Spark plug with

Abstract

To provide a spark plug capable of reducing breakage of an insulator.SOLUTION: The spark plug includes an insulator provided with an axial hole, a metal shell disposed on an outer periphery of the insulator, and a center electrode disposed on a front side of the axial hole, the center electrode includes a shaft portion and a head portion adjacent to a rear side of the shaft portion and having a diameter larger than that of the shaft portion, and the insulator includes a first portion in which the shaft portion is disposed in the axial hole and a second portion adjacent to a rear side of the first portion and in which the head portion is disposed in the axial hole. The portion where the shaft portion and the first portion overlap each other in the radial direction includes a narrow portion where the gap on one side between the shaft portion and the shaft hole is 0.0005 mm or more and 0.024 mm or less, and the narrow portion continues for 1 mm or more in the axial direction, in a portion excluding a root which is a region of 3 mm from the head portion to the tip end side in the shaft portion and a tip end portion which is a region of 1 mm from the tip end of the first portion to the rear end side in the first portion.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a spark plug having a center electrode disposed in an insulator. [Background technology]

[0002] Patent Document 1 discloses a prior art technique for preventing pre-ignition due to overheating of the tip of the insulator in a spark plug for a gasoline engine, in which the gap on one side between the inner circumference of the tip and the outer circumference of the center electrode is set to 0.025 mm or more and 0.075 mm or less, and heat transfer from the tip is achieved by the center electrode. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 5-159853 Summary of the Invention [Problem to be solved by the invention]

[0004] When a spark plug according to the prior art is installed in an engine that uses hydrogen as fuel, water generated by combustion may enter between the inner periphery of the insulator and the outer periphery of the center electrode, and the insulator may be damaged due to the vaporization and expansion of the infiltrated water. This is because a hydrogen-fueled engine may generate a larger amount of water due to combustion than a gasoline engine.

[0005] The present invention has been made to solve this problem, and has as its object to provide a spark plug that can reduce damage to the insulator. [Means for solving the problem]

[0006] A first aspect to achieve this object includes an insulator having an axial hole extending from the front end to the rear end along an axis, a metallic shell arranged on the outer periphery of the insulator, and a center electrode arranged on the front side of the axial hole, wherein the center electrode includes a shank and a head adjacent to the rear end of the shank and having a diameter larger than that of the shank, and the insulator includes a first portion where the shank is arranged in the axial hole and a second portion adjacent to the rear end of the first portion and where the head is arranged in the shank hole. The portion where the shank and the first portion overlap in the radial direction includes a narrowed portion where one side of the gap between the shank and the shank hole is 0.005 mm to 0.024 mm, excluding a base portion of the shank that is a region from the head to the front end of the shank and a front portion of the first portion that is a region from the front end of the first portion to the rear end of the first portion, and the narrowed portion continues for a length of 1 mm or more in the axial direction.

[0007] In the second aspect, in the first aspect, the narrow portion includes an expanding portion in which the gap between the shaft portion and the shaft hole expands toward the tip side. The rate of change obtained by subtracting the gap at the rear end of the narrow portion from the gap at the tip of the expanding portion and dividing the result by the axial length from the rear end of the narrow portion to the tip is 5.1 x 10 -4 Over 2.0 x 10 -2 The following is the result. [Effects of the Invention]

[0008] According to the present invention, the gap on one side between the axial portion of the center electrode and the axial hole of the insulator includes a narrow portion measuring 0.005 mm to 0.024 mm, and the narrow portion continues for a length of 1 mm or more in the axial direction. The narrow portion reduces the intrusion of water, thereby reducing damage to the insulator due to the vaporization and expansion of the intruded water. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a half-sectional view of a spark plug according to a first embodiment. [Figure 2] 4 is a cross-sectional view of a portion where the shaft portion of the center electrode and the insulator overlap in the radial direction. FIG. [Figure 3] FIG. 6 is a cross-sectional view of a spark plug according to a second embodiment. [Figure 4]FIG. 10 is a cross-sectional view of a spark plug according to a third embodiment. [Figure 5] FIG. 10 is a cross-sectional view of a spark plug according to a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a half-sectional view of a spark plug 10 according to a first embodiment, combining an outline view and a full cross-sectional view along the axis X. The lower side of Fig. 1 is the leading end side of the spark plug 10, and the upper side is the trailing end side of the spark plug 10 (the same applies to Figs. 2 to 5). The spark plug 10 includes an insulator 11, a center electrode 15, and a metallic shell 19.

