Spark plugs for internal combustion engines

Abstract

To provide a spark plug for an internal combustion engine, which is excellent in ignitability.SOLUTION: Provided is a spark plug 1 in which a plug cover 5 is formed with injection holes 51. At least one of the injection holes 51 is an axial injection hole 511 formed along a plug axial direction. When viewed from the plug axial direction, the center 511C of the axial injection hole 511 is eccentric to the outside in a plug radial direction with respect to the central axis CC of a center electrode 4, without overlapping a ground electrode 6. When a distance D1 from the central axis CC of the center electrode 4 to the center 511C of the axial injection hole 511 when viewed from the plug axial direction is represented as x mm, and a volume of a sub combustion chamber is represented as Vcc, the spark plug 1 satisfies the following formula (1) and the following formula (2).SELECTED DRAWING: Figure 3

Description

[Technical field]

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

[0002] For example, as disclosed in Patent Document 1, a spark plug for an internal combustion engine having an auxiliary combustion chamber at its tip is known. Such a spark plug forms a flame in the auxiliary combustion chamber by discharge generated in a discharge gap. The flame formed in the auxiliary combustion chamber is then ejected from a nozzle hole that connects the auxiliary combustion chamber with the main combustion chamber, thereby propagating the flame into the main combustion chamber and combusting the air-fuel mixture.

[0003] The spark plug described in Patent Document 1 aims to promote the growth of flame in the main combustion chamber by providing the plug cover with a nozzle hole that is adjacent to the discharge gap in the plug radial direction. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 6,013,973 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the spark plug described in Patent Document 1, although the promotion of flame growth in the main combustion chamber is taken into consideration, the ignition of the mixture in the auxiliary combustion chamber, i.e., 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 in the auxiliary combustion chamber. In addition, although ignition by discharge may be performed in the expansion stroke of the internal combustion engine for the purpose of increasing the catalyst temperature of the exhaust gas purification filter, the ignition in the expansion stroke is not taken into consideration.

[0006] 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, which has excellent ignition properties. [Means for solving the problem]

[0007] 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 nozzle hole (51) that communicates the auxiliary combustion chamber with the outside, The ground electrode protrudes into the auxiliary combustion chamber from a fixed end (61) fixed to the housing or the plug cover, The discharge gap is formed by the tip end of the center electrode and the ground electrode facing each other in the plug axial direction (Z), At least one of the nozzle holes is an axial nozzle hole (511) formed along the plug axial direction, When viewed from the plug axial direction, a center (511C) of the axial injection hole does not overlap with the ground electrode and is eccentric outward in the plug radial direction with respect to a central axis (CC) of the center electrode, A spark plug (1) for an internal combustion engine satisfies the following formulas (1) and (2), where a distance (D1) from a central axis of the central electrode to a center of the axial injection hole as viewed in the plug axial direction is x mm, and a volume of the pre-combustion chamber is Vcc.

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[0008] The spark plug satisfies the above formula (1) and formula (2). Therefore, the discharge generated in the discharge gap during the expansion stroke of the internal combustion engine is easily extended by the airflow toward the axial nozzle hole. As a result, the ignition performance can be improved.

[0009] 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. [Brief description of the drawings]

[0010] [Figure 1] 2 is a cross-sectional view taken along line II in FIG. 2, showing a portion near the tip end of the spark plug according to the first embodiment, taken along an axial direction of the plug; [Diagram 2] FIG. 2 is a view taken along the line II in FIG. [Diagram 3] FIG. 4 is a diagram showing a spark plug according to the first embodiment, viewed from the tip side, illustrating a distance D1 and the like. [Figure 4] FIG. 4 is an explanatory diagram showing an extension region of a ground electrode in the first embodiment. [Diagram 5] 1 is a cross-sectional view of an internal combustion engine in which a spark plug is installed according to a first embodiment. [Figure 6] 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 7] 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 8] 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 9] 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 10]11 is a graph showing the relationship between the length L1 determined by CFD analysis and the retard limit determined by an experiment in Experimental Example 1. [Figure 11] 11 is a graph showing the relationship between distance D1 and length L1 in Experimental Example 2. [Figure 12] 11 is a graph showing the relationship between volume V and length L1 when distance D1 is 1.5 mm in Experimental Example 2. [Figure 13] 11 is a graph showing the relationship between the length L1 calculated from the formula (12) and the length L1 obtained by the CFD analysis in Experimental Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] (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 has 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. 1 and 2. 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 side. 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 plug cover 5 has an injection hole 51 that communicates the auxiliary combustion chamber 50 with the outside.

