Spark plug

A spark plug having a connecting portion that electrically connects a center electrode with a metal terminal within an axial bore in an insulator. The connecting portion of the spark plug includes a resistor. A center electrode-side resistance, which is the resistance of a portion of the resistor that extends from the center thereof toward the center electrode in the axial direction, is larger than a metal terminal-side resistance, which is the resistance of a portion of the resistor that extends from the center thereof toward the metal terminal.

FIELD OF THE INVENTION

The present invention relates to a spark plug, and, more particularly, to a spark plug including a resistor.

BACKGROUND OF THE INVENTION

In recent years, the voltage applied to a spark plug has been increased because of the increasing power of an internal combustion engine. Therefore, the level of radio noise (ignition noise) generated upon occurrence of spark discharge tends to increase. To reduce such radio noise, various techniques have been proposed (see, for example, Japanese Patent Application Laid-Open (kokai) No. H05-152053; Japanese Patent Application Laid-Open (kokai) No. H11-233232; and Japanese Patent Application Laid-Open (kokai) No. 2006-66086).

Generally, the level of radio noise generated by a spark plug can be reduced by increasing the resistance of a resistor disposed in a connecting portion that electrically connects a center electrode of the spark plug with its metal terminal. However, when the resistance of the resistor is increased in order to reduce the level of radio noise, ignition energy decreases, and this may cause deterioration of the sparking performance of the spark plug.

In view of the above problem, an object to be achieved by the present invention is to reduce the level of radio noise generated from a spark plug while suppressing deterioration of its sparking performance.

SUMMARY OF THE INVENTION

The present invention has been made to solve, at least partially, the above problem and can be embodied in the following modes or application examples.

The present invention may be implemented as a spark plug, as described above. Alternatively, the invention may be implemented as a method of producing the spark plug, a resistor in the spark plug, or a method of producing the resistor in the spark plug.

In accordance with a first aspect of the present invention, there is provided a spark plug comprising an insulator having an axial bore extending in an axial direction; a center electrode disposed at one end of the axial bore; a metal terminal disposed at the other end of the axial bore; and a connecting portion that electrically connects the center electrode with the metal terminal within the axial bore, wherein the connecting portion includes a resistor, and a center electrode-side resistance of the resistor is larger than a metal terminal-side resistance of the resistor, the center electrode-side resistance being a resistance of a portion of the resistor that extends from a center thereof toward the center electrode in the axial direction, the metal terminal-side resistance being a resistance of a portion of the resistor that extends from the center thereof toward the metal terminal.

In accordance with a second aspect of the present invention, there is provided a spark plug as described above according to application example 1, wherein a material forming the portion of the resistor that extends from the center thereof toward the center electrode in the axial direction has a resistance larger than a resistance of a material forming the portion of the resistor that extends from the center thereof toward the metal terminal.

In accordance with a third aspect of the present invention, there is provided a spark plug as described above according to application example 1 or 2, wherein the center electrode-side resistance is larger than the metal terminal-side resistance by at least 0.5 kΩ.

In accordance with a fourth aspect of the present invention, there is provided a spark plug as described above according to any one of application examples 1 to 3, wherein the center electrode-side resistance is larger than the metal terminal-side resistance by at least 1.0 kΩ.

In accordance with a fifth aspect of the present invention, there is provided a spark plug as described above according to any one of application examples 1 to 4, wherein the metal terminal-side resistance is 100Ω or larger.

In accordance with a sixth aspect of the present invention, there is provided a spark plug as described above according to any one of application examples 1 to 5, wherein the resistor has a substantially cylindrical shape, and has a diameter of 2.9 mm or smaller.

In the spark plug of application example 1, the center electrode-side resistance of the resistor is larger than the metal terminal-side resistance. This allows the level of radio noise generated upon occurrence of spark discharge to be effectively suppressed. Since it is not necessary to change the overall resistance of the resistor, deterioration of sparking performance can be suppressed.

