Spark plug

A spark plug has an insulator, a center electrode disposed in an axial hole, a resistor disposed in the axial hole and a seal member disposed between the resistor and the center electrode in the axial hole. The insulator includes an inner-diameter decreasing portion and a small inner-diameter portion. The center electrode includes a head portion supported on the inner-diameter decreasing portion of the insulator. The spark plug satisfies the following conditions: 1.8 mm≤L; and Cp≤11 mm where, assuming a region of the insulator from a boundary of the inner-diameter decreasing portion and the small inner-diameter portion to a rear end of the seal member as a specific region, L is a length of the specific region; D1 is an average inner diameter of the axial hole within the specific region; D2 is an average outer diameter of the specific region; and Cp is L/log(D2/D1).

FIELD OF THE INVENTION

The present invention relates to a spark plug.

BACKGROUND OF THE INVENTION

A spark plug is conventionally used for an internal combustion engine. In general, the spark plug has a center electrode and a ground electrode to ignite an air-fuel mixture by the generation of a spark discharge within a gap between the center electrode and the ground electrode as disclosed in international Publication No. 2011/033902, Japanese Laid-Open Patent Publication No. 2009-245716, Japanese Laid-Open Patent Publication No. H09-63745 etc.

Recently, there has been a demand to increase the compression ratio of the air-fuel mixture in the internal combustion engine for the purpose of improvements in engine performance such as fuel efficiency. In such an internal combustion engine, the voltage applied to the spark plug increases with increase in compression ratio. The higher the voltage applied to the spark plug, the larger the amount of current flowing through the spark plug at the spark discharge. This leads to wear of the electrodes.

In view of the above circumstance, an advantage of the present invention is a spark plug capable of suppressing electrode wear.

SUMMARY OF THE INVENTION

The present invention can be embodied as the following application examples (1), (2) and (3). Hereinafter, the term “front” refers to a spark discharge side with respect to the direction of an axis of a spark plug; and the term “rear” refers to a side opposite the front side.

(1) According to one aspect of the present invention, there is provided a spark plug comprising: an insulator having an axial hole formed therein in a direction of an axis of the spark plug; a center electrode disposed in the axial hole, with a front end portion of the center electrode protruding from a front end of the insulator; a resistor disposed in the axial hole at a position closer to a rear end of the spark plug than the center electrode; and a seal member disposed in the axial hole at a position between the resistor and the center electrode so as to connect the resistor and the center electrode to each other, wherein the insulator includes: an inner-diameter decreasing portion having an inner diameter decreasing toward a front end of the spark plug; and a small inner-diameter portion located closer to the front end of the spark plug than the inner-diameter decreasing portion; Wherein the center electrode includes a head portion located at a position closer to the rear end of the spark plug than the small inner-diameter portion of the insulator and supported on the inner-diameter decreasing portion of the insulator; and wherein the spark plug satisfies the following conditions: 1.8 mm≤L; and Cp≤11 mm where, assuming a region of the insulator extending from a boundary of the inner-diameter decreasing portion and the small inner-diameter portion to a rear end of the seal member in the direction of the axis as a specific region, L is a length of the specific region in the direction of the axis; D1 is an average inner diameter of the axial hole within the specific region; D2 is an average outer diameter of the specific region; and Cp is a value given by L/log(D2/D1).

In the spark plug, a part of the insulator surrounding the seal member constitutes a capacitor. By satisfaction of the above specific conditions, it is possible to limit the capacitance of the capacitor and thereby possible to suppress wear of the electrode caused due to spark discharge and improve the durability of the spark plug.

(2) in accordance to a second aspect of the present invention, there is provided a spark plug as described above, wherein the spark plug preferably satisfies the following condition: 2.0≤M/S≤3.0 where S is a maximum cross-sectional area of the axial hole within the specific region as taken perpendicular to the axis; and M is an area of contact between the seal member and the center electrode.

In this case, it is possible to suppress wear of the electrode caused due to spark discharge and improve the durability of the spark plug by optimizing the maximum cross-sectional area S of the axial hole and the contact area M of the seal member and the center electrode.

