Spark plug

Abstract

To suppress the movement of oxygen ions in an insulator that makes up a spark plug, reducing the occurrence of dielectric breakdown.SOLUTION: An air layer forming portion 111 is provided in which the distance between a housing 11 and an insulator 12 in the radial direction is greater than the clearance formed between the housing 11 and the insulator 12, and the air layer forming portion 111 is arranged at a position corresponding to an expanded diameter portion 121.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to spark plugs. [Background technology]

[0002] Conventionally, spark plugs have been used for ignition in internal combustion engines. For example, a spark plug is used that includes an insulator having a through hole, a center electrode at least partially inserted into the front end side of the through hole, a metal terminal at least partially inserted into the rear end side of the through hole, and a seal portion disposed in the through hole and in contact with the inner circumferential surface of the insulator and the center electrode. The seal portion contains, for example, glass. The following Patent Document 1 proposes reducing the Na content of the glass in order to suppress a decrease in the withstand voltage performance of a spark plug that includes a seal portion containing glass. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-205189 A Summary of the Invention [Problem to be solved by the invention]

[0004] In areas where the voltage resistance performance is reduced, the insulator turns black, and when a high voltage is applied to the blackened insulator, dielectric breakdown occurs from the blackened area. The inventors have discovered that the cause of blackening is not the Na component of the glass, but the movement of oxygen ions in the ceramic that constitutes the insulator, and from this perspective have attempted to provide a spark plug that can effectively suppress dielectric breakdown.

[0005] An object of the present disclosure is to suppress the movement of oxygen ions in an insulator that constitutes a spark plug, thereby reducing the occurrence of dielectric breakdown. [Means for solving the problem]

[0006] The present disclosure relates to a spark plug, comprising: an insulator (12) that accommodates a center electrode (13) and is provided with an expanded diameter portion (121) for engaging the center electrode, a housing (11) that accommodates the insulator, and a ground electrode (14) that is disposed with a predetermined gap from the center electrode. An air layer forming portion (111, 122) is provided that increases the distance between the housing and the insulator in the radial direction beyond a clearance formed between the housing and the insulator, and the air layer forming portion is disposed at a position corresponding to the expanded diameter portion. Effect of the Invention

[0007] According to the present disclosure, the movement of oxygen ions in the insulator that constitutes the spark plug can be suppressed, and the occurrence of dielectric breakdown can be reduced. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a partial cross-sectional view of a spark plug according to the present embodiment. [Diagram 2] FIG. 2 is an enlarged partial cross-sectional view of a part of FIG. [Diagram 3] FIG. 3 is a partial cross-sectional view showing a modification of FIG. [Figure 4] FIG. 4 is a contour diagram of the electric field intensity in a portion corresponding to FIG. [Diagram 5] FIG. 5 is a diagram used to explain the thickness of the air layer between the housing and the insulator. [Figure 6] FIG. 6 is a diagram showing the results of the durability test. [Figure 7] FIG. 7 is a diagram showing the results of the durability test. [Figure 8] FIG. 8 is a diagram for explaining the test conditions of FIG. 6 and FIG. [Figure 9] FIG. 9 is a diagram for explaining the test conditions of FIG. 6 and FIG. [Figure 10] FIG. 10 is a diagram showing the results of the durability test. [Figure 11] FIG. 11 is a diagram for explaining the test conditions of FIG. [Figure 12] FIG. 12 is a diagram for explaining the test conditions of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, the present embodiment will be described with reference to the accompanying drawings. In order to facilitate understanding of the description, the same components in each drawing are denoted by the same reference numerals as much as possible, and duplicated description will be omitted.

[0010] A schematic configuration of a spark plug 10 according to this embodiment will be described with reference to Fig. 1. The spark plug 10 is attached to, for example, an engine head block. The spark plug 10 generates a spark discharge based on application of a voltage, and ignites the air-fuel mixture in the cylinder of the engine. The spark plug 10 includes a housing 11, an insulator 12, a center electrode 13, and a ground electrode 14.

