JP7653233B2 - Spark plugs for internal combustion engines - Google Patents

Description

The present invention relates to a spark plug for an internal combustion engine.

For example, as disclosed in Patent Document 1, a spark plug with a pre-combustion chamber at the tip is known. In this spark plug, a pocket portion that is part of the pre-combustion chamber is formed between the insulator and the housing in the radial direction of the spark plug.

DE 10 2018 211 009 A1

However, depending on how the pocket is formed, the scavenging ability of the pre-combustion chamber can easily become poor. If the scavenging ability of the pre-combustion chamber is poor, the combustion of the air-fuel mixture may become unstable in an internal combustion engine equipped with a spark plug.

The present invention was made in consideration of these problems, and aims to provide a spark plug for an internal combustion engine that can stably combust the air-fuel mixture.

One aspect of the present invention is a cylindrical insulator (3),
a center electrode (4) held on the inner circumferential side of the insulator and protruding from the insulator toward the tip side;
a cylindrical housing (2) that holds the insulator on its inner periphery;
A plug cover (5) provided at the tip of the housing;
a sub-combustion chamber (50) surrounded by at least the plug cover, the housing, the insulator, and the center electrode,
a distance (L1) from a tip of the insulator in a plug axial direction (Z) to a base end (211) of an inner circumferential surface (21) of the housing facing the auxiliary combustion chamber is equal to or shorter than a shortest distance (L2) from a plug central axis (C) to the inner circumferential surface of the housing in a plug radial direction,
The auxiliary combustion chamber has a pocket portion (501) between the insulator and the housing in the plug radial direction,
The distance (L1) is equal to or less than the maximum width (L3) of the opening (502) of the pocket portion in the plug radial direction,
a width of the pocket portion in a radial direction of the plug is greater than a quenching distance of a flame formed in the auxiliary combustion chamber,
The spark plug (1) for an internal combustion engine has a relationship between a surface area S ^mm2 of the insulator and the housing facing the pocket portion and a volume V ^mm3 of the pocket portion that satisfies S/V≦10 mm ⁻¹ .

In the above spark plug for an internal combustion engine, the distance in the plug axial direction from the tip of the insulator to the base end of the inner peripheral surface of the housing facing the auxiliary combustion chamber is equal to or shorter than the shortest distance in the plug radial direction from the central axis of the plug to the inner peripheral surface of the housing. This improves the scavenging performance of the auxiliary combustion chamber. As a result, the air-fuel mixture can be burned stably.

As described above, according to the above aspect, it is possible to provide a spark plug for an internal combustion engine that can stably combust an air-fuel mixture.
In addition, the symbols in parentheses described in the claims and the means for solving the problems indicate a correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention.

2 is a cross-sectional view of a tip portion of the spark plug according to the first embodiment. FIG. FIG. 2 is a plan view taken along the line II in FIG. 2 is a cross-sectional view of the tip portion of the spark plug according to the first embodiment, showing distances between each portion, etc. 4 is a cross-sectional view of a tip portion of a spark plug showing a creepage distance of an insulator in the first embodiment. FIG. FIG. 11 is a cross-sectional view of a tip portion of a spark plug according to a second embodiment. FIG. 11 is a cross-sectional view of a tip portion of a spark plug according to a second embodiment, showing a distance of a discharge gap and the like. FIG. 4 is a plan view of a tip portion of a spark plug in a comparative example 1. FIG. 11 is a cross-sectional view of a tip portion of a spark plug according to a second comparative example. FIG. 11 is a cross-sectional view of a tip portion of a spark plug in a comparative embodiment 2, showing distances between each portion. 4 is a graph showing the discharge voltages of spark plugs of Comparative Examples 1 and 2 in Experimental Example 1. FIG. 11 is an analytical diagram of airflow during the compression stroke of an internal combustion engine in which a spark plug of Comparative Example 2 is installed in Experimental Example 2. 13 is a graph showing the relationship between the crank angle and the heat release rate in an internal combustion engine in which the spark plug of the first embodiment or the first comparative example is installed in an experimental example 3. FIG. 4 is a cross-sectional view of a tip portion of a spark plug according to the first embodiment . FIG. 11 is a cross-sectional view of a tip portion of a spark plug according to a third embodiment . FIG. 15 is a plan view taken along the arrow XV in FIG. 14 . FIG. 13 is a cross-sectional view of a tip portion of a spark plug according to a third comparative example. FIG. 11 is a cross-sectional view of a tip portion of a spark plug in a comparative embodiment 3, showing distances between each portion. 13 is a graph showing the relationship between the amount of heat generated and the time elapsed after ignition by a spark plug in Experimental Example 4. 13 is a graph showing the relationship between the ratio of the surface area to the volume of the pocket and the amount of unburned fuel remaining in the pocket in Experimental Example 5. FIG. 13 is an analytical diagram showing the combustion range of fuel immediately after ignition in an internal combustion engine equipped with a spark plug of Comparative Example 3 in Experimental Example 6. FIG. 13 is an analytical diagram showing the combustion range of fuel 2 ms after ignition in an internal combustion engine equipped with a spark plug of Comparative Example 3 in Experimental Example 6. FIG. 13 is an analytical diagram showing a combustion range of fuel immediately after ignition in an internal combustion engine in which the spark plug of the third embodiment is installed in an experimental example 6. FIG. 13 is an analytical diagram showing the combustion range of fuel 2 ms after ignition in an internal combustion engine in which the spark plug of the third embodiment is installed in an experimental example 6. FIG. 11 is a cross-sectional view of a tip portion of a spark plug according to a second embodiment . FIG. 11 is a cross-sectional view of a tip portion of a spark plug, illustrating the flow of air in an auxiliary combustion chamber in the reference embodiment 2 . FIG. 11 is a cross-sectional view of a tip portion of a spark plug according to a third embodiment . FIG. 13 is a cross-sectional view of a tip portion of a spark plug according to a fourth embodiment . A cross-sectional view taken along line XXVIII-XXVIII in Figure 27. FIG. 13 is a cross-sectional view of a tip portion of a spark plug according to a fifth embodiment . FIG. 13 is a cross-sectional view of a tip portion of a spark plug according to a sixth embodiment . FIG. 13 is a cross-sectional view of a tip portion of a spark plug according to a seventh embodiment . FIG. 13 is a cross-sectional view of a tip portion of a spark plug according to a fourth embodiment . 13 is a cross-sectional view of a tip portion of a spark plug according to a fourth embodiment , showing distances between respective portions.

