Spark plugs for internal combustion engines

Abstract

To provide a spark plug for an internal combustion engine capable of suppressing pre-ignition and improving productivity.SOLUTION: In a spark plug 1, a ground electrode 6 protrudes from a fixed end 61, which is fixed in a distal end of a housing 2, into a sub combustion chamber 50. A cover proximal end face 51 of a plug cover 5 and a ground proximal end face 611 of the fixed end 61 are joined to a housing distal end side face 21 in the distal end of the housing 2. An area of the cover proximal end face 51 joined to the housing tip end side face 21 is defined as A, an area of the ground proximal end face 611 joined to the housing distal end side face 21 is defined as B, and an area of the housing distal end side face 21 is defined as C. At such a time, the spark plug 1 satisfies following inequalities (1) to (3): 0.358≤A / C≤0.964...(1), 0.036≤B / C≤0.642...(2), A / C+B / C≤1...(3).SELECTED DRAWING: Figure 1

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 a pre-combustion chamber at its tip is known. The spark plug described in Patent Document 1 specifies the ratio of the total surface area of ​​the inner wall surfaces of the housing and plug cover facing the pre-combustion chamber to the surface area of ​​the outer circumferential surface of the insulator facing the pre-combustion chamber. This is intended to prevent the pre-combustion chamber from becoming too hot, and to prevent the air-fuel mixture from igniting before the spark plug generates a discharge (i.e., pre-ignition). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-184435 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the spark plug described in Patent Document 1, a through hole is provided in the housing, and the ground electrode is inserted and fixed in the through hole. Therefore, it is necessary to provide an insertion hole in the housing for fixing the ground electrode, which tends to reduce productivity.

[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 can suppress pre-ignition and improve productivity. [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 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) joined to the tip end portion 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 a fixed end portion (61) fixed to a tip portion of the housing, A cover base end surface (51) of the plug cover and a ground base end surface (611) of the fixed end portion are joined to a housing front end side surface (21) of the front end portion of the housing, The spark plug (1) for an internal combustion engine satisfies the following formulas (1) to (3), where A is the area of ​​the cover base end face joined to the housing front end side face, B is the area of ​​the ground base end face joined to the housing front end side face, and C is the area of ​​the housing front end side face. 0.358≦A / C≦0.964 (1) 0.036≦B / C≦0.642 (2) A / C+B / C≦1 (3) Effect of the Invention

[0007] The spark plug satisfies the above formulas (1) to (3). In addition, the ground electrode is fixed to the housing by joining the ground base end surface to the front end side surface of the housing. Therefore, overheating of the plug cover and the ground electrode can be suppressed, and the ground electrode can be easily provided. As a result, pre-ignition can be suppressed, and productivity 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 can suppress pre-ignition and improve productivity. 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] 4 is a cross-sectional view taken along line II in FIG. 3, showing a portion near the tip end of the spark plug according to the first embodiment, taken along an axial direction of the plug. FIG. [Diagram 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Diagram 3] FIG. 3 is a view taken along the line III in FIG. [Figure 4] 1 is a cross-sectional view of an internal combustion engine in which a spark plug is installed according to a first embodiment. [Diagram 5] 6 is a graph showing the relationship between the A / C value and the advance amount in Experimental Example 1. [Figure 6] 6 is a graph showing the relationship between the value of B / C and the amount of advance angle in Experimental Example 1. [Figure 7] 7 is a cross-sectional view taken along line VII-VII in FIG. 8, showing a portion near the tip end of the spark plug according to the second embodiment, taken along an axial direction of the plug. [Figure 8] FIG. 8 is a cross-sectional view taken along line VIII-VIII in FIG. 7 . [Figure 9] 11 is a graph showing the relationship between the A / C value and the advance amount in Experimental Example 2. [Figure 10] 11 is a graph showing the relationship between the value of B / C and the amount of advance angle in Experimental Example 2. [Figure 11] 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 12] FIG. 11 is a cross-sectional view taken along the axial direction of a spark plug in the vicinity of a tip portion of a spark plug installed in an internal combustion engine according to a third embodiment. [Figure 13] 11 is a graph showing the relationship between the A / C value and the advance amount in Experimental Example 3. [Figure 14] 11 is a graph showing the relationship between the value of B / C and the amount of advance angle in Experimental Example 3. 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 Figs. 1 to 3, 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 protrudes from the insulator 3 toward the tip side. 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 joined to the tip portion 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 a fixed end portion 61 fixed to the tip portion of the housing 2.