[0011] The housing 11 is formed in a cylindrical shape centered on a plug central axis m10, which is the central axis of the spark plug 10. The housing 11 is formed of a metal material such as carbon steel. The lower end of the insulator 12 is inserted into the inside of the housing 11. A threaded portion 114 is formed on the outer circumferential surface of the lower part of the housing 11. The spark plug 10 can be fastened and fixed to the engine head block by screwing the threaded portion 114 into a screw hole formed in the engine head block. In the following, the direction along the plug central axis m10 is also referred to as the "plug axial direction Da". The circumferential direction centered on the plug central axis m10 is also referred to as the "plug circumferential direction Dc". The ground electrode 14 side is also referred to as the tip side, and the opposite side is also referred to as the base side.

[0012] The insulator 12 is formed in a cylindrical shape centered on the plug central axis m10. The insulator 12 is made of an insulating material such as alumina. The housing 11 is integrally assembled to the outer periphery of the insulator 12. An axial hole 120 is formed inside the insulator 12. The axial hole 120 is formed so as to penetrate the insulator 12 from the tip end side to the base end side along the plug central axis m10. The center electrode 13, the first seal body 15, the resistor 16, the second seal body 17, and the terminal fitting 18 are inserted into the axial hole 120 in this order from the tip end side. The center electrode 13 is inserted into the insulator 12 so as to be exposed from the tip end of the insulator 12. The axial hole 120 is provided with an enlarged diameter portion 121 for engaging the center electrode.

[0013] The center electrode 13 has a center electrode base material 30 and a center electrode tip 40. The center electrode base material 30 is formed in a cylindrical shape centered on the plug central axis m10. The center electrode base material 30 is made of a nickel (Ni) alloy or the like having excellent heat resistance. The center electrode tip 40 is joined to the tip of the center electrode base material 30. The center electrode tip 40 is formed in a cylindrical shape centered on the plug central axis m10. The center electrode tip 40 is made of an iridium alloy or the like. A first seal body 15, a resistor 16, and a second seal body 17 are sandwiched between the base end side of the center electrode 13 and the tip side of the terminal fitting 18.

[0014] The terminal fitting 18 is formed in a substantially cylindrical shape centered on the plug central axis m10. The terminal fitting 18 is formed of a steel material or the like. A terminal portion 180 is provided on the base end side of the terminal fitting 18. The terminal portion 180 is exposed to the outside from the base end side of the insulator 12.

[0015] The ground electrode 14 has a ground electrode base material 50 and a ground electrode tip 60. The ground electrode base material 50 is formed of a nickel (Ni) alloy or the like. The ground electrode base material 50 is joined to a front end surface 110 of the housing 11. The ground electrode base material 50 is formed so as to extend from the front end surface 110 of the housing 11 to a position facing the center electrode tip 40.

[0016] The ground electrode base material 50 has a base end 51 and a tip end 52. The base end 51 is joined to the tip end surface 110 of the housing 11. The base end 51 is formed to extend from the tip end surface 110 of the housing 11 in the plug axial direction Da. The tip end 52 is formed to extend from the tip of the base end 51 to a position facing the center electrode tip 40 of the center electrode 13. The ground electrode base material 50 is formed into a substantially L-shape as a whole by being composed of the base end 51 and the tip end 52.

[0017] The ground electrode tip 60 is joined to the tip portion 52 of the ground electrode base material 50. The ground electrode tip 60 is made of a precious metal alloy such as an iridium alloy or a platinum alloy. The ground electrode tip 60 is disposed so as to face the center electrode tip 40. A gap formed between the center electrode tip 40 and the ground electrode tip 60 is called a spark gap 19.

[0018] In the spark plug 10, an external circuit capable of applying a high voltage is connected to the terminal portion 180 of the terminal fitting 18. When a high voltage is applied to the terminal portion 180 by the external circuit, a spark discharge is generated between the center electrode tip 40 of the center electrode 13 and the ground electrode tip 60 of the ground electrode 14. This spark discharge ignites the air-fuel mixture in the cylinder of the engine, forming a flame, and the mixture is burned.