(3) In accordance to a third aspect of the present invention, there is provided a spark plug as described above, wherein the spark plug preferably satisfies the following condition: D1≤1 mm.

In this case, it is possible to properly limit the capacitance of the capacitor and effectively suppress wear of the electrode caused due to spark discharge by setting the average inner diameter D1 of the axial hole to a small value.

It is herein noted that the present invention can be embodied in various forms such as not only a spark plug but also an internal combustion engine with a spark plug.

Other advantages and features of the present invention will also become understood from the following description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described below with reference to the drawings.

A-1. Structure of Spark Plug

FIG. 1is a cross-sectional view of a spark plug100for an internal combustion engine, such as gasoline engine, according to one embodiment of the present invention. InFIG. 1, a flat cross section of the spark plug100is taken along a center axis CL of the spark plug100. Hereinafter, the direction parallel to the axis CL is referred to as the “direction of the axis CL” or simply referred to as the “axis direction”. The radial direction of a circle about the axis CL is simply referred to as the “radius direction”. The circumferential direction of a circle about the axis CL is simply referred to as the “circumferential direction”. InFIG. 1, the front side is indicated by an arrow “Df”; and the rear side is indicated by an arrow Dfr.

As shown inFIG. 1, the spark plug100includes a substantially cylindrical insulator10having an axial hole12formed therein along the axis CL, a center electrode20disposed in a front end part of the axial hole12, a metal terminal40disposed in a rear end part of the axial hole12, a connection part300disposed between the center electrode20and the metal terminal40within the axial hole12, a metal shell50fixed around an outer circumference of the insulator10and a ground electrode30having a base end joined to a front end face57of the metal shell50and a distal end facing the center electrode20with a gap g left therebetween.

The insulator10includes a large diameter portion19, a front body portion17, a first outer-diameter decreasing portion15, a leg portion13, a second outer-diameter decreasing portion11and a rear body portion18. The large diameter portion19has the largest outer diameter among the respective portions of the insulator10. The front body portion17, the first outer-diameter decreasing portion15and the leg portion13are arranged in this order on the front side with respect the large diameter portion19. The first outer-diameter decreasing portion15has an outer diameter gradually decreasing toward the front. The second outer-diameter decreasing portion11and the rear body portion18are arranged in this order on the rear side with respect to the large diameter portion19. The second outer-diameter decreasing portion11has an outer diameter gradually decreasing toward the rear. Further, the insulator10has an inner-diameter decreasing portion16formed in the vicinity of the first outer-diameter decreasing portion15(in the present embodiment, in the front body portion17). The inner-diameter decreasing portion16has an inner diameter gradually decreasing toward the front. Preferably, the insulator10is made of a material having mechanical strength, thermal strength, electrical strength etc. As such an insulator material, there can be used an alumina-based sintered ceramic material. It is needless to say that any other insulating material may alternatively be used as the material of the insulator10.

The center electrode20has a rod-shaped electrode body27extending along the axis CL and a first tip29fixed to a front end of the electrode body27by e.g., laser welding. A head portion24of large diameter is formed on a rear part of the electrode body27. In the present embodiment, the maximum outer diameter of the head portion24is set larger than the inner diameter of the leg portion13of the insulator10. A front side surface of the head portion24is supported on the inner-diameter decreasing portion of the insulator10. The center electrode20is disposed in the front end part of the axial hole12of the insulator10, with a front end portion of the center electrode20protruding toward the front from a front end of the insulator10. In the present embodiment, the electrode body27has an outer layer21and a core22located inside the outer layer21. The outer layer21is made of e.g. a nickel-based alloy. The core22is made of a material (e.g. copper-based alloy) having higher thermal conductivity than that of the outer layer21. The first tip29is made of a material (e.g. noble metal such as iridium (Ir) or platinum (Pt), tungsten (W), or an alloy of at least one thereof) having higher spark resistance than that of the electrode body27.

The metal terminal40is disposed in the rear end part of the axial hole12of the insulator10, with a rear end portion of the metal terminal40protruding toward the rear from a rear end of the insulator10. The metal shell40is rod-shaped along the axis CL and is made of a conductive material (e.g. metal such as low carbon steel).

