Spark plug for internal combustion engine

A spark plug includes a ground electrode formed with a flat region and a convex curved region on an outer peripheral surface thereof. The flat region is located on a front end of the ground electrode and has a length of 0.2 mm or more from a front end face of the ground electrode in a longitudinal direction of the ground electrode. The ground electrode satisfies the following dimensional condition (1) with respect to first and second cross sections of the ground electrode taken through the convex curved region and the flat region in directions perpendicular to the longitudinal direction of the ground electrode,0.950≦(S2/L2)/(S1/L1)≦0.995   (1)where S1 is the area of the first cross section; L1 is the perimeter of the first cross section; S2 is the area of the second cross section; and L2 is the perimeter of the second cross section.

BACKGROUND OF THE INVENTION

The present invention relates to a spark plug for use in an internal combustion engine. Hereinafter, the term “front” refers to a spark discharge side with respect to the axial direction of a spark plug, and the term “rear” refers to a side opposite the front side.

A spark plug for an internal combustion engine includes a center electrode extending axially of the spark plug, an insulator disposed around the center electrode, a metal shell disposed around the ceramic insulator and a ground electrode joined at a rear end thereof to a front end of the metal shell. In general, the ground electrode is substantially rectangular in cross section and bent in such a manner that a front end of the ground electrode faces a front end of the center electrode to define a spark gap between the front end of the center electrode and the front end of the ground electrode. In some cases, tips of precious metal alloys (precious metal tips) may be joined to the front ends of the center and ground electrodes for improvements in spark wear resistance.

When the spark plug is mounted on a cylinder head of the engine at a position that causes a collision of an air-fuel mixture to an outer (back) surface of the ground electrode, there is a possibility that the ground electrode interferes with the flow of the air-fuel mixture into the spark gap. This results in variations in engine ignition performance.

In order to prevent such ignition performance variations, Japanese Laid-Open Patent Publication No. 11-121142 proposes a spark plug with two or more ground electrodes, each of which is substantially circular in cross section (i.e. substantially cylindrical in shape) so as to allow the air-fuel mixture to easily flow to the inner peripheral side of the ground electrode and then flow to the spark gap even when the spark plug is in a position that causes a collision of the air-fuel mixture to the outer peripheral surface of the ground electrode.

SUMMARY OF THE INVENTION

In recent years, high-compression-ratio, high-output engines have been developed with varying combinations of superchargers and variable valve systems. There have also been developed so-called spray-guide direct-injection engines with injectors to inject fuel directly against highly-compressed air in the engine cylinders. These engines tend to reach a significantly high cylinder temperature. It is conceivable that, by the direct fuel injection under such high-temperature engine conditions, the fuel of relatively low temperature will directly collide against the ground electrode, which has been exposed to high temperature. In this case, the ground electrode gets suddenly cooled by the fuel and thus may suffer a grain defect formation phenomenon (also called a “wormhole phenomenon”) in which some crystal grains fall out of their grain boundaries. The grain defect formation phenomenon is more likely to occur in the case of the cylindrical-shaped ground electrode.

It is therefore an object of the present invention to provide a spark plug for an internal combustion engine, capable of securing improvement in engine ignition performance, without being affected by the inflow direction of an air-fuel mixture, while protecting a ground electrode from grain defect formation under direct fuel injection.

According to an aspect of the present invention, there is provided a spark plug for an internal combustion engine, comprising: a cylindrical metal shell arranged in an axial direction of the spark plug; a cylindrical insulator retained in the metal shell; a column-shaped center electrode disposed in the insulator with a front end thereof protruding from the insulator; and a ground electrode joined a rear end thereof to a front end of the metal shell and bent in such a manner that a front end of the ground electrode extends toward an axis of the spark plug so as to define a spark gap between the front end of the center electrode and the front end of the ground electrode, the ground electrode including a flat region formed on an outer peripheral surface thereof opposite to an inner peripheral surface facing the insulator, the flat region being located on the front end of the ground electrode and having a length of 0.2 mm or more from a front end face of the ground electrode in a longitudinal direction of the ground electrode, any region other than the flat region of the outer peripheral surface of the ground electrode being convex curved, and the ground electrode satisfying the following dimensional condition (1) with respect to a first cross section of the ground electrode taken through the any region other than the flat region in a direction perpendicular to the longitudinal direction of the ground electrode and a second cross section of the ground electrode taken through the flat region in a direction perpendicular to the longitudinal direction of the ground electrode, 0.950≦(S2/L2)/(S1/L1)≦0.995 (1) where S1is the area of the first cross section; L1is the perimeter of the first cross section; S2is the area of the second cross section; and L2is the perimeter of the second cross section.

The other objects and features of the present invention will also become understood from the following description.

DESCRIPTION OF THE EMBODIMENTS

The present invention will be described in detail below by way of the following embodiments, in which like parts and portions are designated by like reference numerals to eliminate repeated explanations thereof.

A spark plug1for an internal combustion engine according to the first embodiment of the present invention will be first explained blow with reference toFIGS. 1 to 10.