[0019] In FIG. 1, an x-axis, a y-axis, and a z-axis are set, which are perpendicular to each other. The z-axis is set to be along the plug central axis m10. The x-axis and the y-axis are set along a plane perpendicular to the z-axis. The y-axis is set to be along the paper surface of FIG. 1. The x-axis is set to be perpendicular to the paper surface of FIG. 1.

[0020] The x-axis is set to have a positive direction from the back to the front in Fig. 1. The y-axis is set to have a positive direction from the left to the right in Fig. 1. The z-axis is set to have a positive direction from the tip end to the base end of the spark plug 10.

[0021] Next, the vicinity of the expanded diameter portion 121 will be described with reference to Fig. 2. As shown in Fig. 2, the expanded diameter portion 121 is a portion where the inner diameter of the insulator 12 expands, and is formed as an R portion. The expanded diameter portion 121 is formed from an R portion uppermost point 121a to an R portion lowermost point 121b. The R portion uppermost point 121a is a point on the base end side of the spark plug 10. The R portion lowermost point 121b is a point on the tip end side of the spark plug 10.

[0022] The line that passes through the R-portion uppermost point 121a and intersects perpendicularly to the plug central axis m10 (see FIG. 1) is defined as the uppermost line L121a. The line that passes through the R-portion lowermost point 121b and intersects perpendicularly to the plug central axis m10 (see FIG. 1) is defined as the lowermost line L121b.

[0023] The insulator 12 and the housing 11 are disposed with a predetermined gap therebetween. The dimension between the outer surface 12a of the insulator 12 and the inner surface 11a of the housing 11 is defined as a clearance distance C12.

[0024] The housing 11 is provided with an air layer forming portion 111. The air layer forming portion 111 is a countersunk groove formed by partially cutting the inner surface 11a of the housing 11. The air layer forming portion 111 is formed from a groove uppermost point 111a to a groove lowermost point 111b. The groove uppermost point 111a is a point on the base end side of the spark plug 10. The groove lowermost point 111b is a point on the tip end side of the spark plug 10. A recessed surface 111c is formed so as to extend from the groove uppermost point 111a to the groove lowermost point 111b. The dimension from the inner surface 11a of the housing 11 to the recessed surface 111c is defined as a groove depth D111. The clearance distance C12, the groove depth D111, and the thickness T12 of the insulator middle portion satisfy the following formula (f01). 0.08≦(C12+D111) / T12≦0.14 (f01)

[0025] The air layer forming portion 111 is provided at a position corresponding to the expanded diameter portion 121. More specifically, the groove uppermost point 111a is provided closer to the base end than the R portion uppermost point 121a. The groove lowermost point 111b is provided closer to the tip end than the R portion lowermost point 121b. The expanded diameter portion 121 is provided so that the uppermost line L121a and the lowermost line L121b pass through the air layer forming portion 111.

[0026] Next, an example in which the air layer forming portion is provided on the insulator 12 side will be described with reference to FIG. 3. As shown in FIG. 3, the insulator 12 is provided with an air layer forming portion 122. The air layer forming portion 122 is a countersunk groove formed by partially cutting the outer surface 12a of the insulator 12. The air layer forming portion 122 is formed from a groove uppermost point 122a to a groove lowermost point 122b. The groove uppermost point 122a is a point on the base end side of the spark plug 10. The groove lowermost point 122b is a point on the tip end side of the spark plug 10. A recessed surface 122c is formed so as to extend from the groove uppermost point 122a to the groove lowermost point 122b. The dimension from the outer surface 12a of the insulator to the recessed surface 122c is defined as a groove depth D122. The clearance distance C12, the groove depth D122, and the thickness T12 of the middle portion of the insulator satisfy the following formula (f02). 0.11≦(C12+D122) / T12≦0.14 (f02)

[0027] The air layer forming portion 122 is provided at a position corresponding to the expanded diameter portion 121. More specifically, the groove uppermost point 122a is provided closer to the base end than the R-portion uppermost point 121a. The groove lowermost point 122b is provided closer to the tip end than the R-portion lowermost point 121b. The expanded diameter portion 121 is provided so that the uppermost line L121a and the lowermost line L121b pass through the air layer forming portion 122.

