JP2007294412A - Spark plug for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure durability and stain resistant property, achieve improvement of ignition characteristics, and make a fuel bridge hard to occur regarding a semi-surface discharge type spark plug. <P>SOLUTION: The spark plug 100 is provided with the main fitting 1, an insulating body 2, the center electrode 3, and two pieces of grounding electrodes 4A, 4B. The tip face 3a of the center electrode 3 has approximately the same face as that of the tip face 2a of the insulating body 2, and the tip faces of the grounding electrodes 4A, 4B are opposing to the tip face outer peripheral face of the insulating body 2. Noble metal chips 11A, 11B are protruded and fixed to the tip faces of the grounding electrodes 4A, 4B. A creeping discharging route along the tip face 2a of the insulating body 2 is included in one part of a spark discharging route between the respective noble metal chips 11A, 11B and the center electrode 3. Protrusion length E1 from the tip face of the grounding electrodes 4A, 4B of the noble metal chips 11A, 11B exceeds 0.2 mm, and the thickness t1 has become 1.0 mm or less, and the width has become 50% or more of an axial pore diameter of the tip face 2a of the insulating body 2. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a spark plug used for an internal combustion engine, and more particularly to a spark plug for an internal combustion engine having a semi-surface discharge gap.

  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-surface discharge type spark plug (hereinafter referred to as a semi-surface discharge type spark plug) is similar to a normal air discharge type spark plug, and includes a center electrode, an insulator provided on the outer periphery thereof, and the insulation. A cylindrical metal shell provided on the outer periphery of the body, and a ground electrode having a base end joined to the tip of the metal shell. When a voltage is applied between the center electrode and the ground electrode, the semi-creeping spark plug has an air discharge path based on a part of the tip of the ground electrode and a part of the tip of the insulator. A spark discharge path (spark discharge gap) composed of a creeping discharge path starting from a part of the tip of the insulator and a part of the central electrode near the tip of the insulator is formed. Has been.

By the way, when a spark plug is used for a long time in an environment where the temperature of the center electrode and the ground electrode is relatively low, the state of so-called “swelling” or “fogging” occurs, and the insulator front end surface is made of carbon or the like. It is known that a state called “fouling”, which is covered with a conductive fouling substance and does not normally fly in the spark discharge gap, is called “fouling”. In this respect, according to the semi-creeping spark plug, spark discharge occurs in the shape of the insulator tip, so that the pollutant is burned out, and the anti-fouling property is improved compared to the air discharge type spark plug. (See, for example, Patent Document 1).
JP 2003-22885 A

  On the other hand, in the spark plug, it is necessary to make the heat of the ground electrode good and to ensure sufficient durability. In order to ensure a predetermined durability with respect to the ground electrode, it is the actual situation that a certain size of the ground electrode must be employed.

  However, if a large size ground electrode is used, the space where the flame spreads is hindered, and the heat of the flame is easily taken away by the ground electrode, so that the flame extinguishing effect is increased. As a result, the ignitability may be reduced.

  Further, compared with the air discharge type spark plug, the semi-creeping spark plug has a large area facing the insulator of the ground electrode, and tends to generate a fuel bridge. In addition, when the size of the ground electrode is increased as described above, the concern about the occurrence of a fuel bridge is further increased.

  In the above-mentioned Patent Document 1, it is described that a noble metal tip is provided on the ground electrode. However, by simply providing the tip, a certain degree of durability can be ensured, but the ignitability is still lowered, and the fuel bridge Concerns about the outbreak are not dispelled.

  The present invention has been made in view of the above circumstances, and an object of the present invention relates to a semi-surface discharge type spark plug, which can improve ignitability while ensuring durability and antifouling properties, and fuel. An object of the present invention is to provide a spark plug for an internal combustion engine that can prevent the occurrence of bridging.

  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 provided with a cylindrical insulator having an axial hole penetrating in the axial direction, a center electrode inserted in the axial hole, and an outer periphery of the insulator, and the insulating plug is provided from its tip. A spark plug for an internal combustion engine comprising a metal shell arranged so that a distal end surface of the body protrudes, and a ground electrode whose base end portion is fixed to the distal end surface of the metal shell,
A tip of the ground electrode is fixed with a noble metal tip protruding in a radial direction toward the center electrode.
A spark discharge path formed by applying a voltage between the tip of the noble metal tip and the tip of the center electrode includes a part of the tip of the noble metal tip and a part of the tip of the insulator. And a creeping discharge path along the tip surface of the insulator with a part of the tip portion of the insulator and a part of the center electrode as a base point.
The protruding length of the noble metal tip from the tip of the ground electrode exceeds 0.2 mm.

