JP2007184299A - Spark plug - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spark plug having a structure suitable for miniaturization without damaging pollution resistant property. <P>SOLUTION: This spark plug in which the diameter of a front end portion 2i of an insulator 2 is reduced due to a circumferentially extending stepped portion thereof to form the stepped portion into an insulator-side engagement portion 2h, and the insulator is inserted into a main fitting 1 from a rear opening thereof. The insulator-side engagement portion 2h engages with a fitting-side engagement portion 1c projecting from an inner circumferential surface of the main fitting 1, and an outer circumferential surface (clearance-forming outer circumferential surface) 2k of the portion 2i positioned ahead of the insulator-side engagement portion 2h of the insulator 2 is formed into a nearly coaxial cylindrical surface-like shape and opposed to an inner circumferential surface (clearance-forming inner circumferential surface) 52 of the fitting-side engagement portion 1c so as to form a predetermined clearance Q in an engagement position. An amount β of clearance in the engagement position expressed by the equation β=(D1-d1)/2 where d1 represents an outer diameter of the clearance-forming outer circumferential surface 2k; and D1 represents an inner diameter of the clearance-forming inner circumferential surface 52, is adjusted not greater than 0.4 mm. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a spark plug.

  In recent years, spark plugs used to ignite internal combustion engines such as gasoline engines for automobiles have become more compact as the engine head structure has become more complex and the space for installing the spark plugs has decreased as the performance of the engine has increased. The demand for conversion is becoming quarreled. Miniaturization of the spark plug means that the diameter of the metal shell on which the mounting portion to the engine head is formed, but the insulator inserted inside it must be reduced in size to ensure withstand voltage. I can't.

  Here, the insulator of the spark plug is assembled in such a manner that the tip side is reduced in diameter by the step portion and the step portion is engaged with the protruding portion formed on the inner peripheral surface of the metal shell. Therefore, in such a structure, when reducing the diameter of the metal shell, there is a limit to the reduction in the outer diameter of the insulator, so the inner peripheral surface of the protruding portion on the metal shell side and the insulator outer surface facing this A method of reducing the gap with the peripheral surface is adopted.

  However, when the gap is reduced, there is a problem that the antifouling property of the spark plug is deteriorated. That is, when the spark plug is used in a low temperature environment where the electrode temperature is 450 ° C. or less, a large amount of unburned gas is generated. For example, if such unburned gas generation continues for a long time, such as during pre-delivery, the insulator becomes a so-called “blow” or “fogging” state, and the surface is contaminated with a conductive material such as carbon, resulting in malfunction. It tends to occur. In particular, when the surface of the insulator is soiled in the gap due to the intrusion of unburned gas, a spark discharge occurs in the gap and normal ignition is impossible.

  An object of the present invention is to provide a spark plug having a structure suitable for downsizing without impairing stain resistance.

Means for Solving the Problems and Effects of the Invention

A center electrode, an insulator provided outside the center electrode, a cylindrical metal shell provided outside the insulator, one end side coupled to the metal shell, and the other end facing the tip of the center electrode A ground electrode that is arranged to form a spark discharge gap with the center electrode,
With the spark discharge gap side in the axial direction of the insulator as the front side and the opposite side as the rear side, the insulator has a front end that is reduced in diameter by a step in the circumferential direction. And is inserted into the metal shell from the rear opening, and the insulator-side engagement portion engages with the metal-side engagement portion protruding from the inner peripheral surface of the metal shell, and the insulator is insulated. The outer peripheral surface (gap forming outer peripheral surface) of the portion located on the front side of the body side engaging portion is a substantially coaxial cylindrical surface, and the inner peripheral surface (gap forming inner peripheral surface) of the metal fitting side engaging portion And an outer diameter of the gap forming outer peripheral surface as d1, and an inner diameter of the gap forming inner peripheral surface as D1,
β = (D1-d1) / 2 (1)
The engagement position gap amount β represented by is adjusted to 0.4 mm or less.