Referring toFIGS. 1 and 2, the spark plug1includes a ceramic insulator2, a metal shell3, a center electrode5with a precious metal tip31, a terminal electrode6, a ground electrode27with a precious metal tip32and a resistor element7.

The ceramic insulator2is formed into a substantially cylindrical shape, with a through hole4thereof extending in the direction of an axis CL1(hereinafter just referred to as “axial direction”) of the spark plug1, and is made of a sintered ceramic material such as sintered alumina. As shown inFIG. 1, the ceramic insulator2includes a flange portion11radially outwardly protruding at around an axially middle position of the ceramic insulator2, a body portion12located on a front side of the flange portion11and having a smaller diameter than that of the flange portion11and a leg portion13located on a front side of the body portion12and having a smaller diameter than that of the body portion12. There is a step14formed at a location between the body portion12and the leg portion13on an outer peripheral surface of the ceramic insulator2.

The metal shell3is formed into a cylindrical shape of a metal material such as iron-based material or stainless steel (e.g. low-carbon steel S15C, S25C etc.) and arranged in the axial direction of the spark plug1around the outer peripheral surface of the ceramic insulator2so as to retain therein the flange portion11, the body portion12and the leg portion13of the ceramic insulator2. In general, the metal shell3includes a male-threaded portion15, a flange portion16radially outwardly protruding on a rear side of the threaded portion15and a tool engagement portion19located on a rear side of the flange portion16as shown inFIG. 1. The threaded portion15is screwed into a plug hole of a cylinder head of the engine to mount the spark plug1onto the engine cylinder head in such a manner that the leg portion13of the ceramic insulator2is exposed to a combustion chamber of the engine. The flange portion16is seated on the engine cylinder head. A gasket18is fitted on a thread neck end17of the threaded portion15and interposed between the flange portion16and the engine cylinder head. The tool engagement portion19is shaped into a hexagonal cross section for engagement with a tool such as a wrench to screw the threaded portion15into the plug hole of the engine cylinder head. Further, there is a step21formed on an inner peripheral surface of the metal shell3so that the step14of the ceramic insulator2is engaged on the step21of the metal shell3. The metal shell3is swaged at a rear end20thereof onto the ceramic insulator2, with a pair of annular rings23and24interposed between the ceramic insulator2and the metal shell3and a talc powder25filled between the annular rings23and24, to hold the ceramic insulator2and ensure the gastightness between the ceramic insulator2and the metal shell3. In order to hermetically seal the combustion chamber and prevent combustion gas leakage from between the leg portion13of the ceramic insulator2and the inner peripheral surface of the metal shell3, an annular plate packing22is interposed between the step14of the ceramic insulator2and the step21of the metal shell3. In this way, the ceramic insulator2is fixed in the metal shell3via the packing22, the annular rings23and24and the talc powder25by engaging the step14of the ceramic insulator2on the step21of the metal shell3and by swaging the rear end20of the metal shell3on the ceramic insulator2.

The center electrode5is generally formed into a cylindrical column (rod) shape and fitted in a front side of the through hole4of the ceramic insulator2in such a manner that a front end of the center electrode5protrudes from a front end of the ceramic insulator2and gradually decreases in diameter toward its flat end face. In the first embodiment, the center electrode5has its body with an inner layer5A of pure copper or copper alloy and an outer layer5B of nickel alloy for efficient heat transfer.

The precious metal tip31is formed into a cylindrical column shape of precious metal alloy e.g. iridium alloy and joined by welding to the front end face of the center electrode5for improvement in spark wear resistance. The welding can be performed by any welding technique such as laser welding or electron-beam welding so as to form a fused joint41between the precious metal tip31and the center electrode5as shown inFIGS. 1 and 2.

The terminal electrode6is fitted in a rear side of the through hole4of the ceramic insulator2in such a manner that a rear end of the terminal electrode6protrudes from a rear end of the ceramic insulator2.

The resistor element7is disposed between the center electrode5and the terminal electrode6within axial through hole4of the ceramic insulator2and electrically connected at front and rear ends thereof to the center electrode5and the terminal electrode6via conductive glass seal layers8and9, respectively.

The ground electrode27is joined at a rear end thereof to a front end face26of the metal shell3and is bent at an angle of approximately 90 degrees in such a manner that a front end of the ground electrode27is directed toward the plug axis CL1and substantially faces the front end of the center electrode5(the precious metal tip31). Namely, the front end of the ground electrode27extends in the radial direction of the spark plug1and substantially faces the front end of the center electrode5(the precious metal tip31) whereas the rear end of the ground electrode27extend in the axial direction of the spark plug1(i.e. in parallel with the plug axis CL1). Preferably, the ground electrode27has its body formed with an inner layer27A of pure copper or copper alloy and an outer layer27B of nickel alloy available under the trademark of e.g. Inconel 600 or Inconel 601 in the first embodiment. The formation of such an inner layer27A enables efficient heat transfer from the inside of the ground electrode27since the copper or copper alloy exhibits higher thermal conductivity than the nickel alloy.

