JP4739281B2 - Semi creeping spark plug - Google Patents

Description

  The present invention relates to a spark plug used for an internal combustion engine, and more particularly to a spark plug capable of performing semi-surface discharge at all times.

  Conventionally, what is called a semi-surface discharge type is known as a spark plug for an internal combustion engine with improved fouling resistance. Such a semi-creeping discharge type spark plug is similar to a normal air discharge type spark plug in that a center electrode, an insulator provided on the outer periphery thereof, and a cylindrical shape provided on the outer periphery of the insulator. And a ground electrode having a base end portion joined to a distal end portion of the metal shell. However, in the semi-surface discharge type spark plug, the front end surface of the insulator is arranged in a positional relationship so as to enter between the ignition part of the center electrode and the ignition part of the ground electrode. Thereby, a spark discharge can occur along the insulator front end surface.

  In general, when a spark plug is used for a long time in a low-temperature environment where the electrode temperature is, for example, 450 ° C. or less, the so-called “swelling” or “fogging” state occurs, and the insulator front surface is a conductive fouling substance such as carbon. It will be covered with and will likely cause malfunction. In this regard, according to the above-mentioned semi-surface discharge type spark plug, spark discharge can occur in the form of scooping the insulator tip surface, so that the pollutant is burned out, and it is more resistant than the air discharge type spark plug. The fouling property is improved.

  However, it is known that spark plugs that can cause creeping discharge frequently generate sparks that scoop the surface of the insulator, so that the surface of the insulator is easily cut into grooves, so-called channeling is likely to occur. As the channeling progresses, there is a risk that the heat resistance of the spark plug is impaired or the reliability is lowered.

As a technique for suppressing such channeling, there is a technique in which an erosion-suppressing layer containing an insulator erosion-suppressing component is formed on the surface of the tip of the insulator along with spark discharge (for example, , See Patent Document 1).
JP 2004-165168 A

  By the way, in a spark plug that can perform semi-surface discharge, air discharge is normally performed between the ground electrode and the center electrode, and when the surface of the insulator is contaminated to some extent, the discharge path includes the surface discharge. There are an intermittent discharge type that is formed and a type that always forms a discharge path including creeping discharge (also referred to as a constant creeping discharge type). When the technology for forming the above-described insulating erosion suppressing layer is adopted, a certain degree of channeling suppression effect can be expected, but the former intermittent discharge type does not exhibit sufficient antifouling property (cleaning effect). There is a fear.

  On the other hand, in a type that constantly performs creeping discharge, channeling suppression and the like are not necessarily sufficient compared to an intermittent discharge type, and channeling concerns due to the fact that the tip surface of the insulator is scraped remain. . Further, depending on the relative positional relationship between the ground electrode and the conductor, there is a possibility that a problem called “penetration” may occur due to the formation of a hole in the insulator side wall.

  The present invention has been made in view of the above circumstances, and an object of the present invention relates to a spark plug that performs semi-surface creeping discharge at all times, and is capable of further suppressing channeling and the like while ensuring antifouling property. The purpose is to provide creeping spark plugs.

  Hereinafter, each configuration suitable for solving the above-described problems will be described in terms of items. In addition, the effect etc. peculiar to the structure which respond | corresponds as needed are added.

Configuration 1. The spark plug of this configuration is
A cylindrical insulator having an axial hole penetrating in the axial direction;
A center electrode inserted in the shaft hole;
A metal shell provided on the outer periphery of the insulator and disposed so that a tip surface of the insulator protrudes from a tip of the insulator;
A plurality of ground electrodes each having a proximal end fixed to the distal end surface of the metal shell,
The front end surface of each ground electrode extends parallel to the axial direction, and creeping discharge along the front end surface of the insulator is part of a spark discharge path between the front end of each ground electrode and the center electrode. A semi-creeping spark plug that includes a route,
The edge portion between the front end surface and the inner side surface of each ground electrode,
On the tip side of a virtual plane including the tip surface of the insulator, and
Located on the tip side of the tip surface of the center electrode,
Located outside the outer periphery of the front end surface of the insulator,
The angle formed between the tip surface and the inner side surface of each ground electrode is an obtuse angle ,
An axis passing through the center is drawn from the base end side and the tip end side of the ground electrode, respectively, and the intersection of both axes is located closer to the base end side than the tip end surface of the insulator .

  The spark plug in Configuration 1 has an essential requirement that a creeping discharge path along the tip surface of the insulator is included in a part of the spark discharge path. In particular, unlike the so-called intermittent discharge type, the spark plug of Configuration 1 can always perform creeping discharge.

