JP2017004932A - Spark plug - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique capable of improving the ignitability of the spark plug with a simple configuration.SOLUTION: A spark plug includes a main metal fitting, an insulator, a center electrode, and a ground electrode. At least one of the center electrode and the ground electrode has a columnar protruding portion which protrudes toward the other electrode and forms a discharge gap, and has a recess in a part of the side surface of the protruding portion. The protruding portion has a columnar noble metal tip located on a discharge gap side, and a fused portion positioned on the side opposite to the discharge gap side of the noble metal tip and formed by melting the noble metal tip and an electrode base metal. The recess is formed in the fused portion portion, and is 0.05 mm≤D≤0.3*R, where D is the depth of the recess from the side surface of the noble metal tip and R is the diameter of the noble metal tip.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a spark plug.

  Conventionally, in order to improve the ignitability of a spark plug, for example, ignition energy applied to the spark plug may be increased (see Patent Document 1).

JP2013-108465A

  However, in order to increase the ignition energy, an ignition coil or the like that can generate large energy is required, and the cost of the entire ignition system increases. Therefore, a technique capable of improving the ignitability of the spark plug with a simple configuration is required.

  The present invention has been made to solve the above-described problems, and can be realized as the following forms.

(1) According to one aspect of the present invention, a spark plug is provided. The spark plug has a cylindrical metal shell; an insulator having at least a part of its outer periphery held by the metal shell and having an axial hole along an axis; a center electrode provided in the axial hole; A ground electrode fixed to the metal shell and forming a discharge gap with the center electrode, wherein at least one of the center electrode and the ground electrode protrudes toward the other electrode and opens the discharge gap. A columnar protrusion to be formed, and a depression in a part of a side surface of the protrusion, the protrusion being a columnar noble metal tip positioned on the discharge gap side, and the discharge gap of the noble metal tip And a melted portion formed by melting the noble metal tip and the electrode base material, and the depression is formed in the melted portion, and is provided on a side surface of the noble metal tip. Said recess depth et D, and the diameter of the noble metal tip when R, and, and satisfies the following condition (1).
0.05 mm ≦ D ≦ 0.3 * R (1)
If it is a spark plug of such a form, since a dent is formed in a part of the side surface of the protruding portion protruding toward the discharge gap, the dent blocks the heat drawn from the tip of the protruding portion, The temperature at the tip of the protrusion is kept high. As a result, the flame extinguishing action by the electrode is reduced, and the ignitability is improved. Therefore, the ignitability of the spark plug can be improved with a simple configuration. Moreover, since the hollow is formed in the fusion | melting part, a hollow can be formed easily. Moreover, since the protrusion part is provided with the noble metal tip, the durability of the electrode can be enhanced. Moreover, the ignitability of the spark plug can be improved satisfactorily when the depth D of the dent and the diameter R of the noble metal tip satisfy the above condition (1).

(2) In the spark plug of the above aspect, the following condition (2) may be satisfied when the dimension T of the opening of the recess in the direction along the central axis of the protrusion is used.
T ≧ 0.2mm (2)
If it is a spark plug of such a form, the ignitability of a spark plug can be improved favorably.

(3) The spark plug according to the above aspect may include a plurality of the recesses in the protruding portion. If it is a spark plug of such a form, the ignitability of a spark plug can be improved more favorably.

(4) In the spark plug according to the above aspect, the protrusion may be provided in a center electrode. If it is a spark plug of such a form, the flame-extinguishing effect | action by a center electrode can be reduced.

(5) In the spark plug of the above aspect, the protrusion may be provided on a ground electrode. With such a form of spark plug, the flame extinguishing action by the ground electrode can be reduced.

  The present invention can be implemented in various forms other than the above-described form as a spark plug, such as a spark plug manufacturing method.

It is a fragmentary sectional view of a spark plug. It is a perspective view of the front-end | tip part of a center electrode. It is a figure explaining the dimension of each part of a projection part. It is a figure which shows the example by which multiple hollows are formed in the fusion | melting part. It is a graph which shows the test result of a 1st ignitability evaluation test. It is a graph which shows the test result of a 2nd ignitability evaluation test. It is a figure which shows the test result of a 1st vibration evaluation test. It is a figure which shows the test result of a 2nd vibration evaluation test. It is a graph which shows the test result of a 3rd ignitability evaluation test. It is a graph which shows the test result of a 4th ignitability evaluation test.