In the spark plug of application example 2, the resistance of the material used for the portion of the resistor that extends toward the center electrode is different from the resistance of the material used for the portion of the resistor that extends toward the metal terminal. Therefore, the resistor can have different resistances in the portion extending toward the center electrode and the portion extending toward the metal terminal.

In the spark plug of application example 3, the center electrode-side resistance is at least 0.5 kΩ larger than the metal terminal-side resistance. In this case, the level of radio noise can be efficiently reduced.

In the spark plug of application example 4, the center electrode-side resistance is at least 1.0 kΩ larger than the metal terminal-side resistance. In this case, the level of radio noise can be efficiently reduced.

In the spark plug of application example 5, the metal terminal-side resistance is at least 100Ω. In this case, the level of radio noise can be reduced. Even when the resistor has a small resistance, the level of radio noise can be reduced by increasing the length of the resistor.

In the spark plug of application example 6, the diameter of the resistor is relatively small, i.e., 2.9 mm or smaller. In this case, the level of radio noise can be significantly reduced by setting the center electrode-side resistance to be larger than the metal terminal-side resistance.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1is a partially sectional view of a spark plug100according to an embodiment of the present invention. InFIG. 1, the right side of an axis O-O represented by a dot-dash line is an exterior front view, and the left side of the axis O-O is a sectional view obtained by cutting the spark plug100along a cross section passing through the center axis of the spark plug100. In the following description, the lower side of the spark plug100inFIG. 1in an axial direction OD is referred to as the front side of the spark plug100, and the upper side is referred to as the rear side.

The spark plug100includes a ceramic insulator10serving as an insulator, a metallic shell50, a center electrode20, a ground electrode30, and a metal terminal40. The metallic shell50has an insertion hole501formed therethrough in the axial direction OD. The ceramic insulator10is inserted into the insertion hole501and held therein. The center electrode20is held in an axial bore12formed in the ceramic insulator10such that the center electrode20extends in the axial direction OD. The front end portion of the center electrode20protrudes frontward from the ceramic insulator10. The ground electrode30is joined to the front end portion of the metallic shell50. The metal terminal40is disposed rearward of the center electrode20, and the rear end portion of the metal terminal40protrudes rearward from the ceramic insulator10. A high-voltage cable (not shown) is connected to the metal terminal40via a plug cap (not shown) to apply a high voltage to the metal terminal40.

As is well-known, the ceramic insulator10is formed by firing, for example, alumina and has a cylindrical tubular shape. The ceramic insulator10has, at its center in the radial direction, the axial bore12extending in the axial direction OD. A flange portion19having the largest outer diameter is formed substantially at the center of the ceramic insulator10in the axial direction OD, and a rear trunk portion18is formed rearward of the flange portion19. A front trunk portion17, that is smaller in outer diameter than the rear trunk portion18, is formed frontward of the flange portion19, and a leg portion13, that is smaller in outer diameter than the front trunk portion17, is formed frontward of the front trunk portion17. The leg portion13is tapered in the frontward direction and exposed to a combustion chamber of an internal combustion engine when the spark plug100is mounted on an engine head200of the engine.

The metallic shell50is a cylindrical metallic member used to secure the spark plug100to the engine head200of the internal combustion engine. The metallic shell50holds the ceramic insulator10so as to surround a portion of the ceramic insulator10that extends from a part of the rear trunk portion18to the leg portion13. More specifically, the spark plug100is configured such that the ceramic insulator10is inserted into the insertion hole501of the metallic shell50, and the front and rear ends of the ceramic insulator10protrude from the front and rear ends, respectively, of the metallic shell50. The metallic shell50is formed of low-carbon steel, and the entire metallic shell50is plated with, for example, nickel or zinc. A hexagonal columnar tool engagement portion51is provided at the rear end portion of the metallic shell50. A spark plug wrench (not shown) is engaged with the tool engagement portion51. The metallic shell50includes a mounting screw portion52having a screw thread that is to be threadingly engaged with a mounting screw hole201of the engine head200disposed in the upper portion of the internal combustion engine.