  When the diameter difference D1-d1 between the outer diameter d1 of the gap forming outer peripheral surface and the inner diameter D1 of the gap forming inner peripheral surface varies depending on the position in the axial direction, the engagement position gap amount β is the smallest in the diameter difference. It shall be represented by the value at a certain position. Moreover, although a metal fitting side engaging part can be made into a cyclic | annular protruding item | line part, for example, if the function as an engaging part can be fulfill | performed, a form will not be limited to this.

  As described above, if it is attempted to reduce the outer diameter of the metal shell without impairing the withstand voltage characteristics of the spark plug, the thickness of the insulator cannot be reduced so much. I must. Conventionally, however, it has been established as one technical common sense that the value of β is set as large as possible in order to avoid flying in this gap as much as possible when fouling. Therefore, reducing the engagement position gap amount β in response to a request for downsizing of the spark plug was considered to be a dilemma from the viewpoint of preventing flying at the time of fouling.

  However, as a result of intensive studies by the present inventors, if the engagement position gap amount β is not set to a large value halfway, but rather is actively reduced beyond a certain boundary, the antifouling property is unexpectedly increased. It has been found that the problem can be remarkably improved and that the problem of flying in the engagement position gap at the time of fouling can be effectively prevented, and the present invention has been completed. Specifically, by adjusting the engagement position gap amount β to 0.4 mm or less, it is possible to reliably block the intrusion of unburned gas into the engagement position gap. It is possible to prevent the insulator surface from being soiled. As a result, it is possible to effectively reduce the size of the spark plug without impairing the stain resistance.

  When the engagement position gap amount β exceeds 0.4 mm, it becomes difficult to prevent the intrusion of unburned gas, and it becomes impossible to prevent the insulator surface from being contaminated in the engagement position gap. Note that when the engagement position gap amount β is extremely small, the entry of the pollutant into the engagement position gap does not occur, but when the fouling substance adheres to the insulator surface extending forward from the engagement position gap. The deposited layer of the fouling substance comes into contact with the metal shell side engaging portion located on the opposite side with the engagement position gap interposed therebetween, so that a short circuit is likely to occur, and the ignitability may be impaired instead. Considering this point, it is desirable to secure the engagement position gap amount β, for example, 0.05 mm or more, and more desirably 0.2 mm or more.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 and 2 show a spark plug 100 as an embodiment of the present invention. FIG. 1 is an overall vertical cross-sectional view, and FIG. 2 is an enlarged view of a main portion on the tip side. The spark plug 100 includes a cylindrical metal shell 1, an insulator 2 fitted inside the metal shell 1 so that the tip 21 protrudes, and an insulator 2 inside the insulator 2 with the tip 3 e protruding. One end of the center electrode 3 and the metal shell 1 are connected to each other by welding or the like, and the other end side is bent back to the side so that the side surface faces the tip of the center electrode 3. The electrode 4 etc. are provided. As shown in FIG. 2, a spark discharge gap g having an interval α is formed between the ground electrode 4 and the center electrode 3. The main body 3a of the ground electrode 4 and the center electrode 3 is made of Ni alloy or the like. A core material 3b made of Cu or Cu alloy is embedded in the main body 3a of the center electrode 3 to promote heat dissipation.

  The metal shell 1 is formed in a cylindrical shape by a metal such as low carbon steel, and constitutes a housing of the spark plug 100. On the outer peripheral surface thereof, a mounting screw for attaching the spark plug 100 to an engine block (not shown) Part 7 is formed. In addition, 1e is a tool engaging part which engages tools, such as a spanner and a wrench, when attaching the metal shell 1, and has a hexagonal axial cross-sectional shape. The insulator 2 is entirely configured as an alumina ceramic sintered body, and has a through hole 6 formed along the axial direction O. A terminal fitting 13 is fixed to one end side of the insulator 2, and the other The center electrode 3 is fixed to the end portion side. A resistor 15 is disposed between the terminal fitting 13 and the center electrode 3 in the through hole 6. Both ends of the resistor 15 are electrically connected to the center electrode 3 and the terminal fitting 13 through the conductive glass seal layers 16 and 17, respectively. The resistor 15 and the conductive glass seal layers 16 and 17 constitute a sintered conductive material portion. In addition, the resistor 15 is comprised by the resistor composition which uses the mixed powder of glass powder and electrically-conductive material powder (and ceramic powder other than glass as needed) as a raw material.