[0011] A cover base end surface 51 of the plug cover 5 and a ground base end surface 611 of the fixed end portion 61 are joined to the housing front end side surface 21 of the front portion of the housing 2. The area of ​​the cover base end surface 51 joined to the housing front end side surface 21 is A, the area of ​​the ground base end surface 611 joined to the housing front end side surface 21 is B, and the area of ​​the housing front end side surface 21 is C. In this case, the spark plug 1 satisfies the following formulas (1) to (3). 0.358≦A / C≦0.964 (1) 0.036≦B / C≦0.642 (2) A / C+B / C≦1 (3)

[0012] In the above formulas (1) to (3), the area C of the housing front end side surface 21 means the area of ​​the housing front end side surface 21 before the cover base end surface 51 and the ground base end surface 611 are joined.

[0013] The spark plug 1 of this embodiment can be used as an ignition means in an internal combustion engine of an automobile, for example. As shown in Fig. 4, the spark plug 1 is attached to an internal combustion engine 10 by screwing the threaded portion 22 of the housing 2 into a female threaded portion of a plug hole 711 of a cylinder head 71. The nominal diameter of the threaded portion 22 of the housing 2 can be, for example, M8 to M18. The housing 2 is in thermal contact with the cylinder head 71.

[0014] 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.

[0015] 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 C refers to the central axis C of the spark plug 1. The plug radial direction refers to the radial direction of a circle centered on the plug central axis C on a plane perpendicular to the plug central axis C. The plug circumferential direction refers to the direction along the circumference centered on the plug central axis C. In this embodiment, the plug central axis C is also the central axis of the center electrode 4.

[0016] When the spark plug 1 is attached to the internal combustion engine 10, the plug cover 5 faces the main combustion chamber 101. In addition, the plug cover 5 separates the auxiliary combustion chamber 50 from the main combustion chamber 101.

[0017] 1, the auxiliary combustion chamber 50 includes a space on the inner circumferential side of the tip portion 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.

[0018] In this embodiment, the plug cover 5 is made of a nickel alloy. From the viewpoint of heat dissipation, the plug cover 5 is more preferably made of beryllium copper or the like, which has a higher thermal conductivity than nickel alloys.

[0019] The plug cover 5 is formed with a nozzle hole 55 that communicates the auxiliary combustion chamber 50 with the outside. The nozzle hole 55 opens at an angle with respect to the Z direction so that it approaches the tip side and moves outward in the plug radial direction. When the spark plug 1 is installed in an internal combustion engine, the nozzle hole 55 communicates between the auxiliary combustion chamber 50 and the main combustion chamber.

[0020] The plug cover 5 has a peripheral wall portion 54 that covers at least a portion of the outer periphery of the auxiliary combustion chamber 50 and is provided with a cover base end surface 51. In this embodiment, the peripheral wall portion 54 covers a portion of the outer periphery of the auxiliary combustion chamber 50. The peripheral wall portion 54 is formed along the Z direction and has a substantially cylindrical shape.

[0021] The plug cover 5 has a bottom wall portion 56 and a corner portion 57. The bottom wall portion 56 is a portion that covers the tip side of the auxiliary combustion chamber 50. The corner portion 57 is a portion that connects the tip of the peripheral wall portion 54 and the outer periphery of the bottom wall portion 56 in a curved shape. The injection hole 55 is formed in the corner portion 57.

[0022] In this embodiment, the plug cover 5 is joined to the housing 2 by welding the cover base end surface 51 in contact with the housing tip side surface 21 in the Z direction. The ground electrode 6 is joined to the housing 2 by welding the ground base end surface 611 in contact with the housing tip side surface 21 in the Z direction. The plug cover 5 and the ground electrode 6 are in thermal contact with the housing 2 via their respective joints with the housing 2.

[0023] The ground electrode 6 has a generally L-shape as a whole. The end of the ground electrode 6 opposite the fixed end 61 faces the tip of the center electrode 4 in the Z direction to form a discharge gap G. In this embodiment, the discharge gap G is formed on the tip side of the tip of the housing 2.

[0024] 2, the cross section of the ground electrode 6 perpendicular to the extending direction thereof has a substantially rectangular shape. In this embodiment, the ground electrode 6 is made of a nickel alloy.

