JP7584371B2 - Spark plugs for internal combustion engines - Google Patents

Description

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

Some spark plugs for internal combustion engines are equipped with a sub-combustion chamber in which a discharge gap is arranged. In some such spark plugs, as shown in Patent Document 1, multiple injection holes provided in the plug cover that forms the sub-combustion chamber are inclined relative to the plug radial direction when viewed from the plug axial direction. This allows a swirl flow, which is an air flow that swirls around the plug axial direction, to be formed in the sub-combustion chamber.

JP 2009-270539 A

However, in the spark plug described in Patent Document 1, it is difficult to generate sufficient airflow near the discharge gap.

That is, although a swirl flow is formed in the auxiliary combustion chamber by the airflow flowing into the auxiliary combustion chamber from the nozzle hole, the airflow is prone to stagnation near the center of the auxiliary combustion chamber. Therefore, it is difficult to form a sufficient airflow near the discharge gap. As a result, it is difficult to extend the discharge formed in the discharge gap by the airflow. Therefore, it can be said that there is room for improvement in the ignition performance of the spark plug described in the above Patent Document 1.

The present invention was made in consideration of these problems, and aims to provide a spark plug for internal combustion engines with excellent ignition properties.

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 ground electrode (6) forming a discharge gap (G) between itself and the center electrode;
a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed,
The plug cover is formed with a plurality of nozzle holes (51) that communicate the auxiliary combustion chamber with the outside,
The ground electrode is formed so as to protrude from an outer periphery of the auxiliary combustion chamber toward a central axis of the plug,
The plurality of nozzle holes are provided so that an eccentric swirl flow (Fs) is formed in the auxiliary combustion chamber by the airflow flowing in from the nozzle holes,
The eccentric swirl flow is a swirling flow having a center of rotation at a position on the protruding side of the ground electrode relative to the discharge gap as viewed in the plug axial direction (Z), in a spark plug (1) for an internal combustion engine.

In the above spark plug for an internal combustion engine, the multiple nozzle holes are arranged so that the airflow flowing in from the nozzle holes creates an eccentric swirl flow in the auxiliary combustion chamber. Therefore, the discharge generated in the discharge gap can be extended by the eccentric swirl flow. This improves ignition performance.

As described above, according to the above aspect, a spark plug for an internal combustion engine having excellent ignition properties can be provided.
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.

1 is a cross-sectional view taken along an axial direction of a spark plug in a vicinity of a tip portion of the spark plug according to a first embodiment; FIG. 2 is a cross-sectional view taken along line II-II of FIG. FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2 . FIG. 2 is a front view of the spark plug attached to the internal combustion engine in the first embodiment. FIG. 4 is an explanatory diagram of an airflow generated in an auxiliary combustion chamber in the first embodiment. FIG. 2 is an explanatory diagram in which an extension region of a first nozzle hole is added to FIG. 1 . FIG. 3 is an explanatory diagram in which an extension region of the first nozzle hole is added to FIG. 2 . FIG. 11 is an explanatory diagram of an analysis result as viewed in a cross section that passes through the first nozzle hole and includes the central axis of the plug in Experimental Example 1. FIG. 4 is an explanatory diagram of an analysis result in Experimental Example 1, viewed in a cross section passing through the discharge gap and perpendicular to the plug axial direction. FIG. 11 is an explanatory diagram of the angle θ in Experimental Example 2. 13 is a diagram showing test results in Experimental Example 2. FIG. 11 is an explanatory diagram of the distance L1 and the inner diameter d in Experimental Example 3. 13 is a diagram showing test results in Experimental Example 3. 13 is a diagram showing test results in Experimental Example 4. FIG. 13 is an explanatory diagram of area S5 in Experimental Example 4. FIG. 13 is an explanatory diagram of the distance L1 and the inner diameter d in Experimental Example 5. 13 is a diagram showing test results in Experimental Example 5. FIG. 13 is an explanatory diagram of one sample in Experimental Example 6. FIG. 13 is an explanatory diagram of another sample in Experimental Example 6. 13 is a diagram showing test results in Experimental Example 6. 13 is a diagram showing test results in Experimental Example 7. FIG. 13 is an explanatory diagram showing the arrangement of nozzle holes in a comparative sample in Experimental Example 7. FIG. 11 is an explanatory diagram showing an arrangement of nozzle holes in a second embodiment. FIG. 11 is an explanatory diagram showing the arrangement of nozzle holes in a third embodiment. FIG. 13 is an explanatory diagram showing the arrangement of nozzle holes in a fourth embodiment. FIG. 13 is an explanatory diagram showing the arrangement of nozzle holes in a fifth embodiment. FIG. 13 is a cross-sectional view taken along the axial direction of a spark plug in a vicinity of a tip portion of a spark plug according to a sixth embodiment. A cross-sectional view taken along line XXVIII-XXVIII in Figure 27. FIG. 13 is a cross-sectional view taken along the axial direction of a spark plug in a vicinity of a tip portion of a spark plug according to a seventh embodiment. FIG. 13 is a cross-sectional view taken along the axial direction of a spark plug in an eighth embodiment, showing the vicinity of a tip portion of the spark plug. FIG. 13 is a cross-sectional view of a spark plug according to a ninth embodiment, taken along a cross section perpendicular to the plug axial direction. FIG. 13 is a cross-sectional view of another spark plug according to the ninth embodiment, taken along a cross section perpendicular to the plug axial direction.