(Embodiment 1)
An embodiment of a spark plug for an internal combustion engine will be described with reference to FIGS.
As shown in Fig. 1, a spark plug 1 for an internal combustion engine according to this embodiment has a cylindrical insulator 3, a center electrode 4, a cylindrical housing 2, a plug cover 5, and an auxiliary combustion chamber 50. The center electrode 4 is held on the inner periphery of the insulator 3 and protrudes from the insulator 3 toward the tip side. The housing 2 holds the insulator 3 on its inner periphery. The plug cover 5 is provided at the tip portion of the housing 2. The auxiliary combustion chamber 50 is surrounded by at least the plug cover 5, the housing 2, the insulator 3, and the center electrode 4.

As shown in FIG. 3, the distance from the tip of the insulator 3 in the plug axial direction Z to the base end 211 of the inner circumferential surface 21 of the housing 2 facing the auxiliary combustion chamber 50 is defined as distance L1. The shortest distance from the plug center axis C to the inner circumferential surface 21 of the housing 2 in the plug radial direction is defined as distance L2. In this embodiment of the spark plug 1, distance L1 is equal to or less than distance L2.

The spark plug 1 of this embodiment can be used as an ignition means in an internal combustion engine for a vehicle such as an automobile. In this specification, the plug central axis C means the central axis C of the spark plug 1. The direction parallel to the plug central axis C is also referred to as the plug axial direction or Z direction, as appropriate. The side connected to the ignition coil (not shown) in the Z direction is referred to as the base end side, and the side disposed in the main combustion chamber is referred to as the tip side. The plug radial direction means the radial direction of a circle centered on the plug central axis C on a plane perpendicular to the plug central axis C.

The center electrode 4 has an approximately cylindrical shape overall. As shown in FIG. 1, the center electrode 4 is arranged along the plug central axis C. The center electrode 4 is formed, for example, from a base material made of a metal material whose main component is nickel.

The tip of the insulator 3 that holds the center electrode 4 tapers toward the tip in the portion facing the auxiliary combustion chamber 50. The tip surface 31 of the tip of the insulator 3 is curved on the outer circumferential side. The tip surface 31 is flat on the inner side of the curved outer circumferential side in the plug radial direction. As shown in FIG. 4, the creepage distance L5 of the insulator 3 between the center electrode 4 and the housing 2 is longer than the length L4 of the discharge gap shown in FIG. 3. The insulator 3 is made of a ceramic such as alumina.

As shown in FIG. 1, a supported portion 33 is formed on a portion of the outer circumferential surface of the insulator 3. The supported portion 33 is formed in a stepped shape with a smaller diameter at the tip end than at the base end. In other words, the diameter of the supported portion 33 decreases toward the tip end so that it approaches the plug central axis C. The supported portion 33 is formed in an annular shape.

The tip of the housing 2 that holds the insulator 3 is located further forward than the tip of the insulator 3. The housing 2 also has a support portion 22 in which a part of the inner circumferential surface 21 protrudes inward in the plug radial direction. The support portion 22 is formed in an annular shape. The housing 2 is made of a metal material such as iron, nickel, an alloy of iron or nickel, or stainless steel.

The support portion 22 supports the supported portion 33 of the insulator 3 in the Z direction. Specifically, the support portion 22 of the housing 2 supports the supported portion 33 of the insulator 3 via an annular gasket 11. The gasket 11 is, for example, made of a metal material formed into an annular shape.

As shown in FIG. 1, a gap is formed between the insulator 3 and the housing 2 in the plug radial direction. The clearance portion 12, which is a gap formed on the tip side of the gasket 11, is connected to the auxiliary combustion chamber 50.

The clearance portion 12 has a width in the plug radial direction that does not allow the flame formed in the auxiliary combustion chamber 50 to penetrate therein. In other words, the width of the clearance portion 12 in the plug radial direction is equal to or less than the quenching distance of the flame formed in the auxiliary combustion chamber 50. In this embodiment, the width of the clearance portion 12 in the plug radial direction is, for example, 1 mm or less.
It should be noted that the clearance portion 12 is not included in the auxiliary combustion chamber 50 .

In this embodiment, the auxiliary combustion chamber 50 has a pocket portion 501 between the insulator 3 and the housing 2 in the plug radial direction, as shown in FIG. 1. As shown in FIG. 3, the distance L1 is equal to or less than the maximum width L3 of the opening 502 of the pocket portion 501 in the plug radial direction.