A. Embodiment:
FIG. 1 is a partial cross-sectional view of a spark plug 100 according to an embodiment of the present invention. The spark plug 100 has an elongated shape along the axis O. In FIG. 1, the right side of the axis O indicated by a dashed line shows an external front view, and the left side of the axis O shows a cross-sectional view passing through the axis O. In the following description, the upper side in FIG. 1 is referred to as the front end side of the spark plug 100, and the lower side in FIG. 1 is referred to as the rear end side.

  The spark plug 100 includes an insulator 10, a center electrode 20, a ground electrode 30, and a metal shell 50. The insulator 10 has at least a part of its outer periphery held by a cylindrical metal shell 50 and has a shaft hole 12 along the axis O. A center electrode 20 is provided in the shaft hole 12. The ground electrode 30 is fixed to the front end surface 57 of the metal shell 50 and forms a discharge gap G between the ground electrode 30 and the center electrode 20.

  The insulator 10 is an insulator formed by firing a ceramic material such as alumina. The insulator 10 is a cylindrical member in which a part of the center electrode 20 is accommodated at the front end side and the shaft hole 12 that accommodates a part of the terminal fitting 40 is formed at the rear end side. At the center in the axial direction of the insulator 10, a central body 19 having a larger outer diameter is formed. A rear end side body portion 18 that insulates between the terminal metal fitting 40 and the metal shell 50 is formed on the terminal metal fitting 40 side of the central body portion 19. A front end side body portion 17 having an outer diameter smaller than that of the rear end side body portion 18 is formed on the center electrode 20 side with respect to the central body portion 19. A leg length portion 13 having a smaller outer diameter and a smaller outer diameter toward the center electrode 20 side is formed.

  The metal shell 50 is a cylindrical metal fitting that surrounds and holds a portion extending from a part of the rear end body portion 18 of the insulator 10 to the long leg portion 13. The metal shell 50 is made of, for example, low carbon steel, and is subjected to a plating process such as nickel plating or zinc plating. The metal shell 50 includes a tool engaging portion 51, a seal portion 54, and a mounting screw portion 52 in order from the rear end side. A tool for attaching the spark plug 100 to the engine head is fitted into the tool engaging portion 51. The attachment screw portion 52 has a thread that is screwed into the attachment screw hole of the engine head. The seal portion 54 is formed in a hook shape at the base of the mounting screw portion 52. An annular gasket 5 formed by bending a plate is fitted between the seal portion 54 and the engine head. The front end surface 57 of the metal shell 50 has a hollow circular shape, and the leg long portion 13 of the insulator 10 and the center electrode 20 protrude from the center thereof.

  A thin caulking portion 53 is provided on the rear end side of the metal shell 50 from the tool engaging portion 51. Further, between the seal portion 54 and the tool engaging portion 51, a compression deformation portion 58 having a small thickness is provided in the same manner as the caulking portion 53. Between the inner peripheral surface of the metal shell 50 from the tool engaging portion 51 to the crimping portion 53 and the outer peripheral surface of the rear end side body portion 18 of the insulator 10, annular ring members 6 and 7 are interposed. Further, talc (talc) 9 powder is filled between the ring members 6 and 7. When the spark plug 100 is manufactured, the compression deformable portion 58 is compressed and deformed by pressing the caulking portion 53 inward so as to be bent inward, and the compression deformation of the compression deformable portion 58 causes the ring members 6, 7 and The insulator 10 is pressed toward the front end side in the metal shell 50 through the talc 9. By this pressing, the talc 9 is compressed in the direction of the axis O, and the airtightness in the metal shell 50 is enhanced.

  In the inner periphery of the metal shell 50, an insulator step portion located at the base end of the leg long portion 13 of the insulator 10 via an annular plate packing 8 on a metal inner step portion 56 formed at the position of the mounting screw portion 52. 15 is pressed. The plate packing 8 is a member that maintains airtightness between the metal shell 50 and the insulator 10, and prevents the combustion gas from flowing out.

  The ground electrode 30 is made of a metal having high corrosion resistance. As the metal having high corrosion resistance, for example, a nickel alloy mainly composed of nickel such as Inconel (trade name) 600 or Inconel 601 is used. The proximal end of the ground electrode 30 is welded to the distal end surface 57 of the metal shell 50. In the present embodiment, the ground electrode 30 is bent at an intermediate portion so that one side surface of the tip portion of the ground electrode 30 faces the center electrode 20. The ground electrode 30 may have a two-layer structure in which the above-described nickel alloy is used as a base material and copper or a copper alloy is embedded therein, and further, a three-layer structure in which nickel or a nickel alloy is embedded in copper or a copper alloy. It is good also as a structure. The main component means that the mass is the largest among the components of the object, and the ratio does not necessarily exceed 50 mass%.