(Embodiment 1)
An embodiment of a spark plug for an internal combustion engine will be described with reference to FIGS.
A spark plug 1 for an internal combustion engine according to this embodiment includes a cylindrical insulator 3, a center electrode 4, a cylindrical housing 2, a ground electrode 6, and a plug cover 5, as shown in FIGS.

As shown in Figures 1 and 3, 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 the inner periphery. The ground electrode 6 forms a discharge gap G between the center electrode 4 and the ground electrode 6. The plug cover 5 is provided at the tip of the housing 2 so as to cover the auxiliary combustion chamber 50 in which the discharge gap G is located.

As shown in FIGS. 1 to 3, the plug cover 5 is formed with a plurality of injection holes 51 that communicate the auxiliary combustion chamber 50 with the outside.
The ground electrode 6 is formed so as to protrude from the outer periphery of the auxiliary combustion chamber 50 toward the center axis of the plug.

The multiple nozzle holes 51 are provided so that the airflow flowing in from the nozzle holes 51 creates an eccentric swirl flow in the auxiliary combustion chamber 50. As shown in FIG. 5, the eccentric swirl flow Fs is a swirling flow that has a swirl center at a position on the protruding side of the ground electrode 6 relative to the discharge gap G when viewed from the plug axial direction Z.

The spark plug 1 of this embodiment can be used as an ignition means in internal combustion engines of automobiles, cogeneration systems, etc. As shown in FIG. 4, one end of the spark plug 1 in the axial direction Z is placed in the combustion chamber of the internal combustion engine. This combustion chamber of the internal combustion engine is called the "main combustion chamber 11" as opposed to the above-mentioned "auxiliary combustion chamber 50."

As shown in Figs. 1 and 4, the housing 2 has a mounting thread 23 on its outer circumferential surface. As shown in Fig. 4, the mounting thread 23 is screwed into the female thread of a plug hole 12 provided in the engine head of the internal combustion engine, thereby mounting the spark plug 1 to the internal combustion engine. The spark plug 1 is mounted to the internal combustion engine with a portion of its tip exposed to the main combustion chamber 11.

In the axial direction Z of the spark plug 1, the side exposed to the main combustion chamber 11 is called the tip side, and the opposite side is called the base side. The central axis of the spark plug 1 along the axial direction Z is called the plug central axis. The circumferential direction of a circle centered on the plug central axis is called the plug circumferential direction.

The plug cover 5 is joined to the tip of the housing 2 by welding or the like. One end of the ground electrode 6 is fixed to the housing 2 or the plug cover 5. The ground electrode 6 protrudes from this fixed end toward the plug central axis. In this embodiment, as shown in FIG. 3, the ground electrode 6 is inclined toward the tip as it moves from the fixed end to the protruding end. However, the ground electrode 6 can also be configured to be disposed perpendicular to the plug axial direction Z.

When the spark plug 1 is installed in the internal combustion engine, the plug cover 5 separates the auxiliary combustion chamber 50 from the main combustion chamber 11. The flame generated in the auxiliary combustion chamber 50 is ejected from the nozzle hole 51 into the main combustion chamber 11.

As shown in FIG. 2, the spark plug 1 has at least the following first and second nozzle holes 511 and 512 as multiple nozzle holes 51. The second nozzle hole 512 is closer to the discharge gap G than the first nozzle hole 511.

As shown in Figures 6 and 7, at least a portion of the extension region 511E along the nozzle hole axis 511L of the first nozzle hole 511 overlaps with the discharge gap G. Furthermore, as shown in Figure 5, the first nozzle hole 511 is formed such that a vector 511v from the outer opening 511o to the inner opening 511i when viewed from the plug axis direction Z satisfies the following requirements. That is, the vector 511v faces toward the side approaching the discharge gap G and forms an angle of 90°±30° with respect to the protruding direction 6v of the ground electrode 6. In this embodiment, the angle of the vector 511v with respect to the protruding direction 6v of the ground electrode 6 is approximately 90°.

As shown in FIG. 3, the nozzle axis 512L of the second nozzle hole 512 does not pass through the discharge gap G. Also, as shown in FIG. 5, when viewed from the plug axial direction Z, the vector 512v from the outer opening 512o to the inner opening 512i of the second nozzle hole 512 faces away from the discharge gap G toward the protruding side of the ground electrode 6.

When viewed from the plug axial direction Z, the second injection hole 512 is arranged such that at least a part of the inner opening 512i is located closer to the protruding side of the ground electrode 6 than the discharge gap G and is located inside the width of the ground electrode 6. More preferably, when viewed from the plug axial direction Z, the second injection hole 512 is arranged such that the center 512c of the inner opening 512i is located at the center of the width of the ground electrode 6. Also, in this embodiment, when viewed from the plug axial direction Z, the entire inner opening 512i of the second injection hole 512 is located inside the width of the ground electrode 6.

As shown in FIG. 2, the spark plug 1 of this embodiment has a third injection hole 513 in addition to the first injection hole 511 and the second injection hole 512 as the injection hole 51. As shown in FIG. 5, the third injection hole 513 is an injection hole 51 in which a vector 513v from the outer opening 513o to the inner opening 513i is directed to the first injection hole 511 when viewed from the plug axial direction Z.