[0011] The insulator 11 is a cylindrical member having an axial hole 12 extending along the axis X, and is made of a ceramic such as alumina, which has excellent insulating properties and mechanical properties at high temperatures. The axial hole 12 extends from the rear end to the front end of the insulator 11. The insulator 11 includes a first portion 13 located at the front end and a second portion 14 adjacent to the rear end of the first portion 13.

[0012] A center electrode 15 is disposed in the axial hole 12 between the first portion 13 and the second portion 14 of the insulator 11. The center electrode 15 is a rod-shaped conductor, and a core material with excellent thermal conductivity is embedded in a base material. Examples of the material of the base material include a Ni-based alloy or a metal made of Ni. Examples of the material of the core material include copper or an alloy mainly composed of copper. The core material can be omitted. A tip containing a precious metal such as Pt, Ir, or Ru can be provided at the tip of the center electrode 15, or the tip can be omitted.

[0013] The center electrode 15 includes a shaft portion 16 and a head portion 17 that is adjacent to the rear end of the shaft portion 16 and has a diameter larger than that of the shaft portion 16. The shaft portion 16 is disposed in the axial hole 12 of the first portion 13 of the insulator 11, and the head portion 17 is disposed in the axial hole 12 of the second portion 14 of the insulator 11. Because the diameter of the axial hole 12 of the first portion 13 is smaller than the diameter of the axial hole 12 of the second portion 14, the position of the tip of the head portion 17 is determined at the boundary between the first portion 13 and the second portion 14.

[0014] A metal terminal 18 is disposed on the rear end side of the axial hole 12 of the insulator 11. The metal terminal 18 is electrically connected to the center electrode 15. The metal terminal 18 is a rod-shaped metal member to which an ignition device (not shown) is connected. An example of the material of the metal terminal 18 is low carbon steel. The front end side of the metal terminal 18 is disposed in the axial hole 12 of the insulator 11, and the rear end side of the metal terminal 18 protrudes from the insulator 11.

[0015] The metallic shell 19 is a substantially cylindrical member made of a conductive metal material (such as low-carbon steel). The metallic shell 19 is disposed on the outer periphery of the insulator 11. The ground electrode 20 is a rod-shaped conductor connected to the metallic shell 19. A spark gap is formed between the ground electrode 20 and the center electrode 15. The ground electrode 20 may be omitted, or a plurality of ground electrodes 20 may be connected to the metallic shell 19.

[0016] Fig. 2 is a cross-sectional view including the axis X of a portion where the shaft portion 16 of the center electrode 15 and the insulator 11 overlap in the radial direction. Fig. 2 omits one side of the axis X, and further omits the front and rear ends of the center electrode 15 and the rear end of the insulator 11, the outer periphery of the insulator 11, and the metallic shell 19 (the same applies to Figs. 3 to 5).

[0017] The radial overlapping portion between the shank 16 of the center electrode 15 and the first portion 13 of the insulator 11 includes a root 21, which is a region of the shank 16 extending from the head 17 to the tip end by 3 mm, and a tip portion 23, which is a region of the first portion 13 extending from the tip of the first portion 13 to the rear end by 1 mm, and includes a narrowed portion 24 in the portion excluding the root 21 and the tip portion 23. The narrowed portion 24 is a portion where the size G1, G2 of the gap 26 between the shank 16 and the axial hole 12 is 0.005 mm or more and 0.024 mm or less, and continues for a length of 1 mm or more in the axial direction.

[0018] The reason why the root 21 of the shaft portion 16 is excluded when determining the position of the narrow portion 24 is that the root 21 has a larger variation in diameter than the other parts of the shaft portion 16. The reason why the tip portion 23 of the first part 13 is excluded when determining the position of the narrow portion 24 is that the tip portion 23 may have a chamfer 22 at the tip of the shaft hole 12 or may have a tip portion with a small diameter of the shaft portion 16, which may result in the gap 26 being an unintended size.

