JP7261687B2 - Spark plug - Google Patents

Description

The present invention relates to spark plugs.

Spark plugs are used as ignition means in internal combustion engines such as vehicle engines and cogeneration systems. Patent Literature 1 discloses a spark plug in which a plug cover that covers a discharge gap that generates spark discharge is attached to a housing, and an auxiliary combustion chamber is formed inside the plug cover.

In such a spark plug, the air-fuel mixture in the combustion chamber of the internal combustion engine is introduced into the sub-combustion chamber through the injection hole formed in the plug cover, and the air-fuel mixture is ignited by performing spark discharge in the discharge gap. A flame is generated in the combustion chamber. Then, a flame jet is ejected from the injection hole into the combustion chamber outside the sub-combustion chamber, and the flame spreads over the entire combustion chamber. As a result, an internal combustion engine with a high combustion speed can be obtained.

JP 2014-159778 A

However, in the spark plug disclosed in Patent Document 1, when the flame grows in the sub-combustion chamber, the housing and the plug cover that constitute the sub-combustion chamber may take heat from the flame and impede the growth of the flame. Therefore, the spark plug described in Patent Document 1 has room for improvement from the viewpoint of improving ignitability.

SUMMARY OF THE INVENTION The present invention has been made in view of such problems, and an object of the present invention is to provide a spark plug for an internal combustion engine that can improve ignitability.

One aspect of the present invention is a center electrode (11),
a ground electrode (12) forming a discharge gap (G) with the center electrode;
a tubular insulator (13) in which the center electrode is arranged;
a housing structure (2) that holds the insulator and surrounds a sub-combustion chamber (14) in which the discharge gap is arranged;
The housing structure includes a low emissivity layer (5) arranged to face the sub-combustion chamber and having a surface emissivity lower than that of the housing structure, and a low emissivity layer laminated on the low emissivity layer having a thermal conductivity A spark plug (1) provided with a low thermal conductivity layer (6) less than air.

In the spark plug of the aspect described above, the housing structure includes a low emissivity layer disposed to face the sub-combustion chamber and having a surface emissivity lower than that of the housing structure, and a low emissivity layer laminated on the low emissivity layer and having a thermal conductivity. A low thermal conductivity layer less than air is provided. Therefore, the flame heat generated in the sub-combustion chamber is likely to be reflected by the surface of the low emissivity layer, and the heat in the sub-combustion chamber is less likely to escape to the outside of the sub-combustion chamber. Furthermore, the housing structure is provided with a low thermal conductivity layer laminated to the low emissivity layer. Therefore, the heat in the sub-combustion chamber does not easily pass through the housing structure and tends to stay in the sub-combustion chamber. Therefore, when the flame grows in the sub-combustion chamber, it is possible to suppress the housing and the plug cover that constitute the sub-combustion chamber from taking heat from the flame, thereby easily improving the ignitability of the spark plug.

As described above, according to the aspect, it is possible to provide a spark plug for an internal combustion engine that can improve ignitability.
It should be noted that the symbols in parentheses described in the claims and the means for solving the problems indicate the corresponding relationship with the specific means described in the embodiments described later, and limit the technical scope of the present invention. not a thing

1 is a cross-sectional view of a spark plug and an internal combustion engine to which it is attached according to Embodiment 1. FIG. FIG. 1 is a cross-sectional view taken along line II-II. FIG. 4 is a cross-sectional view of a spark plug according to sample 2 in an experimental example; FIG. 4 is a cross-sectional view of a spark plug according to sample 3 in an experimental example; 4 is a diagram showing the lean limit A/F of each sample in the experimental example; FIG. Sectional drawing of the spark plug in Embodiment 2. FIG. The front view of the spark plug in Embodiment 2. FIG. Sectional drawing of the spark plug in Embodiment 3. FIG. FIG. 11 is an exploded cross-sectional view of a spark plug according to Embodiment 3; Sectional drawing of the spark plug in Embodiment 4. FIG. FIG. 11 is a cross-sectional view of a spark plug according to Embodiment 5;

(Embodiment 1)
An embodiment of a spark plug will be described with reference to FIGS. 1 and 2. FIG.
A spark plug 1 of this embodiment comprises a center electrode 11, a ground electrode 12, an insulator 13, and a housing structure 2, as shown in FIG.