The spark plug 1 of this embodiment also has a fourth nozzle hole 514 as the nozzle hole 51 in addition to the first nozzle hole 511, the second nozzle hole 512, and the third nozzle hole 513. The fourth nozzle hole 514 is a nozzle hole whose vector 514v from the outer opening 514o to the inner opening 514i is directed toward the third nozzle hole 513 when viewed from the plug axial direction Z.

The inner openings of the injection holes 51 other than the first injection hole 511 are arranged on the protruding side of the ground electrode 6 relative to the discharge gap G. That is, the inner opening 512i of the second injection hole 512, the inner opening 513i of the third injection hole 513, and the inner opening 514i of the fourth injection hole 514 are arranged on the protruding side of the ground electrode 6 relative to the discharge gap G. In this embodiment, the inner opening 511i of the first injection hole 511 is arranged at a position approximately equivalent to the protruding end in the protruding direction 6v of the ground electrode 6.

When viewed from the plug axial direction Z, the injection hole axis 513L of the third injection hole 513 and the injection hole axis 514L of the fourth injection hole 514 are inclined with respect to the plug radial direction. When viewed from the plug axial direction Z, the injection hole axis 513L of the third injection hole 513 is approximately parallel to the injection hole axis 512L of the second injection hole 512, and the vector 513v is in the opposite direction to the vector 512v. When viewed from the plug axial direction Z, the injection hole axis 514L of the fourth injection hole 514 is approximately parallel to the injection hole axis 511L of the first injection hole 511, and the vector 514v is in the opposite direction to the vector 511v.

When viewed from the plug axis direction Z, the nozzle hole axis 511L of the first nozzle hole 511 and the nozzle hole axis 514L of the fourth nozzle hole 514 are substantially perpendicular to the nozzle hole axis 512L of the second nozzle hole 512 and the nozzle hole axis 513L of the third nozzle hole 513.

In this embodiment, the plug cover 5 is formed with four nozzle holes 51: a first nozzle hole 511, a second nozzle hole 512, a third nozzle hole 513, and a fourth nozzle hole 514. Of these four nozzle holes 51, the second nozzle hole 512 has a larger flow passage cross-sectional area than the other nozzle holes 51. The first nozzle hole 511, the second nozzle hole 512, and the third nozzle hole 513 have approximately the same flow passage cross-sectional area. Here, the flow passage cross-sectional area is the area of a cross section perpendicular to the nozzle hole axis of each nozzle hole 51.

For each of the multiple nozzle holes 51, the vector from the outer opening to the inner opening has a vector component that faces the base end. In other words, the vector from the outer opening to the inner opening faces diagonally toward the base end. As shown in FIG. 6, the nozzle hole axis 511L of the first nozzle hole 511 is inclined, for example, by 55° to 65° with respect to the plug axis direction Z. Also, as shown in FIG. 3, the nozzle hole axis 512L of the second nozzle hole 512 is inclined, for example, by 55° to 65° with respect to the plug axis direction Z.

As described above, as shown in Figures 6 and 7, at least a portion of the extension region 511E of the first nozzle hole 511 along the nozzle hole axis 511L overlaps with the discharge gap G. However, the extension regions of the other nozzle holes 51, i.e., the second nozzle hole 512, the third nozzle hole 513, and the fourth nozzle hole 514 along their respective nozzle hole axes do not overlap with the discharge gap G.

In addition, the extension region 511E of the first nozzle hole 511 does not overlap with the extension regions of the other nozzle holes 51, i.e., the second nozzle hole 512, the third nozzle hole 513, and the fourth nozzle hole 514.

By forming the multiple injection holes 51 in the plug cover 5 in the above-mentioned manner, an eccentric swirl flow Fs in the rotational direction as shown in FIG. 5 can be formed in the auxiliary combustion chamber 50 during the compression stroke of the internal combustion engine. That is, in the case of this embodiment, an eccentric swirl flow Fs having a center of rotation is formed at the lower left position of the discharge gap G in FIG. 5. The eccentric swirl flow Fs rotates clockwise in the figure. Therefore, in the vicinity of the discharge gap G, this eccentric swirl flow Fs has a vector component that faces the protruding side of the ground electrode 6. More specifically, the direction of the eccentric swirl flow Fs in the vicinity of the discharge gap G is the lower right direction in the figure. In addition, in the auxiliary combustion chamber 50, a complex air flow is actually formed, but the eccentric swirl flow Fs as described above is formed as part of the air flow, and when focusing on this eccentric swirl flow Fs, the flow direction is as described above.

Next, the effects of this embodiment will be described.
In the spark plug 1 for an internal combustion engine, the multiple nozzle holes 51 are provided so that an eccentric swirl flow Fs is formed in the auxiliary combustion chamber 50 by the airflow flowing in from the nozzle holes 51. Therefore, the discharge generated in the discharge gap G can be elongated by the eccentric swirl flow Fs, thereby improving ignition performance.

This will be explained in more detail by comparing it with the case where a swirl flow is formed in the auxiliary combustion chamber around the central axis of the plug. First, we will consider the case where a swirl flow is formed in the auxiliary combustion chamber around the central axis of the plug by the airflow flowing in from the injection hole.

That is, for example, in the configuration disclosed in the above-mentioned Patent Document 1, during the compression stroke, the airflow flowing from the main combustion chamber into the auxiliary combustion chamber forms a swirl flow centered on the plug central axis in the auxiliary combustion chamber. As a result, the flow speed of the swirl flow is low near the plug central axis, and airflow stagnation is likely to occur. As a result, when performing compression stroke ignition, the discharge generated in the discharge gap located on the plug central axis is not easily extended. Therefore, for example, there is a concern that ignition performance may decrease during operation using EGR (i.e., exhaust gas recirculation) at low loads, etc.