[0019] The spark plug 10 has a narrow portion 24 at a portion where the shaft portion 16 of the center electrode 15 and the first portion 13 of the insulator 11 overlap in the radial direction, and the narrow portion 24 continues for a length of 1 mm or more in the axial direction. The narrow portion 24 blocks the intrusion of water generated by the combustion of fuel supplied to the combustion chamber of the engine (not shown), thereby reducing damage to the insulator 11 due to the vaporization and expansion of the intruded water.

[0020] The reason why the lower limit of the size of the gap 26 in the narrow portion 24 is 0.005 mm is to prevent damage to the ceramic insulator 11 (first portion 13) due to thermal expansion of the metal center electrode 15 (shaft portion 16) and damage to the first portion 13 caused by the shaft portion 16 hitting the first portion 13 due to engine vibration. The reason why the upper limit of the size of the gap 26 in the narrow portion 24 is 0.024 mm and the lower limit of the axial length of the narrow portion 24 is 1 mm is to prevent water from entering due to capillary action. The axial length of the narrow portion 24 is 1 mm or more, and the longer the axial length of the narrow portion 24, the greater the effect of preventing water from entering.

[0021] The position of the narrow portion 24 can be identified by observing a cross section of the spark plug 10 including the axis X with a scanning electron microscope (SEM) or by measuring the size of the gap 26 through tomography of the spark plug 10 with a microfocus X-ray CT (Computed Tomography) device (X-ray CT scan). In the SEM image of the cross section of the spark plug 10 including the axis X, gaps 26 appear on both sides of the axis X. The two gaps 26 appearing on both sides of the axis X are usually different in size. The presence or absence of the narrow portion 24 is examined by focusing on the smaller of the two gaps 26 appearing on both sides of the axis X.

[0022] When measuring the size of gap 26 by X-ray CT scanning, attention is paid to the smallest gap to check for the presence or absence of narrowed portion 24. The reason for focusing on the smaller of the two gaps 26 that appear in the SEM image or the smallest gap measured by X-ray CT scanning is to prevent damage to first part 13 due to thermal expansion or vibration of shaft portion 16 if the gap becomes too small.

[0023] When the engine (not shown) is operating, the temperature of the tip end of the shaft portion 16 and the first portion 13 becomes higher than the temperature of the rear end, and the amount of thermal expansion of the shaft portion 16 and the first portion 13 is therefore greater at the tip end than at the rear end. Therefore, damage to the ceramic first portion 13 due to thermal expansion of the metal shaft portion 16 is more likely to occur at the tip end. In this embodiment, the narrow portion 24 includes an expanding portion 25 in which the gap 26 expands toward the tip end. In the expanding portion 25, the gap 26 continuously expands toward the tip end. The diameter of the shaft portion 16 is constant throughout the entire axial length except for the tip end of the shaft portion 16, but the diameter of the shaft hole 12 in the first portion 13 expands toward the tip end.

[0024] The rate of change of the expansion portion 25, calculated by subtracting the size G1 of the gap 26 at the rear end of the narrow portion 24 from the size G2 of the gap 26 at the front end of the expansion portion 25, is G2-G1, and divided by the axial length from the rear end of the narrow portion 24 to the front end of the expansion portion 25, is 5.1×10 -4 Over 2.0 x 10 -2 The rate of change of the expanded portion 25 is set based on the difference in the amount of thermal expansion due to the temperature gradient. The provision of the expanded portion 25 reduces damage to the tip side of the first portion 13 due to thermal expansion of the shaft portion 16.

[0025] A second embodiment will be described with reference to Figure 3. In the first embodiment, the diameter of the shaft portion 16 is constant over the entire axial length except for the tip of the shaft portion 16, but the diameter of the shaft hole 12 in the first portion 13 increases continuously toward the tip. In the second embodiment, the diameter of the shaft hole 32 in the first portion 31 increases stepwise toward the tip. In the second embodiment, the same parts as those described in the first embodiment are designated by the same reference numerals, and description of these same parts will be omitted.

[0026] 3 is a cross-sectional view of a spark plug 30 including an axis X of a portion where the shank 16 of the center electrode 15 and the insulator 11 overlap in the radial direction. The head 17 of the center electrode 15 is disposed in an axial hole 32 of the second portion 14 of the insulator 11. The first portion 31 of the insulator 11 is adjacent to the tip side of the second portion 14. The shank 16 of the center electrode 15 is disposed in the axial hole 32 of the first portion 31.