[0028] Fig. 3 shows a contour diagram of the electric field strength around the enlarged diameter portion 121 of the insulator 12. In Fig. 3, the electric field strength is in the order of Region S1 > Region S2 > Region S3 > Region S4 > Region S5. It can be seen from Fig. 3 that the electric field strength is highest around the enlarged diameter portion 121, which is the R portion. This is because the density of the electric field lines is high in the enlarged diameter portion 121, which is the R portion. The relationship between the thickness of the insulator 12, the thickness of the air layer, and the electric field strength will be examined.

[0029] FIG. 5 shows a model in which two dielectric layers are laminated. The metal on the right side of FIG. 5 corresponds to the center electrode 13. The metal on the left side of FIG. 5 corresponds to the housing 11. Material A is a dielectric and corresponds to the insulator. Material B is a dielectric and corresponds to the air layer. The dielectric constant of material A is ε A The thickness of material A is d A The electric field strength in the thickness direction of material A is E A The dielectric constant of material B is ε B The thickness of material A is d B The electric field strength in the thickness direction of material A is E B The total thickness of material A and material B is L. The voltage between the metals is V.

[0030] From the theory of electromagnetics, the electric field strength E A is expressed by the following formula (f03), and the electric field strength E B can be calculated using the following equation (f04).

number

number

[0031] If the total thickness L is constant, L=d A +d B Therefore, by substituting this into equation (f03), we get the following equation (f05).

number

[0032] Formula (f05) is applied to the thickness d of material B. B By partially differentiating with respect to , we obtain the following equation (f06). Since the dielectric constant of the insulator made of material A is higher (ε A >ε B ), the following equation (f06) becomes negative.

number

[0033] The clearance between the housing 11 and the insulator 12 also plays a role in aligning the insulator 12 with respect to the housing 11, so it is not possible to increase the thickness of the air layer by expanding the clearance as a whole. However, as described with reference to Fig. 4, the electric field strength is increased around the enlarged diameter portion 121, which is the R portion. Therefore, it was decided to provide the air layer forming portion 111 and / or the air layer forming portion 122 at a position corresponding to the enlarged diameter portion 121.

[0034] When providing the air layer forming portion 111 in the housing 11, if the groove depth D111 of the air layer forming portion 111 is too deep, the strength of the housing 11 decreases. The test results of examining the groove depth D111 for ensuring the strength of the housing 11 are shown in Figs. 6 and 7. In Figs. 6 and 7, W, X, Y, and Z are the dimensions of the parts shown in Figs. 8 and 9. Fig. 8 shows a form similar to the air layer forming portion 111 shown in Fig. 2. Fig. 9 shows a form in which the air layer forming portion 111 shown in Fig. 2 is provided up to the tapered surface. W corresponds to the clearance distance C12 in Fig. 2. X corresponds to the groove depth D111 in Fig. 2. Y corresponds to the thickness T12 in Fig. 2. Z corresponds to the distance from the plug central axis m10 to the receding surface which is the bottom of the air layer forming portion 111.

[0035] Fig. 6 shows the case where the thickness Y is 1.9 mm. Fig. 7 shows the case where the thickness Y is 2.2 mm. In Fig. 6 and Fig. 7, Sample No. 1 to Sample No. 7 are test results for the configuration shown in Fig. 8. In Fig. 6 and Fig. 7, Sample No. 8 to Sample No. 9 are test results for the configuration shown in Fig. 9.