In contrast, in the spark plug 1 of this embodiment, as described above, an eccentric swirl flow is formed by the inflow airflow. Therefore, during the compression stroke, an airflow of sufficient strength is generated in the discharge gap G. In the case of compression stroke ignition, this airflow tends to elongate the discharge. As a result, ignition performance can be improved.

The spark plug 1 of this embodiment has a first injection hole 511 and a second injection hole 512. At least a part of the extension region 511E of the first injection hole 511 overlaps with the discharge gap G (see FIG. 6). Furthermore, the vector 511v is oriented toward the side approaching the discharge gap G as viewed from the plug axial direction Z, and is formed to form an angle of 90°±30° with respect to the protruding direction 6v of the ground electrode 6 (see FIG. 5). The second injection hole 512 has an injection hole axis 512L that does not pass through the discharge gap G (see FIG. 3), and the vector 512v is oriented away from the discharge gap G toward the protruding side of the ground electrode 6 as viewed from the plug axial direction Z (see FIG. 5). This makes it possible to effectively generate an eccentric swirl flow Fs and to obtain a sufficient strength of the air flow in the discharge gap G. Therefore, the discharge generated by the compression stroke ignition can be sufficiently extended.

In addition, during the expansion stroke, airflow flows from the auxiliary combustion chamber to the main combustion chamber through the nozzle hole, but in the case of the configuration disclosed in the above-mentioned Patent Document 1, airflow accompanying the outflow airflow is difficult to form in a position in the auxiliary combustion chamber away from the nozzle hole. Therefore, it is difficult to form a sufficient airflow near the discharge gap, and even in the case of expansion stroke ignition, a sufficient discharge extension effect cannot be expected.
Therefore, for example, when performing a catalyst warm-up operation under low load or the like, there is a concern that ignition performance may decrease.

In contrast, as described above, the spark plug 1 of this embodiment has the second injection hole 512, which is closer to the discharge gap G than the first injection hole 511. Therefore, along with the airflow flowing out from the second injection hole 512, an airflow toward the second injection hole 512 is generated near the discharge gap G. This makes it possible to extend the discharge generated by the expansion stroke ignition. As a result, ignition performance can be improved even in the expansion stroke ignition.

In addition, when viewed from the plug axial direction Z, at least a portion of the inner opening 512i of the second injection hole 512 is positioned closer to the protruding side of the ground electrode 6 than the discharge gap G and on the inside of the width of the ground electrode 6 (see FIG. 2). This makes it easier to extend the discharge generated in the discharge gap G during expansion stroke ignition toward the second injection hole 512.

The spark plug 1 of this embodiment also has a third nozzle hole 513. This allows an eccentric swirl flow to be effectively formed during the compression stroke.

The spark plug 1 of this embodiment also has a fourth nozzle hole 514. This makes it possible to more effectively form an eccentric swirl flow during the compression stroke.

In addition, the inner openings of the nozzle holes 51 other than the first nozzle hole 511 are arranged on the protruding side of the ground electrode 6 rather than the discharge gap G. This makes it easier to form the center of rotation of the eccentric swirl flow on the protruding side of the ground electrode 6 rather than the discharge gap G.

As described above, this embodiment provides a spark plug for an internal combustion engine with excellent ignition properties.

(Experimental Example 1)
8 and 9, this example is an example in which the formation of an airflow in the auxiliary combustion chamber 50 was confirmed by simulation. Note that, among the reference characters used in the experimental examples and embodiments following this example, the reference characters that are the same as those used in the previously described embodiments represent the same components, etc., as those in the previously described embodiments, unless otherwise specified.

In this example, with the spark plug 1 shown in the first embodiment attached to an internal combustion engine, the airflow inside and outside the auxiliary combustion chamber 50 during the compression stroke was analyzed.
The airflow was analyzed by computational flow dynamics (hereinafter referred to as CFD). That is, in the spark plug 1 of the first embodiment, a general simulation analysis was performed by CFD, assuming an airflow that occurs when the spark plug 1 is used in an actual automobile engine.

Figure 8 shows the analysis results viewed in a cross section that passes through the first nozzle hole 511 and includes the plug center axis. Figure 9 shows the analysis results viewed in a cross section that passes through the discharge gap G and is perpendicular to the plug axial direction Z. In these figures, each of the numerous arrows indicates the direction of the airflow at each location, and the longer the arrow, the faster the flow speed.

As can be seen from Figure 8, most of the airflow flowing in from the first nozzle hole 511 heads toward the discharge gap G. Also, in Figure 9, an eccentric swirl flow can be seen centered around the lower left of the discharge gap G. The eccentric swirl flow has its center of rotation at a position on the protruding side of the ground electrode 6 relative to the discharge gap G as viewed from the plug axial direction Z. Furthermore, when focusing on this swirling swirl flow, it becomes a swirling flow that has a vector component that points toward the protruding side of the ground electrode 6 near the discharge gap.

The analysis results of this example confirm that the spark plug 1 of embodiment 1 creates an eccentric swirl flow during the compression stroke.