[0012] The ground electrode 6 protrudes into the auxiliary combustion chamber 50 from a fixed end 61 that is fixed to the housing 2 or the plug cover 5. A 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 axial direction Z.

[0013] At least one of the injection holes 51 is an axial injection hole 511 formed along the plug axial direction Z. As shown in Fig. 2, when viewed from the plug axial direction Z, a center 511C of the axial injection hole 511 does not overlap with the ground electrode 6 and is eccentric outward in the plug radial direction with respect to the central axis CC of the center electrode 4.

[0014] 3, the distance D1 from the central axis CC of the central electrode 4 to the center 511C of the axial injection hole 511 as viewed in the plug axial direction Z is defined as x mm, and the volume of the auxiliary combustion chamber 50 is defined as Vcc. In this case, the spark plug 1 satisfies the following formula (1) and formula (2).

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[0015] Furthermore, the spark plug 1 of this embodiment satisfies the following formula (3) in addition to the above formulas (1) and (2). 1.058≦x≦1.942 (3)

[0016] It is more preferable that the spark plug 1 further satisfies the following formula (4) and formula (5).

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[0017] It is more preferable that the spark plug 1 further satisfies the following formula (6). 1.254≦x≦1.746 (6)

[0018] The spark plug 1 of this embodiment can be used, for example, as an ignition means in an internal combustion engine of an automobile, etc. The spark plug 1 is attached to an internal combustion engine 10 by screwing the threaded portion 21 of the housing 2 into the female threaded portion of a plug hole 711 of a cylinder head 71, as shown in FIG.

[0019] The internal combustion engine 10 includes a piston 74 that reciprocates within a cylinder 70. The volume of the main combustion chamber 101 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.

[0020] One end of the spark plug 1 in the axial direction Z is disposed in the main combustion chamber 101 of the internal combustion engine 10. In the axial direction Z of the spark plug 1, the side exposed to the main combustion chamber 101 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 PC means the central axis PC of the spark plug 1. The plug radial direction means the radial direction of a circle centered on the plug central axis PC on a plane perpendicular to the plug central axis PC. In this embodiment, the center electrode 4 is disposed so that the central axis CC substantially overlaps with the plug central axis PC.

[0021] 1, the plug cover 5 is joined to the tip of the housing 2 by welding or the like. When the spark plug 1 is attached to the internal combustion engine, the plug cover 5 separates the auxiliary combustion chamber 50 from the main combustion chamber. Further, the injection hole 51 communicates the auxiliary combustion chamber 50 with the main combustion chamber.

[0022] The auxiliary combustion chamber 50 includes a space on the inner circumferential side of the tip of the housing 2 around the center electrode 4 protruding from the insulator 3 toward the tip side. The auxiliary combustion chamber 50 also includes a pocket portion 501 which is an annular space formed between the outer circumferential surface of the insulator 3 and the inner circumferential surface of the housing 2. In this embodiment, the volume V of the auxiliary combustion chamber 50 is 0.40 cc or more. It is more preferable that the volume V of the auxiliary combustion chamber 50 is 0.55 cc or more.

[0023] The plug cover 5 has a peripheral wall portion 52, a bottom wall portion 53, and corner portions 54. The peripheral wall portion 52 is a generally cylindrical portion that covers part of the outer periphery of the auxiliary combustion chamber 50. The bottom wall portion 53 is a portion that covers the tip side of the auxiliary combustion chamber 50. The corner portions 54 are portions that connect the tip of the peripheral wall portion 52 and the outer periphery of the bottom wall portion 53 in a curved shape.