The ground electrode 12 forms a discharge gap G with the center electrode 11 . The insulator 13 has a tubular shape, and the center electrode 11 is arranged inside thereof. The housing structure 2 holds an insulator 13 . The housing structure 2 also surrounds a secondary combustion chamber 14 in which a discharge gap G is arranged.

The housing structure 2 is provided with a low emissivity layer 5 and a low thermal conductivity layer 6 . The low-emissivity layer 5 is arranged to face the sub-combustion chamber 14 . The low emissivity layer 5 is a layer whose surface emissivity is lower than that of the housing structure 2 . A low thermal conductivity layer 6 is laminated on the low emissivity layer 5 . The thermal conductivity of the low thermal conductive layer 6 is equal to or lower than that of air.
Hereinafter, this embodiment will be described in detail.

Hereinafter, the center axis of the spark plug 1 will be referred to as a plug center axis C. As shown in FIG. Further, the direction in which the plug central axis C extends is referred to as the plug axial direction X. As shown in FIG. The side on which the auxiliary combustion chamber 14 of the spark plug 1 is located in the axial direction X of the spark plug 1 (for example, the lower side in FIG. 1) is called the distal side, and the opposite side (for example, the upper side in FIG. 1) is called the proximal side. A radial direction of the spark plug 1 is called a plug radial direction, and a circumferential direction of the spark plug 1 is called a plug circumferential direction.

The spark plug 1 can be used, for example, as ignition means in internal combustion engines such as automobiles and cogeneration systems. A base end portion of the spark plug 1 is connected to an ignition coil (not shown), and a distal end portion of the spark plug 1 is arranged in a combustion chamber of an internal combustion engine.

The housing structure 2 has a housing body 3 and a plug cover 4 . The housing main body 3 is made of a heat-resistant metal material such as iron, nickel, iron-nickel alloy, stainless steel, etc., and formed into a cylindrical shape. A body male screw portion 31 is formed on the outer peripheral portion of the housing body 3 . The plug cover 4 is screwed onto the body male screw portion 31 .

The plug cover 4 has a cup shape with an open base end. The plug cover 4 has a shape of a body of revolution with the plug central axis C as the axis of rotation. The plug cover 4 can be made of the same material as the housing body 3, for example. The plug cover 4 includes a cylindrical cover peripheral wall 41 and a cover bottom wall 42 covering the cover peripheral wall 41 from the tip.

The cover peripheral wall 41 is formed in a cylindrical shape. A cover recess 411 is formed at the proximal end of the cover peripheral wall 41 so that the inner peripheral surface thereof is recessed toward the outer peripheral side and is open to the proximal end. A cover female threaded portion 411 a that is screwed into the body male threaded portion 31 of the housing body 3 is formed on the inner peripheral surface of the cover recessed portion 411 . The front end surface of the housing body 3 is in contact with the front end surface 411 b of the cover recess 411 .

A cover male screw portion 412 for screwing the spark plug 1 into the female screw hole 101 of the plug hole of the cylinder head 10 of the internal combustion engine is formed on the outer peripheral portion of the tip portion of the cover peripheral wall 41 . A portion of the cover peripheral wall 41 adjacent to the base end side of the cover male threaded portion 412 protrudes further to the outer peripheral side than the cover male threaded portion 412 , and when the spark plug 1 is attached to the cylinder head 10 , it is attached to the cylinder head 10 . Pressed. Further, when the spark plug 1 is attached to the cylinder head 10, the portion of the spark plug 1 on the tip side of the cover male screw portion 412 is exposed to the combustion chamber.

The cover bottom wall 42 is formed to cover the tip of the cover peripheral wall 41 . As shown in FIG. 2, the cover bottom wall 42 is disc-shaped. A space formed inside the housing body 3 and the plug cover 4 in the spark plug 1 is the auxiliary combustion chamber 14 .