(Experimental Example 2)
In this example, as shown in Figs. 10 and 11, an appropriate range of the angle θ of the vector 511v with respect to the protruding direction 6v of the ground electrode 6 was examined.
That is, a number of types of spark plugs were prepared in which the angle θ was changed by changing the position and orientation of the first injection hole 511. An example of such a spark plug is shown in Fig. 5, in which θ is 90°. Also, an example of a spark plug in which θ is less than 90° is shown in Fig. 10.

Then, each of the different types of spark plugs was attached to a 2L supercharged engine, and the engine was operated at 2000 rpm to measure the EGR combustion limit. The EGR combustion limit was defined as the EGR rate at which the combustion variation rate (hereafter referred to as "COV") could be kept below 3%.

Here, the COV is calculated by the following formula:
COV (%) = (indicated mean effective pressure (standard deviation)) / (indicated mean effective pressure (average value))
The indicated mean effective pressure (average value) is 500 kPa.

The measurement results are shown in FIG. 11 as the relationship between the angle θ and the EGR combustion limit. In the figure, the vertical axis indicates the extension of the EGR combustion limit relative to the EGR combustion limit when the angle θ is 0°.

As can be seen from the figure, when the angle θ is 60° or more, the EGR combustion limit is expanded by 1.5 points or more. When the angle θ is 75° or more, the EGR combustion limit is expanded by 3 points or more. Furthermore, when the angle θ approaches 90°, the EGR combustion limit is expanded even more. Note that the expansion of the EGR combustion limit of 1.5 points and 3 points means that the EGR combustion limit is expanded by 1.5% and 3%, respectively, compared to when the angle θ is 0°.

In addition, when the angle θ exceeds 90°, it is theoretically presumed that the following will occur. That is, when the angle θ is 120°, the EGR combustion limit can be expected to be expanded in a manner similar to when the angle θ is 60°. When the angle θ is 105°, the EGR combustion limit can be expected to be expanded in a manner similar to when the angle θ is 75°.

The results of this example show that it is desirable to set the angle θ to 90°±30°. More preferably, the angle θ is 75° to 105°, i.e., 90°±15°. It is even more preferable for the angle θ to be approximately 90°.

(Experimental Example 3)
In this example, as shown in Figs. 12 and 13, an appropriate range of the distance L1 in the plug radial direction between the center 512c of the inner opening 512i of the second injection hole 512 and the discharge gap G was examined.

First, several types of spark plugs were prepared in which the distance L1 was changed by changing the position of the second injection hole 512. The position of the discharge gap G for determining the distance L1 was determined to be the position where the center electrode 4 and the ground electrode 6 overlap in the plug axial direction Z, and where the distance in the plug axial direction Z is the shortest.

Each spark plug was then attached to a 2L supercharged engine, and the engine was operated at 1500 rpm. The ignition timing was set to 10° ATDC during the expansion stroke. The COV at this time was measured. The measurement results are shown in Figure 13. In this figure, the horizontal axis is L1/(d/2), where d is the inner diameter of the plug cover 5, which is constant for all samples. The vertical axis is COV.

As can be seen from the figure, if L1/(d/2) becomes too large, the COV becomes too large. It can also be seen that by setting L1/(d/2) to 0.5 or less, the COV can be reduced to 20% or less. Furthermore, by satisfying 0.2≦L1/(d/2)≦0.5, the COV can be reduced to 15% or less. Note that a COV of 20% or less is a level at which drivability problems are unlikely to occur. Also, a COV of 15% or less is a level at which drivability problems rarely occur.

From the results of this example, it can be said that ignition performance can be improved by making the center position of the inner opening 512i of the second injection hole 512 not too far away from the discharge gap G in the plug radial direction. It can also be said that ignition performance can be improved by making the center position of the inner opening 512i of the second injection hole 512 slightly away from the discharge gap G in the protruding direction of the ground electrode 6.

(Experimental Example 4)
In this example, as shown in FIG. 14, an appropriate range for the cross-sectional area S2 of the second nozzle hole 512 perpendicular to the nozzle hole axis 512L (see FIG. 3) was examined.

First, several types of spark plugs were prepared with different cross-sectional areas S2 of the second nozzle hole 512. Then, in the internal combustion engines to which each spark plug was attached, the COV was measured in the same manner as in Experimental Example 3. The measurement results are shown in FIG. 14. In the figure, the horizontal axis is S2/S5. Here, S5 is the inner area of the outline of the cross section of the plug cover 5 perpendicular to the plug axial direction Z. In other words, the area of the region with diagonal lines inside the outline 59 in FIG. 15 corresponds to S5. The outline in FIG. 15 (i.e., outline 59) shows the same outline as the outline of the cross section shown in FIG. 2. S5 is constant in all samples. The vertical axis is COV.

As can be seen from the figure, if S2/S5 is too small, the COV becomes too large. It can also be seen that by making S2/S5 0.006 or more, the COV can be reduced to 20% or less. Furthermore, by making S2/S5 0.01 or more, the COV can be reduced to 15% or less. This is thought to be because by increasing the cross-sectional area S2 of the second nozzle hole 512 to a certain extent, the airflow flowing in from the second nozzle hole 512 can sufficiently extend the discharge.

In addition, by setting S2/S5 to 0.028 or less, it is possible to prevent the inflow airflow from the second injection hole 512 from becoming too large during the compression stroke. This makes it possible to ensure the formation of an eccentric swirl flow in the auxiliary combustion chamber 50.
Considering this point of view, together with the results of the above-mentioned experimental examples, it is preferable to satisfy 0.006≦S2/S5≦0.028. It is even more preferable to satisfy 0.01≦S2/S5≦0.028.