  The center electrode 20 is a rod-like member in which a core material 22 having better thermal conductivity than the electrode base material 21 is embedded in the electrode base material 21. The electrode base material 21 is made of a nickel alloy containing nickel as a main component, and the core member 22 is made of copper or an alloy containing copper as a main component.

  In the vicinity of the rear end portion of the center electrode 20, a flange portion 23 having a shape projecting to the outer peripheral side is formed. The flange 23 contacts the shaft hole inner step 14 formed in the shaft hole 12 from the rear end side, and positions the center electrode 20 in the insulator 10. The rear end portion of the center electrode 20 is electrically connected to the terminal fitting 40 via the ceramic resistor 3 and the seal body 4.

  FIG. 2 is a perspective view of the tip portion of the center electrode 20. In the present embodiment, the center electrode 20 includes a columnar protrusion 60 that protrudes toward the ground electrode 30 that is the other electrode and forms a discharge gap G at the tip. The protrusion 60 is positioned on the side opposite to the discharge gap G side of the noble metal tip 61 and the columnar noble metal tip 61 located on the discharge gap G side, and the electrode base material 21 and the noble metal tip 61 are melted. And a melting part 62 formed by the above. The noble metal tip 61 is made of, for example, platinum (Pt), iridium (Ir), ruthenium (Ru), rhodium (Rh), or an alloy thereof. In the present embodiment, the electrode base material 21 has a larger diameter than the protruding portion 60. The tip of the electrode base material 21 is formed in a tapered shape and is connected to the melting part 62.

  A depression 63 is provided on the side surface of the protrusion 60. In the present embodiment, the recess 63 is formed in the melting part 62. The melting part 62 is formed by performing laser irradiation on these boundaries a plurality of times using a pulsed laser device while rotating the electrode base material 21 and the noble metal tip 61 around the central axis C thereof. The In the present embodiment, the depression 63 is formed in the melting portion 62 by increasing the output of any one laser irradiation. In order to form the depression 63 in a circular shape, it is preferable to increase the output of the last laser irradiation. The output of the laser for forming the depression 63 is determined in advance so that the dimension of the depression 63 satisfies the conditions (1) and (2) described later. In the present embodiment, the center axis C of the electrode base material 21 and the noble metal tip 61, that is, the center axis C of the protrusion 60 coincides with the axis O.

  FIG. 3 is a diagram illustrating dimensions of each part of the protrusion 60. The height H of the protrusion 60, that is, the dimension along the central axis C from the front end face 64 of the noble metal tip 61 to the rear end of the fusion part 62 is generally 0.5 mm to 2.5 mm. In form, it is 2.5 mm.

In the present embodiment, when the diameter of the noble metal tip 61 is R, the depth D of the recess 63 preferably satisfies the following condition (1). The depth D of the recess 63 is a depth in a direction from the side surface of the noble metal tip 61 toward the central axis C. The diameter R of the noble metal tip 61 is 0.17 mm or more.
0.05 mm ≦ D ≦ 0.3 * R (1)

Moreover, in this embodiment, when the dimension of the opening part of the hollow 63 in the direction along the central axis C of the protrusion part 60 is set to T, it is preferable that the dimension T satisfy | fills the following conditions (2). Hereinafter, the dimension T is referred to as an opening height T.
T ≧ 0.2mm (2)
The reason why it is preferable that the size of the recess 63 of the protrusion 60 satisfies the above conditions (1) and (2) will be described later.

  According to the spark plug 100 of the present embodiment described above, the depression 63 is formed in a part of the side surface of the protruding portion 60 provided at the tip portion of the center electrode 20. The heat attraction from the tip of 20 is blocked, and the temperature of the tip of the center electrode 20 is kept high. As a result, the flame extinguishing action by the center electrode 20 is reduced, and the ignitability is improved. More specifically, the high-temperature portion 65 (FIG. 3), which is the tip side portion along the central axis C with respect to the depression 63, is blocked by the depression 63 so that the temperature of the electrode surface is locally at that portion. The thermal electron emission is improved. Then, it becomes easy to discharge in the high temperature part 65 vicinity, and an initial flame becomes easy to be formed in the high temperature part 65 vicinity. Then, since the high temperature part 65 is hotter than the other parts, the flame extinguishing effect is reduced and the ignitability is improved. Therefore, according to the present embodiment, the ignitability of the spark plug 100 can be improved by a simple configuration in which the recess 63 is formed in the protrusion 60 provided at the tip portion of the center electrode 20. In addition, in this embodiment, since the depression is not provided in the entire circumference of the side surface of the protruding portion 60 but in a part, it is easy to make the portion where the temperature is locally higher than providing the depression in the entire circumference. Can be generated, and the ignitability can be effectively improved.