As shown in FIGS. 1 and 2, the cover bottom wall 42 is formed with a plurality of injection holes 421 penetrating through the cover bottom wall 42 and communicating the auxiliary combustion chamber 14 with the space outside the spark plug 1 . As shown in FIG. 1 , the injection hole 421 is formed to protrude into the combustion chamber when the spark plug 1 is attached to the cylinder head 10 . The plurality of injection holes 421 are arranged on the outer peripheral side of the plug center axis C in the plug radial direction. A plurality of injection holes 421 are formed at regular intervals in the circumferential direction of the plug. Each injection hole 421 is formed so as to be inclined toward the outer peripheral side in the radial direction of the plug as it goes toward the tip side in the axial direction X of the plug. In addition, the number, shape, arrangement location, etc. of the injection holes 421 are appropriately determined according to a request.

A body-side space 30 in which the low-emissivity layer 5 and the low-thermal-conductivity layer 6 are arranged is formed in the wall of the housing body 3 , and the low-emissivity layer 5 and the low-thermal-conductivity layer 6 are formed in the wall of the plug cover 4 . A cover-side space 40 for arranging is formed.

The body-side space 30 is a closed space that is entirely surrounded by the walls of the housing body 3 . The body-side space 30 is formed in the housing body 3 from a position on the distal end side of a body engaging portion 32 (to be described later) to the vicinity of the distal end surface of the housing body 3 . The body-side space 30 is formed continuously over the entire periphery in the plug circumferential direction. The body-side space 30 is formed at a position closer to the inner peripheral side (that is, the sub-combustion chamber 14 side) than the outer peripheral side of the housing body 3 in the radial direction of the plug.

As shown in FIGS. 1 and 2, the cover-side space 40 is a closed space surrounded by the walls of the plug cover 4 as a whole. As shown in FIG. 1, the cover-side space 40 is formed in the cover peripheral wall 41 from the vicinity of the tip surface 411b of the cover concave portion 411 to substantially the entire cover peripheral wall 41 in the axial direction X of the plug. As shown in FIG. 2, the cover-side space 40 is formed continuously over the entire circumference of the plug. As shown in FIGS. 1 and 2, the body-side space 30 is formed closer to the inner peripheral side (that is, the sub-combustion chamber 14 side) than the outer peripheral side of the plug cover 4 in the radial direction of the plug.

As shown in FIG. 1, the low emissivity layer 5 and the low thermal conductivity layer 6 are laminated in the radial direction of the plug in each of the main body side space 30 and the cover side space 40 .

The low emissivity layer 5 is a layer whose surface emissivity is lower than that of the housing structure 2, as described above. It is generally known that there are heat radiation, heat conduction, and heat transfer as elements of heat transfer. It is for suppression. Thermal radiation is the phenomenon in which heat is emitted as electromagnetic waves. An object with a low thermal emissivity emits little thermal radiation and tends to reflect thermal radiation coming from its surroundings. The low emissivity layer 5 can be made of aluminum, brass, or other materials with a glossy surface. The low heat conductive layer 6 is formed in a thin film.

The low emissivity layer 5 faces the auxiliary combustion chamber 14 in the plug radial direction, which is the thickness direction. The low emissivity layer 5 is formed in a cylindrical shape penetrating in the axial direction X of the plug. That is, the low emissivity layer 5 is formed over the entire circumference of the plug in the circumferential direction. The low emissivity layer 5 is formed in the entire region of the main body side space 30 and the cover side space 40 in the axial direction X of the plug. The low-emissivity layer 5 is arranged on the entire wall surface on the outer peripheral side of each of the body-side space 30 and the cover-side space 40 .

In each of the body-side space 30 and the cover-side space 40 , the low thermal conductivity layer 6 is formed on the inner peripheral side of the low emissivity layer 5 . That is, the low emissivity layer 5 is laminated on the opposite side (ie, the outer peripheral side) of the low heat conductive layer 6 at least from the sub-combustion chamber 14 side (ie, the inner peripheral side).

As described above, the low thermal conductivity layer 6 is a layer whose thermal conductivity is lower than that of air. In this embodiment, the low thermal conductivity layer 6 can be made of, for example, ceramic, air, vacuum, argon, or the like. Since the low thermal conductivity layer 6 has a low thermal conductivity, it is difficult for heat to pass therethrough.