(Experimental Example 5)
In this embodiment, as shown in FIG. 16 and FIG. 17, an appropriate range of the distance h in the plug axial direction Z between the tip of the center electrode 4 and the tip of the auxiliary combustion chamber 50 was examined.

First, several types of spark plugs with different distances h were prepared. Then, in the internal combustion engines equipped with each spark plug, the COV was measured in the same manner as in Experimental Example 3. The measurement results are shown in Figure 17. In the figure, the horizontal axis is h/d, where d is the inner diameter of the plug cover 5, which is constant for all samples. The vertical axis is COV.

As can be seen from the figure, if h/d becomes too large, the COV becomes too large. It can also be seen that by setting h/d to 0.33 or less, the COV can be reduced to 20% or less. Furthermore, by setting h/d to 0.3 or less, the COV can be reduced to 15% or less.

From the viewpoint of preventing interference between the ground electrode 6 and the plug cover 5, it is preferable that the ratio h/d is 0.15 or more.
In light of this viewpoint, and in addition to the results of this experimental example, it is preferable to satisfy 0.15≦h/d≦0.33. It is even more preferable to satisfy 0.15≦h/d≦0.3.

(Experimental Example 6)
In this embodiment, as shown in Figs. 18 to 20, an appropriate range for the ratio of the volume of the region of the auxiliary combustion chamber 50 on the tip side of the central electrode 4 to the volume of the entire auxiliary combustion chamber 50 was examined.

"V/(h x S50)" is used as an index that approximates the ratio of the volume of the entire auxiliary combustion chamber 50 to the volume of the area of the auxiliary combustion chamber 50 that is further forward than the center electrode 4. Here, h is the distance in the plug axial direction Z between the tip of the center electrode 4 and the tip of the auxiliary combustion chamber 50, as shown in FIG. 16 above. S50 is the inner area of the inner circumference of the cross section of the plug cover 5 that is perpendicular to the plug axial direction Z. V is the volume of the auxiliary combustion chamber 50. The auxiliary combustion chamber 50 includes not only the space inside the plug cover 5, but also the space inside the housing 2 that is continuous with its base end side.

In this example, first, as shown in Figures 16, 18, and 19, several types of spark plugs were prepared in which the volume V was changed to change V/(h x S50). The spark plugs shown in Figures 18 and 19 have a larger protruding length of the center electrode 4 from the insulator 3, and the tip of the insulator 3 is located on the base end side. As a result, these spark plugs have a larger spatial volume inside the housing 2, and a larger volume for the auxiliary combustion chamber 50.

Then, in the internal combustion engine equipped with each spark plug, the COV was measured in the same manner as in Experimental Example 3. The measurement results are shown in Figure 20. In this figure, the horizontal axis is V/(h x S50), and the vertical axis is COV.

As can be seen from the figure, if V/(h x S50) becomes too small, the COV becomes too large. It can also be seen that by making V/(h x S50) 2 or more, the COV can be reduced to 20% or less. Furthermore, by making V/(h x S50) 2.5 or more, the COV can be reduced to 15% or less.

From the viewpoint of the manufacturing cost of the spark plug 1, it is preferable to set V/(h×S50) to 6 or less.
Based on this viewpoint, and in combination with the results of this experimental example, 2≦V/(h×S50)≦6
It is preferable that the following relationship is satisfied. It is further preferable that the following relationship is satisfied: 2.5≦V/(h×S50)≦6.

(Experimental Example 7)
In this example, as shown in FIG. 21, the effect of the volume of the auxiliary combustion chamber 50 on the EGR combustion limit was confirmed.

First, as samples 1 to 3, several spark plugs with different volumes of the auxiliary combustion chamber 50 were prepared in addition to the spark plug 1 shown in embodiment 1. The spark plug 1 of embodiment 1 (see Figures 1 to 3) was used as sample 1. Samples 2 and 3 had a larger volume of the auxiliary combustion chamber 50 than sample 1. As with the ones shown in Figures 18 and 19, samples 2 and 3 have a larger protruding length of the center electrode 4 from the insulator 3 and the tip position of the insulator 3 is located on the base end side. As a result, these spark plugs have a larger spatial volume inside the housing 2 and a larger volume of the auxiliary combustion chamber 50. The volumes of the auxiliary combustion chamber 50 are 0.3 cc for sample 1, 0.4 cc for sample 2, and 0.55 cc for sample 3.

As a comparative sample, a spark plug 9 with an arrangement of nozzle holes 951 as shown in FIG. 22 was prepared. In the case of this spark plug 9, a swirl flow centered on the plug central axis is formed in the auxiliary combustion chamber 950 during the compression stroke. Note that the configuration of the comparative spark plug 9 other than the nozzle hole 951 is the same as that of embodiment 1.

The test was conducted in the same manner as in Experimental Example 2, and the EGR combustion limit was measured. The results are shown in Figure 21. As shown in the figure, it was confirmed that the EGR combustion limit was increased in all of Samples 1 to 3 compared to the comparison sample. Note that in the figure, the multiple dashed lines are extensions of each scale of the EGR combustion limit, but the distance between adjacent dashed lines above and below is three points.

(Embodiment 2)
As shown in FIG. 23, this embodiment is an embodiment in which the position and orientation of the injection hole 51 are changed from those shown in the first embodiment.