  As described above, according to the first configuration, since the creeping discharge path along the tip surface of the insulator is included in a part of the spark discharge path, the spark discharge is generated so as to crawl the insulator tip surface. For this reason, the fouling substance is burned out, and the fouling resistance is ensured. In particular, in Configuration 1, the edge portion between the front end surface and the inner side surface of each ground electrode is located closer to the front end side than the front end surface of the center electrode. For this reason, it is unlikely that a spark discharge (air discharge) occurs directly between the front end surface of the ground electrode and the outer peripheral surface of the center electrode. Moreover, in the structure 1, the said edge part is located outside the front end surface outer periphery of an insulator. For this reason, a situation in which a spark discharge directly occurs between the inner surface of the front end of the ground electrode and the front end surface of the center electrode is unlikely to occur. For these reasons, a more reliable realization of creeping discharge along the tip surface of the insulator can be achieved. As a result, the function as a semi-creeping spark plug is sufficiently exhibited, and the antifouling property is excellent. Further, since the edge portion is located outside the outer periphery of the tip end surface of the insulator, a sufficient combustion space is ensured. Therefore, it is possible to prevent a decrease in ignitability due to the ground electrode impeding flame propagation. Note that there is no particular dimensional limitation for semi-surface discharge, but the overall spark discharge gap between the air discharge and surface discharge (total gap, that is, between the center electrode and the ground electrode). The shortest distance) is more preferably 1.8 mm or less.

  In Configuration 1, the edge portion between the front end surface and the inner side surface of each ground electrode is positioned on the front end side with respect to the virtual plane including the front end surface of the insulator. For this reason, the electric field strength is relatively weak as compared with the case where the edge portion is located on the base end side, and the stress of the insulator received by the spark can be relatively small. As a result, the progress of channeling can be suppressed. In addition, there is almost no spark flying on the side surface of the insulator, and it is possible to suppress problems caused by “penetration” in which holes are formed in the insulator side wall as well as channeling.

  In addition, in Configuration 1, the angle formed between the tip surface and the inner side surface of each ground electrode is an obtuse angle. Therefore, the electric field strength at the edge portion between the front end surface and the inner side surface of each ground electrode is further reduced, and the stress of the insulator that is received by the spark can be further reduced. As a result, channeling can be more reliably suppressed.

  Configuration 2. The spark plug of this configuration is characterized in that, in the above configuration 1, the angle formed between the front end surface and the inner side surface of each ground electrode is 110 degrees or more.

  As in the configuration 2, the electric field strength at the edge portion can be more reliably reduced by setting the angle formed between the front end surface and the inner side surface of each ground electrode to 110 degrees or more. As a result, channeling can be more reliably suppressed.

  Configuration 3. In the spark plug of this configuration, in any one of the above configurations 1 or 2, a protruding length in the axial direction of the front end surface of the center electrode with respect to the front end surface of the insulator is +0.5 mm or less.

  When the protrusion length in the axial direction of the tip surface of the center electrode with respect to the tip surface of the insulator exceeds +0.5 mm, the frequency of air discharge between the ground electrode and the center electrode without involving creeping discharge is high. There is a concern that it will become higher, the chance of burning off the pollutant will decrease, and the fouling resistance will decrease. In this regard, in Configuration 3, since the protruding length of the center electrode tip surface with respect to the tip surface of the insulator is +0.5 mm or less, spark discharge can occur more reliably between the insulator tip surface and the insulator. Therefore, the creeping discharge along the tip surface of the insulator can be more reliably realized, and the fouling resistance can be improved.

  The protrusion length of the center electrode tip surface with respect to the insulator tip surface is negative, that is, there is no problem even if the center electrode is immersed in the insulator, but more reliable suppression of channeling (formation of deep channeling) From the viewpoint of (suppressing), it is desirable that the protrusion length is −0.3 mm or more.

  Configuration 4. The spark plug of this configuration is characterized in that, in any one of the above configurations 1 to 3, the edge portion between the tip surface and the inner side surface of each ground electrode is curved or chamfered. In short, the gist is that the edge portion between the tip surface and the inner side surface of each ground electrode is not angular.