[0027] The shaft hole 32 has a step 33. The diameter of the shaft hole 32 on the tip side of the step 33 is constant over the entire axial length, excluding the chamfer 22, and the diameter of the shaft hole 32 on the rear side of the step 33 is constant over the entire axial length. The diameters of the shaft hole 32 differ at each side of the step 33. The diameter of the shaft hole 32 on the tip side of the step 33 is larger than the diameter of the shaft hole 32 on the rear side of the step 33.

[0028] The portion where shaft portion 16 and first portion 31 overlap in the radial direction includes tip portion 34, which is a region of first portion 31 extending from the tip to the rear end of first portion 31 by 1 mm, and root 21 of shaft portion 16, and is provided with narrow portion 35 extending 1 mm or more in the axial direction in the portion excluding root 21 and tip portion 34. Narrow portion 35 is a portion where sizes G1, G2 of gap 37 between shaft portion 16 and shaft hole 32 are 0.005 mm or more and 0.024 mm or less.

[0029] The narrow portion 35 includes an expanding portion 36 in which the gap 37 expands toward the tip side. In the expanding portion 36, the gap 37 expands stepwise toward the tip side. The rate of change of the expanding portion 36, calculated by subtracting the size G1 of the gap 37 at the rear end of the narrow portion 35 from the size G2 of the gap 37 at the tip of the expanding portion 36, is G2-G1, and divided by the axial length from the rear end of the narrow portion 35 to the tip of the expanding portion 36, is 5.1 x 10 -4 Over 2.0 x 10 -2 The second embodiment can reduce damage to the insulator 11 in the same way as the first embodiment.

[0030] A third embodiment will be described with reference to Figure 4. In the first and second embodiments, the case where the diameter of the axial holes 12, 32 of the first portions 13, 31 increases toward the tip side has been described. In the third embodiment, the case where the diameter of the axial portion 43 decreases toward the tip side will be described. In the third embodiment, the same parts as those described in the first embodiment are designated by the same reference numerals, and description of the same parts will be omitted.

[0031] 4 is a cross-sectional view of the spark plug 40, including the axis X, of the portion where the shank 43 of the center electrode 15 and the insulator 11 overlap in the radial direction. The head 17 of the center electrode 15 is disposed in the axial hole 42 of the second portion 14 of the insulator 11. The first portion 41 of the insulator 11 is adjacent to the tip side of the second portion 14. The shank 43 of the center electrode 15 is disposed in the axial hole 42 of the first portion 41. The diameter of the axial hole 42 of the first portion 41 is constant over the entire axial length, except for the chamfer 22. The diameter of the shank 43 continuously decreases toward the tip side.

[0032] The portion where shank 43 and first portion 41 overlap in the radial direction includes a root 44, which is a region of shank 43 extending from head 17 to the tip end by 3 mm, and a tip portion 45, which is a region of first portion 41 extending from the tip of first portion 41 to the rear end by 1 mm, and is provided with a narrowed portion 46 in the portion excluding root 44 and tip portion 45. Narrowed portion 46 is a portion where gap 48 between shank 43 and axial hole 42 has sizes G1, G2 of 0.005 mm or more and 0.024 mm or less, and continues for a length of 1 mm or more in the axial direction.

[0033] The narrow portion 46 includes an expanding portion 47 in which the gap 48 expands toward the tip side. In the expanding portion 47, the gap 48 continuously expands toward the tip side. The rate of change of the expanding portion 47, obtained by subtracting the size G1 of the gap 48 at the rear end of the narrow portion 46 from the size G2 of the gap 48 at the tip of the expanding portion 47, is 5.1×10 -4 Over 2.0 x 10 -2 According to the third embodiment, damage to the insulator 11 can be reduced, similarly to the first embodiment.

[0034] A fourth embodiment will be described with reference to Figure 5. In the third embodiment, the case where the diameter of shaft portion 43 continuously decreases toward the tip side will be described. In the fourth embodiment, the case where the diameter of shaft portion 51 decreases stepwise toward the tip side will be described. In the fourth embodiment, the same parts as those described in the first or third embodiment will be assigned the same reference numerals, and the following description of the same parts will be omitted.