For suppression of electrical noise, a substantially cylindrical column-shaped resistor70is disposed between the metal terminal40and the center electrode20(i.e. at a position closer to the rear end of the spark plug100than the center electrode20) within the axial hole12of the insulator10. The resistor70is made of a composition containing a conductive material (e.g. carbon particles), ceramic particles (e.g. ZrO2particles) and glass particles (e.g. SiO2—B2O3—Li2O—BaO glass particles).

A first conductive seal member60is arranged between the resistor70and the center electrode20, whereas a second conductive seal member80is arranged between the resistor70and the metal terminal40. The seal member60,80is made of a composition containing metal particles (e.g. Cu particles) and glass particles of the same kind as those contained in the resistor70.

The center electrode20and the metal terminal40are electrically connected to each other via the resistor70and the seal members60and80. Thus, these conductive members60,70and80function together as the electrical connection part300. In the present embodiment, the first seal member60corresponds to the claimed seal member.

The metal shell50has a substantially cylindrical shape with a through hole59along the axis CL such that the insulator10is inserted through the through hole59of the metal shell50. The metal shell50is made of a conductive material (e.g. metal such as low carbon steel) and is fixed around the outer circumference of the insulator10, with a front end portion of the insulator10protruding toward the front from a front end of the metal shell50and a rear end portion of the insulator10protruding toward the rear from a rear end of the metal shell50.

The metal shell50includes a shell body55formed with a thread portion52for screwing into a mounting hole of the internal combustion engine and a seat portion54located on the rear side of the shell body55. An annular gasket5is fitted between the thread portion52and the seal portion54. The metal shell50also includes a deformation portion58, a tool engagement portion51and a crimp portion53arranged in this order on the rear side with respect to the seal portion54. The deformation portion58is deformed in such a shape that a middle of the deformation portion58projects radially outwardly (i.e., in a direction apart from the axis CL). The tool engagement portion51is formed into e.g. a hexagonal column shape so as to be engageable with a spark plug wrench. The crimp portion53is formed in a radially inwardly bent shape. In the present embodiment, the crimp portion53is located at a position closer to the rear end of the spark plug100than the second outer-diameter decreasing portion11of the insulator10.

There is a space SP defined by an inner circumferential surface of the metal shell50and an outer circumferential surface of the insulator10at a location between the crimp portion53of the metal shell50and the second outer-diameter decreasing portion11of the insulator10. A first rear-side packing6, a talc (talc powder)9and a second rear-side packing7are disposed, in this order from the rear toward the front, within the space SP. In the present embodiment, the packing6,7is in the form of a C-ring of iron. It is needless to say that the packing6,7may be made of any other material.

Furthermore, the metal shell50includes an inner-diameter decreasing portion56formed on the shell body55and having an inner diameter gradually decreasing toward the front. A front-side packing8is disposed between the inner-diameter decreasing portion56of the metal shell50and the first outer-diameter decreasing portion15of the insulator10. The packing8is also in the form of a C-ring of iron in the present embodiment. It is needless to say that the packing8may be made of any other material (e.g. metal such as copper).

During manufacturing of the spark plug100, the crimp portion53is crimped toward the insulator10so as to be radially inwardly bent while being pressed toward the front. By such crimping, the deformation portion58is compressed and deformed. The insulator10is then pressed toward the front in the metal shell50via the rear-side packings6and7and the talc9. The front-side packing8is consequently compressed between the first outer-diameter decreasing portion15and the inner-diameter decreasing portion to establish a seal between the metal shell50and the insulator10. In this way, the metal shell50is fixed around the insulator10so as to prevent combustion gas from leaking from a combustion chamber of the internal combustion engine to the outside through between the metal shell and the insulator10.