  Here, the “tip portion of the ground electrode” refers to the “tip surface of the ground electrode” when the tip surface of the ground electrode is opposed to the radial direction of the center electrode. On the other hand, when the tip side surface of the ground electrode is opposed to the radial direction of the center electrode, it indicates the “tip electrode side surface”. In addition, the “noble metal tip” only has to have a tip projecting in the radial direction toward the center electrode, and examples of the material include an alloy mainly composed of a noble metal such as platinum or iridium. Further, when the tip surface of the ground electrode is opposed to the radial direction of the center electrode and the “noble metal tip” protrudes from the tip surface of the ground electrode, the “noble metal tip” It needs to be smaller than the area, and further needs to be thinner than the thickness of the front end surface of the ground electrode.

  According to the above configuration 1, since the noble metal tip having its tip protruding in the radial direction toward the center electrode is fixed to the tip of the ground electrode, a spark discharge is generated between the noble metal tip and the center electrode. Occur. Since the noble metal tip is excellent in high-temperature oxidation resistance and spark wear resistance, the wear of itself and the ground electrode does not easily occur, and in this sense, durability can be ensured. Moreover, 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 in a shape that crawls the insulator tip surface. For this reason, the fouling substance is burned out, and the fouling resistance is ensured. Furthermore, since the durability can be ensured even if the size of the ground electrode is not increased so much, the space where the flame spreads is not obstructed as much as the size does not need to be increased, and the heat of the flame is not easily deprived. Therefore, a decrease in ignitability can be suppressed.

  Further, in the configuration 1, since the protruding length of the noble metal tip from the tip of the ground electrode exceeds 0.2 mm, when considering the space near the ignition point, the tip of the ground electrode (main body part) compared to the case where there is no noble metal tip. The position will move away from the ignition point. That is, a noble metal tip having a smaller cross-sectional area exists in the vicinity of the ignition point. Therefore, a combustion space can be easily secured, and it can be expected that the flame spreads more smoothly, and the degree to which the heat of the flame is taken away by the ground electrode can be further reduced. As a result, it is possible to further improve the ignitability. In addition, when protrusion length is 0.2 mm or less, there exists a possibility that the effect about the ignition property improvement mentioned above may not fully be show | played.

  In addition, when considering the area where spark discharge can occur, the protruding end of the noble metal tip is opposed to the tip of the insulator compared to the total area of the surface facing the insulating tip of the ground electrode when there is no noble metal tip. The total area of the surface to be used is small. Therefore, the chance that the fuel with insufficient atomization is held between the insulator tip portion and the ground electrode (precious metal tip) can be reduced, and as a result, the fuel bridge can be suppressed.

  In addition, since the noble metal tip protrudes, the length of the ground electrode can be shortened by that amount. Therefore, the heat receiving area of the ground electrode is reduced, and the thermal load can be reduced. Furthermore, since the weight of the ground electrode (main body part) can be reduced, vibration can be suppressed and the electrode breakage resistance can be improved.

  However, it does not mean that the protrusion length should be longer than that of a dark cloud. That is, the following configuration 2 is more desirable.

  Configuration 2. The spark plug of this configuration is characterized in that, in the above configuration 1, the protruding length of the noble metal tip from the tip of the ground electrode is 5 mm or less.

  In general, the longer the protrusion length of the noble metal tip, the higher the material cost. In this respect, if the protruding length of the noble metal tip from the tip of the ground electrode exceeds 5 mm, the degree of the ignitability improvement effect is small with respect to the increase in material cost. Therefore, the configuration 2 is more desirable.

  Configuration 3. The spark plug of this configuration is characterized in that, in the above configuration 1 or 2, the noble metal tip has a thickness of 1.0 mm or less.

  Here, the “thickness” specifically refers to the most distal end side in the axial direction among the distal end surfaces of the noble metal tip in a plane including the axis and the axis of the noble metal tip in a direction in which the noble metal tip protrudes. Means the distance in the axial direction between the distal end edge located at the end of the noble metal tip and the proximal end edge located closest to the proximal end in the axial direction of the distal end portion of the noble metal tip. Therefore, for example, when the rectangular parallelepiped noble metal tip is arranged extending in the radial direction orthogonal to the axial direction, it indicates the thickness in the axial direction at an arbitrary position, but the noble metal tip is in the axial direction. In the case of protruding non-orthogonally (that is, inclined), the length in the axial direction of the protruding end of the noble metal tip is taken as the thickness.

  According to the configuration 3, since the thickness of the noble metal tip is 1.0 mm or less, the effect of improving the ignitability can be achieved more reliably. In particular, considering that the spark discharge position varies within the thickness range of the noble metal tip, the variation in the spark discharge position occurs only within the thickness range. As a result, by reducing the thickness to 1.0 mm or less, variations in spark discharge position can be suppressed, and stable combustion can be ensured. On the other hand, when the thickness of the noble metal tip exceeds 1.0 mm, the spark discharge position tends to vary, and stable combustion may be hindered and the ignitability may be affected.