  Moreover, in this embodiment, since the hollow 63 is formed in the fusion | melting part 62, the hollow 63 can be easily formed only by adjusting the output of a laser apparatus. Therefore, it is possible to suppress an increase in manufacturing cost for forming the recess 63. In the present embodiment, since the noble metal tip 61 is provided on the center electrode 20, the durability of the center electrode 20 can be improved.

  In the present embodiment, only one recess 63 is formed in the protrusion 60. On the other hand, a plurality of recesses 63 may be formed. FIG. 4 shows an example in which two depressions 63 are formed in the protrusion 60. When a plurality of the depressions 63 are formed, the depressions 63 may be arranged at equal intervals on the side surface of the projection 60 around the central axis C of the projection 60. Further, the recesses 63 may be arranged close to each other so as not to contact each other.

  In the present embodiment, “having a dent 63 on a part of the side surface of the protruding portion 60” means that the portion other than the dent 63 in the entire side surface of the protruding portion 60 has no dent. That is, the portion other than the recess 63 means that the outer diameter is constant or the difference in diameter with respect to the chip diameter R is less than 0.05 mm.

B. Test results:
Hereinafter, the reason why it is preferable that the size of the recess 63 satisfies the above conditions (1) and (2) will be described based on various test results.

B1. First ignitability evaluation test:
FIG. 5 is a graph showing test results of the first ignitability evaluation test. In the first ignitability evaluation test, a plurality of spark plug 100 samples having different depths D of the dents 63 are prepared (depths D = 0 mm, 0.02 mm, 0.04 mm, 0.05 mm, 0.06 mm, Each sample was mounted on a 4-cylinder engine having a displacement of 2.0 L, and the ignition timing was set to MBT (optimum ignition position), and the engine was operated at a rotational speed of 1500 rpm. Then, while gradually increasing the air-fuel ratio, that is, while making the fuel thinner, the combustion fluctuation rate (COV) is measured for each air-fuel ratio, and the air-fuel ratio (A / A) when the combustion fluctuation rate exceeds 5% is measured. F) is shown in the graph as the ignition limit air-fuel ratio. In all the samples used in this test, the diameter R of the noble metal tip 61 is 0.4 mm, the opening height T of the recess is 0.2 mm, and the height H of the protrusion 60 is 2.5 mm. And the number of depressions is one. In addition, each value shown in FIG. 5 has shown the average value which measured 3 times about each sample.

  As shown in FIG. 5, according to the first ignitability evaluation test, the depth D of the depression 63 is greater than 0.05 mm than less than 0.05 mm. Excellent results were obtained.

B2. Second ignitability evaluation test:
FIG. 6 is a graph showing test results of the second ignitability evaluation test. In the second ignitability evaluation test, the first ignitability evaluation test is performed on each sample in which the diameter R of the noble metal tip 61 is 1.2 mm and the other conditions are the same as those of the samples in the first ignitability evaluation test. The same test was conducted. Each value shown in FIG. 6 represents an average value obtained by performing three measurements for each sample.

  As shown in FIG. 6, also in the second ignitability evaluation test, as in the first ignitability evaluation test, the depth D of the depression 63 is ignited more than 0.05 mm rather than less than 0.05 mm. The result was that the critical air-fuel ratio was large and the ignitability was excellent. Therefore, according to the first ignitability evaluation test and the second ignitability evaluation test, it was confirmed that the lower limit value of the depth D of the recess 63 in the condition (1) is preferably 0.05 mm.