In the spark plug 1 of this embodiment, the first injection hole 511 is disposed closer to the fixed end of the ground electrode 6 than the discharge gap G. Also, a part of an inner opening 512i of the second injection hole 512 overlaps with the discharge gap G in the plug axial direction Z. The positions and orientations of the other injection holes 51 are also slightly different from those shown in the first embodiment.
The rest is the same as in the first embodiment.

In the case of this embodiment as well, the formation of an eccentric swirl flow in the compression stroke can be expected.
In addition, the same effects as those of the first embodiment can be obtained.

(Embodiment 3)
As shown in FIG. 24, this embodiment is also an embodiment in which the position and orientation of the injection hole 51 are changed from those shown in the first embodiment.

In the spark plug 1 of this embodiment, the inner opening 511i of the first injection hole 511 is disposed on the protruding side of the ground electrode 6 with respect to the discharge gap G. As a result, the inner openings 511i, 512i, 513i, 514i of all the injection holes 51 are disposed on the protruding side of the ground electrode 6 with respect to the discharge gap G.
The rest is the same as in the first embodiment.

In the case of this embodiment as well, the formation of an eccentric swirl flow in the compression stroke can be expected.
In addition, the same effects as those of the first embodiment can be obtained.

(Embodiment 4)
In this embodiment, as shown in FIG. 25, the number of injection holes 51 is three.
Further, the inner openings of all the injection holes 51 are disposed on the protruding side of the ground electrode 6 relative to the discharge gap G.
The rest is the same as in the first embodiment.

In the case of this embodiment as well, the formation of an eccentric swirl flow in the compression stroke can be expected.
In addition, the same effects as those of the first embodiment can be obtained.
The number of injection holes 51 may be, for example, five or more.

(Embodiment 5)
In this embodiment, as shown in FIG. 26, a center 512c of an inner opening 512i of a second injection hole 512 is shifted from the center of the ground electrode 6 in the width direction.

However, even in this embodiment, the second injection hole 512 is positioned such that at least a portion of the inner opening 512i is closer to the protruding side of the ground electrode 6 than the discharge gap G and is on the inside of the width of the ground electrode 6 (i.e., within the region marked with w in FIG. 26) when viewed from the plug axial direction Z.

In this embodiment, the center 512 c of the inner opening 512 i of the second injection hole 512 is located farther from the first injection hole 511 than the center of the ground electrode 6 in the width direction, and inside the width w of the ground electrode 6 .
The rest is the same as in the first embodiment.
In this embodiment as well, the same effects as those of the first embodiment can be obtained.

(Embodiment 6)
In this embodiment, as shown in Figs. 27 and 28, the width w of the ground electrode 6 facing the discharge gap G is reduced.
For example, the width w of the ground electrode 6 can be made smaller than the diameter of the portion of the center electrode 4 that is held by the insulator 3 .

28, the ground electrode 6 may have an electrode main body 60 fixed to the housing 2 or the plug cover 5, and an extension portion 61 extending from the electrode main body 60 in the protruding direction. In this case, the width of the extension portion 61 may be made smaller than that of the electrode main body 60. The extension portion 61 may be joined to the electrode main body 60 by welding or the like.
The rest is the same as in the first embodiment.

In the case of this embodiment, when an air flow occurs near the discharge gap G, the discharge tends to extend relatively quickly. As a result, the ignition property is easily stabilized. In addition, the quenching effect of the initial flame can be suppressed.
In addition, the second embodiment has the same effects as the first embodiment.

(Embodiment 7)
In this embodiment, as shown in FIG. 29, the discharge surface 42 of the center electrode 4 and the discharge surface 62 of the ground electrode 6 which face each other across a discharge gap G are formed to be approximately parallel to each other.

In this embodiment, since the discharge surface 62 of the ground electrode 6 is inclined with respect to the plug radial direction, the discharge surface 42 of the center electrode 4 is inclined with respect to the plug radial direction accordingly.
The rest is the same as in the first embodiment.

In the present embodiment, it is possible to suppress the expansion of the discharge gap G due to wear of the center electrode 4. Therefore, it is possible to extend the life of the spark plug 1.
In addition, the second embodiment has the same effects as the first embodiment.

(Embodiment 8)
As shown in FIG. 30 , in this embodiment, when viewed from a direction perpendicular to both the extension direction of the ground electrode 6 and the plug axial direction, an extension line of the discharge surface 62 of the ground electrode 6 passes near the inner opening 512i of the second injection hole 512.

As shown in FIG. 30, when viewed from a direction perpendicular to both the extension direction of the ground electrode 6 and the plug axial direction, a straight line connecting the center 512c of the inner opening 512i of the second injection hole 512 and the center of the discharge gap G is defined as a straight line L2. Note that the center of the discharge gap G here is the midpoint of the line segment connecting the ground electrode 6 and the center electrode 4 at the shortest distance.

At this time, the discharge surface 62 is approximately parallel to the straight line L2. For example, the absolute value of the angle between the discharge surface 62 and the straight line L2 can be set to 5° or less.
The rest is the same as in the first embodiment.

In this embodiment, the vector extending from the discharge gap G to the inner opening 512i of the second nozzle hole 512 can be made substantially parallel to the discharge surface 62. Therefore, the discharge generated in the discharge gap G can be more easily extended toward the second nozzle hole 512.
In addition, the second embodiment has the same effects as the first embodiment.

(Embodiment 9)
In this embodiment, as shown in FIGS. 31 and 32, the thickness of the side wall of the plug cover 5 varies depending on the circumferential position of the plug.

