Spark plugs for internal combustion engines

Abstract

To provide a spark plug for an internal combustion engine which allows improvement in ignitability.SOLUTION: In a spark plug 1 for an internal combustion engine, a plug cover 5 is provided at the tip end of a housing 2 such that it covers a sub-combustion chamber 50 where a discharge gap G is located. The plug cover 5 is provided with a plurality of injection holes 51. One injection hole 51 of the plurality of injection holes 51 is a tip end injection hole 510 formed further on the tip end side than the other injection holes 51. An extension line of the central axis of the tip end injection hole 510 does not pass through a ground electrode 6. A distance to the discharge gap G from the center of an outer opening of the tip end injection hole 510 is shorter than a distance to the discharge gap G from the center of an outer opening of any of the injection holes 51 other than the tip end injection hole 510. The shortest distance from the tip end of the ground electrode 6 to an inner circumferential end 512 of an inner opening 511 of the tip end injection hole 510 is shorter than a distance from the tip end of a center electrode 4 to the tip end of the sub-combustion chamber 50 in a plug axis direction Z.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a spark plug for an internal combustion engine. [Background technology]

[0002] A spark plug having a sub-combustion chamber surrounding a discharge gap is disclosed in, for example, Patent Document 1. Such a spark plug forms a flame by igniting the air-fuel mixture in the auxiliary combustion chamber. The flame generated in the auxiliary combustion chamber is then ejected from a nozzle hole that connects the auxiliary combustion chamber to the main combustion chamber. This causes the flame to propagate into the main combustion chamber, burning the air-fuel mixture. Patent Document 1 discloses that the rebound effect of the airflow in the auxiliary combustion chamber is utilized to promote the growth of the flame. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-53370 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the spark plug described in Patent Document 1, although the growth of the flame is taken into consideration, the formation of the initial flame itself is not taken into consideration. In other words, no consideration is given to improving ignition by extending the discharge generated in the discharge gap. In addition, in order to increase the catalyst temperature of the exhaust gas purification filter, etc., ignition by discharge may be performed during the expansion stroke of the internal combustion engine, but ignition during the expansion stroke is also not taken into consideration.

[0005] The present invention has been made in view of the above problems, and has an object to provide a spark plug for an internal combustion engine that is capable of improving ignition performance. [Means for solving the problem]

[0006] 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 having a tip protrusion (41) protruding toward the tip side of the insulator; 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 ground electrode protrudes into the auxiliary combustion chamber from an inner wall surface (501) of the auxiliary combustion chamber, The plug cover is formed with a plurality of nozzle holes (51) that communicate the auxiliary combustion chamber with the outside, Some of the plurality of nozzle holes are tip nozzle holes (510) that are formed closer to the tip than the other nozzle holes, An extension line (510L) of the central axis of the tip nozzle hole does not pass through the ground electrode, a distance (D1) from a center of an outer opening (513) of the tip nozzle hole to the discharge gap is shorter than a distance (D2) from a center of an outer opening of the nozzle hole other than the tip nozzle hole to the discharge gap; The shortest distance (D3) from the tip of the ground electrode to the inner peripheral end (512) of the inner opening (511) of the tip injection hole is shorter than the distance (D4) from the tip of the center electrode to the tip of the auxiliary combustion chamber in the plug axial direction (Z). Ku , the extension line is inclined with respect to the plug axial direction and passes through a base end side of the discharge gap, When viewed from the plug axial direction, the discharge gap and the tip nozzle hole are located on opposite sides of the plug central axis (C), When a straight line L1 is a straight line connecting the tip of the center electrode and the projecting end (62) of the ground electrode at the shortest distance, in a cross section including the straight line L1 and taken along the axial direction of the plug, an angle α4 between the straight line L1 and the extension line is larger than an angle α1 between the straight line L1 and the central axis of the plug. In a spark plug (1) for an internal combustion engine. Effect of the Invention

[0007] The spark plug has a tip nozzle hole as one of a plurality of nozzle holes. The tip nozzle hole is formed in a position and orientation that satisfies the above-mentioned conditions. Therefore, during the expansion stroke of the internal combustion engine, the discharge generated in the discharge gap can be extended by the airflow discharged from the auxiliary combustion chamber through the tip nozzle hole. As a result, ignition performance can be improved.