[0024] A plurality of injection holes 51 are formed in the plug cover 5. In this embodiment, the injection holes 51 include an axial injection hole 511 and an inclined injection hole 512. The inclined injection hole 512 opens 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 FIG. 3, the inclined injection hole 512 is formed such that an injection hole axis 512L substantially passes through the plug central axis PC. The inclined injection hole 512 is formed in the corner portion 54. The axial injection hole 511 is formed in the bottom wall portion 53.

[0025] The inner diameter D2 of the axial nozzle hole 511 is larger than the inner diameter D3 of the oblique nozzle hole 512. The inner diameter D2 can be set to, for example, twice or more the inner diameter D3.

[0026] As shown in Fig. 1, the axial injection hole 511 is formed so that the injection hole axis 511L is along the Z direction. As shown in Figs. 2 to 4, the axial injection hole 511 is arranged so as not to overlap with the ground electrode 6 when viewed from the Z direction.

[0027] In this embodiment, the ground electrode 6 is fixed to the tip end of the housing 2 as shown in Fig. 1. The ground electrode 6 has a generally L-shape. A protruding end 62 of the ground electrode 6, which is opposite to a fixed end 61, faces the tip end of the center electrode 4 in the Z direction, thereby forming a discharge gap G.

[0028] 2 to 4, when viewed from the Z direction, the axial injection hole 511 is formed on the protruding side of the protruding end portion 62. As shown in Fig. 4, when viewed from the Z direction, at least a part of the axial injection hole 511 overlaps with an extension region 6E in which the ground electrode 6 is extended in the protruding direction of the protruding end portion 62. In this embodiment, when viewed from the Z direction, the entire axial injection hole 511 overlaps with the extension region 6E.

[0029] 1 to 4, a protruding end surface 621 is formed on the protruding side of the protruding end portion 62. In this embodiment, the protruding end surface 621 is a flat surface and is formed along the Z direction.

[0030] In this embodiment, as shown in Fig. 3, when viewed from the Z direction, the distance from the protruding end face 621 to the center 511C in the arrangement direction of the discharge gap G and the axial injection hole 511 is substantially the same as the distance D1. Also, when viewed from the Z direction, the distance from the protruding end face 621 to the center 511C in the protruding direction of the protruding end portion 62 is substantially the same as the distance D1. In other words, the protruding end face 621 is disposed so as to substantially overlap with the central axis CC. Also, when viewed from the Z direction, the distance from the plug central axis PC to the center 511C is substantially the same as the distance D1.

[0031] When viewed from the Z direction, a part of the protruding end portion 62 and a part of the tip surface 41 of the center electrode 4 overlap with each other. When viewed from the Z direction, a part of the protruding end surface 621 and a part of the tip surface 41 of the center electrode 4 overlap with each other.

[0032] 1, the discharge gap G is formed by the base end surface 622 of the protruding end portion 62 and the tip surface 41 of the center electrode 4 facing each other in the Z direction. The base end surface 622 is formed to follow the tip surface 41. The base end surface 622 and the tip surface 41 are each a flat surface and are formed to be substantially perpendicular to the Z direction. The base end surface 622 and the tip surface 41 are formed to be substantially parallel to each other.

[0033] In this embodiment, the discharge gap G is formed on the tip side of the tip of the housing 2. The central axis CC and the plug central axis PC pass through the discharge gap G. As shown in Fig. 3, when viewed from the Z direction, the discharge gap G is formed between the fixed end portion 61 and the axial injection hole 511.

[0034] Next, the effects of this embodiment will be described. The spark plug 1 satisfies the above formula (1) and formula (2). Therefore, during the expansion stroke of the internal combustion engine, the discharge generated in the discharge gap G is easily extended by the airflow toward the axial injection hole 511. As a result, the ignition performance can be improved.