The precious metal tip32is formed into a cylindrical column shape of precious metal alloy e.g. platinum alloy containing 20 mass % of rhodium and joined by welding to the front end of the ground electrode27for improvement in spark resistance. The welding can be performed by any welding technique such as laser welding, electron-beam welding or resistance welding so as to form a fused joint42between the precious metal tip32and the ground electrode27as shown inFIGS. 1 and 2.

With such a configuration, there is a spark gap33defined between the front end of the center electrode5and the front end of the ground electrode27, more specifically, between the opposing end faces of the precious metal tips31and32so that the spark plug1generates a spark discharge in the spark gap33approximately in the axial direction of the spark plug1.

Although the precious metal tips31and32are provided on the respective electrodes5and27in the first embodiment, these precious metal tips31and32are not necessarily provided. For example, only the precious metal tip31may be provided on the center electrode5with no precious metal tip on the ground electrode27as shown inFIG. 10. In this case, the spark gap33is defined between the precious metal tip31and the front end of the ground electrode27. Only the precious metal tip32may alternatively be provided on the ground electrode27with no precious metal tip on the center electrode5. In this case, the spark gap33is defined between the front end of the center electrode5and the precious metal tip32. Both of the precious metal tips31and32may not provided on the center and ground electrodes5and27. In this case, the spark gap33is defined between the front ends of the center and ground electrodes5and27.

The materials of the precious metal tips31and32are not limited to the above. Any other precious metal alloys can be used as the materials of the precious metal tips31and32. Each of the cylindrical precious metal tips31and32can be obtained by e.g. preparing an ingot of precious metal, alloying the precious metal ingot with alloying metal, forming the resulting molten alloy into an ingot, subjecting the alloy ingot to hot forging and/or hot rolling (grooved rolling), wiredrawing the alloy ingot into a rod shape and then cutting the alloy ingot to a given length.

Herein, the spark plug1of the first embodiment is characterized in that the ground electrode27is substantially circular in cross section with a flat region51formed on an outer peripheral surface of the ground electrode27, which is opposite to an inner peripheral surface of the ground electrode27facing the center electrode5(ceramic insulator2) and is visually identified when the ground electrode27is viewed from the outside, as shown inFIGS. 3,4and5. Any region, other than the flat region51, of the outer peripheral surface of the ground electrode27is curved into a convex shape, more specifically a circular arc, with a curvature radius of 0.5 to 1.0 mm (hereinafter referred to as “convex curved region”).

The flat region51is located on the front end of the ground electrode27and rectangular-shaped having a length of 0.2 mm or more from a front end face27sof the ground electrode27in the longitudinal axis direction of the ground electrode27(hereinafter occasionally referred to as “longitudinal length”) and a given width of e.g. 0.4 to 1.2 mm, preferably 0.5 to 1.0 mm, more preferably 0.6 to 0.7 mm, in a lateral direction perpendicular to the longitudinal direction of the ground electrode27(hereinafter occasionally referred to as “lateral width”). The method of formation of the flat region51is not particularly limited. The flat region51can be formed by e.g. cutting away or press working a given part of the outer peripheral surface of the front end of the ground electrode27.

In addition, the ground electrode27satisfies the following dimensional condition:
0.950≦(S2/L2)/(S1/L1)≦0.995
with respect to a first cross section of the ground electrode27taken through the convex curved region in a direction perpendicular to the longitudinal direction of the ground electrode27(e.g. along line J-J ofFIG. 6across the rear end of the ground electrode27) and a second cross section of the ground electrode27taken through the flat region51in a direction perpendicular to the longitudinal direction of the ground electrode27(e.g. along line K-K ofFIG. 6across the front end of the ground electrode27) where S1is the area of the first cross section; L1is the perimeter of the first cross section; S2is the area of the second cross section; and L2is the perimeter of the second cross section.

The form of the ground electrode27is not limited to the above. There is no particular limitation on the form of the ground electrode27as long as both of the flat region51and the convex curved region are made on the outer peripheral surface of the ground electrode27to satisfy the dimensional condition of 0.950≦(S2/L2)/(S1/L1)≦0.995. Various modifications of the ground electrode27are possible. For example, the ground electrode27can be modified in such a manner that the outer peripheral surface of the ground electrode27, except for the flat region51, has a circular arc cross-sectional profile and the inner peripheral surface of the ground electrode27has a flat (straight) cross-sectional profile, with flat regions formed on the opposite side surfaces of the ground electrode27, as shown inFIGS. 9A,9B and9C. It is however impractical to modify the ground electrode27into a rounded corner rectangular cross section as shown inFIG. 9Dsince any region, other than the outer rounded corners, of the outer peripheral surface of such a rounded-corner ground electrode is flat and cannot be considered as the convex curved region.

When the spark plug1comes into a position that causes a direct collision of fuel and air against the outer peripheral (back) surface of the ground electrode27, the air-fuel mixture easily flows around the convex curved region of the ground electrode27from the outer peripheral side to the inner peripheral side. It is thus possible to ensure the flow of the air-fuel mixture into the spark gap33for improvements in engine ignition performance and flame propagation characteristics.