[0008] As described above, according to the above aspect, it is possible to provide a spark plug for an internal combustion engine that is capable of improving ignition performance. 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. [Brief description of the drawings]

[0009] [Figure 1] 2. FIG. 3 is a cross-sectional view of the spark plug in the axial direction in the vicinity of the tip portion of the spark plug according to the first embodiment, which corresponds to a cross-sectional view taken along line II in FIG. [Diagram 2] FIG. 2 is a view taken along the arrow II in FIG. [Diagram 3] FIG. 4 is a cross-sectional explanatory view illustrating the relationship between an extension line of the central axis of the tip nozzle hole and the ground electrode in the first embodiment. [Figure 4] FIG. 4 is a cross-sectional view illustrating the distance from the center of the outer opening of the injection hole to the discharge gap in the first embodiment. [Diagram 5] FIG. 4 is an explanatory cross-sectional view for explaining the shortest distance from the tip of the ground electrode to the inner circumferential end of the inner opening of the tip nozzle hole in the first embodiment. [Figure 6] FIG. 4 is an explanatory cross-sectional view illustrating the size of the inner diameter of an injection hole in the first embodiment. [Figure 7] FIG. 4 is an explanatory cross-sectional view illustrating the position of a ground electrode relative to a center electrode in the first embodiment. [Figure 8] FIG. 1 is a cross-sectional explanatory view of an internal combustion engine according to a first embodiment. [Figure 9] 4 is a cross-sectional view of the vicinity of the tip of the spark plug in the first embodiment during the expansion stroke and before the discharge is extended. [Figure 10] 4 is a cross-sectional view of the vicinity of the tip of the spark plug according to the first embodiment when the discharge is extended during the expansion stroke. [Figure 11] 4 is a cross-sectional view of the vicinity of the tip of the spark plug when the discharge extends to the main combustion chamber during the expansion stroke in the first embodiment. FIG. [Figure 12] FIG. 4 is a cross-sectional view taken along the axial direction of a spark plug in the vicinity of a tip portion of the spark plug in a comparative embodiment 1. [Figure 13] 6 is a graph showing the relationship between the shortest distance from the tip of the central electrode to the center of the outer opening of the tip nozzle hole and the retard limit in Experimental Example 1. [Figure 14] 1 is a graph showing the relationship between the discharge length and the discharge sustaining voltage. [Figure 15] 13 is a graph showing the relationship between the shortest distance from the tip of the ground electrode to the inner circumferential end of the inner opening of the tip nozzle hole and COV in Experimental Example 2. [Figure 16] FIG. 11 is a cross-sectional view taken along the axial direction of a spark plug in a vicinity of a tip portion of the spark plug according to a second embodiment. [Figure 17] FIG. 11 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 third embodiment. [Figure 18] FIG. 11 is a cross-sectional view taken along the axial direction of a spark plug in a vicinity of a tip portion of the spark plug according to a fourth embodiment. [Figure 19] FIG. 13 is a cross-sectional view taken along the axial direction of a spark plug in a vicinity of a tip portion of the spark plug according to a fifth embodiment. [Figure 20] FIG. 13 is a cross-sectional view taken along the axial direction of a spark plug in a vicinity of a tip portion of the spark plug according to a sixth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] (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 and Fig. 2, 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 ground electrode 6, and a plug cover 5. The center electrode 4 is held on the inner periphery of the insulator 3, and has a tip protrusion 41 protruding from the tip side of the insulator 3. The housing 2 holds the insulator 3 on its inner periphery. The ground electrode 6 forms a discharge gap G between itself and the center electrode 4. 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 arranged. The ground electrode 6 protrudes into the auxiliary combustion chamber 50 from an inner wall surface 501 of the auxiliary combustion chamber 50.

[0011] A plurality of nozzle holes 51 that communicate the auxiliary combustion chamber 50 with the outside are formed in the plug cover 5. Some of the plurality of nozzle holes 51 are tip nozzle holes 510 that are formed closer to the tip side than the other nozzle holes 51.

[0012] An extension line 510L of the central axis of the tip injection hole 510 does not pass through the ground electrode 6, as shown in Fig. 3. As shown in Fig. 4, a distance D1 from the center of the outer opening 513 of the tip injection hole 510 to the discharge gap G is shorter than a distance D2 from the center of the outer opening 513 of the injection hole 51 other than the tip injection hole 510 to the discharge gap G. Also, as shown in Fig. 5, a minimum distance D3 from the tip of the ground electrode 6 to an inner circumferential end 512 of the inner opening 511 of the tip injection hole 510 is shorter than a distance D4 from the tip of the center electrode 4 to the tip of the auxiliary combustion chamber 50 in the plug axial direction Z.

[0013] The spark plug 1 of this embodiment can be used, for example, as an ignition means in an internal combustion engine of an automobile, cogeneration, etc. The spark plug 1 is attached to an internal combustion engine 10 by screwing a mounting thread portion 21 formed on the outer circumferential surface of a housing 2 into a female thread portion of a plug hole 761 of a cylinder head 76, as shown in Fig. 8 .