[0035] In the expansion stroke, the piston moves to the tip side, and 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. Also, as the gas is discharged through the axial nozzle hole 511, an airflow toward the axial nozzle hole 511 is formed in the auxiliary combustion chamber 50. Here, let us consider the cases where the distance D1 is too small and the case where the distance D1 is too large. When the distance D1 is too small, the airflow toward the axial nozzle hole 511 is obstructed by the ground electrode 6, and the discharge is difficult to extend. Also, the ground electrode 6 is likely to disturb the airflow, and the discharge is likely to be short-circuited. On the other hand, when the distance D1 is too large, the airflow in the discharge gap G is likely to be weak, and the discharge is likely to be short-circuited, and the discharge is difficult to extend. Therefore, the spark plug 1 of this embodiment satisfies the above formula (1) and the above formula (2). Therefore, during the expansion stroke of the internal combustion engine, the airflow toward the axial injection hole 511 is not easily blocked by the ground electrode 6, and an airflow of sufficient strength is easily formed in the discharge gap G. Therefore, the discharge generated in the discharge gap G is not easily short-circuited and is easily extended toward the axial injection hole 511. Therefore, it is possible to improve the ignition performance during the expansion stroke. As a result, it is possible to increase the catalyst temperature of the exhaust gas purification filter in a short period of time. As a result, it is possible to expect improved fuel efficiency and reduced emissions.

[0036] Also, as shown in FIG. 6, the starting point S1 of the discharge S generated in the discharge gap G on the ground electrode 6 side is likely to move toward the axial nozzle hole 511 along the protruding end surface 621 due to the airflow A toward the axial nozzle hole 511 during the expansion stroke, as shown in FIG. 7. Therefore, the discharge is more likely to extend toward the axial nozzle hole 511. As a result, the ignition performance can be further improved. In some cases, the starting point S1 of the discharge S may move from the ground electrode 6 to the inner surface of the axial nozzle hole 511 as shown in FIG. 8. Then, the discharge S is further extended, and it is expected that a part of the discharge S will fly out from the axial nozzle hole 511 toward the main combustion chamber. This can improve the ignition performance of the main combustion chamber.

[0037] In addition, since the discharge S extends toward the axial nozzle hole 511, the initial flame generated by the discharge S is less likely to be cooled by the ground electrode 6. Therefore, the ignition performance can be further improved.

[0038] In addition, the extension of the discharge S toward the axial nozzle hole 511 makes it easier to bring the ignition position closer to the axial nozzle hole 511. This makes it easier for the flame to be ejected from the axial nozzle hole 511 into the main combustion chamber, for example, under operating conditions where the temperature of the auxiliary combustion chamber 50 is low. As a result, it is possible to improve the ignition performance in the main combustion chamber.

[0039] Furthermore, the spark plug 1 satisfies the above formula (3) in addition to the above formulas (1) and (2), and therefore, the ignition performance can be reliably improved.

[0040] Furthermore, if the spark plug 1 further satisfies the above formula (4) and formula (5), the discharge S is more likely to be extended by the airflow A during the expansion stroke of the internal combustion engine. In this case, the discharge S is more likely to extend to the main combustion chamber. Therefore, it is possible to reliably improve the combustion stability in the main combustion chamber. Therefore, it is possible to improve the retard limit. As a result, it is expected that the EURO7 emission regulations will be satisfied.

[0041] Furthermore, if the spark plug 1 further satisfies the above formula (6), the discharge S is more likely to be extended by the airflow A. In this case, the ignition performance can be further improved.

[0042] In this embodiment, the inner diameter D2 (see FIG. 3) is larger than the inner diameter D3 (see FIG. 3). Therefore, the airflow A can be strengthened. As a result, the discharge S can be extended further.

[0043] When viewed from the Z direction, the axial nozzle hole 511 is formed so that the center 511C does not overlap with the ground electrode 6. Therefore, it is easy to reliably generate the airflow A in the discharge gap G. As a result, the discharge S can be reliably extended.