  However, it does not mean that the thickness of the noble metal tip may be extremely reduced. That is, the following configuration 4 is more desirable.

  Configuration 4. The spark plug of this configuration is characterized in that, in the configuration 3, the thickness of the noble metal tip is 0.2 mm or more.

  In general, when the thickness of the noble metal tip is too thin, the volume occupied by the noble metal is reduced, so that the heat capacity is also reduced. For this reason, the noble metal tip is constantly in a high temperature state (overheating) in use, and the wear resistance is deteriorated. Therefore, as in the configuration 4, it is preferable that the thickness is 0.2 mm or more.

  Configuration 5. The spark plug of this configuration is characterized in that, in any one of the above configurations 1 to 4, the width of the noble metal tip is 50% or more of the axial hole diameter of the tip end face of the insulator.

  Here, in detail, the “width” is perpendicular to the radial direction of the front end surface of the noble metal tip when the ground electrode provided with the noble metal tip is viewed in the radial direction from the center electrode, and the The distance between both end edges in the direction orthogonal to the axial direction (the distance between the left and right end edges) is indicated.

  According to the above configuration 5, since the width of the noble metal tip is 50% or more of the axial hole diameter of the front end face of the insulator, a spark will fly over a wide range in the circumferential direction with respect to the center electrode, and the insulator around the center electrode The fouling substance on the tip surface can be burned out more reliably. As a result, the stain resistance can be further improved.

  In addition, it does not necessarily mean that the width of the noble metal tip is extremely large. That is, the following configuration 6 is more desirable.

  Configuration 6. The spark plug of this configuration is characterized in that, in the above configuration 5, the width of the noble metal tip is equal to or less than the outer diameter of the tip end surface of the insulator.

  Since the width of the noble metal tip depends on the clean range of the insulator front end surface, the maximum performance can be obtained if it has a width equal to the outer diameter of the insulator front end surface that can be soiled. For this reason, the width of the chip may be equal to or smaller than the outer diameter of the front end surface of the insulator, but even if the width is approximately the same as the diameter of the shaft hole in the front end surface of the insulator, Since the cleaning performance is ensured, it can be said that it is more preferable that the diameter is equal to or less than the diameter of the shaft hole in the tip surface of the insulator in consideration of the amount of precious metal used and the ignitability.

  Configuration 7. In the spark plug of this configuration, in any one of the above configurations 1 to 6, the distal end edge located closest to the distal end side in the axial direction among the distal end surfaces of the noble metal tip is closer to the distal end side than the distal end surface of the insulator. The base end side edge positioned closest to the base end side in the axial direction is positioned closer to the base end side than the front end surface of the insulator.

  According to the configuration 7, the distal end edge located closest to the distal end in the axial direction among the distal end surfaces of the noble metal tip is located closer to the distal end than the distal end surface of the insulator and located closest to the proximal end in the axial direction. The proximal end edge to be positioned is located on the proximal end side with respect to the distal end surface of the insulator. That is, the protruding side end surface of the noble metal tip and the tip end surface of the insulator are positioned at substantially the same height. For this reason, it is easy to ensure the spark discharge along the front end surface of the insulator. Therefore, it is possible to suppress the problem of channeling due to the exhaustion of the insulator while ensuring the expected antifouling property. On the other hand, when the tip end edge located closest to the tip end in the axial direction among the tip end surfaces of the noble metal tip is positioned closer to the base end side than the tip end surface of the insulator, a spark is likely to hit the insulator side surface. There is a possibility that the insulator is consumed and channeling occurs. Further, when the base end side edge located closest to the base end in the axial direction among the front end surfaces of the noble metal tip is positioned on the front end side with respect to the front end surface of the insulator, a spark along the front end surface of the insulator There is a possibility that the discharge is difficult to be performed and the stain resistance is lowered.

  In addition, in order to exhibit said each effect more reliably, it is desirable to set it as the following structure.

  Configuration 8. The spark plug of this configuration is characterized in that in any of the above configurations 1 to 7, a plurality of ground electrodes to which the noble metal tip is fixed are provided.

  Further, the following configuration can also be adopted.

  Configuration 9 The spark plug of this configuration is provided with a parallel ground electrode in one of the above configurations 1 to 8, wherein one side surface of the center electrode faces the tip surface of the center electrode and forms an air discharge gap along the axial direction. It is characterized by.

  These spark plugs are generally referred to as multipolar semi-creeping plugs or hybrid plugs, but the above-described noble metal tip configuration may be adopted when configuring these spark plugs.

  Furthermore, by adopting the following configuration, it is possible to solve a new problem that has not existed in the past.

  Configuration 10 The spark plug of this configuration is characterized in that, in any one of the above configurations 1 to 9, the screw diameter of the outer periphery of the metal shell is M10 or less.