[0014] The internal combustion engine 10 includes a piston 74 that reciprocates within a cylinder 70. The volume of the main combustion chamber 11 changes due to the reciprocating motion of the piston 74. The internal combustion engine 10 is formed with an intake port 721 and an exhaust port 731, and is provided with an intake valve 72 and an exhaust valve 73, respectively.

[0015] One end of the spark plug 1 in the axial direction Z is disposed in the main combustion chamber 11 of the internal combustion engine 10. In the axial direction Z of the spark plug 1, the side exposed to the main combustion chamber 11 is referred to as the tip side, and the opposite side is referred to as the base side. The axial direction Z of the spark plug 1 is also referred to as the plug axial direction Z or simply as the Z direction, as appropriate. The plug central axis C refers to the central axis C of the spark plug 1. As shown in FIG. 1, the plug central axis C is also the central axis of the center electrode 4 in this embodiment.

[0016] The plug cover 5 is joined by welding or the like to the tip of the housing 2. When the spark plug 1 is attached to an internal combustion engine, the plug cover 5 separates the auxiliary combustion chamber 50 from the main combustion chamber.

[0017] The auxiliary combustion chamber 50 includes a space on the inner circumferential side of the tip of the housing 2 around the tip protrusion 41 of the center electrode 4. Therefore, an inner wall surface 501 of the auxiliary combustion chamber 50 includes the inner surface of the plug cover 5 as well as the inner surface of the tip of the housing 2.

[0018] The plug cover 5 has a peripheral wall portion 52 that covers a part of the outer periphery of the auxiliary combustion chamber 50, a bottom wall portion 53 that covers the tip side of the auxiliary combustion chamber 50, and a corner portion 54 that connects the peripheral wall portion 52 and the bottom wall portion 53. A tip injection hole 510 is formed in the bottom wall portion 53. In the corner portion 54, injection holes 51 other than the tip injection hole 510 are formed.

[0019] 2, five nozzle holes 51 are formed in the plug cover 5, one of which is a tip nozzle hole 510. Each nozzle hole 51 is formed in a substantially cylindrical shape.

[0020] 6, the inner diameter D6 of the tip nozzle hole 510 is larger than the inner diameter D7 of the other nozzle holes 51. In other words, the tip nozzle hole 510 has a larger opening area than the nozzle holes 51 other than the tip nozzle hole 510.

[0021] The inner diameter of the tip nozzle hole 510 can be, for example, 1.5 to 3.0 times the inner diameter of the nozzle holes 51 other than the tip nozzle hole 510. The inner diameter of the tip nozzle hole 510 is preferably 2.0 to 3.0 times the inner diameter of the nozzle holes 51 other than the tip nozzle hole 510.

[0022] In this embodiment, the inner diameter D6 of the tip nozzle hole 510 is larger than the maximum thickness of the bottom wall portion 53 of the plug cover 5. The thickness of the plug cover 5 may be, for example, 0.5 mm to 1.5 mm.

[0023] The injection holes 51 other than the tip injection hole 510 are formed at equal intervals in the plug circumferential direction when viewed from the Z direction, as shown in Fig. 2. The injection holes 51 other than the tip injection hole 510 open at an incline with respect to the Z direction so as to move outward in the plug radial direction toward the tip side, as shown in Figs. 1 and 3 to 7. The plug circumferential direction is a direction along a circumference centered on the plug central axis C. The plug radial direction means a radial direction of a circle centered on the central axis C of the spark plug 1 on a plane perpendicular to the central axis C of the spark plug 1.

[0024] In this embodiment, the tip nozzle hole 510 is formed so that an extension line 510L of the central axis of the tip nozzle hole 510 is along the Z direction, as shown in Fig. 3. Also, the tip nozzle hole 510 is formed so that the extension line 510L overlaps with the plug central axis.

[0025] As shown in FIG. 1, the tip injection hole 510 is formed on the plug central axis C side of the intermediate position between the plug central axis C and the inner wall surface 501 of the peripheral wall portion 52 of the plug cover 5 in the plug radial direction.

[0026] 5, in this embodiment, the shortest distance D5 from the tip of the center electrode 4 to the center of the outer opening 513 of the tip nozzle hole 510 is 1.0 to 2.6 mm. The shortest distance D3 is 0.0 to 0.8 mm. When D3=0.0 mm, the tip of the ground electrode 6 is substantially in contact with the inner wall surface 501 of the bottom wall portion 53 of the plug cover 5.