The ground electrode30has a rod-shaped electrode body37joined at a base end portion thereof to the front end face57of the metal shell50by e.g. resistance welding and a second tip39fixed to a distal end portion of the electrode body37by e.g. laser welding. The electrode body37extends from the metal shell50toward the front and then gets bent toward the axis CL such that the distal end portion31of the electrode body37faces the front end portion of the center electrode20. Accordingly, the first tip29of the center electrode20and the second tip39of the ground electrode30face each other via the gap g. In the present embodiment, the electrode body37has an electrode base35defining a surface of the electrode body37and a core36embedded in the electrode base35. The electrode base35is made of a material (e.g. nickel alloy) having higher oxidation resistance than that of the core36. The core36is made of a material (e.g. pure copper, copper alloy etc.) having higher thermal conductivity than that of the electrode base35.

The spark plug100can be manufactured by the following procedure. The insulator10, the center electrode20, the metal terminal40, the metal shell50, the material compositions of the seal members60and80and the material composition of the resistor70are prepared. The center electrode20is inserted into the axial hole12of the insulator10from a rear end opening12xof the axial hole12and arranged at a predetermined position within the axial hole12by engagement of the head portion24of the center electrode20on the inner-diameter decreasing portion16of the insulator10as mentioned above with reference toFIG. 1. The material composition of the first seal member60, the material composition of the resistor70and the material composition of the second seal member80are, in this order, put into the axial hole12from the rear end opening12xand compacted/molded by insertion of a rod in the axial hole12from the rear end opening12x. After that, a part of the metal terminal40is inserted in the axial hole12from the rear end opening12x. In this state, the insulator10is heated at a predetermined temperature higher than the softening points of the glass components of the respective material compositions while the metal terminal40is pushed toward the front. As a result, the material compositions are compressed and sintered to respectively form the seal members60and80and the resistor70. On the other hand, the ground electrode30is joined to the metal shell50. The metal shell50to which the ground electrode30has been joined is then fixed around the insulator10. Finally, the spark plug100is completed by bending the ground electrode30.

A-2. Specific Region of Insulator

FIG. 2is an enlarged cross-sectional view of a substantive part of the spark plug100in the vicinity of the first seal member60. InFIG. 2, the center electrode20, a part of the insulator10, the first seal member60, a part of the resistor70and a part of the metal shell50are illustrated; and the ground electrode30is omitted from illustration. Further, the inner structure of the center electrode20is omitted from illustration.

As shown inFIG. 2, the insulator10includes a small inner-diameter portion14connected to a front end of the inner-diameter decreasing portion16(i.e. located at a position closer to the front end of the spark plug100than the inner-diameter decreasing portion16) in the present embodiment. The small inner-diameter portion14has an inner diameter smaller than that of the inner-diameter decreasing portion16. An inner circumferential surface of the small inner-diameter portion14is approximately in parallel with the axis CL.

Herein, a region of the insulator10surrounding the first seal member60is defined as a specific region10L as shown inFIG. 2. More specifically, the specific region10L of the insulator10is defined as extending from a boundary P1of the inner-diameter decreasing portion16and the small inner-diameter portion14to a rear end P2of the first seal member60in the direction of the axis CL (e.g. extending between broken lines inFIG. 2). The vicinity of the boundary P1is shown in enlargement in the balloon ofFIG. 2. As shown in the figure, the connection area between the inner-diameter decreasing portion16and the small inner-diameter portion14may be chamfered. In this case, the boundary P1is defined as, in a flat cross section of the insulator10taken through the axis CL, an intersection between the extension of a straight line segment16L representing the inner circumferential surface of the inner-diameter decreasing portion16and the extension of a straight line segment14L representing the inner circumferential surface of the small inner-diameter portion14.

The first seal member60is situated inside the specific region10L. By contrast, the metal shell50is situated outside the specific region10L (i.e., the specific region10L is surrounded by the metal shell50). In such a configuration, the first seal member60and the metal shell50form a capacitor C across the specific region10L. When a high voltage is applied to the spark plug100, the capacitor C accumulates electric charge according to the applied voltage before the generation of a spark discharge. The electric charge accumulated in the capacitor C flows as electric current at the spark discharge. This electric current flows from the center electrode20to the ground electrode30without being regulated by the resistor70because the resistor70lies on the rear side with respect to the first seal member60. There is thus a large current flow caused between the electrodes20and30at the spark discharge in the case where the capacitance of the capacitor C is high. It is more likely that wear of the electrode20,30will occur due to such a large current flow.

