Spark plug for internal combustion engine

A spark plug for an internal combustion engine is provided which has a center electrode and a ground electrode. The ground electrode includes an upright portion extending in a lengthwise direction of the spark plug and an extension bent from the upright portion in a radial direction of the spark plug. The extension has a slant surface which is shaped to be inclined away from the center electrode downstream in a flow of air-fuel mixture within a combustion chamber when the spark plug is mounted in the engine. This results in an increase in in distance by which a starting point on the ground electrode where a spark is created is moved on the slant surface, thereby increasing a length of time the spark is moved downstream and then blown out to increase the probability of successful ignition of the air-fuel mixture.

CROSS REFERENCE TO RELATED DOCUMENT

The present application claims the benefit of priority of Japanese Patent Application No. 2017-190578 filed on Sep. 29, 2017, the disclosure of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

This disclosure relates generally to a spark plug for internal combustion engines.

2. Background Art

Spark plugs are usually used to ignite fuel in internal combustion engines, such as automobile engines. Japanese Patent First Publication No. 2013-98042 discloses a spark plug equipped with a ground electrode and a center electrode. The ground electrode includes a ground electrode body and a ground electrode chip extending from the ground electrode body toward the center electrode. The ground electrode chip protrudes from the center of a width of the ground electrode body. The spark plug, as taught in the above publication, forms a spark gap between a surface of the ground electrode chip which faces the center electrode and the front end of the center electrode.

The end surface of the ground electrode chip which faces the center electrode slants downward toward the head of the length of the spark plug in a direction in which an air-fuel mixture flows through the spark gap. The spark gap, therefore, has the shortest interval between an upstream edge of the end surface of the center electrode chip and the front end of the center electrode and the longest interval between a downstream edge of the end of the center electrode chip and the front end of the center electrode. In other words, the spark gap gradually increases in the direction of the flow of the air-fuel mixture.

With the above arrangements of the spark plug, an initial spark will be created in the shortest interval of the spark gap which is located on the upstream side of the spark gap. This results in an increase in time it takes for the spark to be carried downward and then blown off in order to ensure the stability in igniting the air-fuel mixture using flame.

In the above spark plug, a starting point on the ground electrode where a spark is developed moves in the downstream direction on the end surface of the ground electrode chip which faces the center electrode. This causes a linear distance between the starting points on the center electrode and the ground electrode to increase and the spark to greatly expand in the downstream direction. Such an increase in linear distance between the starting points of the spark minimizes a risk that ends of the expanded spark are shorted and also facilitates the expansion of the spark, which leads to an increase in area of contact between the air-fuel mixture and the spark.

The above spark plug is, however, designed to have the starting point of a spark on the ground electrode which is movable in a range limited to the size of the ground electrode chip affixed to a portion of the width of the ground electrode body, thus having a limitation in expanding the spark. There is, therefore, still room for improvement in expanding the spark to enhance the stability in igniting the air-fuel mixture.

SUMMARY

It is therefore an object of this disclosure to provide a spark plug for an internal combustion engine which has an enhanced ability to ignite an air-fuel mixture.

According to one aspect of this disclosure, there is provided a spark plug for an internal combustion engine which comprises: (a) a hollow cylindrical housing; (b) a hollow cylindrical porcelain insulator which is retained inside the housing; (c) a center electrode which is retained inside the porcelain insulator with a top portion thereof protruding outside the porcelain insulator; and (d) a ground electrode which defines a spark gap between itself and the center electrode.

The ground electrode includes an upright portion which extends from a front end of the housing to a front side of the spark plug and an extension which is bent from the upright portion inwardly in a radial direction of the spark plug.

If a direction which is oriented perpendicular both to an extension lengthwise direction that is a lengthwise direction of the extension and to a plug axial direction that is an axial direction of the spark plug is defined as a lateral direction, and sides opposed to each other in the lateral direction are defined as a Y1side and a Y2side, respectively, the extension has an inner slant portion which faces the center electrode and is inclined away from the center electrode from the Y1side to the Y2side.

The extension has a first and a second edge which are opposed to each other in the lateral direction. The inner slant portion continuously extends from the first edge to the second edge of the extension.