The metallic shell50has a flange-like seal portion54formed between the tool engagement portion51and the mounting screw portion52. An annular gasket5formed by folding a plate is fitted to a screw neck59between the mounting screw portion52and the seal portion54. When the spark plug100is mounted on the engine head200, the gasket5is crushed and deformed between a seat surface55of the seal portion54and a mounting surface205around the opening of the mounting screw hole201. The deformation of the gasket5provides a seal between the spark plug100and the engine head200, and gas leakage from the internal combustion engine through the mounting screw hole201is thereby prevented.

The metallic shell50has a thin-walled crimp portion53extending rearward from the tool engagement portion51. The metallic shell50further has a compression deformable portion58which also has a reduced wall thickness as in the case of the thin-walled crimp portion53and which is disposed between the seal portion54and the tool engagement portion51. Annular ring members6and7are interposed between the inner circumferential surface of the metallic shell50and the outer circumferential surface of the rear trunk portion18of the ceramic insulator10such that they are located in a region extending from the crimp portion53to the tool engagement portion51. The space between the ring members6and7is filled with powder of talc9. When the spark plug100is produced, the crimp portion53is bent inward and pressed frontward, whereby the compression deformable portion58is compressed and deformed. As a result of the compressive deformation of the compression deformable portion58, the ceramic insulator10is pressed frontward in the metallic shell50through the ring members6and7and the talc9. As a result of this pressing, a ledge15of the ceramic insulator10is pressed through an annular sheet packing8against a ledge56formed on the inner circumference of the metallic shell50at a position corresponding to the screw portion52, whereby the metallic shell50and the ceramic insulator10are united together. The compressed sheet packing8maintains airtightness between the metallic shell50and the ceramic insulator10, and outflow of combustion gas is thereby prevented. Also, as a result of the pressing, the talc9is compressed in the axial direction OD, whereby the airtightness of the metallic shell50is improved.

The center electrode20is a rod-like electrode disposed at the front end of the axial bore12and includes an electrode base metal21and a core22embedded therein. The electrode base metal21is formed of nickel or an alloy containing nickel as a main component, such as INCONEL (trademark)600. The core22is formed of copper or an alloy containing copper as a main component, copper and the alloy having higher thermal conductivity than the electrode base metal21.

The ground electrode30is formed of a metal having high corrosion resistance, and a nickel alloy, for example, is used for the ground electrode30. The base end of the ground electrode30is welded to the front end surface of the metallic shell50. The ground electrode30is bent such that its distal end portion and the front end face of the center electrode20face each other on the axis O in the axial direction OD. A spark gap across which spark discharge occurs is formed between the distal end portion of the ground electrode30and the front end portion of the center electrode20.

A connecting portion2for electrically connecting the metal terminal40with the center electrode20is disposed in the axial bore12of the ceramic insulator10. The connecting portion2includes an upper seal member4a, a lower seal member4b, and a cylindrical columnar resistor3sandwiched between these seal members. Each of the upper seal member4aand the lower seal member4bis a well-known seal member which is high in electrical conductivity and has a resistance of 0.1Ω or lower. The upper seal member4aand the lower seal member4bare formed of a material containing powder of a metal such as a copper, tin, or iron, and powder of borosilicate glass. The resistor3has a resistance of, for example, 1Ω or higher and is formed of a material containing zirconia powder, alumina powder, carbon black, glass powder, PVA binder, etc. The upper seal member4a, the lower seal member4b, and the resistor3are formed in the axial bore12in the following manner, for example. The center electrode20is inserted into the axial bore12from its rear end, and the powdery material of the lower seal member4bis placed on the center electrode20and then pressed with a pressing rod. Then, the powdery material of the resistor3is placed on the pressed powdery material of the lower seal member4band pressed with the pressing rod. The powdery material of the upper seal member4ais placed on the pressed powdery material of the resistor3and pressed with the pressing rod. Subsequently, the metal terminal40is inserted into the rear end of the axial bore12. The ceramic insulator10is heated, and then the metal terminal40is pressed into the axial bore12. The powdery materials of the materials of the upper seal member4a, the lower seal member4b, and the resistor3in the axial bore12are thereby melted and then cooled. In this manner, the upper seal member4a, the lower seal member4b, and the resistor3are solidified in the axial bore12, and the center electrode20and the metal terminal40are fixed in the axial bore12.