B3. First vibration evaluation test:
FIG. 7 is a diagram illustrating test results of the first vibration evaluation test. In this first vibration evaluation test, for each sample in which the diameter R of the noble metal tip 61 is 0.4 mm, the remaining ratio of the protruding portion 60 with respect to the diameter R varies by changing the depth D of the recess 63 in various ways. A plurality of samples were prepared (residual rate = 65%, 70%, 75%, 80%, 100%). The remaining rate is the ratio of the value Z (see FIG. 3) to the diameter R obtained by subtracting the depth D of the recess 63 from the diameter R of the noble metal tip 61. In this test, these samples were attached to an ultrasonic vibration tester and vibrated for 600 seconds with a vibration frequency of 27.3 kHz. The frequency of 27.3 kHz is a value determined based on the resonance frequency of a general noble metal tip. As a result of this test, when breakage occurs in the melted portion 62 between the noble metal tip 61 and the electrode base material 21, "X" is shown in FIG. Indicated. In addition, the opening height T of the depression 63 formed in each sample is 0.2 mm, the number of the depressions 63 is one, and the height H of the protrusion 60 is 2.5 mm. In addition, the test results shown in FIG. 7 were all subjected to three tests on the sample under the same conditions, and when breakage occurred even once, “x” was given as the test result.

  As shown in FIG. 7, according to the first vibration evaluation test, when the diameter R of the noble metal tip 61 is 0.4 mm, the remaining rate is 70% or more, that is, the depth D of the recess 63 is the noble metal tip. If the diameter R of 61 was 30% or less, the result that no breakage occurred was obtained.

B4. Second vibration evaluation test:
FIG. 8 is a diagram showing test results of the second vibration evaluation test. In the second vibration evaluation test, a test similar to the first vibration evaluation test was performed on each sample in which the diameter R of the noble metal tip 61 was 1.2 mm. The test results shown in FIG. 8 were tested three times for samples with the same conditions, and when breakage occurred even once, “x” was given as the test result.

  As shown in FIG. 8, according to the second vibration evaluation test, even when the diameter R of the noble metal tip 61 is 1.2 mm, the residual rate is 70% or more, as in the first vibration evaluation test. That is, when the depth D of the recess 63 is 30% or less with respect to the diameter R of the noble metal tip 61, a result that breakage does not occur was obtained. Therefore, according to the first vibration evaluation test and the second vibration evaluation test, it was confirmed that the upper limit value of the depth D of the depression 63 in the condition (1) is preferably 0.3 * R.

  According to the first ignitability evaluation test, the second ignitability evaluation test, the first vibration evaluation test, and the second vibration evaluation test described above, the protrusion 60 is formed so that the recess 63 satisfies the condition (1). It can be understood that the ignitability can be improved satisfactorily while suppressing breakage of the projecting portion 60 by forming the protrusions 60 in the same manner.

B5. Third ignitability evaluation test:
FIG. 9 is a graph showing test results of the third ignitability evaluation test. In this third ignitability evaluation test, samples of a plurality of spark plugs 100 having different opening heights T of the recesses 63 are prepared (T = 0 mm, 0.1 mm, 0.15 mm, 0.2 mm, 0.3 mm, 0 .4 mm, 0.5 mm), each sample was subjected to the same test as the first ignitability evaluation test. In all the samples used in this test, the diameter R of the noble metal tip 61 is 0.4 mm, the depth D of the recess 63 is 0.05 mm, and the height H of the protrusion 60 is 2.5 mm. The number of depressions 63 is one. Each value shown in FIG. 9 represents an average value obtained by performing three measurements for each sample.

  As shown in FIG. 9, according to the third ignitability evaluation test, the opening height T of the recess 63 is 0.2 mm or more, and the ignition limit air-fuel ratio is larger and the ignitability is less than 0.2 mm. The result was excellent. Therefore, it can be understood that when the opening height T of the recess 63 satisfies the above condition (2), the ignitability is improved satisfactorily.

B6. Fourth ignitability evaluation test:
FIG. 10 is a graph showing test results of the fourth ignitability evaluation test. In the fourth ignitability evaluation test, a plurality of samples in which two recesses 63 having the same shape are arranged at symmetrical positions with the central axis C of the protrusion 60 in between are prepared, and the same as in the first ignitability evaluation test. A test was conducted. The conditions for each sample are the same as those for each sample in the first ignitability evaluation test, except for the number of recesses 63. Each value shown in FIG. 10 indicates an average value obtained by performing three measurements for each sample. In addition, in FIG. 10, the test result (the number of dents: 1) of a 1st ignitability evaluation test is also shown for reference with the test result (the number of dents: 2) of the 4th ignitability evaluation test. Yes.

  As shown in FIG. 10, according to the fourth ignitability evaluation test, the number of the recesses 63 formed on the protrusion 60 is preferably a plurality of recesses rather than one, and in particular, the depth of the recesses 63. It was confirmed that D is remarkably preferable when it is 0.05 mm or more. In this test, the two dents 63 are arranged at symmetrical positions with the central axis C of the protrusion 60 in between. However, the dents 63 are not in contact with each other in order to locally generate a high-temperature part. You may arrange | position close to.