The inner slant portion of the ground electrode, as described above, extends continuously from the first edge to the second edge of the inner extension surface in the lateral direction, in other words, fully occupies the width of the ground electrode. This results in an increase in distance by which a starting point on the ground electrode where a spark is created is moved on the inner slant portion, thereby increasing a length of time during which the spark is moved downstream in a flow of the air-fuel mixture and then blown out, to increase the probability of successful ignition of the air-fuel mixture, which improves the ability of the spark plug to ignite the air-fuel mixture. The continuous extending of the inner slant portion between the first and second edges of the inner extension surface in the lateral direction, as described above, results in an increase in linear interval between starting points of the spark on the center electrode and the ground electrode, in other words, an increase in distance the starting point on the ground electrode is moved. This minimizes a risk that the spark is shorted early to ground and results in an increase in elongation of the spark to enhance the ability of the spark plug to ignite the air-fuel mixture.

As apparent from the above discussion, this disclosure provides a spark plug for an internal combustion engine which has enhanced ability to ignite an air-fuel mixture.

In this disclosure, symbols in brackets represent correspondence relation between terms in claims and terms described in embodiments which will be discussed later, but are not limited only to parts referred to in the disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

The spark plug1for an internal combustion engine according to an embodiment will be described below with reference toFIGS. 1 to 8.

The spark plug1includes, as shown inFIGS. 1 to 4, the housing (also called a metal shell)11, the porcelain insulator12, the center electrode2, and the ground electrode3. The housing11is of a hollow cylindrical shape. The porcelain insulator12is, as clearly illustrated inFIG. 1, retained inside the housing11. The porcelain insulator11is of a hollow cylindrical shape. The center electrode2is disposed inside the porcelain insulator12with a head or a top portion extending outside an open end of the porcelain insulator12. The ground electrode3creates a spark gap (also called a discharge gap) G between itself and the center electrode2.

The ground electrode3is, as illustrated inFIGS. 2 to 4, equipped with the upright portion31and the extension32. The upright portion31extends from a front end of the housing11outward in a lengthwise direction of the spark plug1. The extension32is, as clearly illustrated inFIGS. 2 and 4, bent from the upright portion31inwardly in a radial direction of the spark plug1. In this disclosure, a direction in which the extension32extends from the upright portion31is defined as an extension lengthwise direction X. An axial direction (i.e., the longitudinal center line) of the spark plug1is defined as a plug axial direction Z. A direction perpendicular to the extension lengthwise direction X and the plug axial direction Z is defined as a lateral direction Y. One of two sides which are opposed to each other in the lateral direction Y will also be referred to as the Y1side, while the other will be referred to as the Y2side. The extension32, as illustrated inFIGS. 2 to 4, has the inner surface321(which will also be referred to as an inner extension surface) which faces the head of the center electrode2in the plug axial direction Z. The inner surface321has formed thereon the slant portion320(which will also be referred to as an extension slant portion) which is inclined downward, as viewed inFIG. 3, from the Y1side to the Y2side. In the following discussion, one of two sides opposed to each other in the plug axial direction Z will be referred to a top side, while the other will be referred to as a base side. The top side is closer to the head of the spark plug1than the base side is. The inner extension surface321has a width between edges opposed to each other in the lateral direction Y (i.e., the radial direction of the spark plug1). The inner slant portion320is shaped to extend continuously from one (which will also be referred to below as a first edge) of the edges of the inner extension surface321to the other edge (which will also be referred to below as a second edge).

The structure of the spark plug1will also be described below in detail.

In this disclosure, the plug axial direction Z is a longitudinal or lengthwise direction of the spark plug1. The radial direction of the spark plug1will also be referred to below as a plug radial direction. One of sides opposed to each other in the longitudinal direction (i.e., the extension lengthwise direction X) of the extension32will be referred to as the X1side, and the other side will be referred to as the X2side.

The spark plug1is used as an igniter in internal combustion engines mounted in, for example, automotive vehicles or cogeneration systems. The spark plug1has ends opposed to each other in the plug axial direction Z. One of the ends of the spark plug1(which will also be referred to as a base end) is connected to an ignition coil, not shown. The other end of the spark plug1(which will also be referred to as a top end) is disposed inside a combustion chamber of the internal combustion engine. In this disclosure, the base end of the spark plug1connected to the ignition coil will also be referred to as a base end side, while the front end of the spark plug1disposed inside the combustion chamber will also be referred to as a front end side.