  In the middle of the insulator 2 in the axial direction, a protruding portion 2e protruding outward in the circumferential direction is formed in a flange shape, for example. In the insulator 2, in the axial direction O, the side toward the tip 3 e (that is, the spark discharge gap g) of the center electrode 3 is defined as the front side, and the rear side of the protrusion 2 e is formed with a smaller diameter. The main body 2b is made. On the other hand, on the front side of the protruding portion 2e, a first shaft portion 2g having a smaller diameter and a second shaft portion 2i having a smaller diameter than the first shaft portion 2g are formed in this order. In addition, you may form a corrugation part in the outer peripheral surface rear end part of the main-body part 2b.

  On the other hand, the axial sectional diameter of the center electrode 3 is set smaller than the axial sectional diameter of the resistor 15. The through-hole 6 of the insulator 2 has a substantially cylindrical first portion 6a through which the center electrode 3 is inserted, and a substantially cylindrical shape having a larger diameter on the rear side (upper side in the drawing) of the first portion 6a. Second portion 6b. The terminal fitting 13 and the resistor 15 are accommodated in the second portion 6b, and the center electrode 3 is inserted into the first portion 6a. At the rear end portion of the center electrode 3, an electrode fixing convex portion 3c is formed so as to protrude outward from the outer peripheral surface thereof. The first portion 6a and the second portion 6b of the through-hole 6 are connected to each other within the first shaft portion 2g in FIG. 2, and the electrode fixing convex portion 3c of the center electrode 3 is located at the connection position. The convex receiving surface 6c for receiving the tape is formed in a tapered surface or a rounded surface.

  The insulator 2 is inserted into the metal shell 1 from the rear opening, and the connecting portion between the first shaft portion 2g and the second shaft portion 2i is a circumferential step portion, and the step portion is on the insulator side. As the engaging portion 2 h, the ring-shaped plate packing 63 is engaged with a circumferential annular ridge portion 1 c as a fitting-side engaging portion formed on the inner surface of the metal shell 1, so that the axial direction Has been secured. On the other hand, a ring-shaped wire packing 62 that engages with the rear peripheral edge of the flange-shaped protrusion 2e is disposed between the inner surface of the rear opening of the metal shell 1 and the outer surface of the insulator 2, and further On the rear side, a ring-shaped wire packing 60 is disposed via a filling layer 61 such as talc. Then, the insulator 2 is pushed forward toward the metal shell 1, and in this state, the crimping portion 1d is formed by crimping the opening edge of the metal shell 1 toward the packing 60 inward. It is fixed with respect to the insulator 2.

As shown in FIG. 2, the portion of the insulator located on the front side of the insulator-side engaging portion 2h, here, the base end portion outer peripheral surface (gap-forming outer peripheral surface) 2k of the second shaft portion 2i is the metal fitting side. It is opposed to the inner peripheral surface (gap forming inner peripheral surface) 52 of the ridge portion 1c, which is an engaging portion, so as to form a predetermined amount of engaging position gap Q. The outer diameter of the gap forming outer peripheral surface 2k is d1, and the inner diameter of the gap forming inner peripheral surface 52 is D1,
β = (D1-d1) / 2 (formula (1))
Is adjusted to 0.4 mm or less (preferably 0.05 mm or more).

  By adjusting the engagement position gap amount β to 0.4 mm or less, it is possible to reliably block the intrusion of unburned gas into the engagement position gap Q even in a use environment where contamination easily occurs, for example, during pre-delivery. It is possible to prevent the surface of the insulator 2 (gap-forming outer peripheral surface 2k) from being contaminated in the engagement position gap Q. As a result, it is possible to effectively reduce the size of the spark plug 100 without impairing the fouling resistance. For example, even if the nominal size of the mounting screw portion 7 formed on the outer peripheral surface of the front end side of the metal shell 1 is reduced to M12 or less, the stain resistance can be satisfactorily maintained. Specifically, the mounting screw portion 7 can specifically adopt a value such as M12 or M10 (in this specification, the designation of the mounting screw portion is a value defined in ISO 2705 (M12) and ISO 2704 (M10)). Naturally, variation within the dimensional tolerances defined in the standard is allowed). According to the present invention, the engagement position gap Q is set to 0.4 mm or less, which is smaller than the conventional spark plug. Therefore, even when the size of the mounting screw portion 7 is reduced, the metal shell side engagement portion It is not necessary to make the thickness of the insulator 2 at the engagement position so small. Therefore, the anti-fouling property is improved by reducing the engagement position gap Q, and the withstand voltage characteristic of the insulator 2 can be maintained well.