The capacitance of the capacitor C can be determined as follows by approximating the shape of the specific region10L to a cylindrical shape with the assumption that the clearance between the specific region10L and the metal shell50is sufficiently small.

As shown inFIG. 2, it is defined that: L is a length of the specific region10L in the direction of the axis CL; D1 is an average inner diameter of the axial hole12within the specific region10L; and D2 is an average outer diameter of the specific region10L. The average inner diameter D1 refers to e.g. the average of a plurality of inner diameter values measured at intervals of 1 mm over the entire range from the front end to the rear end of the specific region10L in the direction of the axis CL. Similarly, the average outer diameter D2 refers to e.g. the average of a plurality of outer diameter values measured at intervals of 1 mm over the entire range from the front end to the rear end of the specific region10L in the direction of the axis CL. On the assumption that the cylindrical shape of the specific region10L is represented by the length L, the average inner diameter D1 and the average outer diameter D2, the capacitance of the capacitor C is given by 2πεL/log(D2/D1) where the base of log is 10.

The value of L/log(D2/D1), which is the omission of the constant 2πε from the expression 2πεL/log(D2/D1), is herein referred to as the “approximate capacitance evaluation value Cp” or “capacitance evaluation value Cp”. The capacitance of the capacitor C is in proportion to the capacitance evaluation value Cp. Accordingly, the higher the capacitance evaluation value Cp, the larger the electric current caused at the spark discharge, the more likely wear of the electrode20,30will occur. It is thus possible to suppress wear of the electrode20,30by limiting the capacitance evaluation value Cp of the insulator10to a low value.

In view of the above fact, the spark plug100is adapted to satisfy the following specific conditions in the present embodiment (see the after-mentioned examples).
1.8 mm≤L
Cp≤11 mm

It is preferable to satisfy the following condition: D1≤3 mm in order to properly limit the capacitance of the capacitor C.

It is also defined that: M is an area of contact between the first seal member60and the center electrode20(as indicated by a thick line62inFIG. 2); and S is a maximum cross-sectional area of the axial hole12within the specific region10L as taken perpendicular to the axis CL. The thick line62is hereinafter also referred to as “contact line62”.

In order to properly limit the capacitance of the capacitor C, it is further preferable to satisfy the following condition: 2.0≤M/S≤3.0 by optimization of the maximum cross-sectional area S and the contact area M.

In the present embodiment, the center electrode20is symmetric in shape with respect to the axis CL. It means that the cross section of the center electrode20is substantially the same in shape as long as the cross section is taken through the axis CL (i.e. irrespective of the direction of the cross section). In this case, the contact line62, when rotated 180° about the axis CL, outlines a three-dimensional shape which is well approximate to the shape of the contact area M. Namely, the area of the three-dimensional shape well approximates the contact area M.

The contact area M can be thus determined as follows based on the shape of the contact line62.

For example, the contact line62is approximated to a bent line consisting of a plurality of straight line segments of predetermined length (e.g. 0.1 mm).

The areas defined by rotation of the respective line segments are calculated in the same manner as the calculation of a lateral surface area of a truncated cone. The sum of the calculated surface areas is determined as the contact area M. It is feasible to approximate the contact area line60to the bent line by any known method.

B. Evaluation Test

Fifteen types of samples of the spark plug100(sample No. 1 to 15) were produced and each tested by gap test and load lifetime test. The configurations and test results of the respective samples are shown in TABLE 1.

In the samples No. 1 to 15, the parameters D1, D2, L and Cp were determined as defined above (seeFIG. 2). These samples were different in at least one of the parameters D1, D2, L and Cp. The other configurations of the samples were common.

The gap test was performed as follows to test the gap increase reduction rate (%).