  That is, the electric field strength at the edge portion can be reduced by making the angle formed between the front end surface and the inner side surface of each ground electrode an obtuse angle as in Configuration 1 or the like. By making the portion curved or chamfered, the electric field does not concentrate locally, so that the discharge follows various paths and can further contribute to suppression of channeling.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. As shown in FIGS. 1A and 1B, the spark plug 100 of this embodiment includes a metal shell 1, an insulator 2, a center electrode 3, and two ground electrodes 4A and 4B. Yes. The metal shell 1 is formed in a cylindrical shape from a metal such as low carbon steel, and constitutes a housing of the spark plug 100. On the outer peripheral surface thereof, the spark plug 100 is attached to a cylinder head of an engine (not shown). A threaded portion 7 is formed. An insulator 2 is held inside the metal shell 1. The front end surface 2 a of the insulator 2 protrudes from the front end surface of the metal shell 1 to the front end side.

  The insulator 2 is made of a ceramic sintered body such as alumina, for example, and an axial hole 6 is formed in the inside along the axial direction of the insulator 2, and the central electrode 3 is inserted in the axial hole 6. It is fixed. In the present embodiment, the front end surface 3 a of the center electrode 3 is substantially flush with the front end surface 2 a of the insulator 2.

  Further, the ground electrodes 4 </ b> A and 4 </ b> B are provided at symmetrical positions with the center electrode 3 interposed therebetween, and the respective base end surfaces are welded to the front end surface of the metal shell 1. The ground electrodes 4A and 4B are bent (or curved) in the axial direction at intermediate positions in the longitudinal direction.

  The ground electrodes 4A and 4B have a two-layer structure including an outer layer and an inner layer. The outer layer is made of a nickel alloy or the like. On the other hand, the inner layer is made of a metal having good thermal conductivity (for example, a metal material mainly composed of copper or high-purity nickel having higher thermal conductivity than the nickel alloy) than the nickel alloy. The main body of the center electrode 3 also has a two-layer structure of an outer layer and an inner layer.

  In the present embodiment, the tip surfaces 41a and 41b of the ground electrodes 4A and 4B extend parallel to the axial direction, and a spark discharge path is formed between the tip of each ground electrode 4A and 4B and the center electrode 3. ing. And it is set so that the creeping discharge path along the front end surface 2a of the insulator 2 may be included in a part of the spark discharge path. That is, in the gaps GA and GB between the edge portions EA and EB between the front end surfaces 41a and 41b and the inner side surfaces 42a and 42b of the ground electrodes 4A and 4B and the insulator 2, air discharge occurs. Spark discharge is generated between the central electrode 3 and the central electrode 3 in a form of creeping discharge via the tip surface 2a of the insulator 2.

  However, the edge portions EA and EB are located on the tip side of the virtual plane M1 including the tip surface 2a of the insulator 2 and on the tip side of the tip surface 3a of the center electrode 3, and It is located outside the front end surface outer periphery G1 (located in the region β shown in FIG. 2A).

  Further, in the present embodiment, the angle (edge portion angle) θ formed by the tip surfaces 41a and 41b of the ground electrodes 4A and 4B and the inner side surfaces 42a and 42b is an obtuse angle (for example, θ = 120 degrees). .

  Unlike the so-called intermittent discharge type, the spark plug 100 according to the present embodiment can always perform creeping discharge. In particular, in the present embodiment, the protrusion length of the center electrode 3 from the distal end surface 2a of the insulator 2 is as small as 0.5 mm or less as described above (in this embodiment, the protrusion length is 0, that is, the insulator 2 The tip surface 2a and the tip surface 3a of the center electrode 3 are located on the same plane), and the edge portions EA and EB are located on the tip side of the tip surface 3a of the center electrode 3. In addition, it is unlikely that a direct spark discharge (air discharge) occurs between the front end surfaces 41a and 41b of the ground electrodes 4A and 4B and the outer peripheral surface of the center electrode 3 without a creeping discharge. Further, since the edge portions EA, EB are located outside the outer peripheral surface G1 of the insulator 2, the inner side surfaces 42a, 42b of the tips of the ground electrodes 4A, 4B and the end surface 3a of the center electrode 3 It is unlikely that a direct spark discharge will occur. For these reasons, a more reliable realization of creeping discharge along the tip surface 2a of the insulator 2 can be achieved. In particular, the edge portions EA and EB can be cleaned from the edge of the tip surface 2a of the insulator 2 by positioning the edge portions EA and EB outside the tip surface outer periphery G1 of the insulator 2. As a result, the function as a semi-creeping spark plug is sufficiently exhibited, and the antifouling property is excellent.

  Next, in order to confirm the effect of this embodiment regarding suppression of channeling, various samples were produced by changing various conditions, and various evaluations were tried. The experimental results are described below.