  Conventionally, when the screw diameter on the outer periphery of the metal shell is relatively small, such as M10 or less, when attempting to construct a semi-surface discharge type spark plug, bending or the like is performed so that the tip of the ground electrode is aligned with the tip of the insulator. Was practically difficult due to space constraints. In this regard, by adopting the configuration in which the noble metal tip protrudes as described above, it is possible to easily realize semi-creeping discharge even with a small-diameter spark plug having a thread diameter of M10 or less on the outer periphery of the metal shell. Become.

  In addition, the insulator generally has a tip end side surface and a tip end surface connected to each other with a predetermined radius of curvature, and it is more desirable to configure the insulator as follows.

Configuration 11 In the spark plug of this configuration, in any one of the above configurations 1 to 10, the insulator has a tip end side surface and a tip end surface continuously connected with a radius of curvature R, and the relationship with the thickness t1 of the noble metal tip. In
R ≧ 0.2mm
t1 (mm) × R (mm) ≦ 1
It has the relationship of these.

  If the radius of curvature R is too large, the spark discharge position may vary. Similarly, even if the chip thickness t1 is large, the position of the spark discharge tends to vary. Therefore, it can be said that it is more preferable to set the curvature radius R and the thickness t1 of the chip in the above relationship in order to obtain the effect of the upper claim more effectively.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing the overall structure of the spark plug 100 of the present embodiment, and FIG. 2 is a partially broken front view showing the main part. In the following, description will be made mainly with reference to FIG. 2, 4, 9, 10, 11, and 12, the upper side of the paper is the tip direction of the axis, and the lower side of the paper is the rear end direction of the axis. In FIG. 3 and 8 are assumed to be viewed from the front end side in FIGS.

  As shown in FIGS. 2A and 2B, 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 center electrode 3 is conventionally known in the axial hole 6. It is fixed in the inserted state by the method. 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 toward the distal end portion of the center electrode 3 at a middle position in the longitudinal direction. The ground electrodes 4 </ b> A and 4 </ b> B are arranged so that the front end faces the outer peripheral surface of the front end of the insulator 2. These ground electrodes 4A and 4B are made of nickel alloys such as Inconel 600 and 601 (registered trademark).

  As shown in FIGS. 1 to 4, noble metal tips 11 </ b> A and 11 </ b> B protruding toward the tip of the center electrode 3 are fixed to the tip surfaces of the ground electrodes 4 </ b> A and 4 </ b> B, and the noble metal tips 11 </ b> A and 11 </ b> B are fixed. A spark discharge gap is formed between the front end surface of the central electrode 3 and the front end portion of the center electrode 3. When a predetermined voltage is applied to the spark plug 100, a spark discharge occurs in the spark discharge gap. The spark discharge path includes an air discharge path (see gaps GA and GB in the figure) formed between the tip surfaces of the noble metal chips 11A and 11B and the insulator 2, and an insulator 2 of the air discharge path. It consists of a creeping discharge path formed along the front end surface 2a of the insulator 2 between the center electrode 3 and the point that becomes the base point of the side path.

  Further, the noble metal tips 11A and 11B in the present embodiment are made of a noble metal alloy made of, for example, Pt-20Ni. However, the material configuration is merely an example, and is not limited at all. Each noble metal tip 11A, 11B is fixed to the tip of the ground electrodes 4A, 4B by welding. In this case, the noble metal tips 11A and 11B may be fixed by welding while being in contact with the front end surfaces of the ground electrodes 4A and 4B. From the viewpoint of achieving stronger fixation, for example, the ground electrodes It is desirable to form a recess in the front end surfaces of 4A and 4B, fit the chip into the recess, and weld and fix the boundary between the ground electrodes 4A and 4B and the chip. Examples of the welding include laser welding, electron beam welding, resistance welding and the like.

  The ground electrodes 4A and 4B in the present embodiment have a rectangular cross section, a width W of 2.8 mm, and a thickness T of 1.5 mm (see FIG. 4). On the other hand, the protrusion length E1 of the noble metal tips 11A and 11B from the front end surfaces of the ground electrodes 4A and 4B exceeds 0.2 mm and is 5 mm or less (in this embodiment, for example, 0.6 mm). Yes.

  Further, the noble metal tips 11A and 11B have a rectangular parallelepiped shape (thin plate shape), are arranged so as to extend in the radial direction toward the center electrode 3 and have a thickness (axis line). The length (direction) t1 is 0.2 mm or more and 1.0 mm or less (in the present embodiment, for example, 0.4 mm). Further, the width w1 of the noble metal tips 11A and 11B is 50% or more of the shaft hole diameter D of the tip surface 2a of the insulator 2 and not more than the outer diameter of the tip surface 2a of the insulator 2 (for example, 90% in this embodiment). (See FIG. 3).