The test sample was placed in the air of 10 MPa pressure and allowed to repeat spark discharge a frequency of 60 for 20 hours. The gap g between the electrodes20and30was measured with a pin gauge before and after the repeated spark discharge cycles. The difference of these measurement results was calculated as the amount of increase of the gap g (i.e. the amount of wear of the electrode20,30). In this gap test, three samples was used for each sample type. The average of the calculated gap increase amount values of the three respective samples was adopted as the gap increase. The rate of reduction of the gap increase was determined with reference to that of the sample No. 3 by the following formula.

The positive value of the gap increase reduction rate means that the gas increase of the test sample was smaller than that of the reference sample (sample No. 3), that is, the wear of the electrode20,30of the test sample was more suppressed as compared to that of the reference sample (sample No. 3). The lower the gap increase reduction rate, the smaller the gap increase, the more suppressed the wear of the electrode20,30.

The gap test result was evaluated as follows.

D: 0%>Gap increase reduction rate

The load lifetime test was performed as follows according to the clauses 7.13 and 7.14 of JIS B 8031: 2006 “Internal Combustion Engines—Spark Plugs”.

The resistance of the test sample was first measured according to the clause 7.13 of JIS B 8031. The test sample was then subjected to load test operation according to the clause 7.14 of JIS B 8031. In the load test operation, the test sample was allowed to repeat 1.3×107times of spark discharge with the application of a voltage of 20 kV. The resistance of the test sample after the load test was measured according to the clause 7.13 of JIS B 8031. The rate of change of the resistance was determined by subtracting the resistance of the test sample before the load test from the resistance of the sample after the load test. In this load lifetime test, one sample was used for each sample type.

The load lifetime test result was evaluated as: A when the resistance change rate was in the proper range of −30% to +30%; and B when the resistance change rate was out of the proper range.

As shown in TABLE 1, the longer the length L of the specific region10L, the better the load lifetime test result. The reason for this is assumed that, when the length L of the specific region10L was long, the length of the first seal member60was long so that the first seal member60was improved in durability. The load lifetime test result was evaluated as A for the samples where the length L was 1.8 mm, 2.0 mm, 3.0 mm, 4.0 mm, 4.5 mm and 5.0 mm. It has thus been shown that it is possible to improve the durability of the spark plug by satisfaction of 1.8 mm≤L. It is feasible to use any of the above sixth length values other than 1.8 mm as the lower limit of the length L. Further, it is feasible to use any one of the above sixth length values as the upper limit of the length L. For example, the length L may be set shorter than or equal to 5.0 mm. It is needless to say that the length L may be set shorter than 5.0 mm.

Furthermore, the lower the capacitance evaluation value Cp, the better the gap test result, as shown in TABLE 1. The reason for this is assumed that the current flow between the electrodes20and30was more suppressed when the capacitance evaluation value Cp was low than when the capacitance evaluation value Cp was high as mentioned above. The gap test result was evaluated as A or B for the samples where the capacitance evaluation value Cp was 3.5 mm, 4.7 mm, 5.0 mm, 5.4 mm, 7.3 mm, 9.9 mm, 10.4 mm and 11.0 mm. It has thus been shown that it is possible to suppress the wear of the electrode20,30by satisfaction of Cp≤11.0 mm. It is feasible to use any of the above eight capacitance evaluation values other than 11.0 mm as the upper limit of the capacitance evaluation value Cp. It is further feasible to use any one of the above eight capacitance evaluation values as the lower limit of the capacitance evaluation value Cp. For example, the capacitance evaluation value Cp may be set higher than or equal to 3.5 mm. It is needless to say that the capacitance evaluation value Cp may be set lower than 3.5 mm.

Regardless of the shape of the specific region10L, the gap test result was favorable as long as the capacitance evaluation value Cp was lower than or equal to 11.0 mm. It is thus considered that, when the capacitance evaluation value Cp is lower than or equal to 11.0 mm, the amount of electric charge accumulated in the capacitor C is decreased to limit the flow of electric current between the electrodes20and30at the spark discharge and thereby suppress the wear of the electrode20,30regardless of the average inner and outer diameters D1 and D2. The average inner diameter D1 may be thus within or out of the range of D1 of the fifteen test samples (i.e. the range from 2.7 mm to 3.9 mm). Likewise, the average outer diameter D2 may be within or out of the range of D2 of the fifteen test samples (i.e. the range from 6.3 mm to 9.2 mm). However, it is apparent that it is preferable to satisfy D1≤3 mm in view of the fact that the gap test result was better when the average inner diameter D1 was smaller than or equal to 3 mm as shown in TABLE 1.