[0027] 2, the ground electrode 6 is fixed to the housing 2 so as to extend along the plug radial direction when viewed from the Z direction. As shown in FIGS. 1 and 2, the plug central axis C does not pass through the ground electrode 6.

[0028] 1, the base end surface 61 of the ground electrode 6 is inclined toward the tip side as it approaches the protruding end portion 62 of the ground electrode 6. The base end surface 61 of the ground electrode 6 is a flat surface.

[0029] In this embodiment, the tip of the center electrode 4 is located on the base end side of the protruding end portion 62 of the ground electrode 6. The tip of the center electrode 4 is located radially inward of the protruding end portion 62. The tip of the center electrode 4 is located on the tip side of the tip of the housing 2.

[0030] 1 and 2, a flat tip surface 411 is formed at the tip of the center electrode 4. As shown in Fig. 2, the ground electrode 6 is fixed so that the tip surface 411 and the ground electrode 6 do not overlap each other when viewed from the Z direction.

[0031] 7, a straight line L1 is a line that connects the tip of the center electrode 4 and the protruding end portion 62 of the ground electrode 6 at the shortest distance. In this embodiment, as shown in FIG 7, an angle α1 between the straight line L1 and the plug central axis C in a cross section along the Z direction that includes the straight line L1 is an acute angle.

[0032] A plane that passes through the tip of the center electrode 4 and is perpendicular to the plug central axis C is defined as a plane P. In this embodiment, as shown in Fig. 7, an angle α2 between the plane P and the base end surface 61 of the ground electrode 6 is an acute angle in a cross section that includes the plug central axis C and is along the protruding direction of the ground electrode 6. The angle α2 can be, for example, 0° to 90°.

[0033] 7, in the present embodiment, in a cross section along the Z direction, the angle α3 between the line L1 and the plane P is an acute angle, as shown in FIG. 7. As will be described in the second and third embodiments, the angle α3 can be, for example, 0° to 90°.

[0034] In this embodiment, the discharge gap G is formed in the vicinity of the plug central axis C. As shown in Fig. 1 , the discharge gap G is formed closer to the plug central axis C in the plug radial direction than a midpoint between the plug central axis C and an inner wall surface 501 of the peripheral wall portion 52 of the plug cover 5. The discharge gap G is formed further outward in the plug radial direction than the plug central axis C.

[0035] The discharge gap G is formed by the tip of the tip protrusion 41 of the center electrode 4 and the ground electrode 6 facing each other at an inclination with respect to the plug axial direction Z. The discharge gap G is formed along a straight line L1 as shown in FIG. 7. It is also possible to join tips to the tip of the tip protrusion 41 and the ground electrode 6 that face each other (not shown). In other words, the discharge gap G can be formed between a tip joined to the tip of the tip protrusion 41 and a tip joined to the ground electrode 6. The tip can be made of a precious metal such as iridium or platinum, or an alloy mainly composed of these metals.

[0036] Next, the effects of this embodiment will be described. The spark plug 1 for an internal combustion engine has a tip nozzle hole 510 as one of the multiple nozzle holes 51. The tip nozzle hole 510 is formed in a position and direction that satisfies the above-mentioned conditions. Therefore, during the expansion stroke of the internal combustion engine, the discharge generated in the discharge gap G can be extended by the airflow guided from the auxiliary combustion chamber 50 through the tip nozzle hole 510. As a result, the ignition performance can be improved.

[0037] In the expansion stroke, the piston moves to the tip side, so that the main combustion chamber becomes negative pressure relative to the auxiliary combustion chamber 50. As a result, gas is discharged from the auxiliary combustion chamber 50 to the main combustion chamber through the nozzle hole 51. Then, as shown in Figs. 9 to 11, the gas is discharged through the tip nozzle hole 510, so that an airflow A toward the tip nozzle hole 510 is formed in the auxiliary combustion chamber 50. Here, in the spark plug 1 of this embodiment, an extension line 510L of the central axis of the tip nozzle hole 510 does not pass through the ground electrode 6. Therefore, the airflow A is sufficiently formed in the discharge gap G and its periphery. Therefore, as shown in Fig. 9, the discharge S generated in the discharge gap G is easily extended by the airflow A as shown in Fig. 10. As a result, the ignition property is easily improved in the expansion stroke. In addition, the catalyst temperature of the exhaust gas purification filter can be increased in a short period of time by improving the ignition property in the expansion stroke. Therefore, improvement in fuel efficiency and reduction in emissions can be expected.