It is however conceivable that, while the front end of the ground electrode27becomes the highest in temperature, the fuel of relatively low temperature will directly collide with the outer peripheral surface of the front end of the ground electrode27. In such a case, the front end of the ground electrode27gets suddenly and locally cooled and subjected to large thermal shock upon the direct fuel collision.

In the case of using a ground electrode81having its whole peripheral surface convex curved with no flat region, it is likely that the thermal shock vectors of the fuel will be concentrated on one point by such a curved peripheral surface of the ground electrode81as shown inFIG. 8B. As a result, the ground electrode81suffers a grain defect formation phenomenon (also called “wormhole phenomenon”) in which some crystal grains fall out of their grain boundaries due to local and sudden cooling.

By contrast, the flat region51is formed on the outer peripheral surface of the front end of the ground electrode27as explained above in the first embodiment. Even when the fuel directly collides against the outer peripheral side of the front end of the ground electrode27, the flat region51prevents the thermal shock vectors of the fuel from being concentrated on one point as shown inFIG. 8A. It is thus possible to prevent the occurrence of grain defects (wormhole phenomenon) in the ground electrode27due to local and sudden cooling of the ground electrode27by the fuel. When the longitudinal length of the flat region51is less than 0.2 mm or when the condition of 0.950≦(S2/L2)/(S1/L1)≦0.995 is not satisfied, the flat region51may not produce a sufficient grain defect prevention effect so that the grain defects are likely to occur in the ground electrode27upon the direct fuel collision. The grain defect prevention effect of the flat region51can be ensured sufficiently and assuredly when the longitudinal length of the flat region51is 0.2 mm or longer and, at the same time, the condition of 0.950≦(S2/L2)/(S1/L1)≦0.995 is satisfied.

Furthermore, the convex curved region of the outer peripheral surface of the ground electrode27is in circular arc form with a curvature radius of 0.5 to 1.0 mm as explained above. This allows the air-fuel mixture to flow around the convex curved region of the ground electrode27more easily and efficiently from the outer peripheral side to the inner peripheral side and reach the spark gap33for further improvements in engine ignition performance and flame propagation characteristics. When the curvature radius of the convex curved region is less than 0.5 mm, the distance between the longitudinal axis and the peripheral surface of the ground electrode27is so small that the front end of the ground electrode27does not become so high in temperature by heat radiation from its peripheral surface. When the curvature radius of the convex curved region exceeds 1.0 mm, there is not so large difference between the convex curved region and the flat region51so that the concentration of the thermal shock vectors of the fuel is unlikely occur even on the convex curved region. For these reasons, the grain defect formation phenomenon is originally unlikely to occur by the direct fuel collision when the curvature radius of the convex curved region is less than 0.5 mm and when the curvature radius of the convex curved region exceeds 1.0 mm. In other words, the grain defect prevention effect of the flat region51becomes evident and pronounced when the curvature radius of the convex curved region is 0.5 to 1.0 mm.

The above spark plug1can be manufactured by the following procedure.

The metal shell3is first produced in a semifinished form by preparing a cylindrical metal piece, forming an axial hole by cold forging through the metal piece, and then, cutting the outside shape of the metal piece.

On the other hand, the ground electrode27is produced in a straight cylindrical column form by preparing a core metal material and a bottomed cylindrical metal material, inserting the core material in the cylindrical metal material, forming the resulting two-layer cup material into a thin rod shape by cold forming e.g. wiredrawing using a die etc. or by extrusion using a mold and optionally swaging etc, and then, cutting the rod-shaped electrode material to a given length.

The produced straight ground electrode27is joined by e.g. resistance welding to the front end face26of the metal shell3. After the welding, weld shear drops are removed from the joint between the metal shell3and the ground electrode27. It is alternatively feasible to, after cold forming the ground electrode27into a thin rod shape, weld the ground electrode27to the metal shell3, subject the ground electrode27to swaging and then cut the ground electrode27to a given length. In such a case, the swaging step can be performed by inserting the ground electrode27into a swager (swaging die) from the front end side while holding the metal shell3. This eliminates the trouble of setting the length of the ground electrode27to a longer length so as to secure a portion of the ground electrode27to be held at the swaging step.

The thread portion15is formed at a given position on the metal shell3by component rolling. The thus-obtained subassembly unit of the metal shell3and the ground electrode27(hereinafter just referred to as “metal shell subassembly unit”) is given zinc plating or nickel plating. The metal shell subassembly unit may be further treated by chromating for corrosion resistance improvement.

The front end of the ground electrode27is subjected to cutting or press forming, thereby forming the flat region51on the outer peripheral surface of the front end of the ground electrode27. This cutting or press forming step may alternatively be performed before the component rolling of the thread portion15and before or after the welding of the ground electrode27to the metal shell3.