Next, ten types of other samples of the spark plug100(sample No. 16 to 25) were produced and each tested by impact resistance test and productivity test. The configurations and test results of the respective samples are shown in TABLE 2.

In the samples No. 16 to 25, the parameters M, S and M/S were determined as defined above (seeFIG. 2). Among these ten types of samples, each of seven samples No. 16 to 22 had the same configurations as those of sample No. 10 of TABLE 1, except for the shape of the rear end face28of the center electrode20. The parameters D1, D2 and L of sample No. 16 to 22 were the same as those of sample No. 10. (The parameters M, S and M/S of sample No. 16 were the same as those of sample No. 10.) Each of three samples No. 23 to 25 had the same configurations as those of sample No. 11 of TABLE 1, except for the shape of the rear end face28of the center electrode20. The parameters D1, D2 and L of sample No. 23 to 25 were the same as those of sample No. 11. (The parameters M, S and M/S of sample No. 23 were the same as those of sample No. 11.) The shape of the rear end face28of the center electrode20was changed to vary the contact area M. In each sample, the rear end face28of the center electrode20was depressed toward the front. The contact area M was varied by adjusting the amount of depression of the rear end face28of the center electrode20.

The impact resistance test was performed as follows.

The test sample was subjected to the same test operation as in the gap test. After that, the test sample was subjected to impact resistance test operation three times according to the clause 7.4 of JIS B 8031. The test sample was then tested for whether or not the center electrode20was firmly fixed in position relative to the insulator10.

The impact resistance result was evaluated as: A when the center electrode20was firmly fixed in position relative to the insulator10; and B when the center electrode20was movable relative to the insulator10.

The productivity test was performed by counting the number of occurrence of defective samples during production of thirty test samples. Herein, the sample was judged as defective when the electrical resistance between the center electrode20and the metal terminal40was higher than a threshold value. The threshold value was set as a value higher than the upper limit of a predetermined proper resistance range.

The productivity test result was evaluated as: A when the number of occurrence of defective samples was 0 (zero); B when the number of occurrence of defective samples was 1; and C when the number of occurrence of defective samples was 2 or more.

As shown in TABLE 2, the higher the ratio M/S, the better the impact resistance test result. The reason for this is assumed that, when the ratio M/S was high, the contact area M between the first seal member60and the center electrode20was large relative to the respective outer diameters of the center electrode20and the first seal member60so that the adhesion of the center electrode20and the first seal member60was improved. The impact resistance test result was evaluated as A for the samples where the ratio M/S was 2.0, 2.5, 2.7, 2.8, 3.0, 3.1 and 3.2. It has thus been shown that the ratio M/S is preferably higher than or equal to 2.0. It is feasible to use any arbitrary one of the above seven ratio values higher than 2.0 as the lower limit of the ratio M/S.

On the other hand, the lower the ratio M/S, the better the productivity rest result, as shown in TABLE 2. The reason for this is assumed as follows. The rear end face28of the center electrode20was more depressed when the ratio M/S was high than when the ratio M/S was low. As the rear end face28of the center electrode20was more depressed, it was difficult to introduce the material of the first seal member60to the bottom of the depressed rear end face28of the center electrode20so that there was a clearance formed between the center electrode20and the first seal member60. The formation of such a clearance became a cause of poor conduction between the center electrode20and the first seal member60. The productivity test result was evaluated as A for the samples where the ratio M/S was 1.8, 1.9, 2.0, 2.5, 2.7, 2.8 and 3.0. It has thus been shown that the ratio M/S is preferably lower than or equal to 3.0. It is feasible to use any arbitrary one of the above seven ratio values lower than 3.0 as the upper limit of the ratio M/S.