The porcelain insulator12is, as illustrated inFIG. 1, disposed in the housing11and has a front end portion extending outside the front end of the housing11and a base end portion extending outside the base end of the housing11. The porcelain insulator12has the center electrode2retained inside the front end thereof.

The center electrode2is arranged to have a center axis substantially aligned with the center axis of the spark plug1. The center electrode2is of a cylindrical shape as a whole.

The ground electrode3is joined to the front end surface of the housing11. The upright portion31is, as clearly illustrated inFIGS. 2 to 4, rectangular in cross section and has a length extending in the plug axial direction Z. The upright portion31has a given thickness in the extension lengthwise direction X. The upright portion31, as can be seen inFIG. 4, has the joint311mechanically attached to the housing11. Specifically, a base end of the upright portion31is joined to the front end of the housing11to form the joint311.

The extension32, as clearly illustrated inFIGS. 2 and 4, extends from the front end of the upright portion31inwardly in the plug radial direction. The extension32is rectangular in cross section and has a given length oriented in the extension lengthwise direction X. The extension32has a given thickness in the plug axial direction Z.

The extension32is, as can be seen inFIG. 2, shaped to have a rectangular-triangular transverse section (also called a right-angled triangular cross section). In this embodiment, the whole of the inner surface321of the extension32may be shaped as the inner slant portion320. The inner slant portion320has a flat surface. The dimension (i.e., the width) of the inner slant portion320is, as illustrated inFIGS. 2 and 3, substantially identical with that of the ground electrode3in the lateral direction Y. The inner slant portion320extends to an edge of the end of the extension32on the X1side. In other words, an outer edge of the inner slant portion320in the radial direction (i.e., the extension lengthwise direction X) of the spark plug1coincides with that of the inner surface321in the radial direction of the spark plug1.

Specifically, the edge of the inner slant portion320on the X1side, as clearly illustrated inFIGS. 2 to 4, includes the edge E1that is a boundary between the inner slant portion320and the end surface33of the ground electrode3(i.e., the extension32) on the X1side. The edge E1is, like the inner slant portion320, inclined at a given angle (excluding zero degree) to the extension lengthwise direction X from a corner of the end surface33closest to the center electrode2toward the tip of the spark plug1in the plug axial direction Z.

When viewed in the extension lengthwise direction X inFIG. 3, the center C1of the inner surface321of the inner slant portion320in the lateral direction Y (i.e., a center line extending in the plug axial direction Z through the middle between edges of the width of the extension32opposed to each other in the lateral direction Y) is offset to the Y2side from the center C2of the front end surface21of the center electrode2in the lateral direction Y (i.e., a longitudinal center line of the center electrode2extending in the axial direction of the spark plug1). In other words, when viewed in the extension lengthwise direction X, the inner surface321is offset from the front end surface21of the center electrode2to the Y2side, so that the center C1is in misalignment from the center C2in the lengthwise direction of the spark plug1. When viewed in the extension lengthwise direction X inFIG. 3, the edge390of the inner slant portion320(i.e., the inner surface321) on the Y1side lies in alignment with the edge290of the front end surface21of the center electrode2on the Y1side in the plug axial direction Z. The edge390may alternatively be located closer to the center C1(i.e., the Y2side) than the edge290is. The edge290of the front end surface21illustrated inFIG. 3is a portion of a circumferential outer corner of the center electrode2which is located most outward away from the center C1of the inner slant portion320in the width-wise direction of the extension32of the ground electrode3(i.e., the lateral direction Y). The inner slant portion320, as clearly illustrated inFIG. 3, has the edge390and the edge395which are opposed to each other in the lateral direction Y. The edge390will also be referred to below as an upstream edge or a first edge, while the edge395will also be referred to below as a downstream edge or a second edge. The spark gap G in which sparks are created is formed between the front end surface21of the center electrode2and an end portion of the inner slant portion320on the Y1side.

The extension32, as illustrated inFIG. 3, has the extension side surface322that is one of side surfaces thereof which lies on the Y1side, in other words, is located closer to the center C2than the other side surface. The extension side surface322extends perpendicular to the lateral direction Y.