The precious metal tip32is then joined to the front end of the ground electrode27by laser welding, electron-beam welding or resistance welding while being pressed against the front end of the ground electrode27. For reliable welding, it is feasible to remove the plating of the front end of the ground electrode27prior to the welding step or to mask the front end of the ground electrode27at the plating step. Either of the joint faces the precious metal tip32and the ground electrode27may be subjected to any appropriate processing so that these joint faces suit with each other. The precious metal tip32may be welded to the front end of the ground electrode27after the following assembling (bending) step.

Further, the ceramic insulator2is separately produced by e.g. preparing a granulated powder mixture of alumina and binder etc., molding the ceramic power mixture into a cylindrical shape with a rubber press, shaping the ceramic mold by grinding and sintering the ceramic mold in a furnace.

The center electrode5is also separately produced by forging the nickel alloy layer5B and forming the copper or copper alloy layer5A in the center of the nickel alloy layer5B.

The precious metal tip31is joined to the front end of the center electrode5by laser welding or the like.

The ceramic insulator2, the center electrode5with the precious metal tip31, the resistive element7and the terminal electrode6are assembled together into a unit (hereinafter referred to as “insulator subassembly unit”). The resistive element7is inserted into the through hole4of the ceramic insulator2followed by preparing glass seal materials from borosilicate glass and metal powder and filling the glass seal materials into the through hole4to sandwich the resistive element7between the glass seal materials. After that, the center electrode5and the terminal electrode6are fitted in the front and rear sides of the though hole4. The glass seal layers8and9are formed by baking the glass seal materials in a furnace with the center and terminal electrodes5and6placed under pressure. At this time, a glaze layer may be applied to the rear end portion of the ceramic insulator2concurrently. The glaze layer may alternatively be applied in advance to the rear end portion of the ceramic insulator2.

The metal shell and insulator subassembly units are assembled and fixed together by cold crimping or hot crimping the relatively-thin rear end of the metal shell3onto the ceramic insulator2so that the metal shell3surrounds and retains therein the ceramic insulator2.

Finally, the ground electrode27is bent in such a manner as to define the spark gap33between the precious metal tips31and32.

As described above, the spark plug1is able to ensure the flow of the air-fuel mixture into the spark gap33for improvements in engine ignition performance and flame propagation characteristics, without being affected by the inflow direction of the air-fuel mixture, and to prevent the occurrence of grain defects in the ground electrode27even at the direct collision of the fuel against the outer peripheral surface of the front end of the ground electrode27by forming the flat region51and the convex curved region on the outer peripheral surface of the ground electrode27.

A spark plug100according to the second embodiment of the present invention will be next explained below with reference toFIGS. 11 to 18. The spark plug100of the second embodiment is structurally similar to the spark plug1of the first embodiment, except for the positional relationship of the center electrode5, the ground electrode27and the precious metal tips31and32.

As shown inFIGS. 11 and 12, the ground electrode27is bent in such a manner that the front end face27sof the ground electrode27faces the outer peripheral surface of the precious metal tip31. The precious metal tip32is made smaller in diameter than the front end face27sof the ground electrode27and welded to the center of the front end face27sof the ground electrode27in such a manner as to protrude toward the axis CL1of the spark plug100from the front end face27sof the ground electrode27as shown inFIGS. 11 to 13. With such a configuration, the spark gap33is defined between the outer peripheral surface of the precious metal tip31and the end face of the precious metal tip32so that the spark plug100generates a spark discharge in the spark gap33approximately in the radial (lateral) direction of the spark plug100for improvements in engine ignition performance and flame propagation characteristics. Although the precious metal tip31is joined to the front end of the center electrode5in the second embodiment, the precious metal tip31is not necessarily provided. In this case, the spark gap33is defined between the outer peripheral surface of the front end of the center electrode5and the end face of the precious metal tip32.

In the case of using a cylindrical ground electrode81′ with no flat region, however, there is a possibility that the air-fuel mixture, when collides diagonally with the outer peripheral surface of the front end of the ground electrode81′, flows to the inner peripheral surface of the ground electrode81′ and does not reach a proper discharge point z in the spark gap33as shown inFIG. 16B. This results in engine ignition performance deterioration.

In the second embodiment, both of the flat region51and the convex curved region are formed on the outer peripheral surface of the ground electrode27. The convex curved region allows the air-fuel mixture to easily flow therearound from the outer peripheral side to the inner peripheral side and then into the spark gap33. Further, the flat region51produces the effect of not only preventing a concentration of the thermal shock vectors of the fuel but also guiding the air-fuel mixture to a proper discharge point a in the spark gap33without causing the flow of the air-fuel mixture to the inner peripheral side as shown inFIG. 16Aeven when the air-fuel mixture collides diagonally with the outer peripheral surface of the front end of the ground electrode27. It is thus possible in the second embodiment to ensure the flow of the air-fuel mixture into the spark gap33and prevent the occurrence of grain defects in the ground electrode27at the direct collision of the fuel against the outer peripheral surface of the front end of the ground electrode27, as is the case with the first embodiment, by the formation of the flat region51and the convex curved region on the outer peripheral surface of the ground electrode27.