In this specification, the pocket portion 501 is a space formed between the insulator 3 and the housing 2 in the plug radial direction, and indicates a part of the auxiliary combustion chamber 50. The width of the pocket portion 501 in the plug radial direction is, for example, greater than 1 mm. In other words, the clearance portion 12 is not included in the pocket portion 501.

In other words, distance L1 is the length from the tip to the base end of the pocket portion 501 in the Z direction. In this embodiment, distance L1 is 1/2 or less of the maximum diameter of the auxiliary combustion chamber 50 in the plug radial direction. Also, distance L1 is less than the distance from the tip of the insulator 3 to the tip of the auxiliary combustion chamber 50 in the Z direction. Also, distance L1 is 1/2 or less of distance L2.

The ratio of the surface area of the insulator 3 and the housing 2 facing the pocket portion 501 to the volume of the pocket portion 501 (hereinafter referred to as the "surface area-to-volume ratio of the pocket portion 501") is 10 or less. It is more preferable that the surface area-to-volume ratio of the pocket portion 501 is 7 or less. The surface area-to-volume ratio of the pocket portion 501 refers to the ratio of the sum of the surface area of the insulator 3 facing the pocket portion 501 and the surface area of the housing 2 facing the pocket portion 501 to the volume of the pocket portion 501.

The tip surface 31 of the insulator 3 has an exposed surface 36 facing the auxiliary combustion chamber 50, as shown in Figs. 1 and 3. As shown in Fig. 3, in a cross section including the plug central axis C, the smaller angle α between the plug central axis C and the least squares approximation straight line S1 of the outline of the exposed surface 36 between the outer peripheral surface of the center electrode 4 and the inner peripheral surface 21 of the housing 2 is greater than 45°.

The spark plug 1 of this embodiment also has a ground electrode 6 that forms a discharge gap G between itself and the center electrode 4. As shown in FIG. 3, the length L4 of the discharge gap G is 0.6 mm or less.

The ground electrode 6 is joined to the housing 2. The ground electrode 6 protrudes from the inner peripheral surface 21 of the housing 2 toward the plug central axis C. The ground electrode 6 and the central electrode 4 face each other in the Z direction, and a discharge gap G is formed between them. The discharge gap G is located in the auxiliary combustion chamber 50.

A plug cover 5 is provided at the tip of the housing 2. The plug cover 5 covers the auxiliary combustion chamber 50. The plug cover 5 has a plurality of injection holes 51 formed therein. The injection holes 51 are formed to connect the auxiliary combustion chamber 50 to the outside.

In this embodiment, as shown in Figs. 1 and 2, the plug cover 5 is formed with the injection holes 51, which are an axial injection hole 511 formed along the Z direction and a plurality of inclined injection holes 512 inclined with respect to the Z direction. The axial injection hole 511 is formed along the plug central axis C. As shown in Fig. 1, the inclined injection holes 512 open at an incline with respect to the Z direction so as to move outward in the plug radial direction as they move toward the tip side.

Next, the effects of this embodiment will be described.
In the spark plug of this embodiment, the distance L1 is equal to or shorter than the distance L2, which improves the scavenging performance of the auxiliary combustion chamber 50. As a result, the air-fuel mixture can be combusted stably.

In other words, if the distance L1 is too long, unburned fuel or gas after combustion is likely to remain at the base end of the auxiliary combustion chamber 50. If unburned fuel remains in the auxiliary combustion chamber 50, it may cause pre-ignition or the like in the next combustion cycle. In addition, if gas after combustion remains in the auxiliary combustion chamber 50, the ignition ability of the auxiliary combustion chamber 50 may decrease. On the other hand, in this embodiment, the distance L1 is equal to or less than the distance L2. Therefore, the scavenging ability of the auxiliary combustion chamber 50 can be improved. Therefore, unburned fuel or gas after combustion is unlikely to remain in the auxiliary combustion chamber 50. As a result, the occurrence of pre-ignition and the like can be suppressed, and the decrease in ignition ability in the auxiliary combustion chamber 50 can be suppressed. As a result, the mixture can be burned stably.

The distance L1 is equal to or less than the maximum width L3 of the opening 502 of the pocket portion 501 in the plug radial direction. This improves the scavenging performance of the auxiliary combustion chamber 50. As a result, the air-fuel mixture can be burned stably.

The ratio of the surface area to the volume of the pocket portion 501 is 10 or less. Therefore, unburned fuel is less likely to remain in the pocket portion 501. As a result, the mixture can be burned stably.

In other words, if the ratio of the surface area to the volume of the pocket portion 501 is too large, the flame is likely to lose heat energy in the pocket portion 501. In other words, the flame is likely to be cooled by the surfaces of the housing 2 and the insulator 3 facing the pocket portion 501. Therefore, the fuel in the pocket portion 501 is less likely to burn, and unburned fuel is likely to remain. As a result, there is a risk of pre-ignition, etc. Also, if the flame loses heat energy in the pocket portion 501, the momentum of the flame ejected from the auxiliary combustion chamber 50 to the main combustion chamber through the nozzle hole 51 is likely to weaken, and the combustion of the internal combustion engine may become unstable. On the other hand, in this embodiment, since the ratio of the surface area to the volume of the pocket portion 501 is small, the flame is less likely to lose heat energy in the pocket portion 501. Therefore, unburned fuel is less likely to remain in the pocket portion 501. As a result, the mixture of the internal combustion engine can be burned stably.