  In the present embodiment, the outer peripheral surface of the first shaft portion 2g is substantially cylindrical, while the outer peripheral surface of the base end portion of the second shaft portion 2i forming the gap forming outer peripheral surface 2k is in the axial direction O. The engagement position gap Q is a substantially cylindrical surface that is substantially coaxial with the gap forming inner circumferential surface 52 so that the engagement position gap Q is substantially constant (and the minimum value). Further, the outer peripheral surface of the second shaft portion 2i on the front end side with respect to that is a conical surface with a diameter decreasing toward the front end.

When attempting to reduce the nominal size of the mounting screw portion 7 as described above, the front end portion of the insulator extending forward from the engagement position gap Q together with the engagement position gap amount β, that is, the second shaft portion 2i. It should be noted that the gap formed between the outer peripheral surface and the inner peripheral surface of the metal shell 1, that is, the width J of the gas volume portion GV is likely to be small. If the width J becomes too small, even if the inside of the engagement position gap Q is clean, if the second shaft portion 2i is contaminated on the front side, the inner peripheral surface of the metal shell 1 and the second shaft The problem of so-called side-fire, which causes a fire with the outer peripheral surface of the portion 2i, is likely to occur. Therefore, in order to prevent this, the inner diameter of the front end surface side opening of the metal shell 1 is D2, and the outer diameter of the insulator 2 (second shaft portion 2i) at the front end surface position is d2.
E = (D2-d2) / 2 (2)
The end face width E of the gas volume represented by
α <1.1E (3)
It is effective to adjust so as to satisfy the above.

  The insulator 2 is likely to concentrate an electric field in the vicinity of the tip near the spark discharge gap g, and an edge that tends to concentrate an electric field is formed on the inner peripheral edge of the metal shell 1. The problem tends to occur at the position of the front end surface of the metal shell 1. However, by setting the width of the gas volume GV at this position, that is, the end face width E of the gas volume, to be larger than the interval α of the spark discharge gap g which is the normal spark position, the insulator 2 (second Even when the surface of the shaft portion 2i is soiled, the problem of side fire can be effectively suppressed. In the present specification, as shown in the equation (2), the end face width E of the gas volume portion is defined using the difference in diameter between the metal shell 1 and the insulator 2, but for example, the insulator 2 is mainly used. If a slight eccentricity occurs when the metal fitting 1 is assembled, the actual distance between the inner peripheral surface of the metal shell 1 and the outer peripheral surface of the insulator 2 (second shaft portion 2i) is locally reduced. It is also possible that the occurrence of a side fire is a problem. Therefore, in order to absorb the influence, the value of E is set to a value that allows a slight margin as shown in equation (3). However, if it is possible to reliably prevent eccentricity during assembly, α <E may be satisfied.

Further, the side fire due to the contamination of the tip of the insulator 2 (second shaft portion 2i) does not always occur at the end face position of the metal shell 1, and depending on the width of the gas volume portion GV, the metal shell is slightly deepened. It can also occur at different locations. Therefore, in order to prevent such a side fire, a cross section of the insulator 2 by a virtual plane orthogonal to the axis O is provided in front of the insulator side engaging portion 2h and in front of the insulator side engaging portion 2h. When the outer diameter is d3 and the inner diameter of the metal shell 1 at the corresponding position is D3, at an arbitrary position in the section from the front end surface position of the metal shell 1 to at least 7 mm,
α <(D3-d3) / 2 (4)
In other words, it is effective to satisfy α <(D3-d3) / 2 in the section L in which 7 mm or more is secured from the position of the front end face of the metal shell 1.