The low thermal conductivity layer 6 faces the auxiliary combustion chamber 14 in the plug radial direction, which is the thickness direction thereof. The low thermal conductivity layer 6 is formed in a cylindrical shape that penetrates in the axial direction X of the plug. That is, the low thermal conductivity layer 6 is formed over the entire circumference of the plug in the circumferential direction. The low thermal conductivity layer 6 is formed in the entire regions of the body-side space 30 and the cover-side space 40 in the axial direction X of the plug.

Each of the housing body 3 and the plug cover 4 has a space (body-side space 30, cover-side space 40) and a low radiation layer 5 and a low thermal conductivity layer 6 in their walls by manufacturing, for example, with a 3D printer. It is possible to form

A ground electrode 12 is connected to the cover peripheral wall 41 of the plug cover 4 . The ground electrode 12 is formed on the inner peripheral side of the cover-side space 40 of the plug cover 4 . In the axial direction X of the plug, the connecting portion of the ground electrode 12 to the plug cover 4 is located substantially in the center of the cover-side space 40 . The ground electrode 12 is formed in a bar shape extending in the radial direction of the plug. For example, the ground electrode 12 is formed by a 3D printer at the same time as the plug cover 4, or is manufactured separately from the plug cover 4, is drilled in the plug cover 4, is inserted into the hole, and is welded to the plug cover 4. It is possible to assemble to The end portion of the ground electrode 12 farther from the connecting portion with the plug cover 4 is formed up to the vicinity of the central axis C of the plug, forming a discharge gap G with the tip of the center electrode 11 .

The center electrode 11 is formed so that its center axis coincides with the center axis C of the plug. The tip portion of the center electrode 11 is located on the tip side of the tip of the housing body 3 . Further, the tip of the center electrode 11 is arranged at a position overlapping the cover-side space 40 in the radial direction of the plug. Accordingly, the low emissivity layer 5 and the low thermal conductivity layer 6 laminated in the cover-side space 40 are arranged at least in the region where the discharge gap G is formed. The center electrode 11 is inserted and held inside the insulator 13 , and the tip of the center electrode 11 protrudes from the insulator 13 .

The insulator 13 is formed by forming alumina or the like into a cylindrical shape. As shown in FIG. 1, the insulator 13 is held inside the housing body 3 . The tip portion of the insulator 13 protrudes further to the tip side than the tip surface of the housing body 3 .

The insulator 13 has an insulator leg portion 131 whose outer peripheral surface diameter decreases toward the tip side, and an insulator stepped portion 132 adjacent to the base end side of the insulator leg portion 131 . The insulator stepped portion 132 is formed in a stepped shape such that the outer peripheral surface of the insulator stepped portion 132 in the insulator 13 on the tip side is smaller in diameter than the outer peripheral surface of the insulator stepped portion 132 on the base end side. The insulator stepped portion 132 is supported by the body locking portion 32 formed on the inner peripheral surface of the housing body 3 from the base end side of the body locking portion 32 . The body locking portion 32 is formed on the entire circumference of the housing body 3 so as to protrude inward. A space between the housing and the insulator 13 on the tip end side from the supporting portion between the main body locking portion 32 and the insulator stepped portion 132 constitutes the auxiliary combustion chamber 14 .

Next, the effects of this embodiment will be described.
In the spark plug 1 of the present embodiment, the housing structure 2 includes a low emissivity layer 5 arranged to face the sub-combustion chamber 14 and having a surface emissivity lower than that of the housing structure 2, and a low emissivity layer 5 laminated on the low emissivity layer 5. A low thermal conductivity layer 6 having a thermal conductivity lower than that of air is provided. Therefore, the heat of the flame generated within the sub-combustion chamber 14 is likely to be reflected by the surface of the low emissivity layer 5 , and the heat within the sub-combustion chamber 14 is less likely to escape to the outside of the sub-combustion chamber 14 . Furthermore, the housing structure 2 is provided with a low thermal conductivity layer 6 laminated on the low emissivity layer 5 . Therefore, the heat in the sub-combustion chamber 14 does not easily pass through the housing structure 2 and tends to stay inside the sub-combustion chamber 14 . Therefore, when the flame grows in the sub-combustion chamber 14, the housing and the plug cover 4 forming the sub-combustion chamber 14 can suppress the quenching effect of taking heat from the flame. Easy to improve.