  When the spark plug 100 is viewed from the front (see FIG. 2), the noble metal tips 11A and 11B are located on the tip side of the noble metal tips 11A and 11B on the tip side with respect to the virtual plane γ including the tip surface 2a of the insulator 2. The distal end side edge α located closest to the distal end in the axial direction of the surface is located, and the proximal end edge β located closest to the proximal end in the axial direction among the distal end surfaces of the noble metal tips 11A and 11B is located on the proximal end side. Is formed to be positioned. That is, the virtual plane γ including the distal end surface 2a of the insulator 2 is located between the proximal end edge α and the distal end edge β (see FIG. 2B).

  As described above, according to the present embodiment, the noble metal tips 11A and 11B protruding toward the tip of the center electrode 3 are fixed to the tip surfaces of the ground electrodes 4A and 4B. Spark discharge occurs between the center electrode 3 and the center electrode 3. Since the noble metal tips 11A and 11B are excellent in high-temperature oxidation resistance and spark wear resistance, the wear of itself and the ground electrodes 4A and 4B hardly occur. In this sense, durability is ensured. Further, since a creeping discharge path along the tip surface 2a of the insulator 2 is included in a part of the spark discharge path, a spark discharge is generated in a shape that crawls the tip surface 2a. For this reason, the fouling substances adhering to the surface of the insulator 2 due to scouring or fouling are burned out, and the fouling resistance is ensured. In particular, the width w1 of the noble metal tips 11A and 11B is 50% or more (for example, 90% in the present embodiment) of the shaft hole diameter D of the distal end surface 2a of the insulator 2, and thus is wider than the center electrode 3. A spark will fly, and the fouling substance on the front end surface 2a of the insulator 2 around the center electrode 3 can be burned out more reliably. As a result, the stain resistance can be further improved.

  Furthermore, since durability can be ensured even if the size of the ground electrodes 4A and 4B is not increased so much, the space where the flame spreads is hardly obstructed as much as it can be kept small, and the heat of the flame is not easily taken away. Therefore, a decrease in ignitability can be suppressed. In particular, in the present embodiment, the protruding length E1 of the noble metal tips 11A, 11B from the tip surfaces of the ground electrodes 4A, 4B exceeds 0.2 mm, and the thickness t1 is 1.0 mm or less. Thus, the combustion space is more easily secured, and as a result, the ignitability can be further improved.

  Further, in consideration of a region where spark discharge can occur, the projecting ends of the noble metal tips 11A and 11B having a small cross-sectional area as compared to the total area of the opposing surfaces of the ground electrode and the insulator tip when there is no noble metal tip. And the total area of the surfaces of the insulator 2 facing each other can be reduced. Therefore, it is difficult to cause a situation in which fuel with insufficient atomization is held between the tip of the insulator 2 and the noble metal tips 11A and 11B. As a result, the generation of fuel bridges can be suppressed.

  In addition, since the noble metal tips 11A and 11B are configured to protrude, the length of the ground electrodes 4A and 4B can be shortened accordingly. Therefore, the heat receiving area of the ground electrodes 4A and 4B is reduced, and the thermal load can be reduced. Furthermore, since the weight of the ground electrodes 4A and 4B can be reduced, vibration can be suppressed and electrode breakage resistance can be improved.

  In addition, a virtual plane γ including the distal end surface 2a of the insulator 2 is located between the proximal end side edge α and the distal end side edge β in the axial direction of the projecting side end surfaces of the noble metal tips 11A and 11B. . That is, the protruding side end surfaces of the noble metal tips 11A and 11B and the tip end surface 2a of the insulator 2 are located at substantially the same height. As described above, since the thickness of the noble metal tips 11A and 11B is as thin as 1.0 mm or less, the base point on the insulator 2 side of the air discharge generated on the tip surface of the noble metal tips 11A and 11B is the tip surface 2a of the insulator 2. It becomes a neighborhood. In order to improve the fouling resistance (cleanliness) due to creeping discharge, it is desirable to set the base point of air discharge, that is, the base point of creeping discharge, on the proximal side from the tip surface 2a. There is also a demerit that penetration of the insulator 2 is likely to occur. However, by adopting the configuration of the present embodiment, channeling can be suppressed or reduced while improving stain resistance.

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

  First of all, it has the same general shape as the spark plug 100 of FIGS. 1 and 2, and the protruding length of the noble metal tip is 0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, and 1.0 mm. Samples (spark plugs) were prepared, the number of misfires was measured for each sample, and the ratio between the number of misfires and the number of misfires of the conventional product (no tip, ie, protrusion length 0 mm) was measured. Except for the protruding length, those having a width of 1.5 mm and a thickness of 0.8 mm were employed. When measuring the number of misfires, each sample (spark plug) to be evaluated is attached to a DOHC in-line 6-cylinder engine with a displacement of 2000 cc to obtain a predetermined air-fuel ratio on the lean side, and the engine at 2000 rpm. The number of misfires within a predetermined time when rotating is measured. FIG. 5 shows the result of calculating the ratio of the number of misfires between the sample with the tip protrusion and the conventional product.