The extension32, as illustrated inFIGS. 3 and 5, has the outer extension surface323which is opposed to the inner extension surface321and faces outwardly in the plug axial direction Z. The outer extension surface323extends perpendicular to the plug axial direction Z.

The ground electrode3is made of a metallic elongated plate. The ground electrode3is formed by bending the metallic elongated plate in a thickness-wise direction thereof and then cutting a portion of the plate to form the inner slant portion320. More specifically, the ground electrode3is produced by bending a given portion of a length of the metallic plate which has a rectangular transverse section at right angles and cutting an end portion of the metallic plate to shape the inner slant portion320. This also forms the upright portion31and the extension32which are located on opposite sides of the bend of the ground electrode3.

After being made in the above way, the ground electrode3is joined at the upright portion31to the front end of the housing11.

The spark plug1also includes, as illustrated inFIG. 1, the resistor14arranged above the base end of the center electrode2through the electrically conductive glass seal13awithin the porcelain insulator12. The resistor14is formed by heating a mixture of resistor material, such as carbon or ceramic powder, and glass powder and sealing it in the porcelain insulator12. The resistor14may alternatively be implemented by a cartridge type resistor inserted into the porcelain insulator12. The glass seal13ais made of copper glass formed by mixing copper powder with glass. The spark plug1also includes the terminal15disposed above the base end of the resistor14through the glass seal13b.The glass seal13bis made of copper glass. The terminal15is made of, for example, iron alloy.

An ignition device which is equipped with the spark plug1mounted in an internal combustion engine will be described below.

The spark plug1of the ignition device is, as demonstrated inFIG. 6, mounted in the internal combustion engine to have the inner slant portion320oriented to slant away from the top end of the center electrode2in a direction in which an air-fuel mixture flows through the spark gap G. In other words, the inner slant portion320is inclined away from the center electrode2in the plug axial direction Z from an upstream side to a downstream side of a flow F of the air-fuel mixture (which will also be referred to below as mixture flow f). In the illustrated example, the Y1side is the upstream side of the spark gap G, while the Y2side is the downstream side of the spark gap G. Unless otherwise specified, “upstream side”, as referred to in this disclosure, represents the upstream side of the mixture flow F moving through the spark gap G, while “downstream side” represents the downstream side of the mixture flow F moving through the spark gap G.

The flow F of air-fuel mixture around the spark gap G will be described below in detail with reference toFIG. 6.

The mixture flow F moves in the lateral direction Y on the upstream side of the spark gap G. Upon passage of the air-fuel mixture through the spark gap G, the mixture flow F smoothly moves along the inner slant portion320. In other words, when passing through the spark gap G, the mixture flow F curves or slants toward the tip of the spark plug1, that is, away from the top of the center electrode2as the air-fuel mixture advances to the Y2side.

Elongation of a spark S developed in the spark gap G resulting from the mixture flow F will be described below with reference toFIGS. 6 to 8.

The spark S is initially developed in the spark gap G when voltage is applied between the center electrode2and the ground electrode3. At the initial stage of the spark discharge in the spark gap G, the spark S usually occurs, as demonstrated inFIG. 6, between the edge390of the inner slant portion320of the ground electrode3and the front end surface21of the center electrode2. This is because an electrical field usually concentrates in a minimum interval between the center electrode2and the ground electrode3around the edge E1.

The spark S developed initially is then, as illustrated inFIGS. 7 and 8, curved or elongated by the mixture flow F to the downstream side (i.e., the Y2side). When passing through the spark gap G, the mixture flow F, as described above, gradually slants toward the tip of the spark plug1along the inner slant portion320, thereby causing the spark S to be biased to the tip of the spark plug1as well as elongated to the downstream side in the lateral direction Y.

While the spark S is being elongated to the downstream side, a starting point on the ground electrode3(which will be referred to below as a ground starting point S1) where the spark S is developed is moved by the mixture flow F from the edge290(i.e., the end of the edge E1) to the downstream side. The movement of the ground starting point S1, as can be seen inFIGS. 6 to 8, results in an increase in linear interval between the ground starting point S1and a starting point on the center electrode2. A point-to-point line between the ground starting point S1and the starting point on the center electrode2is also elongated obliquely to the tip of the spark plug1. During such elongation, the air-fuel mixture is ignited by the spark S.