In addition, the low emissivity layer 5 is laminated on at least the side opposite to the sub-combustion chamber 14 side with respect to the low thermal conductivity layer 6 . Therefore, the heat in the sub-combustion chamber 14 reaches the low-emissivity layer 5 by passing through the low heat-conducting layer 6, which is difficult to pass through. It is possible to suppress the temperature inside the chamber 14 from becoming low, thereby suppressing the occurrence of the quenching effect. Furthermore, even if the heat in the sub-combustion chamber 14 passes through the low heat-conducting layer 6 and reaches the low-emissivity layer 5 , the heat is reflected by the low-emissivity layer 5 toward the sub-combustion chamber 14 side. Therefore, it is possible to further suppress the temperature inside the sub-combustion chamber 14 from becoming low, thereby further suppressing the occurrence of the quenching effect.

The laminated low emissivity layer 5 and low thermal conductivity layer 6 are arranged in a region in which at least the discharge gap G is formed in the axial direction X of the plug. Therefore, it is possible to prevent the flame from depriving the housing structure 2 of heat at the beginning of the flame growth process in the sub-combustion chamber 14 . That is, the flame generated by the spark discharge to the discharge gap G grows so as to spread radially around the discharge gap G, and tends to collide with the portion of the housing structure 2 on the outer peripheral side of the discharge gap G first. Therefore, by stacking the low emissivity layer 5 and the low thermal conductivity layer 6 on at least the region where the discharge gap G is formed in the axial direction X of the plug, the heat of the flame is absorbed by the housing structure 2 at the initial stage of the flame growth process. It is possible to suppress the cracking and promote the growth of the flame.

As described above, according to the present embodiment, it is possible to provide a spark plug for an internal combustion engine that can improve ignitability.

(Experimental example)
This example is an example of evaluating the ignitability of the spark plug 1 of the first embodiment.

In this example, the sample 1, which is the spark plug 1 of the first embodiment shown in FIG. 1, and, as shown in FIG. Sample 2, which is a spark plug 92 entirely filled with a low thermal conductivity layer 6 (that is, does not include a low emissivity layer), and, as shown in FIG. A sample 3, which is a spark plug 93 without a side space, was prepared. In Samples 1 to 3, other configurations were the same. Then, each of Samples 1 to 3 was evaluated for ignitability.

Ignition performance was evaluated using the lean limit A/F as an index. That is, in the internal combustion engine to which each sample was attached, the air-fuel ratio (A/F) of the air-fuel mixture was gradually changed, and the limit air-fuel ratio (that is, the lean limit A/F) at which ignition was possible was measured. It can be said that the larger the lean limit A/F value, the better the ignitability.

The conditions of the internal combustion engine in this test were a displacement of 2400 cc, an engine speed of 1200 rpm, and an indicated mean effective pressure of 0.58 MPa. The air-fuel ratio at which the combustion variation rate (that is, the variation rate of the indicated mean effective pressure) is 3% was defined as the lean limit A/F.

The results are shown in FIG. In FIG. 5, the measurement result of sample 1 is indicated by line L1, the measurement result of sample 2 is indicated by line L2, and the measurement result of sample 3 is indicated by line L3. Also, the lean limit A/F of sample 1 is indicated by ARF1, the lean limit A/F of sample 2 is indicated by ARF2, and the lean limit A/F of sample 1 is indicated by ARF3.

As can be seen from FIG. 5, the lean limit A/F (ARF1) of sample 1, which is the spark plug 1 of Embodiment 1, is the lean limit A/F (AFR2) of sample 2, the lean limit A/F of sample 3 ( It is higher than AFR3). Therefore, the spark plug 1 of Embodiment 1 shown in FIG. 1 is similar to that of FIG. It can be seen that the spark plug 92 shown in FIG. 4 and the spark plug 1 shown in FIG.