In order for the flat region51to guide the air-fuel mixture to the spark gap33more stably and efficiently and thereby secure improved ignition performance assuredly, it is preferable to control an angle θ of the front edge of the ground electrode27defined by the flat region51and the front end face27sas appropriate in consideration of the air-fuel mixture inflow direction. It is particularly preferable that the angle θ which the flat region51forms with the front end face27sof the ground electrode27is in the range of 70 to 100 degrees. In the second embodiment, the flat region51is substantially orthogonal (perpendicular) to the front end face27sof the ground electrode27, with the flat region51oriented in the radial direction of the spark plug100and the front end face27sof the ground electrode27oriented in the axial direction of the spark plug100, so that the edge angle θ between the flat region51and the front end face27sof the ground electrode27is about 90 degrees.

In order for the flat region51to guide the air-fuel mixture to the spark gap33more stably and efficiently and thereby secure improved ignition performance assuredly, it is also preferable that the flat region51satisfies the following dimensional conditions:
A×B≧0.2; and
B≧0.2
where A (mm) is the longitudinal length of the flat region51in the longitudinal direction of the ground electrode27and B (mm) is the lateral width of the flat region51as shown inFIGS. 14A and 14B.

In order to achieve further improvement in ignition performance and secure the durability of the precious metal tip32, it is further preferable that the spark plug100satisfies the following dimensional conditions:
E≧2×Dwhen 0.3≦D≦C/4+0.8;
E≧0.6 when D<0.3; and
F≦1.6
where C (mm) is the minimum distance of the spark gap33in the radial direction of the spark plug100; D (mm) is the distance from a midpoint a of the shortest line connecting a front edge of the end face of the precious metal tip31and a front edge of the end face of the precious metal tip32(in the case of no precious metal tip on the center electrode5, a midpoint of the shortest line connecting an edge of the front end face of the center electrode5and a front edge of the end face of the precious metal tip32) to the outer peripheral surface of the front end of the ground electrode27in the axial direction of the spark plug100; E (mm) is the distance from the midpoint a to the front end face27sof the ground electrode27in the radial direction of the spark plug100; and F (mm) is the length of protrusion of the precious metal tip32from the front end face27sof the ground electrode27as shown inFIG. 15. By controlling the protrusion length F of the precious metal tip32to 1.6 mm or smaller, the precious metal tip32can be effectively prevented from deterioration in heat transfer performance. In the case of D<0.3 (mm), the above-mentioned effect of the flat region51can be obtained more assuredly by satisfying the condition of E≧0.6. In the case of D≧0.3 (mm), the above-mentioned effect of the flat region51can also be obtained more assuredly by satisfying the condition of E≧2×D. In this case, the upper limit of the distance D is set to C/4+0.8 (mm) since the equation 2D−C/2≦1.6 (mm) holds based on the equations F≧1.6 and F=E−C/2.

The form of the ground electrode27can be modified as appropriate in the second embodiment. For example, flat regions52and53may also be formed on the opposite side surfaces of the front end of the ground electrode27as shown inFIG. 17Aso as to guide the air-fuel mixture to the spark gap33more stably when the air-fuel mixture flows diagonally against the ground electrode27. As shown inFIG. 17B, the ground electrode27may be formed into a substantially semicylindrical shape with a flat inner surface27f. In the case of the ground electrode27being in semicylindrical form with the flat inner surface27f, a rectangular precious metal tip321may be partly arranged on, or embedded in, and joined by e.g. resistance welding to the flat inner surface27fof the ground electrode27so as to protrude from the front end face of the ground electrode27toward the spark plug axis as shown inFIG. 18.

The present invention will be described in more detail by reference to the following examples. It should be however noted that the following examples are only illustrative and not intended to limit the invention thereto.

Test samples of the spark plug1(as Examples) were produced by varying the longitudinal length A of the flat region51, the area S1and perimeter L1of the first cross section of the ground electrode27and the area S2and perimeter L2of the second cross section of the ground electrode27.

Each of the test samples was subjected to durability test. The durability test was herein conducted by mounting the test sample in a 2.0-L direct-injection engine, driving the engine continuously for 920 hours according to a highway driving simulation pattern (corresponding to about 100,000 km driving). Before and after the durability test, the cross section of the ground electrode27(up to 2 mm in length from the front end face27sof the ground electrode27) was monitored by CT scanning to measure the cross-section area of the ground electrode27. The ratio of the cross-section area ratio of the ground electrode27after the durability test to the cross-section area of the ground electrode27before the durability test was calculated for evaluation of the minimum cross-section area ratio. It can be said that, the smaller the cross-section area ratio, the higher degree of wear, i.e., the likelier the grain defect formation phenomenon (wormhole phenomenon) is to occur in the ground electrode27. The test results are indicated inFIG. 20.

Test samples of comparative spark plugs (as Comparative Examples) were produced and subjected to durability test in the same manner, except for the longitudinal length of the flat region and the condition of the areas and perimeters of the first and second cross sections of the ground electrode. The test results are also indicated inFIG. 20.