In addition, in this embodiment, the ratio of the surface area to the volume of the pocket portion 501 is 7 or less. Therefore, unburned fuel is less likely to remain in the pocket portion 501.

In addition, the length L4 of the discharge gap G is 0.6 mm or less. Therefore, the distance L1 can be shortened. As a result, the scavenging performance of the auxiliary combustion chamber 50 can be further improved.

In other words, as the length of the discharge gap G increases, the required voltage for generating a discharge in the discharge gap G tends to increase. In addition, in order to suppress the generation of a creeping discharge along the surface of the insulator 3, the creeping distance of the insulator 3 between the center electrode 4 and the housing 2 needs to be increased as the length of the discharge gap G increases. In other words, in order to reliably generate a discharge in the discharge gap G, it is necessary to set the creeping distance of the insulator 3 between the center electrode 4 and the housing 2 to an appropriate distance according to the length of the discharge gap G. Therefore, when the length of the discharge gap G is long, the creeping distance of the insulator 3 between the center electrode 4 and the housing 2 also increases, and the length of the pocket portion 501 in the Z direction tends to increase. In other words, as the length of the discharge gap G increases, the distance L1 also tends to increase. On the other hand, in this embodiment, the length L4 of the discharge gap G is 0.6 mm or less. Therefore, the distance L1 can be shortened. As a result, the scavenging performance of the auxiliary combustion chamber 50 can be improved.

In addition, in a cross section including the plug center axis C, the angle α is greater than 45°. Therefore, the distance L1 is likely to be equal to or less than the distance L2. As a result, it is easy to improve the scavenging performance of the auxiliary combustion chamber 50.

The tip surface 31 of the insulator 3 is curved on its outer periphery. This ensures that the insulator 3 is not damaged during assembly.

As described above, this embodiment provides a spark plug 1 for an internal combustion engine that can stably combust an air-fuel mixture.

(Embodiment 2)
As shown in Figs. 5 and 6, this embodiment is an embodiment in which the length of the discharge gap G is shorter than that of the first embodiment.

As shown in FIG. 6, in this embodiment, the length L4 of the discharge gap G is 0.3 mm or less.
The rest is the same as in embodiment 1. Note that, among the reference symbols used in embodiment 2 and onwards, the reference symbols that are the same as those used in the above-mentioned embodiments represent the same components, etc. as those in the above-mentioned embodiments, unless otherwise specified.

In this embodiment, the length L4 of the discharge gap G is 0.3 mm or less. Therefore, the creepage distance of the insulator 3 between the center electrode 4 and the housing 2 can be further shortened. Therefore, the distance L1 can be further shortened. As a result, the scavenging performance of the auxiliary combustion chamber 50 can be further improved.
In addition, the second embodiment has the same effects as the first embodiment.

(Comparative Example 1)
As shown in FIG. 7, the spark plug 9 of this embodiment does not have a sub-combustion chamber.

In this embodiment, the ground electrode 6 is connected to the tip of the housing 2 as shown in FIG. 7. The ground electrode 6 has an erect portion 61 formed along the Z direction, an opposing portion 62 formed along the plug radial direction, and a bent portion 63 connecting the erect portion 61 and the opposing portion 62. In other words, the ground electrode 6 in this embodiment is substantially L-shaped.

The opposing portion 62 of the ground electrode 6 and the center electrode 4 face each other in the Z direction. A discharge gap G is formed between the opposing portion 62 and the center electrode 4.

Furthermore, the spark plug 9 of this embodiment does not have a plug cover that forms a sub-combustion chamber, so that when the spark plug 9 of this embodiment is installed in an internal combustion engine, the discharge gap G is disposed in a state where it is exposed to the main combustion chamber.
The rest is the same as in the first embodiment.

(Comparative Example 2)
As shown in FIGS. 8 and 9, a spark plug 91 according to this embodiment is a spark plug 91 having one injection hole 51 formed in a plug cover 5 .

As shown in Figures 8 and 9, the plug cover 5 has one axial injection hole 511 formed along the Z direction.

As shown in FIG. 9, in this embodiment, the distance L1 is longer than the distance L2. Also, the distance L1 is longer than the maximum width L3 of the opening 502 of the pocket portion 501 in the plug radial direction.

In this embodiment, the length from the tip of the insulator 3 to the tip of the auxiliary combustion chamber 50 is longer than the maximum width of the auxiliary combustion chamber 50 in the plug radial direction.

In the spark plug 91 of this embodiment, a part of the plug cover 5 serves as the ground electrode 6. In other words, a discharge gap G is formed between the center electrode 4 and the base end of the inner circumferential surface of the axial injection hole 511. The distance of the discharge gap G in the spark plug 91 of this embodiment is smaller than the distance of the discharge gap in the spark plug 9 of the first comparative embodiment.
The rest is the same as in the first embodiment.

(Experimental Example 1)
This example is an example of analyzing the relationship between the time elapsed from application of voltage to the spark plug and the discharge voltage in an internal combustion engine equipped with a spark plug 9 of Comparative Example 1 or a spark plug 91 of Comparative Example 2, as shown in Fig. 10. In this example, a naturally aspirated in-line 4-cylinder internal combustion engine with a displacement of 2000cc was used, and the analysis was performed under a condition of full throttle. Voltage was applied to each spark plug during the compression stroke. Fig. 10(a) shows the analysis result of Comparative Example 1, and Fig. 10(b) shows the analysis result of Comparative Example 2.