  If the width J (≡ (D3−d3) / 2) of the gas volume GV at a position in the axial direction O is larger than the interval α of the spark discharge gap g, the side-fire at that position is essentially It becomes difficult to occur. On the other hand, the electric field strength on the surface of the insulator that affects the occurrence of side-fire is high near the tip near the spark discharge gap g, but gradually decreases as it is separated rearward in the axial direction O. However, according to a study by the present inventors using electric field strength distribution simulation by the finite element method, the electric field strength on the insulator surface is somewhat high in the section from the front end face position of the metal shell to about 7 mm in the axial direction. It was expected that this would occur, and there was concern about the occurrence of side fire. Therefore, at least in this section, if the gas volume portion width J is adjusted to be larger than the interval α of the spark discharge gap g, which is a normal discharge location, a side fire at the position where the metal shell 1 is recessed is caused. Actually, it can be effectively suppressed.

Next, a cross section by a virtual plane including the axis O of the insulator 2 (here, also coincides with the axis of the metal shell 1) on the gap forming inner peripheral surface 52 in the protruding strip portion 1 c which is the metal fitting side engaging portion. The outline has a flat portion 52a facing the gap forming outer peripheral surface 2k, and an inclined portion 52b descending from the front end of the flat portion 52a toward the inner peripheral surface of the metal shell 1. And the angle θ formed by the flat portion 52a and the inclined portion 52b is
140 ° ≦ θ ≦ 160 ° (5)
Is satisfied. An edge portion is formed at the intersection between the flat portion 52a and the inclined portion 52b. If the angle θ between them is set slightly larger as shown in (5), the edge portion is formed. Excessive electric field concentration can be avoided, and the withstand voltage performance can be further improved. However, when θ is less than 140 °, the effect is small, and when θ exceeds 160 °, the inclined portion 52b has a long and long skirt toward the inner peripheral surface of the metal shell 1, and the electric field strength in the gas volume portion GV. In some cases, the high-voltage region extends to the tip of the insulator 2 (second shaft portion 2i) having a small thickness, and the withstand voltage performance may be impaired. In addition, since the section where the width J of the gas volume portion GV becomes smaller becomes longer, it may be disadvantageous in terms of preventing side jumping. In the present embodiment, the flat portion 52a forms a cylindrical surface coaxial with the outer peripheral surface 2k of the base end portion of the second shaft portion 2i, while the inclined portion 52b is formed in a conical shape.

  Hereinafter, various modifications that can be added to the spark plug 100 will be described (in addition, the same reference numerals are given to the same parts as those in FIGS. 1 and 2 and detailed description thereof will be omitted). First, in FIG. 3, the cylindrical base of the second shaft portion 2i is set so that the length of the section L where the width J of the gas volume portion GV is larger than the interval α of the spark discharge gap g can be made as large as possible. The tip body portion 2s is connected to the end portion 2r via the reduced diameter portion 2j. In the present embodiment, the diameter-reduced portion 2j is conical (tapered) in order to make it difficult to produce sharp edges that easily concentrate electric fields.

  In the embodiment of FIG. 4 (a), the gap forming inner peripheral surface 52 of the convex portion 1c serving as the metal fitting side engaging portion has a cross-sectional outline line with a virtual plane including the axis O, and the gap forming outer peripheral surface 2k. It has the flat part 52a which opposes, and the inclination part 52b which goes down toward the inner peripheral surface of the metal shell 1 from the front side edge part of this flat part 52a. And the chamfering part 52c is formed in the crossing position of these flat parts 52a and the inclination part 52b (FIG.4 (b) shows an enlarged view). With this configuration, it is difficult for electric field concentration to occur at the intersection between the flat portion 52a and the inclined portion 52b, and the same effect as increasing the angle θ between the flat portion 52a and the inclined portion 52b can be achieved. Also in the embodiment of FIG. 4, the second shaft portion 2i of the insulator 1 has a configuration in which the distal end main body portion 2s is connected to the cylindrical base end portion 2r via the reduced diameter portion 2j, as in FIG. Have. In FIG. 3, the outer peripheral surface of the tip main body portion 2 s has a conical shape, but in FIG. 4, the tip of the gas volume GV is widened as much as possible to the deeper position of the metal shell 1. The outer peripheral surface of the main body portion 2s is a cylindrical surface. In addition, as shown in FIG.4 (c), it may replace with the chamfering part 52c and may provide the round part 52r.

