitted so that the first enlarging portion53and the second enlarging portion55are connected to each other. Also, in the first embodiment, the third straight portion56may, of course, be omitted and the second enlarging portion55may be extended toward the front so that the second enlarging portion55is in contact with the back ends47of the inner open ends46. Also, in the first embodiment, one of the first enlarging portion53and the second enlarging portion55may, of course, be omitted or an additional enlarging portion may, of course, be provided.

In addition, in the third embodiment, a straight portion may, of course, be provided between the enlarging portion84and the back ends47of the inner open ends46. Also, in the third embodiment, a plurality of perpendicular surfaces83may, of course, be provided at different positions in the axial line direction.