[0038] In addition, since the airflow A is sufficiently formed in the discharge gap G, the discharge S tends to extend toward the tip nozzle hole 510 as shown in Fig. 10. This makes it easier to bring the ignition position closer to the tip nozzle hole 510, so that, for example, under operating conditions where the temperature of the auxiliary combustion chamber 50 is low, cooling loss of the initial flame is suppressed and the flame tends to eject from the tip nozzle hole 510 into the main combustion chamber. In addition, the discharge S tends to be attracted from the tip nozzle hole 510 toward the main combustion chamber. As a result, the ignition ability in the main combustion chamber can be improved.

[0039] Also, the shortest distance D3 (see FIG. 5) is shorter than the distance D4 (see FIG. 5). Therefore, as shown in FIG. 9, the starting point SP of the discharge S on the ground electrode 6 side generated in the discharge gap G is likely to move along the protruding end portion 62 by the airflow A so as to approach the tip nozzle hole 510 as shown in FIG. 10. Therefore, the discharge S is more likely to extend toward the tip nozzle hole 510. As a result, the ignition performance can be further improved. In some cases, as shown in FIG. 11, the starting point SP of the discharge S may move from the protruding end portion 62 to the inner surface of the tip nozzle hole 510. The flow rate of the airflow A is likely to become faster as it approaches the tip nozzle hole 510. Then, the starting point SP of the discharge S is likely to move to the outer opening 513 of the tip nozzle hole 510 by the airflow A with a high flow rate, and the discharge S is more likely to extend. Therefore, it is expected that a part of the discharge S will fly out from the tip nozzle hole 510 to the main combustion chamber side. This makes it possible to improve the ignition performance of the main combustion chamber.

[0040] The shortest distance D5 (see FIG. 5) is 1.0 to 2.6 mm. Therefore, an airflow toward the tip nozzle hole 510 can be reliably formed in the discharge gap G and its periphery. As a result, the discharge generated in the discharge gap G can be reliably extended toward the tip nozzle hole 510.

[0041] Furthermore, since the shortest distance D5 is within the above range, the airflow formed in and around the discharge gap G tends to have a high flow rate. Therefore, the discharge generated in the discharge gap G tends to extend further toward the tip nozzle hole 510. As a result, the ignition performance can be further improved.

[0042] The shortest distance D3 is 0.0 to 0.8 mm. Therefore, the airflow discharged through the tip nozzle hole 510 makes it easier for the starting point of the discharge generated in the discharge gap G to move from the ground electrode 6 to the inner surface of the tip nozzle hole 510. Therefore, the discharge is easier to extend toward the main combustion chamber. As a result, the ignition performance of the main combustion chamber can be further improved.

[0043] An inner diameter D6 of the tip nozzle hole 510 is larger than an inner diameter D7 of the other nozzle holes 51 (see FIG. 6). Therefore, the airflow discharged from the auxiliary combustion chamber 50 through the tip nozzle hole 510 is likely to be strong. Therefore, a strong airflow toward the tip nozzle hole 510 is likely to be generated in the discharge gap G and its periphery. As a result, the airflow can effectively extend the discharge.

[0044] The base end surface 61 of the ground electrode 6 is inclined toward the tip side as it approaches the protruding end portion 62. Therefore, during the expansion stroke, the airflow formed in the discharge gap G and its periphery is easily guided toward the tip nozzle hole 510 by the base end surface 61. Therefore, the discharge generated in the discharge gap G is more likely to extend toward the tip nozzle hole 510. As a result, the ignition performance can be further improved.

[0045] As described above, according to this embodiment, it is possible to provide a spark plug 1 for an internal combustion engine that can improve ignition performance.

[0046] (Comparative form 1) As shown in FIG. 12, the spark plug 9 of this embodiment is a spark plug 9 in which the shortest distance D3 and the distance D4 are equal to each other.

[0047] As shown in FIG. 12, in this embodiment, the tip of the center electrode 4 and the tip of the ground electrode 6 are positioned at the same position in the Z direction.

[0048] In this embodiment, the discharge gap G is formed by the tip end of the center electrode 4 and the ground electrode 6 facing each other in the plug radial direction. The rest is the same as in embodiment 1. Note that, among the reference characters used in Fig. 12, the same reference characters used in embodiment 1 represent the same components, etc. as in embodiment 1, unless otherwise specified.