[0044] In this embodiment, the volume V of the auxiliary combustion chamber 50 is 0.40 cc or more. Therefore, it is easy to generate a sufficiently strong airflow A during the expansion stroke. As a result, the discharge S can be reliably extended.

[0045] Furthermore, if the volume V of the auxiliary combustion chamber 50 is 0.55 cc or more, the airflow A tends to become stronger during the expansion stroke. In this case, the discharge S can be extended further.

[0046] When viewed from the Z direction, the axial injection hole 511 is formed on the protruding side of the protruding end portion 62. Also, when viewed from the Z direction, a part of the protruding end face 621 and a part of the tip end face 41 overlap each other. Therefore, during the expansion stroke, the airflow A is more easily formed in the discharge gap G. As a result, the discharge S can be further extended.

[0047] The protruding end surface 621 is disposed so as to substantially overlap the central axis CC. Therefore, during the expansion stroke, the airflow A is more easily formed in the discharge gap G. As a result, the discharge S can be further extended.

[0048] When viewed from the Z direction, at least a portion of the axial nozzle hole 511 overlaps with the extension region 6E (see FIG. 4). Therefore, the discharge S can be easily and reliably extended by the airflow A during the expansion stroke.

[0049] Moreover, when viewed from the Z direction, the entire axial nozzle hole 511 overlaps with the extension region 6E. Therefore, the discharge S can be more reliably extended by the airflow A during the expansion stroke.

[0050] The discharge gap G is formed by the base end surface 622 of the protruding end portion 62 and the tip surface 41 of the center electrode 4 facing each other in the Z direction. The base end surface 622 is formed to fit the tip surface 41. This makes it easy to disperse the starting positions of the discharge S on the ground electrode 6 side and the center electrode 4 side. This makes it possible to suppress localized wear of the ground electrode 6 and the center electrode 4 and to suppress the increase in the distance of the discharge gap G. As a result, the life of the spark plug 1 can be extended.

[0051] As described above, according to this embodiment, it is possible to provide a spark plug 1 for an internal combustion engine having excellent ignition properties.

[0052] (Comparative form 1) As shown in FIG. 9, this embodiment differs from the first embodiment in that it does not have a plug cover.

[0053] When the spark plug 9 of this embodiment is attached to an internal combustion engine, the discharge gap G is disposed in the main combustion chamber (not shown). The rest is the same as in embodiment 1. Note that, among the reference characters used in Fig. 9, the same reference characters used in embodiment 1 represent the same components, etc. as in embodiment 1, unless otherwise specified.

[0054] (Experimental Example 1) In this example, as shown in the graph of FIG. 10, a relationship between the maximum length L1 of the discharge calculated by CFD analysis (abbreviation of "Computational Fluid Dynamics analysis") and the retard limit confirmed by an experiment was analyzed using a plurality of spark plugs having the same basic structure as in the first embodiment but different values ​​of the distance D1. The test conditions were as follows: the volume V of the auxiliary combustion chamber was 0.40 cc, the number of axial nozzle holes was one, and the number of oblique nozzle holes was four for each spark plug. The inner diameter of the axial nozzle hole was 1.5 mm, and the inner diameter of the oblique nozzle hole was 0.7 mm. An in-line four-cylinder engine was used as the internal combustion engine, with a displacement of 2.5 L, a rotation speed of 1500 rpm, and a torque of 20 Nm.

[0055] In general, it is considered that the longer the discharge generated in the discharge gap, the more the ignition timing can be retarded. Here, when an experiment was conducted on the retard limit under the above conditions for a spark plug having the same basic structure as Comparative Example 1, the retard limit was ATDC (abbreviation for top dead center of compression) 0°CA (abbreviation for crank angle). From the above analysis results, the length L1 of the discharge at which the retard limit is ATDC 0°CA or more was obtained. In addition, the length L1 at which the retard limit is ATDC 18°CA or more, which is expected to satisfy the EURO7 emission regulations, was also obtained.

[0056] In the graph of Figure 10, the circles indicate plots of the experimental results and analytical results of each spark plug. The graph of Figure 10 also shows straight lines approximating these plots.