[0035] 5 is a cross-sectional view of a spark plug 50 including an axis X of a portion where the shank 51 of the center electrode 15 and the insulator 11 overlap in the radial direction. The head 17 of the center electrode 15 is disposed in an axial hole 42 of the second portion 14 of the insulator 11. The shank 51 of the center electrode 15 is adjacent to the tip side of the head 17. The shank 51 is disposed in the axial hole 42 of the first portion 41 of the insulator 11.

[0036] Shank 51 has a step 52. The diameter of shank 51 on the tip side of step 52 is constant over the entire axial length except for the tip of shank 51, and the diameter of shank 51 on the rear side of step 52 is constant over the entire axial length. The diameter of shank 51 varies at each side of step 52. The diameter of shank 51 on the tip side of step 52 is smaller than the diameter of shank 51 on the rear side of step 52.

[0037] The portion where shank 51 and first portion 41 overlap in the radial direction includes root 53, which is a region of shank 51 extending from head 17 to 3 mm toward the tip, and tip 45 of first portion 41, and is provided with narrowed portion 54 that continues for a length of 1 mm or more in the axial direction in the portion excluding root 53 and tip 45. Narrowed portion 54 is a portion where sizes G1, G2 of gap 56 between shank 51 and axial hole 42 are 0.005 mm or more and 0.024 mm or less.

[0038] The narrow portion 54 includes an expanding portion 55 in which the gap 56 expands toward the tip side. The expanding portion 55 expands the gap 56 in stages toward the tip side. The rate of change of the expanding portion 55, calculated by subtracting the size G1 of the gap 56 at the rear end of the narrow portion 54 from the size G2 of the gap 56 at the tip of the expanding portion 55, is 5.1×10 -4 Over 2.0 x 10 -2 According to the fourth embodiment, damage to the insulator 11 can be reduced, similarly to the first embodiment. [Example]

[0039] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0040] (Test 1) Sample No. 1-7 was produced, which had the same basic configuration as the spark plug 10 of the first embodiment. The difference between the spark plug 10 of the first embodiment and Sample No. 1-7 was that, while the spark plug 10 of the first embodiment had a diameter of the axial hole 12 in the first portion 13 of the insulator 11 that increased toward the tip, the diameter of the axial hole 12 in the first portion 13 of Sample No. 1-7 was constant over the entire axial length except for the chamfer 22.

[0041] The tester prepared seven types of center electrodes 15 with different diameters of the shank 16, while keeping the dimensions and materials of other parts the same, to create multiple samples No. 1-7 with different sizes of gap 26 between the shank 16 and the axial hole 12 of the first portion 13. The length of the first portion 13 of Sample No. 1-7 was 5 mm, and the length of the shank 16 was 5 mm.

[0042] The tester cut Sample No. 1-7 and measured the size of the gap 26 in the portion (1 mm in the axial direction) between the axial hole 12 of the first part 13 and the shank 16, excluding the base 21 (3 mm in the axial direction) and the tip 23 (1 mm in the axial direction), through SEM observation of the cross section including the axis X. The size of the gap 26 on one side of Sample No. 1-7 is shown in Table 1.

[0043] [Table 1]

[0044] The tester immersed the tip 23 of an uncut sample in water for 24 hours at room temperature while the sample was upright, using capillary action to allow water to seep into the space between the axial hole 12 and the axial portion 16 of the first portion 13. After wiping off the moisture from the surfaces of the sample's insulator 11, center electrode 15, and ground electrode 20, the tester attached the sample to a 2.0L inline 4-cylinder naturally aspirated gasoline engine and started the engine. The test was conducted for 40 cycles, with the engine running at 4,000 rpm for 2 minutes, idling for 10 minutes, then stopping and resting for 2 hours.

[0045] After the test, the samples were disassembled and inspected for the presence or absence of cracks in the insulator 11. Samples with no cracks in the insulator 11 were rated A, samples with cracks in the tip portion 23 of the insulator 11 were rated B, and samples with cracks in the second portion 14 of the insulator 11 were rated C. The results are shown in Table 1.

[0046] According to Table 1, samples No. 1 and 2, in which the size of the gap 26 was less than 0.005 mm, were rated B. Because the gap 26 in samples No. 1 and 2 was small, it is presumed that a crack occurred in the tip 23 of the first part 13 due to thermal expansion of the shaft part 16 and the first part 13, or due to the impact that the shaft part 16 applied to the first part 13 due to engine vibration.