[0049] (Experimental Example 1) In this example, as shown in FIG. 13, the relationship between the shortest distance D5 (see FIG. 5 and FIG. 12) and the retardation limit was analyzed for a spark plug having the same basic structure as that of the first embodiment and a spark plug having the same basic structure as that of the first comparative embodiment. The test conditions were a supercharged engine with a displacement of 2 L, a compression ratio of 10, and a rotation speed of 1500 r / m. The shortest distance D5 at which the retardation limit is 18° CA (abbreviation of crank angle) or more ATDC (abbreviation of compression top dead center) was obtained. In this example, the retardation limit was determined based on a combustion variation rate (hereinafter referred to as COV) of 15%. In other words, the ignition timing at which the retardation is possible the most within a range in which the COV does not exceed 15% was obtained.

[0050] 13, overall, the embodiment 1 showed a result in which the angle could be retarded more than the comparative embodiment 1. Specifically, for example, when the shortest distance D5 was set to 2.0 mm, the angle could be retarded from ATDC 30° CA in the embodiment 1, whereas the angle fell short of the reference ATDC 18° CA in the comparative embodiment 1. Also, from the results of this example, the shortest distance D5 at which the retard limit is ATDC 18° CA or more in the embodiment 1 was 2.6 mm or less.

[0051] In general, it is considered that the more the ignition performance of the spark plug is improved, the more the ignition timing can be retarded. In other words, it is considered that the more the discharge generated in the discharge gap is easily extended, the more the ignition timing can be retarded. In addition, in general, the discharge generated in the discharge gap has a higher sustaining voltage as the length of the discharge increases. Therefore, in this example, as shown in FIG. 14, the length of the discharge was calculated from the sustaining voltage of the discharge detected based on the result of previously calculating the relationship between the length of the discharge and the sustaining voltage of the discharge. Specifically, when the length of the discharge was calculated for each of the first embodiment and the first comparative embodiment when the shortest distance D5 was set to 2.0 mm, the length of the discharge was 5.0 mm in the first embodiment shown by the symbol Sy1 in FIG. 13. Also, the length of the discharge was 1.5 mm in the first comparative embodiment shown by the symbol Sy2. From these results, it is considered that the first embodiment has a higher ignition performance and the discharge is more easily extended than the first comparative embodiment. Therefore, it is considered that the first embodiment was able to retard the discharge more than the first comparative embodiment.

[0052] That is, in the first embodiment, the shortest distance D3 (see FIG. 5) is shorter than the distance D4 (see FIG. 5). Therefore, it is considered that in the first embodiment, the starting point of the discharge on the ground electrode side is more likely to move to the inner surface of the tip nozzle hole than in the first comparative embodiment, in which the shortest distance D3 and the distance D4 are equivalent. Therefore, in this example, it is considered that the discharge is more likely to extend and the timing is more likely to be retarded in the first embodiment than in the first comparative embodiment. In other words, it is considered that the first embodiment is more likely to increase the catalyst temperature of the exhaust gas purification filter by ignition during the expansion stroke than the first comparative embodiment.

[0053] In addition, in the first embodiment, under the condition indicated by the symbol Sy3 where the shortest distance D5 exceeds 2.6 mm, the retard limit was lower than ATDC18°CA. Also, under the condition indicated by the symbol Sy3, the length of the discharge was 2.0 mm. From these results, it is considered that when the shortest distance D5 exceeds 2.6 mm, the distance between the tip nozzle hole and the discharge gap becomes longer, so that the discharge is less likely to extend compared to when the shortest distance D5 is 2.6 mm or less. In other words, when the shortest distance D5 exceeds 2.6 mm, it is considered that the airflow generated in the discharge gap and its periphery is more likely to be weaker compared to when the shortest distance D5 is 2.6 mm or less. As a result, it is considered that the extension distance of the discharge is also shorter.

[0054] (Experimental Example 2) In this example, as shown in Fig. 15, the relationship between the shortest distance D3 (see Fig. 5) and the COV was analyzed for a spark plug having the same basic structure as that of the first embodiment. The test conditions were as follows: shortest distance D5 was 2.0 mm, ignition timing by discharge was ATDC20°CA, and other conditions were the same as those of the first experimental example.

[0055] In the results of this example, the shortest distance D3 at which the COV was 15% or less was 0.8 mm or less, as shown in FIG.