From the results of this example, it was confirmed that the discharge voltage was maintained higher in the comparative form 2 in FIG. 10(b) than in the comparative form 1 in FIG. 10(a) after the voltage was applied. It is believed that the high discharge voltage is due to the increase in the resistance value caused by the discharge being extended. Therefore, it is believed that the discharge was extended more in the comparative form 2 in which the discharge gap was arranged in the auxiliary combustion chamber than in the comparative form 1 in which the discharge gap was exposed to the main combustion chamber. As will be explained in the experimental example 2 described later, in an internal combustion engine in which a spark plug having an auxiliary combustion chamber is installed, gas flows from the main combustion chamber to the auxiliary combustion chamber through the injection hole during the compression stroke. As a result, the air flow is more likely to be stronger in the auxiliary combustion chamber than in the main combustion chamber. Therefore, it is believed that the discharge generated in the discharge gap is more likely to be extended in the spark plug in which the discharge gap is arranged in the auxiliary combustion chamber as in the comparative form 2 or the embodiment 1 than in the spark plug in which the discharge gap is exposed to the main combustion chamber as in the comparative form 1. Therefore, it is believed that a spark plug with a discharge gap located inside the auxiliary combustion chamber can improve ignition performance by extending the discharge, even if the length of the discharge gap is short.

In this way, a spark plug equipped with a sub-combustion chamber is more likely to extend the discharge and improve ignition performance. This means that it is easier to shorten the discharge gap while ensuring ignition performance. By shortening the discharge gap, it is possible to shorten the creepage distance between the center electrode and the housing along the surface of the insulator. Therefore, it is possible to make the pocket shallower, for example, as in embodiment 1.

(Experimental Example 2)
In this example, as shown in FIG. 11, the spark plug 91 of Comparative Example 2 is installed in an internal combustion engine 910, and the strength of the airflow in each of the main combustion chamber 101 and the auxiliary combustion chamber 50 is analyzed. The analysis of the airflow was calculated using computational flow dynamics (hereinafter referred to as CFD). That is, in the internal combustion engine 910 equipped with the spark plug 91 of Comparative Example 2, a general simulation analysis was performed using CFD, assuming the airflow generated when the engine is used as an actual automobile engine. FIG. 11 shows the analysis result of the airflow at 20° CA (crank angle) before BTDC (compression top dead center) in the compression stroke. In addition, in FIG. 11, the flow velocity of the airflow is represented by the shade of color. In other words, the darker the color, the faster the flow velocity. The other experimental conditions are the same as those of Experimental Example 1.

As shown in Figure 11, it was confirmed that the airflow around the discharge gap G in the auxiliary combustion chamber 50 is stronger than the airflow in the main combustion chamber 101 near the tip of the spark plug 91. From this result, it is considered that the discharge generated in the discharge gap is more likely to be extended by the airflow in a spark plug whose discharge gap is located in the auxiliary combustion chamber as in Comparative Example 2 or Embodiment 1 than in a spark plug whose discharge gap is exposed to the main combustion chamber as in Comparative Example 1.

(Experimental Example 3)
In this example, as shown in Fig. 12, the combustion speed of the air-fuel mixture was analyzed in an internal combustion engine equipped with the spark plug 1 of the first embodiment. As a comparison, an internal combustion engine equipped with the spark plug of the comparative embodiment 1 was also analyzed in the same manner. In this example, an internal combustion engine with a displacement of 2500cc and an in-line 4-cylinder engine was used. The length of the discharge gap in the first embodiment was 0.5 mm, and the length of the discharge gap in the comparative embodiment 1 was 1.1 mm.

In addition, in both embodiment 1 and comparative embodiment 1, discharge occurs before the top dead center (TDC) of compression. Also, discharge occurs earlier in comparative embodiment 1 than in embodiment 1. In the graph of FIG. 12, the horizontal axis represents the crank angle, and the vertical axis represents the heat generation rate in the internal combustion engine.

The results of this example confirmed that, despite the delayed ignition timing, the heat generation rate increases earlier in embodiment 1 than in comparative embodiment 1. The results of experimental examples 1 to 3 suggest that in the case of a spark plug with a discharge gap in the pre-combustion chamber, the discharge is likely to be extended by the strong air current in the pre-combustion chamber, so it is believed that ignition performance can be improved even if the length of the discharge gap is shortened. Therefore, it is believed that the combustion speed of the mixture is faster in embodiment 1 compared to comparative embodiment 1.

( Reference Form 1 )
As shown in FIG. 13, this embodiment does not have a pocket portion.

As shown in FIG. 13, in this embodiment, a clearance portion 12 is formed between the tip of the insulator 3 and the housing 2 in the plug radial direction, rather than a pocket portion.

Further, the tip end surface 31 of the insulator 3 is an entirely flat surface. The tip end surface 31 is formed in an annular shape.
The rest is the same as in the first embodiment.

The spark plug 1 of this embodiment does not have a pocket portion, which further improves the scavenging performance of the auxiliary combustion chamber 50. As a result, the air-fuel mixture can be combusted more stably.
In addition, the second embodiment has the same effects as the first embodiment.

( Embodiment 3 )
In this embodiment, as shown in FIGS. 14 and 15, a protrusion 52 is formed on the plug cover 5. As shown in FIG.

In this embodiment, as shown in FIG. 14, the plug cover 5 is formed with a protruding portion 52 that protrudes from the tip end of the plug cover 5 toward the base end. The protruding portion 52 is tapered toward the base end.