  Further, in FIG. 5, a noble metal ignition part 31 having a diameter of 1 mm or less, whose main component is Ir or Pt, is fixed to the front end face of the center electrode 3. If the diameter of the electrode tip is reduced to 1 mm or less, the electric field can be concentrated on the electrode tip facing the spark discharge gap g, so that the discharge voltage can be lowered. Moreover, by using the electrode tip portion as the noble metal ignition portion 31, spark consumption is suppressed and the life of the spark plug can be extended. And even if the insulator 2 is somewhat thinned by reducing the diameter of the mounting screw portion 7 of the metal shell 1, the discharge voltage is lowered, so that the withstand voltage performance can be afforded accordingly. However, from the viewpoint of suppressing the progress of spark consumption due to excessive electric field concentration, it is desirable that the diameter of the noble metal firing portion 31 is 0.2 mm or more.

  In the present embodiment, a noble metal ignition part 31 made of an Ir alloy (alloy component such as Rh, Pt or Ni) is fixed to the tip of the center electrode 3 by laser welding, and the ground electrode 4 is opposed to the ignition part 31. In this manner, the ignition part 32 made of Pt or a Pt alloy (alloy component is Ni, for example) is fixed by resistance welding, and the gap between the ignition part 31 and the opposing ignition part 32 is a spark discharge gap. g.

In order to confirm the effect of the present invention, the following experiment was conducted.
Example 1
In the spark plug shown in FIGS. 1 and 2, the mounting screw portion 7 is called M12, the interval α of the spark discharge gap g is 1.1 mm, and the ratio E / α of the gas volume end face width E to α is 1.4. For the gas volume portion width J, the length of the section L where J> α is 7 mm, the angle θ between the flat portion 52a and the inclined portion 52b of the ridge portion 1c is 150 °, and the value of the engagement position gap amount β is Those set to various values of 0.1 to 0.6 mm were prepared as test products. And in order to investigate the fouling resistance of each spark plug, a pre-delivery durability test was performed under the following conditions. That is, the spark plug is attached to a test vehicle (displacement: 1500 cc, in-line 4-cylinder) with a voltage application polarity in which the ground electrode 4 side is positive and the center electrode 2 side is negative, and the travel exemplified in JIS: D1606 The pattern (test room temperature: −10 ° C.) is taken as one cycle, and this is repeated until the insulation resistance of the spark plug drops to 10 MΩ or less. Depending on the number of cycles, 10 cycles or more are “◯” and 8-9 cycles are “△” , 6 cycles or less were determined as “×” (◯ and Δ are acceptable, × is not possible) The results are shown in Table 1.

  According to this, it can be seen that the antifouling property of the spark plug is remarkably improved by setting the value of the engagement position gap amount β to 0.4 mm or less.

(Example 2)
In the spark plug shown in FIGS. 1 and 2, the mounting screw portion 7 is called M12, the interval α of the spark discharge gap g is 1.1 mm, and the ratio E / α of the gas volume end face width E to α is 1.4. The angle L between the flat portion 52a and the inclined portion 52b of the ridge portion 1c is 150 °, the value of the engagement position gap amount β is 0.4 mm, and the length of the section L where J> α with respect to the gas volume portion width J What set this to various values of 5-8.3 mm was prepared as a test article. And in order to investigate the low temperature startability of each spark plug, it tested on the following conditions. That is, a spark plug is attached to a test vehicle (displacement: 1500 cc, inline 4 cylinder) with a voltage application polarity with the ground electrode 4 side being positive and the center electrode 3 side being negative, and a cycle of idling 30 seconds + stop 30 minutes The test which calculates | requires the number of cycles until it becomes impossible to start was performed on two conditions of room temperature -30 degreeC and -10 degreeC. In any case, 5 cycles or more were determined as “◯” and 4 cycles or less as “×” (◯ is acceptable, × is not possible) depending on the number of cycles. The results are shown in Table 2.