In the present embodiment, when the resistor3is formed in the manner described above, the powdery material of the resistor3is placed in the axial bore12while the amount of carbon black in the powdery material is appropriately controlled to generate a resistance distribution in the axial direction OD. More specifically, the ratio of carbon black mixed into the powdery material is increased in the axial direction OD from the front side to the rear side, so that the resistance increases toward the front side in the axial direction OD.

FIG. 2is a diagram showing an example of the resistance distribution in the resistor3. An enlarged cross section around the connecting portion2(the upper seal member4a+the resistor3+the lower seal member4b) of the spark plug100is shown in the lower part ofFIG. 2. A graph showing the resistances in the connecting portion2at different positions in the axial direction OD is shown in the upper part ofFIG. 2. The horizontal axis of the graph represents different positions in the axial bore12in the axial direction OD, and the vertical axis represents the resistance of a portion of the resistor3extending from the lower seal member4bdisposed frontward of the resistor3in the axial direction OD to each of the different portions.

In the present embodiment, the resistance of the resistor3gradually increases in the axial direction OD from the lower seal member4bto the upper seal member4a, as shown inFIG. 2. In addition, the gradient of the resistance in a portion extending from an interface A between the lower seal member4band the resistor3to the center B of the resistor3is different from the gradient of the resistance in a portion extending from the center B of the resistor3to an interface C between the upper seal member4aand the resistor3. Specifically, the gradient in the former portion is steeper, and the gradient in the latter portion is less steep. More specifically, in the resistor3, the resistance of the portion extending from the center B to the interface A (this portion is hereinafter referred to as a “center electrode-side resistor portion3b,” and this resistance is referred to as a “center electrode-side resistance R1”) is larger than the resistance of the portion extending from the center B to the interface C (this portion is hereinafter referred to as a “metal terminal-side resistor portion3a,” and this resistance is referred to as a “metal terminal-side resistance R2”). In the present embodiment, the “interface A” is a radial cross section of the axial bore12at the frontmost end of a portion in which the resistor3occupies at least 80% of the cross sectional area. The “interface C” is a radial cross section of the axial bore12at the rearmost end of the portion in which the resistor3occupies at least 80% of the cross sectional area. The positions of the interfaces A and C can be determined by image analysis of cross-sectional images of the connecting portion2. In the example of the resistance distribution shown inFIG. 2, the center electrode-side resistance R1is about 3 kΩ, and the metal terminal-side resistance R2is about 2 kΩ. Therefore, the overall resistance of the resistor3is about 5 kΩ. Preferably, the metal terminal-side resistance R2is at least 100 Ω.

To measure the resistance of the resistor3at an arbitrary cross section, first, the resistor3is ground from the side toward the center electrode20and from the side toward the metal terminal40to obtain cross sections to be used for the measurement of the resistance. Subsequently, silver paste is applied to these cross sections, and the resistance between the cross sections is measured. In this manner, the resistance of the resistor3at an arbitrary cross section can be measured. The center B of the resistor3can be determined by performing grinding from the side toward the center electrode20until the interface A appears, also performing grinding from the side toward the metal terminal40until the interface C appears, and then determining the position of the center between the interfaces A and C on the axis O. The resistances of the center electrode-side resistor portion3band the metal terminal-side resistor portion3acan be measured as follows. For example, a silver paste is applied to the cross sections at the interfaces A and C, and the overall resistance of the resistor3is measured. Then the resistor3is ground from the side toward the center electrode20to the center B. The silver paste is applied to the cross section at the center B, and the resistance between the opposite ends of the remaining resistor3(i.e., the metal terminal-side resistor portion3a) is measured. The metal terminal-side resistance R2can be measured in this manner. The center electrode-side resistance R1can be determined by subtracting the metal terminal-side resistance R2from the overall resistance of the resistor3that has been measured prior to the measurement of the metal terminal-side resistance R2. In this example, the resistor3is ground from the side toward the center electrode20to the center B. However, the resistor3may be ground from the side toward the metal terminal40to the center B to determine the center electrode-side resistance R1and the metal terminal-side resistance R2. In addition, the resistor3may be cut at the center B to separate the metal terminal-side resistor portion3aand the center electrode-side resistor portion3bfrom each other, and their resistances may be measured independently.