As seen fromFIG. 20, the cross-section area ratio was significantly small when A<0.2 mm (e.g. A=0.1 mm). The cross-section area ratio was large when A≧0.2 mm and, in particular, remained relatively large when A≧0.2 mm and 0.950≦(S2/L2)/(S1/L1)≦0.995. When (S2/L2)/(S1/L1)<0.950 or (S2/L2)/(S1/L1)>0.995, there was some decrease in the cross-section area ratio regardless of whether A≧0.2 mm. It has been thus shown by this experiment that the occurrence of the grain defect formation phenomenon in the ground electrode27can be prevented effectively by forming the flat region51on the ground electrode27under the conditions of A≧0.2 mm and 0.950≦(S2/L2)/(S1/L1)>0.995.

Further, the likelihood of occurrence of preignition due to a decrease in the cross-section area ratio was tested on each of the test samples. The test was conducted by mounting the test sample in a 2.0-L six-cylinder engine, driving the engine continuously at full throttle and detecting the ignition timing (° CA) at which the preignition occurred. When the grain defect formation phenomenon occurs (i.e. the cross-section area ratio becomes decreased), the edges of the grain defects are pointed. Such pointed edges are likely to accumulate heat and thereby become high in temperature so that ignition combustion occurs, prior to a given ignition timing, staring from these pointed edges. It can be thus said that the preignition resistance decreases as the cross-section area ratio becomes small. The test results are indicated inFIG. 21.

As seen fromFIG. 21, the ignition timing at which the preignition occurred remained around BTDC 33 degrees (corresponding to the full-throttle load) when the cross-section area ratio was larger than or equal to 0.995. When the cross-section area ratio was smaller than 0.995, however, the ignition timing at which the preignition occurred was retarded. Namely, the preignition occurred even under more moderate conditions when the cross-section area ratio was smaller than 0.995. It has been shown that the preignition resistance can be prevented from deterioration when the cross-section area ratio of the ground electrode27becomes 0.995 or larger by satisfaction of the conditions of A≧0.2 mm and 0.950≦(S2/L2)/(S1/L1)≦0.995.

Test samples of the spark plug1(as Examples) were produced and subjected to durability test in the same manner as in Experiment 1 by varying the radius R of the ground electrode27(the curvature radius of the outer peripheral surface of the ground electrode27) to 0.4 mm, 0.5 mm, 0.8 mm, 1.0 mm and 1.1 mm while fixing the longitudinal length A of the flat region51at 0.3 mm. Test samples of comparative spark plugs (as Comparative Examples) were also produced and subjected to durability test in the same manner, except for the condition of the areas and perimeters of the first and second cross sections of the ground electrode. The test results are indicated inFIG. 22.

As seen fromFIG. 22, the cross-section area ratio was prevented from decreasing when 0.5 mm≦R≦1.0 mm and 0.950≦(S2/L2)/(S1/L1)≦0.995. It has been confirmed that the occurrence of grain defect formation (wormhole phenomenon) in the ground electrode27can be prevented more effectively by satisfaction of the conditions of 0.5 mm≦R≦1.0 mm and 0.950≦(S2/L2)/(S1/L1)≦0.995. Regardless of whether 0.950≦(S2/L2)/(S1/L1)≦0.995, there was no or less decrease in the cross-section area ratio when R<0.5 mm and when R>1.0 mm. This leads to the assumptions that: when R<0.5 mm, the distance between the longitudinal axis and the peripheral surface of the ground electrode27was so small that the front end of the ground electrode27did not become so high in temperature by heat radiation from its peripheral surface; and that there was not so large difference between the convex curved region and the flat region51as to cause the concentration of the thermal shock vectors of the fuel even on the convex curved region when R>1.0 mm. It can be concluded that the grain defects are originally unlikely to occur so that the significance of forming the flat region51to satisfy the condition of 0.950≦(S2/L2)/(S1/L1)≦0.995 is small when R<0.5 mm and when R>1.0 mm.

A test sample of the spark plug100(as Example) was produced. In the test sample, the ground electrode27was circular in cross section with a diameter of 1.6 mm. The flat region51was formed with a longitudinal length A of 1.0 mm and a lateral width B of 0.4 mm on the outer peripheral surface of the front end of the ground electrode27. Further, the dimensions of the test sample were controlled to C=0.9 mm, D=0.425 mm and E=1.45 mm.

The test sample was subjected to ignition performance test. The ignition performance test was conducted by placing the test sample in a pressure chamber with a pressure sensor, injecting gasoline (as fuel) toward the test sample at various angles and checking the occurrence of ignition under the conditions of an initial chamber pressure of 1 MPa, a fuel injection pressure of 20 MPa and an air-fuel ratio (A/F) of 25. Fuel injection models at injection angles of −15 degrees, 0 degree and 20 degrees are illustrated inFIGS. 23A,23B and23C, respectively. The occurrence of ignition was judged based on the waveform of the pressure sensor. The ignition rate was determined as the number of times the ignition occurred when the test was repeated 30 times. The test results are indicated inFIG. 24.