[0025] Further, the housing front end side surface 21, the cover base end surface 51, and the ground base end surface 611 are each flat surfaces and are formed to be substantially perpendicular to the Z direction. The housing front end side surface 21 and the cover base end surface 51 are each formed to be annular.

[0026] Next, the effects of this embodiment will be described. The spark plug 1 satisfies the above formulas (1) to (3). Moreover, the ground electrode 6 is fixed to the housing 2 by joining the ground base end surface 611 to the housing front end side surface 21. Therefore, overheating of the plug cover 5 and the ground electrode 6 can be suppressed, and the ground electrode 6 can be easily provided. As a result, pre-ignition can be suppressed, and productivity can be improved.

[0027] In order to prevent overheating of the plug cover 5 and the ground electrode 6, it is important to make it easy to dissipate the heat of both the plug cover 5 and the ground electrode 6 to the outside via the housing 2. Therefore, the spark plug 1 of this embodiment satisfies the above formulas (1) to (3). Therefore, both the plug cover 5 and the ground electrode 6 can effectively dissipate heat to the outside via the joint with the housing 2. Therefore, overheating of both the plug cover 5 and the ground electrode 6 can be prevented. As a result, pre-ignition can be suppressed.

[0028] The discharge gap G is formed on the tip side of the tip of the housing 2. Therefore, it is easy to check the discharge gap G formed between the ground electrode 6 and the center electrode 4 fixed to the housing 2 before joining the plug cover 5 to the housing 2. This makes it easy to adjust the discharge gap G. As a result, the spark plug 1 can be manufactured efficiently.

[0029] As described above, according to the present embodiment, it is possible to provide the spark plug 1 for an internal combustion engine, which is capable of suppressing pre-ignition and improving productivity.

[0030] (Experimental Example 1) In this example, as shown in the graphs of FIG. 5 and FIG. 6, a plurality of spark plugs having the same basic structure as in the first embodiment but different A / C or B / C values ​​were used to analyze the relationship between the A / C or B / C value and the ignition timing at which pre-ignition occurs. Generally, the more the ignition timing is advanced, the more likely pre-ignition occurs. Therefore, in this example, the ignition timing was gradually advanced for each spark plug installed in the internal combustion engine, and the ignition timing at which pre-ignition occurs was analyzed. The test conditions were that the internal combustion engine in which the spark plug was installed was a single-cylinder internal combustion engine, the throttle was fully opened, and the rotation speed was 6500 rpm. Here, the case where the advance amount at which pre-ignition occurs is BTDC (abbreviation for before top dead center) 29° CA (abbreviation for crank angle) or more is set as the standard for sufficient suppression of pre-ignition when used as a spark plug for a general automobile engine. Therefore, the values ​​of A / C and B / C that satisfy the standard were obtained from the above analysis results.

[0031] The graph in Figure 5 is an analysis of the relationship between the A / C value and the advance angle at which pre-ignition occurs when the B / C value of each spark plug is set to 0.036. The graph in Figure 6 is an analysis of the relationship between the B / C value and the advance angle at which pre-ignition occurs when the A / C value of each spark plug is set to 0.358. In the graphs in Figures 5 and 6, the circles indicate plots of the analysis results for each spark plug. The graphs in Figures 5 and 6 also show approximation curves for these plots.

[0032] From the graph in Figure 5, it can be seen that when the A / C value is relatively small, the amount of advance tends to become relatively large as the A / C value increases. On the other hand, when the A / C value is around 0.4 or more, it can be seen that the amount of advance does not tend to become relatively large even if the A / C value increases. From this result, it is considered that when the A / C value is relatively small, pre-ignition mainly caused by the plug cover occurred. Therefore, when the A / C value is relatively small, it is considered that when the A / C value is increased, the heat dissipation of the plug cover is likely to improve, and the amount of advance tends to become relatively large. On the other hand, when the A / C value is around 0.4 or more, it is considered that pre-ignition mainly caused by the ground electrode occurred. Therefore, it is considered that even if the A / C value is increased, the amount of advance does not tend to become relatively large.