The beneficial advantages offered by the spark plug1will be described below.

The inner slant portion320of the ground electrode3, as described above, extends continuously from the edge390to the other edge395of the inner surface321in the lateral direction Y, in other words, fully occupies the width of the ground electrode3. This results in an increase in distance by which the ground starting point S1where the spark S is created is moved on the inner slant portion320, thereby increasing a length of time the spark S is moved downstream and then blown out to increase the probability of successful ignition of the air-fuel mixture, which improves the ability of the spark plug1to ignite the air-fuel mixture. The continuous occupation of the inner slant portion320between the edges of the inner surface321in the lateral direction Y (i.e., the width-wise direction of the ground electrode3), as described above, results in an increase in linear interval between the starting points on the center electrode2and the ground electrode3, in other words, an increase in distance the ground starting point S1is moved. This minimizes a risk that the spark S is rapidly shorted to ground and results in an increase in elongation of the spark S to enhance the ability of the spark plug1to ignite the air-fuel mixture.

When viewed in the extension lengthwise direction X, the center C1of the width of the inner surface321of the ground electrode3is, as described above, offset from the center C2of the diameter of the front end surface21of the center electrode2to the Y2side (i.e., the downstream side), so that the edge395of the inner surface321facing the Y2side (i.e., the downstream side) is located farther away from the center electrode2, thereby resulting in an increase in linear distance between the starting points of the spark S on the center electrode2and the ground electrode3to enhance the ability of the spark plug1to ignite the air-fuel mixture. The edge390of the inner surface321is located closer to the center electrode2, so that the edge390of the inner surface321which faces the upstream side and lies closest to the base end of the spark plug1in the plug axial direction Z is located close to the center electrode2, thereby resulting in a decreased size of the spark gap G, which enables the voltage required to initially develop the spark S to be lowered to reduce mechanical wear of the center electrode2and the ground electrode3.

As apparent from thee above discussion, this embodiment provides the spark plug1which is capable of facilitating the ignition of the air-fuel mixture.

Second Embodiment

FIGS. 9 to 12illustrates the spark plug1according to the second embodiment which is different in configuration of the extension32from the first embodiment.

The outer extension surface323of the extension32, as illustrated inFIGS. 9 and 10, includes the outer slant portion323awhich is inclined from the edge380closer to the center electrode2(i.e., the upstream side) away from the top surface of the center electrode2toward the edge385closer to the downstream side. The outer slant portion323ais of a planar shape and extends parallel to the inner slant portion320. The outer slant portion323aextends from the upstream edge380of the outer extension surface323, but has the downstream edge385, as clearly illustrated inFIG. 10, located slightly closer to the Y1side (i.e., the upstream side) than the edge395of the outer extension surface323is. Note that the edge395coincides with downstream edges of the inner extension surface320and the outer extension portion323a.

The ground electrode3is made of a metallic elongated plate. The ground electrode3is formed by bending the metallic elongated plate in the thickness-wise direction thereof and then cutting opposed portions of the plate to form the inner slant portion320and the outer slant portion323a.

The flow F of air-fuel mixture around the spark gap G will be described below in detail with reference toFIG. 11.

The mixture flow F moves in the lateral direction Y on the upstream side of the spark gap G. Upon passage of the air-fuel mixture through the spark gap G, the mixture flow F smoothly moves along the inner slant portion320and the outer slant portion323a.In other words, when passing through the spark gap G, the mixture flow F curves or slants toward the tip of the spark plug1in the form of a mixture flow F1, that is, away from the top of the center electrode2as the air-fuel mixture advances to the Y2side (i.e., the downstream side). Additionally, the outer slant portion323aproduces a mixture flow F1which curves or slants toward the tip of the spark plug1, that is, away from the top of the center electrode as the air-fuel mixture advances to the Y2side. In brief, the extension32of the ground electrode3works to split the mixture flow F existing upstream of the spark gap G into two streams: the mixture flow F1and the mixture flow F2and direct them obliquely downstream away from the top of the center electrode2.

Other arrangements or operations of the spark plug1are identical with those in the first embodiment, explanation thereof in detail will be omitted here.