FIG. 3is a table showing the results of an evaluation test which was performed for 35 sample spark plugs100in order to investigate the relation between radio noise and the difference in resistance between the center electrode-side resistor portion3band the metal terminal-side resistor portion3a(R1-R2). As shown inFIG. 3, in samples Nos.1to9, the difference between the center electrode-side resistance R1and the metal terminal-side resistance R2was varied from 1.5 kΩ to −1.5 kΩ, with the overall resistance of the resistor3maintained at 2 kΩ. In samples Nos.10to18, the difference between the center electrode-side resistance R1and the metal terminal-side resistance R2was varied from 4 kΩ to −4 kΩ, with the overall resistance of the resistor3maintained at 5 kΩ. In samples Nos.19to35, the difference between the center electrode-side resistance R1and the metal terminal-side resistance R2was varied from 6 kΩ to −6 kΩ, with the overall resistance of the resistor3maintained at 10 kΩ.

FIG. 3shows the results of a test called the box method defined in CISPR12 and performed to evaluate the radio noise performance of each sample. More specifically, a “two-double circle (indicating a rating of very good)” was given to a sample with radio noise reduced by at least 5 dB as compared with reference radio noise generated by a sample in which the difference between the center electrode-side resistance R1and the metal terminal-side resistance R2was 0Ω. A “double circle (indicating a rating of good)” was given to a sample with radio noise reduced by 2.5 dB or more as compared with the reference radio noise, and a “circle (indicating a rating of fair)” was given to a sample with radio noise reduced by 1.5 dB or more as compared with the reference radio noise. In addition, a “cross (indicating a rating of poor)” was given to a sample with radio noise increased by 1.5 dB or more as compared with the reference radio noise. As can be found from the evaluation results for the samples inFIG. 3, irrespective of whether the overall resistance of the resistor3is 2 kΩ, 5 kΩ, or 10 kΩ, the level of radio noise can be effectively reduced as compared with the radio noise generated by a sample with no difference in resistance, so long as the center electrode-side resistance R1is larger than the metal terminal-side resistance R2by at least 0.5 kΩ, preferably at least 1.0 kΩ. Therefore, in the spark plug100of the present embodiment, the resistance of the center electrode-side resistor portion3bof the resistor3is set to be larger than the resistance of the metal terminal-side resistor portion3aby at least 0.5 kΩ, preferably at least 1.0 kΩ.

FIG. 4is a graph showing attenuation of radio noise versus frequency for different samples. Four representative samples (samples Nos.23,24,27, and30) selected from the samples shown inFIG. 3were evaluated at different frequencies using the box method described above. As can be seen from the evaluation results shown inFIG. 4, when the center electrode-side resistance R1was larger than the metal terminal-side resistance R2as in samples Nos.23and24, the amount of attenuation of radio noise was larger than that in sample No.27with no difference in resistance over the entire frequency range of 0 to 1,000 MHz. For example, in sample No.24in which the difference in resistance was 1 kΩ, the amount of attenuation at around 400 to 600 MHz was larger by up to 2.5 dB than that in sample No.27with no difference in resistance. In sample No.23in which the difference in resistance was 2 kΩ, the amount of attenuation at around 400 to 600 MHz was larger by up to 5 dB than that in sample No.27with no difference in resistance. However, in sample No.30in which the center electrode-side resistance R1was smaller than the metal terminal-side resistance R2, the amount of attenuation of radio noise was smaller than that in sample No.27with no difference in resistance over the entire frequency range.