[0057] From the results of this example, it can be seen that the longer L1 is, the better the retard limit is. Also, from the approximation line shown in the graph of Figure 10, it was estimated that the length L1 at which the retard limit is 0° CA ATDC or more is 10 mm or more. Also, it was estimated that the length L1 at which the retard limit is 18° CA ATDC or more is 26 mm or more.

[0058] (Experimental Example 2) In this example, as shown in the graph of Fig. 11, a CFD analysis was performed on the relationship between the distance D1 and the maximum length L1 of the discharge by using a plurality of spark plugs having the same basic structure as in the first embodiment but different distances D1 and volumes V of the auxiliary combustion chambers. The other test conditions were the same as in the first experimental example.

[0059] From the graph in FIG. 11, it can be seen that the value of the length L1 peaks when the distance D1 is 1.5 mm, regardless of the size of the volume V. Specifically, the length L1 becomes smaller as the distance D1 decreases from 1.5 mm, and the length L1 becomes smaller as the distance D1 increases from 1.5 mm. From the CFD analysis of this example, it is considered that when the distance D1 is too small, the airflow toward the axial nozzle hole is obstructed by the ground electrode, and the discharge does not extend sufficiently. It is also considered that the airflow is easily disturbed by the ground electrode, and the discharge is easily short-circuited. On the other hand, when the distance D1 is too large, the airflow toward the axial nozzle hole in the discharge gap is easily weakened, and the discharge is easily short-circuited, and therefore the discharge is not extended sufficiently.

[0060] Moreover, the graph in Fig. 11 has a shape that is close to a Gaussian distribution, with the value of length L1 peaking when distance D1 is 1.5 mm. Based on this result, the following formula (7) was derived, which is based on the Gaussian distribution formula and is used to calculate length L1 when distance D1 is x mm. In formula (7), σ is the standard deviation. Furthermore, in order to add an element of the peak value of length L1, the right-hand side of formula (7) is multiplied by coefficient a to obtain formula (8).

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[0061] Moreover, the graph in Fig. 12 shows the relationship between the volume V and the length L1 when the distance D1 is 1.5 mm in the analysis results shown in the graph in Fig. 11. As shown in the graph in Fig. 12, even if the distance D1 is the same, it can be seen that the value of the length L1 differs depending on the volume V.

[0062] The graph in Fig. 12 shows an approximation curve in the plot of the analytical values. The equation of the approximation curve is the following equation (9). Moreover, by replacing the numbers in the following equation (9) with b, c, and d, the following equation (9) is generalized to obtain the following equation (10). L1=-776.55V 2 +767.24V-155.98 ···(9) L1=-bV 2 +cV-d (10)

[0063] Moreover, in order to add an element of volume V to the above formula (8), the right side of the above formula (8) is multiplied by the right side of the above formula (10) to obtain the following formula (11).

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[0064] Next, the values ​​of a, b, c, d, and σ in the above formula (11) were calculated by the least squares method using the analytical values ​​of the graph in Figure 11. In this case, a was "27.1", b was "0.0", c was "3.2", d was "0.6", and σ was "0.4". Then, by substituting these values ​​into the above formula (11), the following formula (12) was obtained.

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[0065] The graph in Figure 13 is a graph analyzing the relationship between the length L1 calculated by substituting values ​​for the distance D1 and the volume V in the above formula (12) and the length L1 obtained by the CFD analysis. The correlation coefficient between the length L1 calculated using the above formula (12) and the length L1 obtained by the CFD analysis was found to be 0.7 or more. This result shows that there is a strong correlation between the length L1 calculated using the above formula (12) and the length L1 obtained by the CFD analysis.