[0036] The test results shown in Figures 6 and 7 are the results of conducting thread breakage strength tests and durability tests on sample spark plugs prepared with the dimensions and shapes shown in Sample No. 1 to Sample No. 9. The thread breakage strength test is a test in which a sample spark plug is attached to a jig simulating a cylinder head, and an excessive tightening torque is applied to the mounting and dismounting bolt to confirm the torque at which the plug breaks. The "Fracture" column shows the cases in which the housing 11 broke before other parts broke as a result of the thread breakage strength test.

[0037] The durability test is performed by placing the sample spark plug in a furnace at 100°C, applying a potential difference of 35 kV across the gap, and holding it there while discharging. There were two samples for each sample, and the potential difference was applied in a pulse waveform of 60 Hz for 500 hours. The results of the durability test, including whether black deterioration occurred, are shown in the "Black Deterioration" column.

[0038] As shown in the test results of Figures 6 and 7, from the occurrence or non-occurrence of "fracture" in the thread breakage strength test results, it was found that the groove depth X is preferably 0.25 mm or less. In this case, the wall thickness of the housing 11 is 1.45 mm or more. Also, from the occurrence or non-occurrence of "black deterioration" in the durability test results, it was found that the groove depth X is preferably 0.10 mm or more.

[0039] 6 and 7, it was found that the ratio of the sum of the clearance distance W and the groove depth X to the thickness Y, that is, (W+X) / Y, is preferably 0.08 or more and 0.14 or less. From these results, the above formula (f01) is derived.

[0040] When providing the air layer forming portion 122 in the insulator 12, if the groove depth D122 of the air layer forming portion 122 is too deep, the strength of the insulator 12 decreases. If the strength of the insulator 12 decreases, it will break during a heating test in an engine. Therefore, FIG. 10 shows the results of a heating test in an engine instead of the screw breakage strength test in FIG. 6 and FIG. 7. In FIG. 10, W, X, Y, and Z are the dimensions of the parts shown in FIG. 11 and FIG. 12. FIG. 11 shows a form similar to the air layer forming portion 122 shown in FIG. 3. FIG. 12 shows a form in which the air layer forming portion 122 shown in FIG. 3 is provided up to the tapered surface. W corresponds to the clearance distance C12 in FIG. 3. X corresponds to the groove depth D122 in FIG. 3. Y corresponds to the thickness T12 in FIG. 3. Z is the distance from the plug center axis m10 to the receding surface which is the bottom of the air layer forming portion 122.

[0041] In Fig. 10, Samples No. 1 to No. 5 show the test results in the configuration shown in Fig. 11. In Fig. 10, Samples No. 6 to No. 7 show the test results in the configuration shown in Fig. 12.

[0042] The test results shown in Fig. 10 are the results of performing engine heating tests and durability tests on sample spark plugs prepared with the dimensions and shapes shown in Sample No. 1 to Sample No. 7. The engine heating test was performed by mounting the sample spark plugs on a 4-cylinder, 1800cc, non-supercharged gasoline engine. The engine heating test consisted of advancing the ignition at 7000 rpm, heating the engine to a high temperature just before pre-ignition, and then idling the engine, repeating this process five times. The "Fracture" column shows cases in which the insulator 12 broke as a result of the engine heating test.

[0043] The durability test is performed by placing the sample spark plug in a furnace at 100°C, applying a potential difference of 35 kV across the gap, and holding it there while discharging. There were two samples for each sample, and the potential difference was applied in a pulse waveform of 60 Hz for 500 hours. The results of the durability test, including whether black deterioration occurred, are shown in the "Black Deterioration" column.

[0044] As shown in the test results in Figure 10, from the occurrence of "fracture" as a result of the engine heating test, it was found that the insulator wall thickness dimension, calculated by subtracting groove depth X from thickness Y, is preferably 1.8 mm or more. Also, from the occurrence of "black deterioration" as a result of the durability test, it was found that the groove depth X is preferably 0.15 mm or more.