[0047] Samples No. 6 and 7, in which the size of the gap 26 was larger than 0.024 mm, were rated C. Because Samples No. 6 and 7 had large gaps 26, a large amount of water entered between the head 17 and the second part 14 due to capillary action, and it is presumed that cracks occurred in the second part 14 due to the evaporation and expansion of the water, etc.

[0048] Sample No. 3-5, which had a narrow portion 24 (1 mm in axial length) with a gap 26 size of 0.005 mm or more and 0.024 mm or less, was rated A. It is presumed that in Sample No. 3-5, the narrow portion 24 prevented water from entering and also reduced the impact applied by the shaft portion 16 to the first portion 13, so no cracks occurred in the first portion 13 or the second portion 14. According to the examples, it was revealed that the presence of a narrow portion 24 with a gap 26 size of 0.005 mm or more and 0.024 mm or less and an axial length of 1 mm or more can reduce damage to the first portion 13 or the second portion 14.

[0049] (Test 2) Samples were fabricated that had the same basic configuration as the spark plug 40 of the third embodiment. The tester prepared 28 types of center electrodes 15 that differed in the diameter of the shank 43 and the rate at which the shank 43 narrowed toward the tip (taper angle), while keeping the dimensions and materials of the other parts the same, thereby fabricating multiple samples of 28 types that differed in the size G1 of the gap 48 between the axial hole 42 of the first portion 41 and the shank 43 and the rate of change of the enlarged portion 47. The length of the first portion 41 of each sample was 5 mm, and the length of the shank 43 was 5 mm.

[0050] The tester cut the sample and, through SEM observation of the cross section including the axis X, measured the size G1 of the rear end of the gap 48 in the portion (1 mm in axial length) between the axial hole 42 and the shank 43 of the first part 41, excluding the base 44 (3 mm in axial length) and the tip part 45 (1 mm in axial length), and the size G2 of the tip of the gap 48. The rate of change of the expanded part 47 was calculated by subtracting the size G1 from the size G2 of the gap 48, and dividing the value G2-G1 by the axial distance (1 mm) between the part where G2 was measured and the part where G1 was measured.

[0051] For an uncut sample, the tester let water enter between the shaft hole 42 and the shaft portion 43 of the first part 41 in the same manner as in Test 1, wiped off the water on the surface of the sample, then attached the sample to the same engine as in Test 1, and started the engine. One cycle consisted of a series of operations: the engine was operated at 4000 rpm for 2 minutes, then stopped after idling for 10 minutes, and then rested for 2 hours; tests were conducted in which 40 cycles were performed and tests in which 60 cycles were performed.

[0052] After the test, the samples were disassembled and inspected for the presence or absence of cracks in the insulator 11. Samples that had no cracks in the insulator 11 after 60 cycles of testing were rated S. Samples that had no cracks in the insulator 11 after 40 cycles of testing but had cracks in the second portion 14 of the insulator 11 after 60 cycles of testing were rated A. Samples that had cracks in the tip portion 45 of the insulator 11 after 40 cycles of testing were rated B, and samples that had cracks in the second portion 14 of the insulator 11 after 40 cycles of testing were rated C. The size G1 of the gap 48 of the sample, the rate of change in the enlarged portion 47, and the rating are shown in Table 2.

[0053] [Table 2]

[0054] According to Table 2, samples that had a narrow portion 46 (length 1 mm in the axial direction) with a size G1 of the gap 48 of 0.005 mm or more and 0.024 mm or less were judged as S or A. Of these, the samples that were judged as S had a rate of change in the expanded portion 47 of 5.1 × 10 -4 Over 2.0 x 10 -2 According to the example, the rate of change of the expanded portion 47 was 5.1×10 -4 Over 2.0 x 10 -2 It has been revealed that if the thickness is less than or equal to the thickness of the first portion 41 or the second portion 14, damage to the first portion 41 or the second portion 14 can be further reduced.