[0066] Moreover, the above formula (1) and formula (2) are obtained by setting the value of the length L1 of the left side of formula (12) to 10 mm and changing "=" to "≦" or "≧" in the formula expressed as a formula for x. That is, formula (1) and formula (2) are formulas showing the range of distance D1 when length L1 is 10 mm or more. In other words, formula (1) and formula (2) are formulas showing the range of distance D1 when the retard limit is ATDC0°CA or more based on the analysis result of Experimental Example 1. Therefore, it is presumed that a spark plug having the same basic structure as that of the first embodiment and satisfying formula (1) and formula (2) above has ignition performance in the expansion stroke that is equal to or better than that of a spark plug having the same basic structure as that of the first comparative embodiment.

[0067] Moreover, the above formula (4) and formula (5) are obtained by setting the value of the length L1 of the left side of the above formula (12) to 26 mm and changing "=" to "≦" or "≧" in the formula expressed as a formula for x. That is, the above formula (4) and formula (5) are formulas that indicate the range of the distance D1 when the length L1 is 26 mm or more. In other words, the above formula (4) and formula (5) are formulas that indicate the range of the distance D1 when the retard limit is 18° CA ATDC or more based on the analysis result of Experimental Example 1. Therefore, it is presumed that a spark plug that satisfies the above formula (4) and formula (5) and has a basic structure similar to that of the first embodiment has ignition performance in the expansion stroke to a degree that can satisfy the EURO7 emission regulations.

[0068] Also, from the graph of FIG. 11, it is considered that when the volume V of the auxiliary combustion chamber is set to 0.40 cc or more, the length L1 can be reliably set to 10 mm or more by adjusting the distance D1. Therefore, the above formula (3) is obtained by substituting "0.40" for the volume V in the above formulas (1) and (2) and showing the range of x. In other words, it is considered that when the volume V is set to 0.40 cc or more, it is possible to reliably set the length L1 to 10 mm or more by setting x in the range of the above formula (3). As a result, it is considered that it is possible to reliably improve the ignition performance in the expansion stroke.

[0069] Also, from the graph in Figure 11, it is considered that when the volume V of the auxiliary combustion chamber is set to 0.55 cc or more, the length L1 can be reliably set to 26 mm or more by adjusting the distance D1. Therefore, the above formula (6) is obtained by substituting "0.55" for the volume V in the above formulas (4) and (5) and showing the range of x. In other words, it is considered that when the volume V is set to 0.55 cc or more, it is possible to reliably set the length L1 to 26 mm or more by setting x in the range of the above formula (6). As a result, it is considered that the ignition performance in the expansion stroke can be further improved.

[0070] 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]

[0071] 1...spark plug, 2...housing, 3...insulator, 4...center electrode, 5...plug cover, 50...auxiliary combustion chamber, 51...nozzle hole, 511...axial nozzle hole, 511C...center, 6...ground electrode, 61...fixed end, D1...distance, G...discharge gap, Z...plug axial direction, CC...center axis

Claims

1. 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 nozzle hole (51) that communicates the auxiliary combustion chamber with the outside, The ground electrode protrudes into the auxiliary combustion chamber from a fixed end (61) fixed to the housing or the plug cover, the discharge gap is formed by a tip end of the center electrode and the ground electrode facing each other in the plug axial direction (Z), At least one of the nozzle holes is an axial nozzle hole (511) formed along the plug axial direction, When viewed from the plug axial direction, a center (511C) of the axial injection hole does not overlap with the ground electrode and is eccentric outward in the plug radial direction with respect to a central axis (CC) of the center electrode, A spark plug (1) for an internal combustion engine, which satisfies the following formulas (1) and (2), where a distance (D1) from a central axis of the central electrode to a center of the axial injection hole as viewed in the plug axial direction is x mm, and a volume of the auxiliary combustion chamber is Vcc. [0010] [0025]

2. 2. The spark plug for an internal combustion engine according to claim 1, further satisfying the following formula (3): 1.058≦x≦1.942 ... (3)

3. 3. The spark plug for an internal combustion engine according to claim 1 or 2, further satisfying the following formula (4) and the following formula (5): [0030] [0045]

4. 4. A spark plug for an internal combustion engine according to claim 3, further satisfying the following formula (6): 1.254≦x≦1.746 ... (6)

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