In the second embodiment and following embodiments, the same reference numbers as employed in the first embodiment refer to the same parts unless otherwise specified.

The outer extension surface323is, as described above, equipped with the outer slant portion323awhich is inclined away from the top of the center electrode2from the upstream edge380to the downstream edge385. This facilitates guiding a stream of air-fuel mixture to the top of the spark plug1through the spark gap G. This causes, as demonstrated inFIG. 12, the spark S to be elongated downstream farther away from the top of the center electrode2, thereby minimizing a risk that heat of a flame, as created by ignition of the air-fuel mixture by the spark S, absorbed by the engine head to facilitate growth of the flame.

Other beneficial advantages offered by the spark plug1of the second embodiment are identical with those in the first embodiment.

Third Embodiment

FIGS. 13 to 15illustrates the spark plug1according to the third embodiment which is different in structure of the ground electrode3from the first embodiment.

When viewed in the extension lengthwise direction X inFIG. 14, the inner slant portion320is offset from the joint311of the ground electrode3in the lateral direction Y. In other words, the inner slant portion320is located out of alignment with the joint311of the ground electrode3in the plug axial direction Z.

The joint311of the upright portion31, as illustrated inFIG. 15, has an end surface entirely attached to the end surface of the housing11.

The upright portion31of the ground electrode3is, as clearly illustrated inFIGS. 13 to 15, inclined from the joint311to the Y2side (i.e., the downstream side). In other words, the upright portion31is inclined downstream at a given angle to the longitudinal center line (i.e., the length) of the spark plug1. The extension32, as illustrated inFIG. 15, has a length extending parallel to a direction in which the joint311of the ground electrode3and the center electrode2are arranged adjacent each other. The upstream edge390of the inner slant portion320is, as can be seen inFIG. 14, offset from the upstream edge290of the front end surface21of the center electrode2to the Y2side (i.e., the downstream side).

Other arrangements are identical with those in the first embodiment.

When viewed in the extension lengthwise direction X, the joint311is offset from the inner slant portion320in the lateral direction Y. In other words, when viewed in the extension lengthwise direction X, the center C1of the width of the inner extension surface321of the ground electrode3is, as illustrated inFIG. 3, offset from the center C2of the diameter of the front end surface21of the center electrode2to the Y2side (i.e., the downstream side), but it permits the location of the joint311to be changed as needed. For example, the joint311may be designed, as illustrated inFIG. 14, to have a surface entirely facing the front end surface of the housing11, thereby ensuring a required degree of strength of joint between the ground electrode3and the housing11.

The spark plug1of this embodiment offers substantially the same other beneficial advantages as in the first embodiment.

Fourth Embodiment

FIGS. 16 and 17illustrate the spark plug1according to the fourth embodiment which is different in configuration of the ground electrode3from the first embodiment.

The ground electrode3, as can be seen inFIG. 16, has a transverse section whose shape remains unchanged over a length of the ground electrode3. Specifically, the ground electrode3has a right-angled triangular cross section taken in a direction perpendicular to the length of the ground electrode3. The ground electrode3has a surface which defines a hypotenuse of the right-angled triangular cross section and forms a portion of the inner slant portion320. Such a surface is even and extends entirely between ends of the length of the ground electrode3.

The inner slant portion320is, like in the first embodiment, shaped to be inclined from the upstream edge390away from the top of the center electrode2to the downstream edge395. The extension side surface322extends perpendicular to the lateral direction Y. The outer extension surface323extends perpendicular to the plug axial direction Z.

The ground electrode3is made by bending a metallic elongated plate in a thickness-wise direction thereof. The metallic elongated plate has a right-angled triangular cross section. The ground electrode3is bent to orient the extension32in the above described direction and joined to the housing11.

Other arrangements are identical with those in the first embodiment.

The configuration of the ground electrode3improves the productivity thereof (i.e., the spark plug1).