The protruding portion 52 is formed in a cylindrical shape. In other words, the protruding portion 52 has an axial injection hole 511 formed therein. The diameter of the axial injection hole 511 decreases toward the base end.

The base end of the protruding portion 52 is formed with a flat base end surface 521 facing the base end side. The base end surface 521 of the protruding portion 52 is formed in an annular shape on the outer periphery of the opening of the axial injection hole 511 on the auxiliary combustion chamber 50 side.

The center electrode 4 has a small diameter portion 41 at its tip. The small diameter portion 41 has a smaller diameter than the portion of the center electrode 4 that is closer to the base end than the small diameter portion 41. The small diameter portion 41 has a flat tip surface that faces the tip side.

In this embodiment, the tip surface of the small diameter portion 41 and the base end surface 521 of the protruding portion 52 are formed so as to be perpendicular to the Z direction. The tip surface of the small diameter portion 41 and the base end surface 521 of the protruding portion 52 are formed on approximately the same plane.

14 and 15 , the discharge gap G is formed between the small diameter portion 41 of the center electrode 4 and the base end of the protruding portion 52. In other words, the discharge gap G is formed between the small diameter portion 41 and the base end of the inner circumferential surface of the axial injection hole 511. The discharge gap G is also formed along the plug radial direction.
The other configurations and effects are the same as those of the first embodiment.

(Comparative Example 3)
As shown in FIGS. 16 and 17, a spark plug 92 of this embodiment is a spark plug 92 in which the length from the tip end to the base end of a pocket portion 501 in the Z direction is longer than that of the third embodiment .

17, in this embodiment, the distance L1 is longer than the distance L2. Also, the distance L1 is longer than the maximum width L3 of the opening 502 of the pocket portion 501 in the plug radial direction.
The rest is the same as in the third embodiment .

(Experimental Example 4)
This example is an example in which the relationship between the amount of heat generated in an internal combustion engine and the time elapsed after ignition by the spark plug discharge was analyzed for an internal combustion engine equipped with the spark plug 1 of the third embodiment or the spark plug 92 of the comparative embodiment 3, as shown in Fig. 18. The test for this example was performed under conditions of an atmospheric pressure of 2.0 MPa and propane fuel. In the graph of Fig. 18, the horizontal axis represents the time elapsed after ignition by the spark plug, and the vertical axis represents the amount of heat generated in the internal combustion engine.

18, the amount of heat generated increases more quickly after ignition in the internal combustion engine equipped with the spark plug 1 of the third embodiment than in the internal combustion engine equipped with the spark plug 92 of the comparative embodiment 3. From the results of this example, it is considered that the mixture in the main combustion chamber can be burned earlier by reducing the depth of the pocket portion 501 in the Z direction or by eliminating the pocket portion 501. In other words, it is considered that the growth of the flame is accelerated and the mixture can be burned earlier by reducing or eliminating the cooling loss of the flame generated in the auxiliary combustion chamber 50 due to the pocket portion 501.

(Experimental Example 5)
As shown in FIG. 19, this example is an example in which the relationship between the surface area/volume ratio of the pocket portion and the remaining amount of unburned fuel after ignition is analyzed for a spark plug having the same basic structure as that of the third embodiment. The length of the pocket portion of the spark plug in the plug radial direction and the length of the pocket portion in the plug axial direction are changed, and the ratio of the surface area/volume is changed. Then, for each spark plug in which the surface area/volume ratio of the pocket portion is changed, the remaining amount of unburned fuel after ignition by the discharge of the spark plug is confirmed. The test of this example was performed in an internal combustion engine in which the spark plug was installed, under conditions of an atmospheric pressure of 2.0 MPa and propane fuel. In the graph of FIG. 19, the horizontal axis indicates the length of the pocket portion in the plug radial direction, and the vertical axis indicates the length of the pocket portion in the plug axial direction.

In FIG. 19, region A1 indicates the region where the ratio of the surface area to the volume of the pocket is 7 or less. Region A2 indicates the region where the ratio of the surface area to the volume of the pocket is 10 or less and greater than 7. Region A3 indicates the region where the ratio of the surface area to the volume of the pocket is 20 or less and greater than 10. In addition, for each spark plug with a different surface area to volume ratio of the pocket, a symbol indicating the degree of remaining unburned fuel is plotted on the graph in FIG. 19.

As shown in Figure 19, the spark plug in region A1 had almost no residual unburned fuel. The spark plug in region A2 had very little residual unburned fuel. The spark plug in region A3 had residual unburned fuel. In other words, the smaller the ratio of the surface area to the volume of the pocket, the less residual unburned fuel there was. From these results, it is believed that the smaller the ratio of the surface area to the volume of the pocket, the less cooling loss caused by the pocket for the flame generated in the auxiliary combustion chamber.

(Experimental Example 6)
As shown in Figs. 20 to 23, this example is an example in which the residual amount of unburned fuel F1 was analyzed for an internal combustion engine 10, 920 in which the spark plug 1 of the third embodiment or the spark plug 92 of the comparative embodiment 3 was installed. Each spark plug was installed in the cylinder head 102 of the internal combustion engine so that the outer surface of the plug cover 5 was exposed to the main combustion chamber 101. The test of this example was performed using a naturally aspirated internal combustion engine with a displacement of 2000 cc, a rotation speed of 2000 rpm, and a fully open throttle. Figs. 20 and 21 show the analysis results of the comparative embodiment 3, and Figs. 22 and 23 show the analysis results of the third embodiment . Figs. 20 and 22 show the situation immediately after ignition by the discharge of the spark plug, and Figs. 21 and 23 show the situation 2 ms after ignition by the spark plug. The gas F2 after combustion indicates gas containing fuel that has been burned to 2000°C or higher.