FIG. 5is a graph showing the rate of improvement in radio noise performance versus the seal diameter D of the resistor3in a middle frequency range (100 MHz). The rate of improvement in radio noise performance was determined for five representative samples (samples Nos.23,24,25,26, and27) selected from the samples shown inFIG. 3, while the seal diameter D representing the diameter of the resistor3(seeFIG. 2) was varied. Each value shown inFIG. 5represents the rate of improvement in attenuation of radio noise with respect to the attenuation in sample No.27with a seal diameter D of 3.9 mm and no difference between the center electrode-side resistance R1and the metal terminal-side resistance R2. As can be found from the evaluation results shown inFIG. 5, when the seal diameter D was 2.9 mm or smaller, the rate of improvement in radio noise performance increased significantly as the difference in resistance increased. For example, even in sample No.27with no difference in resistance, the radio noise performance was improved by about 7% by merely decreasing the seal diameter from 3.9 mm to 2.9 mm. However, even in the case where the difference in resistance was relatively small (0.2 kΩ) as in the case of sample No.26, the rate of improvement was at least twice that in sample No.27with no difference in resistance. According to the above evaluation results, the diameter (seal diameter D) of the resistor3in the spark plug100of the present embodiment is set to 2.9 mm or smaller.

In the spark plug100of the embodiment described above, the resistor3disposed in the axial bore12has a resistance distribution, and the resistance of the center electrode-side resistor portion3blocated close to the spark gap is set to be larger than the resistance of the metal terminal-side resistor portion3a. This setting effectively suppresses the level of radio noise generated upon occurrence of spark discharge. Since it is not necessary to change the overall resistance of the resistor3, deterioration of sparking performance can be suppressed. It is conventionally known that low-frequency radio noise is reduced by increasing the overall resistance of the resistor3and that high-frequency radio noise is reduced by increasing the length of the resistor3in the axial direction OD. However, in the present embodiment, the resistance of the center electrode-side resistor portion3bis set to be larger than the resistance of the metal terminal-side resistor portion3a. This setting reduces the level of radio noise not in a local frequency range but in a wide frequency range, as shown inFIG. 4.

In the spark plug100of the present embodiment, the center electrode-side resistance R1is larger than the metal terminal-side resistance R2by at least 0.5 kΩ, preferably at least 1.0 kΩ. In addition, the metal terminal-side resistance is set to 100Ω or larger, and the diameter of the resistor3is set to 2.9 mm or smaller. This setting more effectively reduces the level of radio noise.

Although the embodiment of the present invention has been described, the present invention is not limited to the embodiment, and various other configurations can be used within the spirit of the invention. For example, the following modifications are possible.

In the above embodiment, the connecting portion2is configured to include the resistor3and the glass seal members (the upper seal member4aand the lower seal member4b) disposed on opposite sides of the resistor3. However, for example, the upper seal member4amay be omitted from the connecting portion2. In this configuration, the resistor3is in direct contact with the metal terminal40. Alternatively, the lower seal member4bmay be omitted. In this configuration, the resistor3is in direct contact with the center electrode20. Alternatively, both the upper seal member4aand the lower seal member4bmay be omitted. In this configuration, the resistor3is in direct contact with the metal terminal40and also in direct contact with the center electrode20. When the upper seal member4ais omitted, the interface C may be a cross section at the frontmost end of a region in which the resistor3is in contact with the metal terminal40. When the lower seal member4bis omitted, the interface A may be a cross section at the rearmost end of a region in which the resistor3is in contact with the center electrode20.

In the above embodiment, the resistance of the resistor3gradually increases from the interface A toward the interface C. However, the section extending from the interface A to the interface C may include a portion in which the resistance decreases, so long the resistance of the center electrode-side resistor portion3bis larger than the resistance of the metal terminal-side resistor portion3a.

In the above embodiment, the resistor3has a substantially cylindrical columnar shape, and the seal diameter thereof is constant in the axial direction OD. However, the seal diameter of the metal terminal-side resistor portion3amay be different from the seal diameter of the center electrode-side resistor portion3b. In this case, the resistance of the center electrode-side resistor portion3bcan be larger than the resistance of the metal terminal-side resistor portion3awithout changing the material of the resistor3.

DESCRIPTION OF REFERENCE NUMERALS