C. Variations:
<First Modification>
In the above embodiment, the protrusion 60 is provided with the columnar noble metal tip 61. On the other hand, the noble metal tip 61 may not be cylindrical. Further, the protrusion 60 may not be provided with the noble metal tip 61. In this case, the protrusion 60 is made of the electrode base material 21, and the recess 63 is formed on the side surface of the electrode base material 21.

<Second Modification>
In the said embodiment, the hollow 63 of the protrusion part 60 is formed at the time of laser welding. On the other hand, the dent 63 may be formed by cutting using a drill or the like, or may be formed by punching.

<Third Modification>
In the above embodiment, the central electrode 20 is provided with the protrusion 60. On the other hand, for example, a protrusion may be provided on the ground electrode 30 side. Specifically, for example, an intermediate member made of the same material as the base material of the ground electrode 30 is provided on the inner surface of the ground electrode 30 as an electrode base material, and a noble metal tip is welded to the intermediate member, A fusion zone is formed at the junction with the noble metal tip. And the hollow of the shape similar to FIG. 2 and FIG. 3 is formed in the fusion | melting part. With such a structure, since the temperature of the tip of the ground electrode 30 is kept high, the flame extinguishing action by the ground electrode 30 is reduced, and the ignitability is improved. In addition, not only one of the front end portion of the center electrode 20 and the front end portion of the ground electrode 30, projecting portions may be provided on both of them.

  The present invention is not limited to the above-described embodiments and modifications, and can be realized with various configurations without departing from the spirit thereof. For example, the technical features of the embodiments and the modified examples corresponding to the technical features in the embodiments described in the summary section of the invention are intended to solve part or all of the above-described problems or to achieve the above-described effects. In order to achieve part or all of the above, replacement or combination can be performed as appropriate. Further, if the technical feature is not described as essential in the present specification, it can be deleted as appropriate.

DESCRIPTION OF SYMBOLS 3 ... Ceramic resistance 4 ... Sealing body 5 ... Gasket 6, 7 ... Ring member 8 ... Plate packing 9 ... Talc 10 ... Insulator 12 ... Shaft hole 13 ... Leg long part 14 ... Shaft hole inner step part 15 ... Insulator step part 17 ... Front end side body part 18 ... Rear end side body part 19 ... Central body part 20 ... Center electrode 21 ... Electrode base material 22 ... Core material 23 ... Gutter part 30 ... Ground electrode 40 ... Terminal metal fitting 50 ... Main metal fitting 51 ... Tool engagement Numeral 52 ... Mounting screw part 53 ... Clamping part 54 ... Sealing part 56 ... Bracket inner step part 57 ... Tip surface 58 ... Compression deformation part 60 ... Projection part 61 ... Precious metal tip 62 ... Melting part 63 ... Depression 64 ... Tip face 65 ... High temperature part 100 ... Spark plug

Claims (5)

A cylindrical metal shell,
An insulator having at least a part of its outer periphery held by the metal shell and having an axial hole along an axis;
A center electrode provided in the shaft hole;
A grounding electrode fixed to the metal shell and forming a discharge gap with the center electrode;
At least one of the center electrode and the ground electrode has a columnar protrusion that protrudes toward the other electrode and forms the discharge gap,
Having a depression in a part of the side surface of the protrusion,
The protrusion is
A columnar noble metal tip located on the discharge gap side;
A melted portion formed by melting the noble metal tip and the electrode base material, located on the side opposite to the discharge gap side of the noble metal tip,
The depression is formed in the melting part,
A spark plug that satisfies the following condition (1), where D is the depth of the recess from the side surface of the noble metal tip, and R is the diameter of the noble metal tip.
0.05 mm ≦ D ≦ 0.3 * R (1) The spark plug according to claim 1,
A spark plug that satisfies the following condition (2), where T is the dimension of the opening of the recess in the direction along the central axis of the protrusion.
T ≧ 0.2mm (2) The spark plug according to claim 1 or 2, wherein
A spark plug comprising a plurality of the recesses in the protruding portion. The spark plug according to any one of claims 1 to 3, wherein
The spark plug is characterized in that the protrusion is provided on the center electrode. The spark plug according to any one of claims 1 to 3, wherein
The spark plug, wherein the protrusion is provided on the ground electrode.

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