  First, samples (spark plugs) were prepared in which the height of the edge portion relative to the insulator tip surface was varied, and a field strength simulation experiment was performed on each sample. FIG. 3 shows the relationship of the electric field strength with respect to the height of the edge portion relative to the insulator surface. In the graph, the value on the horizontal axis is 0 when the edge portion and the insulator front end surface have the same height.

  As shown in FIG. 3, the electric field strength tends to be weaker when the edge portion is located on the tip side than the tip surface of the insulator. Therefore, as in the present embodiment, by positioning the edge portion EA (EB) closer to the front end side than the front end surface 2a of the insulator 2, it can be said that the electric field strength can be weakened, thereby contributing to the suppression of channeling. .

  Next, the edge portion EA (EB) is a sample (see FIG. 2A) in which the edge portion EA (EB) is located in the region β on the tip side of the insulator 2 and on the outer side of the tip surface outer periphery G1. Samples (spark plugs) with different angles (angles of the edge portions) between the front end surface and the inner side surface of each ground electrode were prepared, durability tests were performed on each sample, and the channeling depth was measured. For the durability test and measurement, each sample (spark plug) was attached to a DOHC inline 6 cylinder engine with a displacement of 2000 cc, and the engine was rotated at a fixed rotational speed (5000 rpm in this case). The depth of the deep channeling part was calculated. The depth and the channel of the conventional product (as shown in FIG. 2B), the angle θ2 of the edge portion E2 is a right angle and the edge portion E2 is located on the proximal side of the distal end surface 22a of the insulator 22. The ratio with the depth of the ring was calculated. The result is shown in FIG.

  As shown in FIG. 4, when the angle of the edge portion is an obtuse angle, the channeling depth is smaller than when it is a right angle, and it can be seen that channeling can be more reliably suppressed. In particular, when the angle is 110 degrees or more (particularly 120 degrees or more), it can be said that channeling can be more reliably suppressed.

  Next, samples (spark plugs) were prepared in which the protrusion length in the axial direction of the tip surface of the center electrode with respect to the tip surface of the insulator was varied, and the probability of a spark to the tip surface of the insulator was measured. In this measurement, each sample to be evaluated (spark plug) assembled in the ignition device is attached to the test chamber, and the test chamber is filled with an evaluation mixture obtained by mixing air and propane. Then, the spark plug was discharged, the presence or absence of a spark on the tip surface of the insulator was determined based on the photographed image, the determination was performed a predetermined number of times (here, 100 times), and the probability of a spark was counted. FIG. 5 is a graph showing the relationship between the protrusion length of the center electrode and the probability of a spark to the insulator tip surface.

  As shown in FIG. 5, when the protrusion length exceeds +0.5 mm, the frequency of air discharge between the ground electrode and the center electrode exceeds 10%, and the stain resistance is reduced. Concerned. On the other hand, when the protrusion length is +0.5 mm or less, it can be seen that a spark is more reliably scattered on the insulator front end surface. Accordingly, it can be said that the creeping discharge along the tip surface 2a of the insulator 2 can be more surely realized and the antifouling property can be improved by setting the protrusion length to +0.5 mm or less.

  Subsequently, as shown in FIG. 2A, samples (spark plugs) in which the positions of the edge portions EA (EB) are respectively positioned in the three regions α, β, γ are prepared. Evaluation was made on heat resistance deterioration and penetration. The region α indicates a region on the tip side of the virtual plane M1 including the tip surface 2a of the insulator 2 and on the inner side of the tip surface outer periphery G1 of the insulator 2, and the region β indicates the virtual plane M1. The region γ is a region closer to the distal end than the outer periphery G1 of the distal end surface, and the region γ is closer to the proximal end than the virtual plane M1 including the distal end surface 2a of the insulator 2 and from the outer periphery G1 of the distal end surface. Also refers to the outer region.

  For the evaluation of ignitability, each sample (spark plug) is attached to an DOHC inline 6 cylinder engine having a displacement of 2000 cc, and the engine is rotated at a constant rotational speed under predetermined conditions on the air-fuel ratio lean side. The number of misfires was measured. Further, the misfire is caused more than the conventional product (a sample in which the angle θ2 of the edge portion E2 is a right angle and the edge portion E2 is located on the proximal side of the distal end surface 22a of the insulator 22 as shown in FIG. 2B). For those with a large number, ×, and for those with a small number, ○. The results are shown in Table 1.