[0055] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0056] In the embodiments, the spark plugs 10, 30, 40, and 50 are described as including the enlarged portions 25, 36, 47, and 55, but the present invention is not necessarily limited to this. As in Sample Nos. 1-7 on which Test 1 was performed in the examples, it is of course possible to make the diameter of the axial bore 12 of the first portion 13 constant over the entire axial length excluding the chamfer 22, make the diameter of the shank 16 constant over the entire axial length excluding the tip of the shank 16, and have the narrow portion 24 not include the enlarged portion 25.

[0057] In the embodiment, the case where the chamfer 22 is applied to the tip of the axial hole 12, 32, 42 of the insulator 11 has been described, but this is not necessarily limited to this. It is of course possible to round the tip of the axial hole 12, 32, 42 instead of the chamfer 22, and it is of course possible to omit the chamfer 22 or the roundness.

[0058] In the second embodiment, a case where one step 33 is provided on the inner circumference of the first portion 31 is described, and in the fourth embodiment, a case where one step 52 is provided on the outer circumference of the shaft portion 51 is described, but this is not necessarily limited to this. It is of course possible to provide a plurality of steps 33 on the inner circumference of the first portion 31 or a plurality of steps 52 on the outer circumference of the shaft portion 51.

[0059] In the embodiments, the enlarged portions 25, 36, 47, 55 are described as having an inner diameter of the insulator 11 that increases toward the tip while the outer diameter of the center electrode 15 is constant, or as having an outer diameter of the center electrode 15 that decreases toward the tip while the inner diameter of the insulator 11 is constant, but this is not necessarily limited to this. It is of course possible to form an enlarged portion by increasing the inner diameter of the insulator 11 toward the tip while decreasing the outer diameter of the center electrode 15 toward the tip.

[0060] In the embodiments, the spark plugs 10, 30, 40, 50 are described in which the ground electrode 20 is exposed to the combustion chamber when the spark plugs 10, 30, 40, 50 are attached to an engine (not shown), but the present invention is not necessarily limited to this. It is of course possible to apply the configurations of the embodiments to a spark plug in which the ground electrode 20 is covered with a cap having a through hole (a spark plug in which an auxiliary chamber is provided in the combustion chamber).

[0061] In the embodiments, the spark plugs 10, 30, 40, and 50 in which a spark discharge occurs between the center electrode 15 and the ground electrode 20 have been described, but the present invention is not necessarily limited to this. It is naturally possible to apply the configuration of the embodiments to a spark plug that utilizes non-equilibrium plasma generated around the center electrode 15. It is also naturally possible to apply the configuration of the embodiments to a spark plug that omits the ground electrode 20 and generates a discharge between the center electrode 15 and the metallic shell 19. [Explanation of symbols]

[0062] 10, 30, 40, 50 spark plugs 11 Insulators 12, 32, 42 shaft holes 13,31,41 Part 1 14 Part 2 15 Center electrode 16,43,51 Shaft 17 Head 19 Metal body 21,44,53 Base 23,34,45 Tip 24,35,46,54 Narrow part 25, 36, 47, 55 Enlarged section 26, 37, 48, 56 gaps X axis

Claims

1. an insulator having an axial hole extending from a front end side to a rear end side along an axis; a metallic shell disposed on the outer periphery of the insulator; a center electrode disposed on a tip side of the axial hole, the center electrode includes a shaft portion and a head portion adjacent to a rear end side of the shaft portion and having a diameter larger than that of the shaft portion, the insulator includes a first portion in which the stem portion is disposed in the axial hole, and a second portion adjacent to a rear end side of the first portion in which the head portion is disposed in the axial hole, The portion where the shank and the first portion overlap in the radial direction includes a narrow portion in which the gap on one side between the shank and the axial hole is 0.005 mm or more and 0.024 mm or less, excluding a root portion which is a region of the shank that is 3 mm from the head portion toward the tip end side, and a tip portion which is a region of the first portion that is 1 mm from the tip end of the first portion toward the rear end side, The narrow portion extends for a length of 1 mm or more in the axial direction of the spark plug.

2. the narrow portion includes an expanding portion in which the gap between the shaft portion and the shaft hole expands toward the tip side, The rate of change obtained by dividing the value obtained by subtracting the gap at the rear end of the narrow portion from the gap at the front end of the expanded portion by the length of the narrow portion from the rear end to the front end in the axial direction is 5.1×10 －４ Above 2.0 x 10 －２ 2. The spark plug according to claim 1, wherein:

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