Although the impact resistance and productivity of the spark plug were largely influenced by the contact area M between the first seal member60and the center electrode20as shown in TABLE 2, it is considered from the test results that the influence of the other factors (average inner diameter D1, average outer diameter D2 and length L) on the impact resistance and productivity of the spark plug is small. In fact, for example, both of the samples No. 16 to 22 and the samples No. 23 to 25 had high impact resistance and productivity even though the average inner diameter D1, average outer diameter D2 and length L of the samples No. 16 to 22 (corresponding to those of the sample No. 10 of TABLE 1) were respectively different from the average inner diameter D1, average outer diameter D2 and length L of the samples No. 23 to 25 (corresponding to those of the sample No. 11 of TABLE 1). It is also considered that: when the ratio M/S is high, the impact resistance is improved as the adhesion of the center electrode20and the first seal member60is increased regardless of the shape of the surface of the center electrode20in contact with the first seal member60; and, when the ratio M/S is low, the productivity is improved as it becomes less difficult to introduce the material of the first seal member60to the surface of the center electrode20. The above preferable range of the ratio M/S is thus applicable to varying combinations of D1, D2 and L and to varying shapes of the surface of the center electrode20in contact with the first seal member60. It is needless to say that the ratio M/S may be out of the above preferable range.

The configurations of the spark plug100are not limited to those ofFIGS. 1 and 2. Although a part of the specific region10L of the insulator10located rear of the inner-diameter decreasing portion16is made constant in inner diameter in the above embodiment, the specific region10L of the insulator10is not limited to such a diameter. The inner diameter of the part of the specific region10L of the insulator10located rear of the inner-diameter decreasing portion16may be changed depending on the position in the direction of the axis CL. The outer diameter of the specific region10L of the insulator10may be changed depending on the position in the direction of the axis CL. Further, the inner and outer circumferential surfaces of the specific region10of the insulator10may be different in shape. In this way, it is feasible to change the size of the clearance between the specific region10L and the metal shell50depending on the position in the direction of the axis CL. In general, the capacitance of the capacitor C is lower than the value of 2πεL/log(D2/D1) when the clearance between the specific region10L and the metal shell50is larger than 0 (zero). It is thus possible to, as long as the capacitance resistance value Cp (=L/log(D2/D1)), suppress wear of the electrode20,30even though the respective configurations of the insulator10and the metal shell50(in particular, the specific region10L of the insulator10and the part of the metal shell50facing the specific region10) are different from those of the above embodiment.

A part of the surface of the center electrode20in contact with the first seal member60may be knurled or be formed with either or both of pits and projections for increase of the contact area M.

The spark discharge gap g may be defined between the a side surface of the center electrode20(in parallel to the axis CL) and the ground electrode30rather than between the front end face of the center electrode20and the ground electrode30.

The center electrode20may be of any shape other than that of the above embodiment. Likewise, the ground electrode30may be of any shape other than that of the above embodiment.

The entire contents of Japanese Patent Application No. 2015-244915 (filed on Dec. 16, 2015) are herein incorporated by reference.

Although the present invention has been described with reference to the above specific embodiments and modifications, the above embodiments and modifications are intended to facilitate understanding of the present invention and are not intended to limit the present invention thereto. Without departing from the scope of the present invention, various changes and modifications can be made to the present invention; and the present invention includes equivalents thereof. The scope of the invention is defined with reference to the following claims.

DESCRIPTION OF REFERENCE NUMERALS

10L: Specific region

12x: Rear end opening

13: Leg portion

14L: Straight line segment

16L: Straight line segment

17: Front body portion

18: Rear body portion

19: Large diameter portion

20: Center electrode

24: Head portion

28: Rear end face

29: First tip

31: Distal end portion

39: Second tip

40: Metal terminal

50: Metal shell

51: Tool engagement portion

52: Thread portion

54: Seat portion

55: Body part

57: Front end face

59: Through hole

60: First seal member

62: Contact line

80: Second seal member

300: Connection part

Df: Front side

Dfr: Rear side