[0056] In general, it is believed that the more the ignition performance of a spark plug is improved, the more the COV can be reduced. In the case of the shortest distance D3 where the COV is 15% or less, as shown by the symbol Sy4 in FIG. 15, the length of the discharge was 5.0 mm. In addition, in the case of the shortest distance D3 where the COV significantly exceeds 15%, as shown by the symbol Sy5, the length of the discharge was 1.5 mm. From these results, it is believed that when the shortest distance D3 is 0.8 mm or less, the discharge is more likely to extend, and therefore the value of the COV is 15% or less. Specifically, it is believed that when the shortest distance D3 is 0.8 mm or less, the starting point of the discharge on the ground electrode side is more likely to move to the inner surface of the tip nozzle hole. Therefore, it is believed that the discharge is more likely to extend, and part of the discharge is more likely to fly out from the tip nozzle hole toward the main combustion chamber, which allows the mixture to be burned stably. On the other hand, when the minimum distance D3 exceeds 0.8 mm, it is considered that the starting point of the discharge does not easily move to the inner surface of the tip nozzle hole, and the extension distance of the discharge becomes shorter, compared with the case where the minimum distance D3 is 0.8 mm or less. As a result, it is considered that the COV exceeds 15%.

[0057] (Embodiment 2) As shown in FIG. 16, this embodiment is an embodiment in which the position of the ground electrode 6 is changed from that of the first embodiment.

[0058] 16, in a cross section taken along the Z direction, an angle α3 between the line L1 and a plane P is approximately 90°. In this embodiment, the discharge gap G is formed along the Z direction. The rest is the same as in embodiment 1. Note that, among the symbols used in embodiment 2 and onwards, the same symbols used in the previous embodiments represent the same components, etc. as in the previous embodiments, unless otherwise specified. This embodiment also has the same effects as the first embodiment.

[0059] (Embodiment 3) As shown in FIG. 17, this embodiment is an embodiment in which the position of the ground electrode 6 is changed from that of the first embodiment.

[0060] 17, the ground electrode 6 is fixed so that the straight line L1 overlaps with the plane P. That is, in a cross section including the straight line L1 and taken along the Z direction, the angle between the straight line L1 and the plane P is 0°. Also, the discharge gap G is formed in the plug radial direction. The other configurations and effects are the same as those of the first embodiment.

[0061] (Embodiment 4) As shown in FIG. 18, this embodiment is an embodiment in which the positional relationship between the tip nozzle hole 510 and the ground electrode 6 is changed from that of the first embodiment.

[0062] The tip nozzle hole 510 is formed so that an extension region 510E extending from the tip nozzle hole 510 in the opening direction does not pass through the ground electrode 6, as shown in FIG.

[0063] In this embodiment, the protruding end portion 62 of the ground electrode 6 is located outside the tip nozzle hole 510 in the plug radial direction. The rest is the same as in the first embodiment.

[0064] The tip nozzle hole 510 is formed so that the extension region 510E does not pass through the ground electrode 6. Therefore, the airflow discharged from the auxiliary combustion chamber 50 through the tip nozzle hole 510 is less likely to be disturbed by the ground electrode 6. Therefore, it is easier to form an airflow toward the tip nozzle hole 510 in the discharge gap G and its periphery. Therefore, the discharge generated in the discharge gap G is more likely to extend toward the tip nozzle hole 510. As a result, the ignition performance can be further improved. In addition, the same effects as those of the first embodiment are achieved.

[0065] (Embodiment 5) As shown in FIG. 19, this embodiment is an embodiment in which the position at which the tip nozzle hole 510 is formed is changed from that of the first embodiment.

[0066] 19, the tip injection hole 510 is formed in a state in which an extension line 510L of the central axis of the tip injection hole 510 is shifted from the plug central axis C. In addition, the tip injection hole 510 is formed so that the extension line 510L does not pass through the tip protruding portion 41.

[0067] The discharge gap G and the tip nozzle hole 510 are located on opposite sides to each other across the plug center axis C in the plug radial direction. The other configurations and effects are the same as those of the first embodiment.

[0068] (Embodiment 6) As shown in FIG. 20, this embodiment is an embodiment in which the opening direction of the tip nozzle hole 510 is changed from that of the fifth embodiment.

[0069] 20, an extension line 510L of the central axis of the tip nozzle hole 510 is inclined with respect to the Z direction. That is, the opening direction of the tip nozzle hole 510 is inclined with respect to the Z direction. In addition, the extension line 510L passes through the base end side of the discharge gap G.

[0070] In this embodiment, the extension line 510L passes through the tip protruding portion 41. Moreover, the extension line 510L substantially passes through the plug central axis C.

[0071] 20, in a cross section including the straight line L1 and taken along the Z direction, the angle α4 formed between the straight line L1 and the extension line 510L is larger than the angle α1 formed between the straight line L1 and the plug central axis C. That is, in this embodiment, unlike the fifth embodiment, the tip nozzle hole 510 is formed so that the angle formed between the direction in which the discharge gap G is formed and the extension line 510L approaches 90°. The rest is the same as in the fifth embodiment.