[0045] From the test results in Figure 10, it was found that it is desirable for the ratio (W+X) / Y, which is the ratio of the sum of the clearance distance W and the groove depth X to the thickness Y, to be 0.11 or more and 0.14 or less. From these results, the above formula (f02) is derived.

[0046] [Note] Notes 1 to 5 below can be combined in any way as long as there is no technical contradiction.

[0047] [Appendix 1] an insulator 12 that houses a center electrode 13 and has an expanded diameter portion 121 for engaging the center electrode 13; A housing 11 that accommodates an insulator 12; a ground electrode 14 disposed with a predetermined gap from the center electrode 13; Air space forming portions 111, 122 are provided in which the distance between the housing 11 and the insulator 12 in the radial direction is greater than the clearance formed between the housing 11 and the insulator 12, In the spark plug 10, the air layer forming portions 111 and 122 are disposed at positions corresponding to the expanded diameter portion 121.

[0048] According to Supplementary Note 1, the air space forming portions 111 and 122 reduce the electric field applied to the enlarged diameter portion 121, suppressing the movement of oxygen ions in the insulator 12 and reducing the occurrence of dielectric breakdown.

[0049] [Appendix 2] 2. The spark plug according to claim 1, wherein the air space forming portion is formed by separating the housing from the insulator radially outward.

[0050] [Appendix 3] 2. The spark plug according to claim 1, wherein the air layer forming portion is formed by separating the insulator from the housing in a radially inward direction.

[0051] [Appendix 4] 3. The spark plug 10 according to claim 2, wherein a ratio of a distance (D111+C12) between the insulator 12 and the housing 11 at the air layer forming portion 111 to a wall thickness (T12) of the insulator 12 above the expanded diameter portion 121 is 0.08 or more and 0.14 or less.

[0052] [Appendix 5] 4. The spark plug 10 according to claim 3, wherein the ratio of the distance (D122+C12) between the insulator 12 and the housing 11 at the air layer forming portion 122 to the wall thickness (T12) of the insulator 12 above the expanded diameter portion 121 is 0.11 or more and 0.14 or less.

[0053] The present embodiment has been described above with reference to specific examples. However, the present disclosure is not limited to these specific examples. Any design modifications made by a person skilled in the art to these specific examples are also included within the scope of the present disclosure as long as they have the features of the present disclosure. The elements of each of the above-mentioned specific examples and their arrangements, conditions, shapes, etc. are not limited to those exemplified and can be changed as appropriate. The combination of each of the elements of each of the above-mentioned specific examples can be changed as appropriate as long as no technical contradiction occurs. [Explanation of symbols]

[0054] 11: Housing 12: Insulator 13: Center electrode 14: Ground electrode 111, 122: Air layer forming section

Claims

1. an insulator (12) that houses a center electrode (13) and has an expanded diameter portion (121) for engaging the center electrode; A housing (11) for accommodating the insulator; a ground electrode (14) disposed with a predetermined gap relative to the center electrode, an air space forming portion (111, 122) is provided in which a distance between the housing and the insulator in a radial direction is wider than a clearance formed between the housing and the insulator, The air layer forming portion is disposed at a position corresponding to the enlarged diameter portion.

2. 2. The spark plug according to claim 1, wherein the air space forming portion (111) is formed by separating the housing from the insulator radially outward.

3. 2. The spark plug according to claim 1, wherein the air layer forming portion (122) is formed by spacing the insulator radially inward from the housing.

4. 3. The spark plug according to claim 2, wherein a ratio of a distance (D111+C12) between the insulator and the housing at the air layer forming portion to a wall thickness (T12) of the insulator above the expanded diameter portion is 0.08 or greater and 0.14 or less.

5. 4. The spark plug according to claim 3, wherein a ratio of a distance (D122+C12) between the insulator and the housing at the air layer forming portion to a thickness (T12) of the insulator above the expanded diameter portion is 0.11 or greater and 0.14 or less.

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