(Embodiment 2)
As shown in FIGS. 6 and 7, this embodiment has the same basic structure as that of the first embodiment, but also includes a cover-side space 40 in the cover bottom wall 42 of the plug cover 4, and the low-emissivity layer 5 and This is a form in which a low thermal conductivity layer 6 is formed. In this embodiment, simply referring to the cover-side space 40, the low-emissivity layer 5, and the low-thermal-conductivity layer 6 means the cover-side space 40, the low-emissivity layer 5, and the low-thermal-conductivity layer 6 provided in the cover bottom wall 42. and

As shown in FIG. 7, the cover-side space 40 is formed in a disc shape having a thickness in the axial direction X of the plug. The cover-side space 40 is formed substantially entirely on the cover bottom wall 42 on the inner peripheral side of the nozzle hole 421 . As shown in FIG. 6, the cover-side space 40 is formed at a position closer to the auxiliary combustion chamber 14 than the space on the tip side of the plug cover 4 (the side on which the combustion chamber is arranged) in the axial direction X of the plug. .

In the cover-side space 40, the low radiation layer 5 and the low thermal conductivity layer 6 are laminated in the axial direction X of the plug. The low emissivity layer 5 is arranged on the entire wall surface of the cover-side space 40 on the tip side. A low heat conductive layer 6 is disposed on the base end side of the cover-side space 40 in the axial direction X of the plug, that is, on the sub-combustion chamber 14 side.

Others are the same as those of the first embodiment.
Note that, of the reference numerals used in the second and subsequent embodiments, the same reference numerals as those used in the previous embodiments represent the same components as those in the previous embodiments, unless otherwise specified.

In this embodiment, the low radiation layer 5 and the low thermal conductivity layer 6 are laminated also on the cover bottom wall 42 of the plug cover 4 . Therefore, the amount of heat that escapes from the auxiliary combustion chamber 14 through the cover bottom wall 42 to the tip side of the plug cover 4 can be suppressed. As a result, it is possible to further suppress the temperature inside the sub-combustion chamber 14 from lowering, thereby further suppressing the occurrence of the quenching effect.
In addition, it has the same effects as those of the first embodiment.

(Embodiment 3)
In this embodiment, as shown in FIGS. 8 and 9, the arrangement of the low emissivity layer 5 and the low thermal conductivity layer 6 is changed from the first embodiment.

In this embodiment, the distal end portion of the housing body 3 has a body distal end portion 33 that protrudes from the body male threaded portion 31 toward the distal end side and has an outer peripheral surface smaller in diameter than the body male threaded portion 31 . The body tip portion 33 is formed in a cylindrical shape.

The outer peripheral surface of the cover concave portion 411 of the cover peripheral wall 41 of the plug cover 4 includes a cover female screw portion 411a to which the main body male screw portion 31 is screwed, and a straight cylindrical tip formed on the tip side from the cover female screw portion 411a. and a concave surface 411c. The tip concave surface 411c faces the main body tip portion 33 over the entire periphery in the plug circumferential direction via the space s. The space s is formed as a closed space inside the housing structure 2 with the housing body 3 and the plug cover 4 .

A cylindrical low-emissivity layer 5 is arranged in the space s. The low emissivity layer 5 is arranged on the tip concave surface 411 c of the plug cover 4 . A low thermal conductivity layer 6 is arranged on the inner peripheral side of the low emissivity layer 5 in the space.

In this embodiment, the body-side space, the low-radiation layer 5, and the low-thermal-conductivity layer 6 are not formed in the housing body 3, but they may be formed.
Others are the same as those of the first embodiment.

In this embodiment, a low radiation layer 5 and a low thermal conductivity layer 6 are laminated between the housing body 3 and the plug cover 4 . Therefore, it is easy to form the low radiation layer 5 and the low thermal conductivity layer 6 in the housing structure 2, and the productivity of the spark plug 1 can be easily improved.
In addition, it has the same effects as those of the first embodiment.

(Embodiment 4)
As shown in FIG. 10, this embodiment is a form in which the shape of the cover-side space 40 is changed with respect to the first embodiment.

In this embodiment, the cover-side space 40 is formed only near the outer peripheral side of the discharge gap G in the cover peripheral wall 41 of the plug cover 4 . The cover-side space 40 is formed with such a length that both ends in the axial direction X of the plug protrude slightly outward from both sides of the discharge gap G. As shown in FIG. The cover-side space 40 is formed with a length equal to or less than half the length of the cover peripheral wall 41 in the axial direction X of the plug.