  As shown in FIG. 5, when the tip protrusion length was 0.2 mm, the number of misfires was the same as before, but when it exceeded 0.2 mm, it was confirmed that the number of misfires tends to be smaller than before. It was. From this, it can be said that when the protruding length of the noble metal tip exceeds 0.2 mm, the combustion space is more easily secured and the ignitability can be improved.

  Next, samples (spark plugs) having various thicknesses of noble metal tips of 0.5 mm, 0.8 mm, 1.0 mm, 1.2 mm, and 1.5 mm are prepared, and a stable combustion region is prepared for each sample. Was measured, and the ratio of this area to the stable combustion area of the conventional product (without tip) was measured. Except for the thickness, the width was 1.5 mm, and the protruding length was 0.4 mm. For measurement of the stable combustion region, each sample (spark plug) to be evaluated is attached to a DOHC inline 4-cylinder, direct-injection engine with a displacement of 1800 cc, the engine is rotated at 3000 rpm, ignition timing and fuel The area where no misfire occurred when the injection timing was variously changed was determined as the area. Here, the area where no misfire occurred was calculated as follows. First, the ignition timing is plotted on the horizontal axis, the air-fuel ratio is plotted on the vertical axis, the spark timing is changed by variously changing the ignition timing and the air-fuel ratio, and normal ignition is plotted. By repeating this, the boundary between the case of ignition and the case of misfire can be obtained. What is necessary is just to measure the area enclosed by this calculated | required boundary. FIG. 6 shows the result of calculating the ratio of the area of the stable combustion region between the sample with tip protrusion and the conventional product.

  As shown in FIG. 6, it was confirmed that when the tip thickness was 1.0 mm or less, a stable combustion area wider than the conventional one could be obtained. Thereby, it can be said that the combustion space is more easily secured and the ignitability can be improved by setting the thickness of the chip to 1.0 mm or less. In addition, it can be said that the fouling resistance can be improved from the viewpoint that combustion is ensured even under a condition in which sag is likely to occur.

  Subsequently, samples (spark plugs) were prepared in which the width of the noble metal tip was varied to 30%, 50%, 80%, 100%, 150%, 200% with respect to the axial hole diameter of the insulator end face. Then, the stable combustion region was measured for each sample, and the ratio between the region and the stable combustion region of the conventional product (no chip) was measured. The width other than the width was 0.8 mm, and the protruding length was 0.4 mm. For measurement of the stable combustion region, each sample (spark plug) to be evaluated is attached to a DOHC inline 4-cylinder, direct-injection engine with a displacement of 1800 cc, the engine is rotated at 3000 rpm, ignition timing and fuel The area where no misfire occurred when the injection timing was variously changed was determined as the area. The calculation method of this region is the same as the calculation method of the stable combustion region of the tip thickness described above. FIG. 7 shows the result of calculating the ratio of the area of the stable combustion region between the sample with the tip protrusion and the conventional product. However, the horizontal axis of the graph of FIG. 7 represents the ratio of the width of the chip to the diameter of the axial hole on the tip surface of the insulator.

  As shown in FIG. 7, when the ratio of the tip width to the axial hole diameter of the tip of the insulator was 50%, the stable combustion area was the same as the conventional one, but the ratio exceeding 50% It has been confirmed that a stable combustion region can be obtained wider than before. Thereby, it can be said that the contamination resistance can be further improved by setting the ratio of the chip width to the axial hole diameter of the tip surface of the insulator to be 50% or more, particularly more than 50%.

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

  (A) In the above-described embodiment, the noble metal tips 11A and 11B whose projecting side end surfaces are flat surfaces are employed. However, as shown in FIG. Center electrode 3) Noble metal tips 31A and 31B having concave curved surfaces along the outer peripheral surface may be employed. In this case, it is desirable to adopt an average value of the protruding length of the side portion in the width direction and the protruding length of the central portion as the protruding length of the noble metal tips 31A, 31B from the tips of the ground electrodes 4A, 4B. Need to exceed 2 mm.

  In the above embodiment, the ground electrodes 4A and 4B whose cross-sectional shape does not change in the longitudinal direction are employed. However, as shown in FIG. 8B, the width on the protruding end side tapers in a tapered shape. The grounding electrodes 41A and 41B may be used. By comprising in this way, a combustion space is easy to be ensured more and it can aim at the further improvement of ignitability.

  Furthermore, in the above embodiment, the width W of the ground electrodes 4A and 4B is larger than the width w1 of the noble metal tips 11A and 11B. However, as shown in FIG. When 42B is adopted, there is no problem even if the width is the same as the width of the noble metal tips 11A and 11B. However, even in this case, the thickness of the noble metal tips 11A and 11B needs to be smaller than the thickness of the ground electrodes 42A and 42B.