表１に示すように、エッジ部分が領域αに位置する場合には、従来よりも着火性が低下してしまった。これに対し、エッジ部分が領域β，γに位置する場合には、従来に比べて着火性が低下しなかった。このことから、エッジ部分ＥＡ，ＥＢが、絶縁体２の先端面外周Ｇ１よりも内側の領域に位置すると、着火性が低下してしまうのに対し、エッジ部分ＥＡ，ＥＢを絶縁体２の先端面外周Ｇ１よりも外側の領域に位置させることで着火性の低下を防止できることがわかる。

As shown in Table 1, when the edge portion is located in the region α, the ignitability is lower than in the prior art. On the other hand, in the case where the edge portion is located in the regions β and γ, the ignitability did not deteriorate compared to the conventional case. For this reason, when the edge portions EA and EB are located in a region inside the tip end outer periphery G1 of the insulator 2, the ignitability deteriorates, whereas the edge portions EA and EB are changed to the tip of the insulator 2. It can be seen that the ignitability can be prevented from being lowered by being positioned in a region outside the outer periphery G1.

  For evaluation of heat resistance deterioration, each sample (spark plug) was attached to a DOHC in-line 6 cylinder engine with a displacement of 2000 cc, the engine was rotated at a constant rotation speed (5000 rpm in this case), and the heat resistance after 500 hours had elapsed. The presence or absence of deterioration was determined. And when there was no heat deterioration, it evaluated as (circle), and when there was heat deterioration (it recognized), it evaluated as x. If there is heat deterioration, the pre-ignition occurrence advance angle tends to be on the retard side. Therefore, here, as one index of heat deterioration, it is determined that there is heat deterioration (×) when the pre-ignition occurrence advance angle is delayed by 5 ° or more, and that there is no heat deterioration (○) otherwise. .

  Furthermore, for evaluation of penetration, each sample (spark plug) was attached to a DOHC in-line 6 cylinder engine with a displacement of 2000 cc, and the engine was rotated at a constant rotational speed to determine the presence or absence of insulation penetration after 500 hours. did. And when there was no penetration, it evaluated as (circle), and when penetration was recognized, it evaluated as x. It shows in Table 2 about each evaluation result.

表２に示すように、エッジ部分が領域γに位置する場合には、チャンネリング及び貫通が発生してしまった。これに対し、エッジ部分が領域α，βに位置する場合には、耐熱劣化及び貫通のいずれについても満足のいくものであった（軽度のチャンネリングで耐熱劣化、貫通はほとんど生じなかった）。このことから、エッジ部分ＥＡ，ＥＢが、絶縁体２の先端面２ａを含む仮想平面Ｍ１よりも基端側に位置すると、チャンネリングや貫通が発生してしまうのに対し、エッジ部分ＥＡ，ＥＢを絶縁体２の先端面２ａを含む仮想平面Ｍ１よりも先端側の領域に位置させることで耐熱劣化を抑制でき、チャンネリング及び貫通の発生を防止できることがわかる。

As shown in Table 2, when the edge portion is located in the region γ, channeling and penetration have occurred. On the other hand, when the edge portion is located in the regions α and β, both the heat resistance deterioration and the penetration were satisfactory (the heat resistance deterioration and the penetration were hardly caused by mild channeling). For this reason, when the edge portions EA and EB are located on the base end side with respect to the virtual plane M1 including the distal end surface 2a of the insulator 2, channeling and penetration occur, whereas the edge portions EA and EB are generated. It can be seen that the heat resistance deterioration can be suppressed and the occurrence of channeling and penetration can be prevented by locating in the region closer to the tip side than the virtual plane M1 including the tip surface 2a of the insulator 2.

  In addition, it is not limited to the description content of embodiment mentioned above, For example, you may implement as follows.

  (A) In the embodiment described above, the edge portions EA and EB between the tip surfaces 41a and 41b and the inner side surfaces 42a and 42b of the ground electrodes 4A and 4B have an angular shape. Is possible. For example, as shown in FIG. 6 (a), the ground electrodes 4A and 4B may be provided with curved edge portions (having an R surface) EA1 (EB1), or as shown in FIG. 6 (b). The ground electrodes 4A and 4B may be provided with edge portions EA2 (EB2) having a chamfered shape (having a C surface). As in the above embodiment, the electric field strength of the edge portion can be reduced by setting the angle θ of the edge portions EA and EB to an obtuse angle. However, as in the modification example, the edge having a curved shape or a chamfered shape is used. By setting the portions EA1 (EB1) and EA2 (EB2), the firing points are dispersed, and the electric field strength is further reduced. Therefore, it can contribute to further suppression of channeling.