A test sample of comparative spark plug (as Comparative Example) was produced and subjected to ignition performance test in the same manner, except that the comparative test sample had no flat region on the ground electrode and had a dimension of D=0.45 mm. The test results are also indicated inFIG. 24.

As seen fromFIG. 24, the ignition rate of Comparative Example was deteriorated when the fuel injection angle was −20 to 10 degrees. On the other hand, there was not so large ignition rate deterioration in Example even when the fuel injection angle was −20 to 10 degrees. The ignition rate of Example was much higher than that of Comparative Example. The greatest difference in ignition rate between Example and Comparative Example was observed when the fuel injection angle was −10 degrees. It has been thus shown that the engine ignition performance can be improved significantly even at a fuel injection angle of −20 to 10 degrees by forming the flat region51on the ground electrode27.

Test samples of the spark plug100were produced by varying the longitudinal length A and lateral width B of the flat region51. Each of the test samples was tested for ignition rate at a fuel ignition angle of −10 degrees in the same manner as in Experiment 3. The test results are indicated inFIG. 25.

As seen fromFIG. 25, the ignition rate was relatively low, regardless of whether A≧0.2 mm, when B<0.2 mm. The ignition rate was also relatively low when the surface area A×B of the flat region51was smaller than 0.2 mm2. It has been shown by this experiment that the effect of the flat region51can be obtained assuredly by satisfaction of the conditions of A×B≧0.2 mm2and B≧0.2 mm.

Test samples of the spark plug100were produced by varying the protrusion length F of the precious metal tip32. Each of the test samples was subjected to durability test to evaluate the amount of increase of the spark gap33due to wear of the precious metal tip32. The durability test was conducted by mounting the test sample in a 2.0-L six-cylinder engine, driving the engine continuously for 100 hours at 5000 rpm (full load) and measuring the amount of increase of the spark gap33during the test. The test results are indicated inFIG. 26.

As seen fromFIG. 26, the amount of wear of the precious metal tip32significantly increased so that the gap increase amount exceeded its consumption limit of 2.0 mm when F>1.6 mm. It has been confirmed that the heat transfer performance becomes insufficient as the protrusion length F (length dimension) of the precious metal tip32increases.

Test samples of the spark plug100were produced by varying the dimensions D and E of the ground electrode27. In the test samples, the longitudinal length A and lateral width B of the flat region51were controlled to 1.0 mm and 0.4 mm, respectively. Each of the test samples was tested for ignition rate at a fuel ignition angle of −10 degrees in the same manner as in Experiment 3. The test results are indicated inFIG. 27.

As seen fromFIG. 27, the ignition rate was high when D<0.3 mm or E≧0.6 mm and when D≧0.3 mm and E≧2×D mm. In view of the fact that the equation 2D−C/2≦1.6, i.e., D≦C/4+0.8 holds based on the equations F≧1.6 and F=E−C/2, the upper limit of the distance D is set to C/4+0.8. It has been confirmed that the effect of the flat region51can be obtained assuredly by satisfaction of the conditions of D<0.3 mm and E≧0.6 mm or by satisfaction of the conditions of 0.3≦D≦C/4+0.8 mm and E≧2×D mm.

The entire contents of Japanese Patent Application No. 2007-327314 (filed on Dec. 19, 2007) and No. 2007-336219 (filed on Dec. 27, 2007) are herein incorporated by reference.

Although the present invention has been described with reference to the above specific embodiment, the invention is not limited to this exemplary embodiment. Various modification and variation of the embodiment described above will occur to those skilled in the art in light of the above teachings.

For example, the spark plug1,100can alternatively be provided with two or more ground electrodes27although the spark plug1,100has a single ground electrode27in the above embodiment.

The center electrode5and the ground electrode27are not limited to the above two-layer structures. Each of the center electrode5and the ground electrode27may have a multi-layer structure of three or more layers. In this case, it is preferable that the metal material of the inner electrode layer exhibits higher thermal conductivity than the metal material of the outer electrode layer for efficient heat transfer. For example, the center electrode5and the ground electrode27can be formed with a three-layer structure having an inner layer of pure nickel, an intermediate layer of pure copper or cupper alloy and an outer layer of nickel etc. Alternatively, each of the center electrode5and the ground electrode27may have a single-layer structure of e.g. nickel.

The flat region51may be not exactly flat but may be nearly flat and slightly concave as long as the flat region51is capable of guiding the air-fuel mixture to the spark gap33without causing a concentration of thermal shock vectors of the fuel. Further, the form of the flat region51is not limited to the rectangular. The ground electrode27may have a flat region151of any shape other than rectangular as shown inFIG. 19. In this case, the longitudinal length A of the flat region151is defined as the length from the front end face27sof the ground electrode27to a point of the flat region151located farthest away from the front end face27sof the ground electrode27in the longitudinal direction of the ground electrode27; and the lateral width B of the flat region151is defined as the width along the front end face27sof the ground electrode27.

The scope of the invention is defined with reference to the following claims.