  According to this result, no problem occurred in any of the test products at −10 ° C., but in the test at −30 ° C., which is a severe condition at a lower temperature, good results were obtained in a test product in which L was secured at 7 mm or more. Obtained. In addition, it is thought that the reason why the non-startable cycle is small in the test product with L of less than 7 mm is that side fire is likely to occur due to the progress of the contamination of the insulator.

(Example 3)
In the spark plug shown in FIGS. 1 and 2, the mounting screw portion 7 is called M12, the spark discharge gap g has an interval α of 1.1 mm, and the angle θ between the flat portion 52a and the inclined portion 52b of the ridge portion 1c is 150. The ratio E / α of the gas volume portion end face width E to α is 0.9 by changing the engagement position gap amount β to 0.4 mm and changing the inclination angle of the outer peripheral surface of the second shaft portion 2i in various ways. Prepared as test products adjusted to various values of ~ 1.7. These spark plugs are ignited in advance and attached to a visible chamber. The spark pressure is set by setting the air pressure in the chamber to 0.4 MPa, and how many side fires occur on the metal fittings during 1000 discharges. The occurrence frequency of side fire was examined by visually checking whether it occurred. The results are shown in FIG. According to this, it can be seen that by setting E / α to 1.1 or more, the side fire occurrence frequency is remarkably reduced.

Example 4
In the spark plug shown in FIGS. 1 and 2, the mounting screw portion 7 is called M12, the interval α of the spark discharge gap g is 1.1 mm, and the ratio E / α of the gas volume end face width E to α is 1.4. As for the gas volume portion width J, the length of the section L where J> α is set to 7 mm, and the angle θ between the flat portion 52a and the inclined portion 52b of the ridge portion 1c is set to 135 to 170 ° as a test product. did. Further, instead of setting the angle θ to 120 °, a test product provided with a chamfered portion 52c (chamfer width 0.5 mm) as shown in FIG. 4 was also prepared.

  Using the dimensions and shapes of these test products as initial conditions, the electric field strength distribution in the gas volume GV when the applied voltage level to the center electrode 3 is 10 kV is simulated by a finite element method using commercially available software, The electric field strength at the position closest to the intersection between the flat portion 52a and the inclined portion 52b was read. The results are shown in Table 3.

  According to this, it can be seen that the electric field strength is remarkably reduced when the angle θ is 140 ° or more or when chamfered. FIG. 7A shows the simulation results for θ = 135 ° and FIG. 7B shows the simulation results for θ = 150 °. The brighter the region in the figure, the higher the electric field strength. According to this, it can be clearly seen that the electric field concentration portion appears remarkably near the intersection in the former with a small angle θ, whereas the degree of electric field concentration is reduced in the latter with a large θ.

  Next, the ground electrode is removed from each test product, and the opening side of the metal shell is immersed in a liquid insulating medium such as silicon oil in this state, so that the gap between the outer surface of the insulator and the inner surface of the metal shell is Filled and insulated with liquid insulating medium. In this state, an AC high voltage or a pulsed high voltage is applied between the metal shell and the center electrode 3 by a high voltage power source, and the voltage waveform is recorded with an oscilloscope or the like, and the voltage waveform is penetrated to the insulator. The voltage value at the time when the breakdown occurred was read as the penetration breakdown voltage. Each test product was tested at n = 40, and an average value and a minimum value of withstand voltage were obtained. The results are shown in Table 4.

  According to this, it can be seen that those with an angle θ of 140 ° to 160 ° or chamfered have high withstand voltage average values and stable withstand voltage performance. . On the other hand, when the angle θ is less than 140 °, both the average value and the minimum value of the withstand voltage are lowered, and the withstand voltage performance tends to be relatively lowered. Further, when the angle θ exceeds 160 °, the average value of the withstand voltage is relatively good, but the minimum value is lowered, and the withstand voltage performance tends to vary.