In this embodiment, when viewed from the plug axial direction Z, the outer peripheral contour 59 of the plug cover 5 is circular and centered on the plug central axis. The discharge gap G exists on the plug central axis. On the other hand, the inner peripheral contour 58 of the plug cover 5 is circular and centered on a point offset from the plug central axis. In other words, the inner peripheral contour 58 is eccentric with respect to the outer peripheral contour 59. As a result, when viewed from the plug axial direction Z, the center of the auxiliary combustion chamber 50 is offset from the discharge gap G.

31 and 32 are examples of this embodiment, showing different arrangements of the multiple injection holes 51. In the spark plug 1 shown in FIG. 31, the multiple injection holes 51 are oriented and positioned so that the incoming airflow creates a swirl flow that follows the inner wall of the auxiliary combustion chamber 50 (i.e., the inner circumferential contour 58 of the plug cover 5). This swirl flow is an eccentric swirl flow Fs that has its center of rotation on the protruding side of the ground electrode 6 relative to the discharge gap G.

The spark plug shown in FIG. 32 also has multiple injection holes 51 so that the incoming airflow creates an eccentric swirl flow Fs. In the spark plug 1 shown in the figure, one of the multiple injection holes 51 is located near the discharge gap G.

In this embodiment, the discharge generated in the discharge gap G can be extended by the eccentric swirl flow Fs. This improves ignition performance.

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.

Reference Signs List 1 Spark plug for internal combustion engine 2 Housing 3 Insulator 4 Center electrode 5 Plug cover 50 Pre-combustion chamber 51 Injection hole 6 Ground electrode G Discharge gap

Claims (10)

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 ground electrode (6) forming a discharge gap (G) between itself and the center electrode;
a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed,
The plug cover is formed with a plurality of nozzle holes (51) that communicate the auxiliary combustion chamber with the outside,
The ground electrode is formed so as to protrude from an outer periphery of the auxiliary combustion chamber toward a central axis of the plug,
The plurality of nozzle holes are provided so that an eccentric swirl flow (Fs) is formed in the auxiliary combustion chamber by the airflow flowing in from the nozzle holes,
The eccentric swirl flow is a swirling flow having a center of swirl at a position on the protruding side of the ground electrode relative to the discharge gap as viewed in the plug axial direction (Z). The plurality of nozzle holes includes at least a first nozzle hole (511) and a second nozzle hole (512) as described below,
the second nozzle hole is closer to the discharge gap than the first nozzle hole;
the first nozzle hole is formed such that at least a part of an extension region (511E) along a nozzle hole axis (511L) overlaps with the discharge gap, and a vector extending from an outer opening (511o) to an inner opening (511i), as viewed in the plug axial direction, faces toward the side approaching the discharge gap and forms an angle of 90°±30° with respect to a protruding direction of the ground electrode,
The second nozzle hole has an injection hole axis (512L) that does not pass through the discharge gap, and a vector extending from an outer opening (512o) to an inner opening (512i) as viewed in the plug axial direction faces a side that moves away from the discharge gap toward the protruding side of the ground electrode.
2. A spark plug for an internal combustion engine according to claim 1. The spark plug for an internal combustion engine according to claim 2, wherein at least a portion of the inner opening of the second nozzle hole is disposed on the protruding side of the ground electrode relative to the discharge gap and on the inner side of the width of the ground electrode, as viewed in the plug axial direction. When the distance in the plug radial direction between the center (512c) of the inner opening of the second injection hole and the discharge gap is L1 and the inner diameter of the plug cover is d,
0≦L1/(d/2)≦0.5
4. The spark plug for an internal combustion engine according to claim 3, which satisfies the above. When the cross-sectional area of the second nozzle hole in a cross section perpendicular to the nozzle hole axis is S2 and the inner area of the outline of the cross section of the plug cover in a cross section perpendicular to the plug axial direction is S5,
0.006≦S2/S5≦0.028
5. The spark plug for an internal combustion engine according to claim 2, wherein A spark plug for an internal combustion engine according to any one of claims 2 to 5, comprising, as the nozzle holes, in addition to the first nozzle hole and the second nozzle hole, a third nozzle hole (513) whose vector from the outer opening to the inner opening is directed to the first nozzle hole when viewed in the plug axial direction. The spark plug for an internal combustion engine according to claim 6, further comprising, as the nozzle holes, a fourth nozzle hole (514) whose vector from the outer opening to the inner opening is directed to the third nozzle hole when viewed in the plug axial direction, in addition to the first nozzle hole, the second nozzle hole, and the third nozzle hole. A spark plug for an internal combustion engine according to any one of claims 2 to 7, wherein the inner openings of the nozzle holes other than the first nozzle hole are arranged closer to the protruding side of the ground electrode than the discharge gap. When the distance in the plug axial direction between the tip of the center electrode and the tip of the auxiliary combustion chamber is h and the inside diameter of the plug cover is d,
0.15≦h/d≦0.33
9. The spark plug for an internal combustion engine according to claim 1, which satisfies the above. Let h be the distance in the plug axial direction between the tip of the center electrode and the tip of the auxiliary combustion chamber, S50 be the inner area of the inner circumference of the cross section of the plug cover perpendicular to the plug axial direction, and V be the volume of the auxiliary combustion chamber.
2≦V/(h×S50)≦6
10. The spark plug for an internal combustion engine according to claim 1, which satisfies the above.

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