As shown in FIG. 21, 2 ms after ignition, the remaining unburned fuel F1 was confirmed in the auxiliary combustion chamber 50 of the spark plug 92 of the comparative embodiment 3. It was also confirmed that the unburned fuel F1 mainly remained in the pocket portion 501. On the other hand, in the spark plug 1 of the embodiment 3 , as shown in FIG. 23, 2 ms after ignition, the majority of the interior of the auxiliary combustion chamber 50 was occupied by the gas F2 after combustion, and the remaining unburned fuel F1 was very small. From the results of this example, it is considered that the remaining unburned fuel F1 in the auxiliary combustion chamber 50 can be reduced by shortening the length of the pocket portion 501 in the Z direction or by eliminating the pocket portion 501. It is considered that this can suppress the occurrence of pre-ignition, etc.

( Reference Form 2 )
In this embodiment, as shown in FIGS. 24 and 25, a recess 32 is formed at the tip of the insulator 3. As shown in FIG.

As shown in FIG. 24, the tip surface 31 of the insulator 3 has a recessed portion 32 that is recessed toward the base end from the outer peripheral end 311 of the tip surface 31.

In this embodiment, the recess 32 is formed so as to decrease in diameter toward the base end side. That is, the recess 32 is formed so as to have a substantially conical shape. In this embodiment, no pocket is formed.
The rest is the same as in the third embodiment .

In this embodiment of the spark plug 1, the tip surface 31 of the insulator 3 has a recessed portion 32. Therefore, the growth of the flame is less likely to be hindered in the auxiliary combustion chamber 50. As a result, the mixture can be burned stably.

In addition, the recess 32 allows the volume of the auxiliary combustion chamber 50 to be expanded. In other words, the recess 32 allows the amount of air-fuel mixture in the auxiliary combustion chamber 50 to be increased. Therefore, the flame in the auxiliary combustion chamber 50 can be forcefully ejected into the main combustion chamber through the nozzle hole 51. As a result, the air-fuel mixture can be burned stably.

In addition, by forming the recessed portion 32, the creepage distance between the center electrode 4 and the housing 2 in the insulator 3 can be increased. Therefore, even if the length of the discharge gap G is increased, the occurrence of creepage discharge between the center electrode 4 and the housing 2 can be reliably suppressed. Therefore, even if the length of the discharge gap G is increased, discharge can be reliably generated in the discharge gap G. Therefore, the ignition ability of the auxiliary combustion chamber 50 can be improved. As a result, the mixture can be burned stably.

The recessed portion 32 is formed so as to decrease in diameter toward the base end side. Therefore, as shown in Fig. 25, the airflow AF flowing from the main combustion chamber into the auxiliary combustion chamber 50 through the injection hole 51 flows along the recessed portion 32, and the gas in the auxiliary combustion chamber 50 can be circulated efficiently. Therefore, the scavenging performance of the auxiliary combustion chamber 50 can be improved. As a result, the air-fuel mixture in the auxiliary combustion chamber 50 can be burned stably.
In addition, the present embodiment has the same effects as the third embodiment .

( Reference Form 3 )
As shown in FIG. 26, this embodiment is an embodiment in which the shape of the recessed portion 32 is changed from that of the second embodiment .

In this embodiment, as shown in FIG. 26, the recess 32 is formed so as to move away from both the inner circumferential surface 21 of the housing 2 and the outer circumferential surface of the center electrode 4 as it moves toward the base end. In other words, the recess 32 is recessed furthest toward the base end at a position approximately in the center between the inner circumferential surface 21 of the housing 2 and the outer circumferential surface of the center electrode 4 in the plug radial direction.

In addition, the inner peripheral end of the tip surface 31 of the insulator 3 and the outer peripheral end 311 of the tip surface 31 are positioned at substantially the same position in the Z direction. In other words, the recessed portion 32 is formed without increasing the exposure of the inner peripheral surface 21 of the housing 2 and the outer peripheral surface of the center electrode 4 to the auxiliary combustion chamber 50.
The rest is the same as in the second embodiment .

In this embodiment, the recessed portion 32 is formed so as to move away from both the inner circumferential surface 21 of the housing 2 and the outer circumferential surface of the center electrode 4 toward the base end. Therefore, the recessed portion 32 can be formed without increasing the exposure of the inner circumferential surface 21 of the housing 2 and the outer circumferential surface of the center electrode 4 to the auxiliary combustion chamber 50. Therefore, it is possible to suppress the flame in the auxiliary combustion chamber 50 from being cooled by the housing 2 and the center electrode 4 made of a metallic material. As a result, the mixture in the auxiliary combustion chamber 50 can be stably combusted.
In addition, it has the same effects as the second embodiment .

( Reference Form 4 )
In this embodiment, as shown in FIGS. 27 and 28, a groove 34 is formed in the tip end surface 31 of the insulator 3. In the embodiment shown in FIG.

As shown in Figures 27 and 28, the spark plug 1 of this embodiment has a plurality of grooves 34 formed in the tip surface 31 of the insulator 3. The grooves 34 are formed by receding the tip surface 31 toward the base end. As shown in Figure 28, when the tip portion of the insulator 3 is viewed from the tip side, each of the grooves 34 is formed in an annular shape. Furthermore, when the tip portion of the insulator 3 is viewed from the tip side, each of the grooves 34 is formed in a substantially concentric shape with the plug central axis C as the center.
The rest is the same as in the second embodiment .