  (B) In the above embodiment, the case where the noble metal tips 11A and 11B protrude from the center in the axial direction of the ground electrodes 4A and 4B is embodied, but for example, as shown in FIG. The noble metal tips 11A and 11B may protrude from the axial base end portion of 4B, and as shown in FIG. 9B, the noble metal tips 11A and 11B protrude from the axial end portions of the ground electrodes 4A and 4B. May be.

  (C) In the above embodiment, the case where the ground electrodes 4A and 4B are bent toward the distal end portion of the center electrode 3 at the intermediate position in the longitudinal direction is embodied, but as shown in FIG. 9C, for example. The straight electrodes 43A and 43B (which do not have a bent portion) extending in the axial direction can also be employed. In this case, the noble metal tips 11A and 11B protrude from the tip side surfaces of the ground electrodes 43A and 43B.

  (D) The shape of the insulator 2 is not necessarily limited to that of the above embodiment. Therefore, for example, as shown in FIG. 9D, an insulator 22 having a tapered surface 21 and having a tapered shape toward the distal end may be employed.

  (E) Further, FIG. 8B illustrates the ground electrodes 41A and 41B having a tapered width on the protruding end side. However, as shown in FIG. 10A, the ground electrodes 41A and 41B are tapered in the thickness direction. The ground electrodes 44A and 44B (thinned toward the protruding end side) may be employed. Even when configured in this way, a combustion space is more easily secured, and further improvement in ignitability can be achieved.

  (F) Moreover, in the said embodiment, although the noble metal chip | tip 11A, 11B which protrudes in parallel with the front end surface 2a of the insulator 2 is employ | adopted, it does not necessarily need to be parallel. For example, as shown in FIG. 10B, it is possible to employ ground electrodes 45A and 45B having a large extension amount toward the tip end, and noble metal tips 32A and 32B projecting obliquely from the tip. In this case as well, a combustion space is more easily secured, and the ignitability can be further improved. Further, as shown in FIG. 10C, ground electrodes 46A and 46B that are inclined obliquely may be employed, and noble metal tips 33A and 33B that may obliquely project from the tips thereof may be employed. Furthermore, as shown in FIG. 10 (d), a noble metal portion 3 b may be provided above the center electrode 3.

  The configuration in which the ground electrode as shown in FIG. 10C is shaped like a straight rod without bending is particularly advantageous when manufacturing a spark plug having a small diameter of M10 or less. As described above, as the spark plug becomes smaller in diameter, it becomes difficult to bend the ground electrode and direct its tip toward the center electrode. In this regard, if the configuration shown in FIG. 10C is employed, the difficulty in the manufacturing process can be solved.

  (G) In the above embodiment, the noble metal tips 11A and 11B having a rectangular parallelepiped shape are illustrated, but for example, as shown in FIG. 11A, noble metal tips 34A and 34B that are thinner toward the protruding end side are adopted. May be.

  (H) In the above embodiment, the tip surface 3 a of the center electrode 3 is substantially flush with the tip surface 2 a of the insulator 2. On the other hand, as shown in FIG. 11 (b), a center electrode 51 whose tip surface protrudes from the tip surface 2 a of the insulator 2 may be used. Conversely, the distal end surface of the center electrode may be located closer to the proximal end than the distal end surface of the insulator.

  (I) 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. In addition to the ground electrodes 4A and 4B capable of performing creeping discharge, as shown in FIG. 12, one side surface of the center electrode 52 faces the tip surface of the center electrode 52, and an air discharge gap along the axial direction is formed. A parallel ground electrode 47 to be formed may be provided. In this case, a spark discharge gap 48 is provided between the tip side surface of the parallel ground electrode 47 and the tip surface of the center electrode 52.

  (J) As described in the above section (f), the metal shell 1 having a thread diameter of the thread portion 7 of M10 or less can be used positively.

  Conventionally, when trying to construct a semi-surface discharge type spark plug, as the screw diameter on the outer periphery of the metal shell becomes smaller than M10, bending or the like is performed so that the tip of the ground electrode follows the tip of the insulator. Was practically difficult due to space constraints. On the other hand, by adopting the configuration in which the noble metal tips 11A and 11B are projected as in the present embodiment, even if the screw diameter of the outer periphery of the metal shell 1 is a relatively small diameter of M10 or less, semi-surface discharge can be easily performed. It can be realized.

  (K) In the above embodiment, the ground electrodes 4A and 4B are made of a nickel alloy such as Inconel 600 or 601 (registered trademark). On the other hand, you may employ | adopt the ground electrode which consists of a two-layer structure of an inner layer and an outer layer. In this case, the outer layer is composed of the above nickel alloy, and the inner layer is a metal having better thermal conductivity than the nickel alloy (for example, a metal material mainly composed of copper or high-purity nickel having higher thermal conductivity than the nickel alloy). It is illustrated that it is configured by. The main body portion of the center electrode 3 may also have a two-layer structure of an outer layer and an inner layer.