The configuration of the ground electrode3(i.e., the extension32) assures an increased distance the starting point of a spark moves on the ground electrode3even when the mixture flow F passing through the spark gap G, as illustrated inFIG. 17, crosses the width of the extension32diagonally toward the X2side in the extension lengthwise direction X. The surface of the ground electrode3which defines the hypotenuse of the right-angled triangular cross section of the ground electrode3is, as described above, even and extends entirely between ends of the length of the ground electrode3without any corners, so that the starting point of a spark on the ground electrode3may be moved by the mixture flow F along a path, as indicated by a broken line BL inFIG. 17. This facilitates the movement of the starting point of the spark on the ground electrode3, thereby resulting in an increase in time it takes for the spark to be carried downstream and then blown off and also resulting in an increase in linear distance between the starting points on the center electrode2and the ground electrode3. This enhances the ability of the spark plug1to ignite the air-fuel mixture.

The spark plug1of this embodiment offers substantially the same other beneficial advantages as in the first embodiment.

Fifth Embodiment

FIGS. 18 to 20illustrate the spark plug1according to the fifth embodiment which is substantially identical in structure with that in the first embodiment, but has the inner slant portion320equipped with the convex portion34protruding to the top of the center electrode2.

The convex portion34, as illustrated inFIG. 19, has a length extending continuously from the upstream edge390to the downstream edge395of the inner extension surface321in the lateral direction Y. The convex portion34is, as clearly illustrated inFIGS. 18 to 20, made of a rectangular elongated bar extending in a direction in which the inner slant portion320is inclined. The convex portion34has a surface which faces the center electrode2and has the edges E2. The edges E2define a width thereof and are opposed to each other in the extension lengthwise direction X. The edges E2are inclined away from the top of the center electrode2to the Y2side (i.e., the downstream side). The convex portion34is, as can be seen inFIGS. 18 and 20, located substantially at the middle of the width of the inner extension surface321in the extension lengthwise direction X.

The convex portion34may be made of material different from that of the ground electrode3. For example, the ground electrode3is made Ni alloy mainly containing nickel. The convex portion34is made of a noble metal such as Ir or Pt. The convex portion34is welded to the material of the ground electrode3.

Other arrangements are identical with those in the first embodiment.

The convex portion34facilitates concentration of electrical field around the edges E2, thereby ensuring the stability of movement of the ground starting point S1of the spark S, thereby resulting in an increase in time it takes for the spark S to be carried downstream and then blown off and also resulting in an increase in linear distance between the starting points on the center electrode2and the ground electrode3. This enhances the ability of the spark plug1to ignite the air-fuel mixture.

The use of the high-stiffness noble metal as material of the convex portion34minimizes mechanical wear thereof.

The spark plug1of this embodiment offers substantially the same other beneficial advantages as in the first embodiment.

Sixth Embodiment

FIGS. 21 to 23illustrate the spark plug1according to the sixth embodiment which is basically identical in structure with that in the first embodiment, but has the groove35formed in the inner slant portion320. The groove35is followed away from the top of the center electrode2. InFIG. 22, the bottom of the groove35is indicated by a broken line.

The groove35continuously extends, as illustrated inFIG. 21, from the upstream edge390to the downstream edge395of the inner extension surface321in the lateral direction Y. The groove35has a given length oriented in a lengthwise direction of the inner slant portion320. The groove35has openings lying at the upstream and downstream edges390and395of the inner slant portion320. The groove35has side walls with edges E3which lie flush with the inner extension surface321and face the top of the center electrode2in the plug axial direction Z. The edges E3are opposed to each other in the extension lengthwise direction X. The edges E3are inclined away from the top of the center electrode2to the Y2side (i.e., the downstream side).

Other arrangements are identical with those in the first embodiment.

The groove35serves to facilitate concentration of electrical field around the edges E3, thereby ensuring the stability of movement of the ground starting point S1of the spark S, thereby resulting in an increase in time it takes for the spark S to be carried downstream and then blown off and also resulting in an increase in linear distance between the starting points on the center electrode2and the ground electrode3. This enhances the ability of the spark plug1to ignite the air-fuel mixture.

The spark plug1of this embodiment offers substantially the same other beneficial advantages as in the first embodiment.

For instance, in each embodiment, the inner slant portion320may be formed in the shape of a concave curve, as illustrated inFIG. 24, hollowed away from the top of the center electrode2or a convex curve, as illustrated inFIG. 25, bulging toward the top of the center electrode2. Similarly, the outer slant portion323ain the second embodiment may be curved.