1 is a longitudinal sectional view showing an overall configuration of a spark plug according to an embodiment of the present invention. The longitudinal cross-sectional view which expands and shows the front end side principal part of FIG. The principal part longitudinal cross-sectional view which shows the 1st modification of the spark plug of FIG. The principal part longitudinal cross-sectional view which shows a 2nd modification similarly. The whole longitudinal cross-sectional view which shows a 3rd modification similarly. 10 is a graph showing experimental results of Example 3. FIG. 10 is a diagram illustrating some simulation results of Example 4.

Claims (7)

A center electrode (3), an insulator (2) provided outside the center electrode (3), a cylindrical metal shell (1) provided outside the insulator (2), and one end side A ground electrode coupled to the metal shell (1) and having the other end faced to the front end of the center electrode (3) to form a spark discharge gap (g) with the center electrode (3) (4)
In the axial direction (O) of the insulator (2), the side where the spark discharge gap (g) is located is the front side, and the opposite side is the rear side, and the insulator (2) is the front end (2i). Is reduced in diameter by a step portion in the circumferential direction so that the step portion serves as an insulator-side engagement portion (2h) and is inserted into the metal shell (1) from a rear-side opening, and the insulator-side engagement portion (2h) engages with the metal fitting side engaging portion (1c) protruding from the inner peripheral surface of the metal shell (1), and more than the insulator side engaging portion (2h) of the insulator (2). The outer peripheral surface (hereinafter referred to as a gap forming outer peripheral surface) (2k) of the portion (2i) located on the front side is an inner peripheral surface (hereinafter referred to as a gap forming inner peripheral surface) of the metal fitting side engaging portion (1c) ( 52) is substantially coaxial with the cylindrical surface and is opposed to form a predetermined amount of engagement position gap (Q). In the gap forming the outer peripheral surface of the outer diameter of the (2k) d1, the inside diameter of the gap forming the peripheral surface (52) as D1,
β = (D1-d1) / 2
The spark plug is characterized in that the engagement position gap amount β represented by is adjusted to 0.4 mm or less. The inner diameter of the front end face side opening of the metal shell (1) is D2, and the outer diameter of the insulator (2) at the front end face position is d2.
E = (D2-d2) / 2
The end face width E of the gas volume part represented by the following formula is defined, where α is the interval of the spark discharge gap (g):
α <1.1E
The spark plug according to claim 1, wherein: On the front side of the insulator-side engagement portion (2h), the diameter of the cross-sectional outline of the insulator (2) by a virtual plane orthogonal to the axis (O) is d3, and the metal shell at a position corresponding thereto When the inner diameter of (1) is D3,
In an arbitrary position in a section from the front end face position of the metal shell (1) to at least 7 mm,
α <(D3-d3) / 2
The spark plug according to claim 1 or 2, wherein The gap forming inner peripheral surface (52) of the metal fitting side engaging portion (1c) has a flat portion (a cross-sectional outline line formed by a virtual plane including the axis (O) is opposed to the gap forming outer peripheral surface (2k)). 52a), and an inclined portion (52b) that descends from the front end of the flat portion (52a) toward the inner peripheral surface of the metal shell (1). The flat portion (52a) and the inclined portion ( 52b) makes an angle θ
The spark plug according to claim 1, wherein 140 ° ≦ θ ≦ 160 ° is satisfied.   The gap forming inner peripheral surface (52) of the metal fitting side engaging portion (1c) has a flat portion (a cross-sectional outline line formed by a virtual plane including the axis (O) is opposed to the gap forming outer peripheral surface (2k)). 52a), and an inclined portion (52b) that descends from the front end of the flat portion (52a) toward the inner peripheral surface of the metal shell (1), and the flat portion (52a) and the inclined portion The spark plug according to any one of claims 1 to 4, wherein a chamfered portion (52c) or a rounded portion (52r) is formed at a position intersecting with the portion (52b).   The spark plug according to any one of claims 1 to 5, wherein a noble metal ignition portion (31) having a diameter of 1 mm or less, whose main component is Ir or Pt, is fixed to the front end face of the center electrode (3).   The spark plug according to any one of claims 1 to 6, wherein a mounting screw portion (7) having a nominal size of M12 or less is formed on a front end side outer peripheral surface of the metal shell (1).