(L) Although not specifically mentioned in the above embodiment, the insulator 2 has its tip end side surface and the tip end surface 2a connected to each other with a predetermined radius of curvature (the end portion of the insulator 2 is a curved surface). In the relationship between the curvature radius R and the thickness t1 of the noble metal tips 11A and 11B,
R ≧ 0.2mm
t1 (mm) × R (mm) ≦ 1
It is desirable to have this relationship. If the radius of curvature R is too large, the spark discharge position may vary, and even if the tip thickness t1 is large, the spark discharge position tends to vary. Therefore, it can be said that it is more preferable to set the curvature radius R and the thickness t1 of the chip in the above relationship in order to obtain the effect of the embodiment more effectively.

It is a partially broken front view which shows the whole structure of the spark plug of this embodiment. (A) is a partially broken front view which shows the structure of the principal part of the spark plug of this embodiment, (b) is further the enlarged view of the principal part. It is a top view which shows the state which looked at the spark plug from the front end side. It is a perspective view which shows the front-end | tip part of a ground electrode. It is a graph which shows the relationship of the misfire number ratio with respect to the protrusion length of a noble metal tip. It is a graph which shows the relationship of the ratio of the width of the stable combustion area | region with respect to the thickness of a noble metal tip. It is a graph which shows the relationship of the ratio of the width of the stable combustion area | region with respect to the width | variety of a noble metal tip. (A)-(c) is a plane schematic diagram of the center electrode and ground electrode in another embodiment. (A)-(d) is a partially broken front view of the center electrode and ground electrode in another embodiment. (A)-(d) is a partially broken front view of the center electrode and ground electrode in another embodiment. (A), (b) is the partially broken front view of the center electrode and ground electrode in another embodiment. It is a partially broken front view of the center electrode and ground electrode in another embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Metal fitting, 2 ... Insulator, 3, 51, 52 ... Center electrode, 4A, 4B, 41A, 41B, 42A, 42B, 43A, 43B, 44A, 44B, 45A, 45B, 46A, 46B ... Ground electrode, 11A, 11B, 31A, 31B, 32A, 32B, 33A, 33B, 34A, 34B ... precious metal tip, 6 ... shaft hole, 7 ... threaded portion.

Claims (11)

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 spark plug for an internal combustion engine comprising a ground electrode having a base end fixed to a front end surface of the metal shell,
A tip of the ground electrode is fixed with a noble metal tip protruding in a radial direction toward the center electrode.
A spark discharge path formed by applying a voltage between the tip of the noble metal tip and the tip of the center electrode includes a part of the tip of the noble metal tip and a part of the tip of the insulator. And a creeping discharge path along the tip surface of the insulator with a part of the tip portion of the insulator and a part of the center electrode as a base point.
A spark plug for an internal combustion engine, wherein a protruding length of the noble metal tip from the tip of the ground electrode exceeds 0.2 mm.   2. The spark plug for an internal combustion engine according to claim 1, wherein a protruding length of the noble metal tip from the tip of the ground electrode is 5 mm or less.   The spark plug for an internal combustion engine according to claim 1 or 2, wherein the noble metal tip has a thickness of 1.0 mm or less.   The spark plug for an internal combustion engine according to claim 3, wherein the thickness of the noble metal tip is 0.2 mm or more.   The spark plug for an internal combustion engine according to any one of claims 1 to 4, wherein a width of the noble metal tip is 50% or more of a shaft hole diameter of a tip surface of the insulator.   6. The spark plug for an internal combustion engine according to claim 5, wherein a width of the noble metal tip is equal to or smaller than an outer diameter of a front end surface of the insulator. Of the tip surface of the noble metal tip,
The distal end edge located closest to the distal end in the axial direction is located closer to the distal end than the distal end face of the insulator, and the proximal end edge located closest to the proximal end in the axial direction is closer to the distal end face of the insulator. The spark plug for an internal combustion engine according to any one of claims 1 to 6, wherein the spark plug is also located on a proximal end side.   The spark plug for an internal combustion engine according to any one of claims 1 to 7, wherein a plurality of ground electrodes to which the noble metal tip is fixed are provided.   The internal combustion engine according to any one of claims 1 to 8, further comprising a parallel ground electrode that faces one end surface of the center electrode and that forms an air discharge gap along the axial direction. For spark plug.   The spark plug for an internal combustion engine according to any one of claims 1 to 9, wherein a screw diameter of the outer periphery of the metal shell is M10 or less. The insulator has a tip side surface and a tip surface of which are connected with a radius of curvature R, and in relation to the thickness t1 of the noble metal tip,
R ≧ 0.2mm
t1 (mm) × R (mm) ≦ 1
The spark plug for an internal combustion engine according to any one of claims 1 to 10, wherein

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