[0072] Angle α4 is larger than angle α1. Therefore, an airflow toward the tip nozzle hole 510 is more likely to be formed in and around the discharge gap G. As a result, the discharge generated in the discharge gap G is more likely to extend toward the tip nozzle hole 510. In addition, the same effects as those of the fifth embodiment are achieved.

[0073] In the above-mentioned embodiments 1 to 6, the tip of the ground electrode 6 does not abut against the plug cover 5. However, the ground electrode can be fixed to the housing or the plug cover with its tip abutted against the plug cover. The ground electrode can also be fixed to the plug cover so as to be adjacent to the inner circumferential end of the inner opening of the tip injection hole. That is, the ground electrode can be provided so as to protrude into the auxiliary combustion chamber from a position adjacent to the inner circumferential end of the inner opening of the tip injection hole. With these configurations, the starting point of the discharge generated in the discharge gap on the ground electrode side is more likely to move to the inner surface of the tip injection hole.

[0074] In the above embodiments 1 to 6, the extension line 510L of the central axis of the tip nozzle 510 does not pass through the discharge gap G. However, the tip nozzle can also be formed, for example, such that the extension line of the central axis of the tip nozzle passes through the discharge gap G. This makes it even easier to generate an airflow toward the tip nozzle in the discharge gap and its periphery.

[0075] The present invention is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the gist of the present invention. [Explanation of symbols]

[0076] 1...spark plug, 2...housing, 3...insulator, 4...center electrode, 41...tip protrusion, 5...plug cover, 50...auxiliary combustion chamber, 501...inner wall surface of auxiliary combustion chamber, 51...nozzle, 511...inner opening, 512...inner peripheral edge of inner opening, 510...tip nozzle, 6...ground electrode, 510L...extension of central axis of tip nozzle, Z...plug axial direction, G...discharge gap, D1...distance from center of outer opening of tip nozzle to discharge gap, D2...distance from center of outer opening of nozzle other than tip nozzle to discharge gap, D3...shortest distance from tip of ground electrode to inner peripheral edge of inner opening of tip nozzle, D4...distance from tip of center electrode to tip of auxiliary combustion chamber in plug axial direction

Claims

1. A cylindrical insulator (3); a center electrode (4) held on the inner circumferential side of the insulator and having a tip protrusion (41) protruding toward the tip side of the insulator; 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 ground electrode protrudes into the auxiliary combustion chamber from an inner wall surface (501) of the auxiliary combustion chamber, The plug cover is formed with a plurality of nozzle holes (51) that communicate the auxiliary combustion chamber with the outside, Some of the plurality of nozzle holes are tip nozzle holes (510) formed on a tip side relative to the other nozzle holes, An extension line (510L) of the central axis of the tip nozzle hole does not pass through the ground electrode, a distance (D1) from a center of an outer opening (513) of the tip nozzle hole to the discharge gap is shorter than a distance (D2) from a center of an outer opening of the nozzle hole other than the tip nozzle hole to the discharge gap; a shortest distance (D3) from the tip of the ground electrode to an inner circumferential end (512) of an inner opening (511) of the tip injection hole is shorter than a distance (D4) from the tip of the center electrode to a tip of the auxiliary combustion chamber in the plug axial direction (Z); the extension line is inclined with respect to the plug axial direction and passes through a base end side of the discharge gap, When viewed from the plug axial direction, the discharge gap and the tip nozzle hole are located on opposite sides of the plug central axis (C), a spark plug for an internal combustion engine, wherein, when a straight line L1 is defined as a straight line connecting the tip of the center electrode and the projecting end portion (62) of the ground electrode at the shortest distance, in a cross section which includes the straight line L1 and is taken along the plug axial direction, an angle α4 formed by the straight line L1 and the extension line is larger than an angle α1 formed by the straight line L1 and a central axis of the plug.

2. 2. The spark plug for an internal combustion engine according to claim 1, wherein a shortest distance (D5) from the tip of said center electrode to a center of an outer opening of said tip nozzle hole is 1.0 to 2.6 mm.

3. 3. A spark plug for an internal combustion engine according to claim 1, wherein a minimum distance from the tip of said ground electrode to an inner circumferential end of an inner opening of said tip nozzle hole is 0.0 to 0.8 mm.

4. 4. The spark plug for an internal combustion engine according to claim 1, wherein an inner diameter (D6) of said tip nozzle hole is larger than inner diameters (D7) of said other nozzle holes.

5. 5. A spark plug for an internal combustion engine according to claim 1, wherein the tip nozzle hole is formed such that an extension region (510E) extending from the tip nozzle hole in an opening direction does not pass through the ground electrode.

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