  (B) Although not particularly mentioned in the above embodiment, a noble metal tip (not shown) may be bonded to at least the edge portions EA and EB. Thus, durability can be improved by providing a noble metal tip.

  (C) The shape of the insulator 2 is not necessarily limited to that of the above embodiment. Therefore, for example, an insulator without a stepped portion may be used, or an insulator having a gently tapered surface and tapering toward the tip end side may be employed.

  (D) In the above embodiment, the distal end surface 2a of the insulator 2 and the distal end surface 3a of the center electrode 3 are substantially flush, but the distal end surface 3a of the center electrode 3 may protrude somewhat. However, based on the experimental results, it is desirable that the protruding length in the axial direction of the tip surface 3a of the center electrode 3 with respect to the tip surface 2a of the insulator 2 is +0.5 mm or less. On the other hand, the protrusion length of the tip surface 3a of the center electrode 3 with respect to the tip surface 2a of the insulator 2 is negative, that is, there is no problem even if the center electrode 3 is immersed in the insulator 2, but channeling is more reliable. From the viewpoint of suppression (suppressing formation of deep channeling), it is desirable that the protrusion length be −0.3 mm or more.

  (E) In the embodiment described above, the two ground electrodes 4A and 4B are provided at symmetrical positions with the center electrode 3 interposed therebetween. On the other hand, three or more ground electrodes may be provided.

(F) Further, as shown in FIG. 7, axes L1 and L2 passing through the centers are drawn from the base end side and the tip end side of the ground electrode 4A (4B), respectively, and the intersection Z of both axis lines L1 and L2 is an insulator. than 2 of the distal end surface 2a is to be positioned on the proximal side.

(A) is a partially broken front view which shows the structure of the spark plug of this embodiment, (b) is an enlarged view of the principal part. (A) is a schematic diagram for demonstrating the area | region in which an edge part is located, (b) is a schematic diagram which expands and shows the principal part of a conventional product. It is a graph which shows the relationship of the electric field strength with respect to the height with respect to the insulator surface of an edge part. It is a graph which shows the relationship of the ratio with the conventional product of the depth of channeling with respect to the angle of an edge part. It is a graph which shows the relationship of the flying probability to the insulator front end surface with respect to the protrusion length of a center electrode. (A), (b) is a schematic diagram which shows the ground electrode in another embodiment. It is a schematic diagram which shows the ground electrode in another embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Metal fitting, 2 ... Insulator, 2a ... Tip surface, 3 ... Center electrode, 3a ... Tip surface, 4A, 4B ... Ground electrode, 41a, 41b ... Tip surface, 42a, 42b ... Inner side surface, EA, EB, EA1, EB1, EA2, EB2... Edge portion.

Claims (4)

A cylindrical insulator having an axial hole penetrating in the axial direction;
A center electrode inserted in the shaft hole;
A metal shell provided on the outer periphery of the insulator and disposed so that a tip surface of the insulator protrudes from a tip of the insulator;
A plurality of ground electrodes each having a proximal end fixed to the distal end surface of the metal shell,
The front end surface of each ground electrode extends parallel to the axial direction, and creeping discharge along the front end surface of the insulator is part of a spark discharge path between the front end of each ground electrode and the center electrode. A semi-creeping spark plug that includes a route,
The edge portion between the front end surface and the inner side surface of each ground electrode,
On the tip side of a virtual plane including the tip surface of the insulator, and
Located on the tip side of the tip surface of the center electrode,
Located outside the outer periphery of the front end surface of the insulator,
The angle formed between the tip surface and the inner side surface of each ground electrode is an obtuse angle ,
A semi-creeping spark plug characterized in that an axis passing through the center is drawn from the proximal end side and the distal end side of the ground electrode, and the intersection of both axes is located on the proximal end side with respect to the distal end surface of the insulator .   2. The semi-creeping spark plug according to claim 1, wherein an angle formed between a front end surface and an inner side surface of each ground electrode is 110 degrees or more.   3. The semi-creeping spark plug according to claim 1, wherein a protruding length of the tip surface of the center electrode in the axial direction with respect to the tip surface of the insulator is +0.5 mm or less.   4. The semi-creeping spark plug according to claim 1, wherein an edge portion between the front end surface and the inner side surface of each ground electrode is curved or chamfered. 5.

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