In the spark plug 1 of this embodiment, a groove 34 is formed in the tip end surface 31 of the insulator 3. This makes it possible to further increase the creepage distance of the insulator 3 between the center electrode 4 and the housing 2. As a result, even if the length of the discharge gap G is increased, discharge can be more reliably generated in the discharge gap G.
In addition, it has the same effects as the second embodiment .

( Reference Form 5 )
As shown in FIG. 29, this embodiment is an embodiment in which the shape of the groove 34 is modified from that of the fourth embodiment .

In this embodiment, as shown in FIG. 29, the groove 34 is formed so that its width in the plug radial direction decreases toward the base end side.
The other configurations and effects are the same as those of the fourth embodiment .

( Reference Form 6 )
In this embodiment, as shown in FIG. 30, a step 35 is formed at the tip of the insulator 3.

30, the step 35 is formed by recessing the tip end surface 31 of the insulator 3 toward the base end. The step 35 is formed so as to expose a part of the outer circumferential surface of the center electrode 4. In other words, an annular space is formed in the plug radial direction between the part of the outer circumferential surface of the center electrode 4 and the surface of the insulator 3 that forms the step 35. The width of the annular space in the plug radial direction is, for example, 1 mm or less.
The rest is the same as in the second embodiment .

In this embodiment, a step 35 is formed at the tip end of the insulator 3. Therefore, the creepage distance of the insulator 3 between the center electrode 4 and the housing 2 can be further increased.
In addition, it has the same effects as the second embodiment .

( Reference Form 7 )
As shown in FIG. 31, this embodiment is different from the sixth embodiment in that the position at which the step portion 35 is formed is different.

In this embodiment, as shown in Fig. 31 , the step portion 35 is formed so as to expose a part of the inner circumferential surface 21 of the housing 2. An annular space is formed between the part of the inner circumferential surface 21 of the housing 2 and the surface of the insulator 3 that forms the step portion 35 in the plug radial direction. The width of the annular space in the plug radial direction is, for example, 1 mm or less. In other words, the annular space is not a part of the auxiliary combustion chamber 50 but is the clearance portion 12.
The other configurations and effects are the same as those of the sixth embodiment .

( Embodiment 4 )
In this embodiment, as shown in FIGS. 32 and 33, a small diameter portion 37 is formed at the tip end of the insulator 3.

As shown in Figures 32 and 33, the small diameter portion 37 has a smaller diameter than the portion of the insulator 3 closer to the base end than the small diameter portion 37. The portion of the small diameter portion 37 closer to the base end is formed so that the diameter increases toward the base end. A pocket portion 501 is formed between the small diameter portion 37 in the plug radial direction and the inner circumferential surface 21 of the housing 2.

In the spark plug 1 of this embodiment, the distance L1 is equal to or less than the distance L2 as shown in Fig. 33. The distance L1 is also equal to or less than the maximum width L3 of the opening 502 of the pocket portion 501 in the plug radial direction.
The other configurations and effects are the same as those of the third embodiment .

The present invention is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the spirit of the present invention.

1...spark plug, 2...housing, 21...inner surface, 3...insulator, 4...center electrode, 5...plug cover, 50...auxiliary combustion chamber, C...plug center axis, Z...plug axial direction, 211...base end of inner surface of housing facing auxiliary combustion chamber, L1...distance from tip of insulator in plug axial direction to base end of inner surface of housing facing auxiliary combustion chamber, L2...shortest distance from plug center axis to inner surface of housing in plug radial direction

Claims (4)

A cylindrical insulator (3);
a center electrode (4) held on the inner circumferential side of the insulator and protruding from the insulator toward the tip side;
a cylindrical housing (2) that holds the insulator on its inner periphery;
A plug cover (5) provided at the tip of the housing;
a sub-combustion chamber (50) surrounded by at least the plug cover, the housing, the insulator, and the center electrode,
a distance (L1) from a tip of the insulator in a plug axial direction (Z) to a base end (211) of an inner circumferential surface (21) of the housing facing the auxiliary combustion chamber is equal to or shorter than a shortest distance (L2) from a plug central axis (C) to the inner circumferential surface of the housing in a plug radial direction,
The auxiliary combustion chamber has a pocket portion (501) between the insulator and the housing in the plug radial direction,
The distance (L1) is equal to or less than the maximum width (L3) of the opening (502) of the pocket portion in the plug radial direction,
a width of the pocket portion in a radial direction of the plug is greater than a quenching distance of a flame formed in the auxiliary combustion chamber,
A spark plug (1) for an internal combustion engine , wherein a relationship between a surface area S ^mm2 of the insulator and the housing facing the pocket portion and a volume V ^mm3 of the pocket portion satisfies S/V≦10 mm ⁻¹ . 2. The spark plug for an internal combustion engine according to claim 1 , further satisfying S/V≦7 mm ⁻¹ . A ground electrode (6) is provided between the center electrode and the ground electrode to form a discharge gap (G),
3. The spark plug for an internal combustion engine according to claim 1, wherein the length (L4) of said discharge gap is 0.6 mm or less. 4. The spark plug for an internal combustion engine according to claim 3 , wherein the length of said discharge gap is 0.3 mm or less.

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