At both ends of the cover-side space 40 in the axial direction X of the plug, protruding spaces 401 protruding radially inward of the plug are formed. As a result, the length between the cover-side space 40 and the inner wall surface of the plug cover 4 in the radial direction of the plug is the shortest at the portion of the cover-side space 40 where the projecting space 401 is formed. The projecting space 401 is formed at a position different from the discharge gap G in the axial direction X of the plug.

A cylindrical low-emissivity layer 5 is disposed on the wall surface of the plug cover 4 located on the outer peripheral side of the cover-side space 40, and a low heat-conducting layer is disposed on the inner peripheral side of the low-emissivity layer 5 in the cover-side space 40. 6 is formed. Due to the formation of the protruding spaces 401, the length of the low thermal conductivity layer 6 in the plug radial direction at both ends in the plug axial direction X (that is, the portions where the protruding spaces 401 are formed) is equal to that of the other portions of the plug. It is larger than the length in the radial direction. As a result, the low thermal conductivity layer 6 has a heat transmission coefficient in the plug radial direction at both end portions in the plug axial direction X that is smaller than the heat transmission coefficient in the plug radial direction at other portions.
Others are the same as those of the first embodiment.

In this embodiment, the low thermal conductivity layer 6 is formed in the region where the discharge gap G is formed in the axial direction X of the plug, and the thickness in the plug radial direction at both ends in the axial direction X of the plug is greater than that of other portions. It is larger than the thickness in the radial direction of the plug. Therefore, the thermal resistance in the plug radial direction at both ends of the low thermal conductive layer 6 in the plug axial direction X is greater than the thermal resistance in the plug radial direction at other portions of the low thermal conductive layer 6 . Therefore, the heat in the subcombustion chamber 14 is less likely to escape from the low heat conductive layer 6 to both outer sides in the axial direction X of the plug, and tends to remain more inward than both ends of the low heat conductive layer 6 in the axial direction X of the plug. Therefore, it is possible to further suppress the temperature inside the sub-combustion chamber 14 from lowering, thereby further suppressing the occurrence of the quenching effect.
In addition, it has the same effects as those of the first embodiment.

(Embodiment 5)
As shown in FIG. 11, this embodiment has the same basic structure as the first embodiment, but has a low-emissivity layer 5 and a low-emissivity It is a form in which a low heat conductive layer 6 laminated on each of both sides of the layer 5 is provided. That is, in this embodiment, three layers of the low thermal conductivity layer 6, the low radiation layer 5, and the low thermal conductivity layer 6 are laminated in this order in the radial direction of the plug in each of the main body side space 30 and the cover side space 40. .
Others are the same as those of the first embodiment.

This embodiment also has the same effect as the first embodiment.

The present invention is not limited to the embodiments described above, and can be applied to various embodiments without departing from the scope of the invention. For example, the housing structure 2 is configured by assembling the separate housing main body 3 and the plug cover 4, but the present invention is not limited to this.

Reference Signs List 1 spark plug 11 center electrode 12 ground electrode 13 insulator 14 auxiliary combustion chamber 2 housing structure 5 low radiation layer 6 low heat conduction layer

Claims (4)

a center electrode (11);
a ground electrode (12) forming a discharge gap (G) with the center electrode;
a tubular insulator (13) in which the center electrode is arranged;
a housing structure (2) that holds the insulator and surrounds a sub-combustion chamber (14) in which the discharge gap is arranged;
The housing structure includes a low emissivity layer (5) arranged to face the sub-combustion chamber and having a surface emissivity lower than that of the housing structure, and a low emissivity layer laminated on the low emissivity layer having a thermal conductivity A spark plug (1) provided with a low thermal conductivity layer (6) less than air. 2. The spark plug according to claim 1, wherein said low emissivity layer is laminated on at least a side opposite to said sub-combustion chamber side with respect to said low thermal conductivity layer. 3. The spark plug according to claim 2, wherein the housing structure is provided with the low emissivity layer and the low thermal conductivity layers laminated on both sides of the low emissivity layer. 4. The laminated low-emissivity layer and low-thermal-conductivity layer are arranged in an axial direction (X) of the spark plug at least in a region where the discharge gap is formed. The spark plug described in .

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