[0033] In addition, from the graph of FIG. 6, it can be seen that when the value of B / C is relatively small, the amount of advance tends to become relatively large as the value of B / C increases. On the other hand, when the value of B / C is around 0.07 or more, it can be seen that even if the value of B / C increases, the amount of advance does not tend to become relatively large. From this result, it is considered that when the value of B / C is relatively small, pre-ignition mainly caused by the ground electrode occurred. Therefore, when the value of B / C is relatively small, it is considered that when the value of B / C is increased, the heat dissipation of the ground electrode is likely to improve, and the amount of advance tends to become relatively large. On the other hand, when the value of B / C is around 0.07 or more, it is considered that pre-ignition mainly caused by the plug cover occurred. Therefore, it is considered that even if the value of B / C is increased, the amount of advance does not tend to become relatively large.

[0034] Furthermore, from the approximation curve shown in the graph of FIG. 5, the value of A / C at which the advance angle at which pre-ignition occurs is 29° CA BTDC or more is estimated to be 0.358≦A / C. Furthermore, from the approximation curve shown in the graph of FIG. 6, the minimum value of B / C at which the advance angle at which pre-ignition occurs is 29° CA BTDC or more is estimated to be 0.036. Here, when the value of B / C is 0.036, the maximum value of A / C is 0.964. Therefore, from these results, it is believed that by setting 0.358≦A / C≦0.964, pre-ignition can be sufficiently suppressed when used as a spark plug for a general automobile engine.

[0035] Furthermore, from the approximation curve shown in the graph of FIG. 6, the value of B / C at which the advance angle at which pre-ignition occurs is BTDC 29° CA or more is estimated to be 0.036≦B / C. Furthermore, from the approximation curve shown in the graph of FIG. 5, the minimum value of A / C at which the advance angle at which pre-ignition occurs is BTDC 29° CA or more is estimated to be 0.358. Here, when the value of A / C is 0.358, the maximum value of B / C is 0.642. Therefore, from these results, it is believed that by setting 0.036≦B / C≦0.642, pre-ignition can be sufficiently suppressed when used as a spark plug for a general automobile engine.

[0036] (Embodiment 2) In this embodiment, as shown in FIGS. 7 and 8, a part of the ground electrode 6 is disposed inside a cover recess 53 formed in the plug cover 5.

[0037] In this embodiment, the ground electrode 6 has a standing portion 62 and an extension portion 63, as shown in Fig. 7. The standing portion 62 has a fixed end portion 61, and stands from the housing front end side surface 21 toward the front end along the plug axial direction Z. The extension portion 63 extends from the front end of the standing portion 62 toward the inside in the plug radial direction. A discharge gap G is formed between the extension portion 63 and the center electrode 4.

[0038] 7 and 8, the plug cover 5 has a cover recess 53 formed by part of the inner circumferential surface 52 being recessed outward in the plug radial direction. The cover recess 53 is open on the base end side. At least a part of the erected portion 62 is disposed inside the cover recess 53.

[0039] Furthermore, the spark plug 1 satisfies the following formulae (4) and (5) in addition to the formulae (1) to (3). A / C≦0.94 (4) 0.042≦B / C (5)

[0040] In this embodiment, the cover recess 53 is formed from the base end of the plug cover 5 to the corner portion 57. The entire standing portion 62 is disposed inside the cover recess 53.

[0041] As shown in FIG. 7, the thickness T2 of the bottom wall portion 56 is smaller than the thickness T1 of the peripheral wall portion 54.

[0042] In this embodiment, the ground electrode 6 is formed by bending a rod-shaped electrode material (not shown). Specifically, first, a substantially rectangular prism-shaped electrode material is joined to the housing 2 before the plug cover 5 is joined. When joining the electrode material, the longitudinal direction of the rod-shaped electrode material is aligned along the Z direction, and the base end surface of the electrode material and the housing front end side surface 21 are brought into contact with each other in the Z direction and welded. Thereafter, the electrode material joined to the housing 2 is bent inward in the plug radial direction to form the ground electrode 6 having the erected portion 62 and the extended portion 63. The longitudinal length of the electrode material before bending can be, for example, 8 to 13 mm. 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.

[0043] At least a portion of the erected portion 62 is disposed inside the cover recess 53. This makes it possible to increase the bonding area of ​​the cover base end surface 51 relative to the housing front end side surface 21, and also makes it easy to reliably ensure the bonding area of ​​the ground base end surface 611 relative to the housing front end side surface 21. This makes it possible to further suppress overheating of the plug cover 5, and also to reliably suppress overheating of the ground electrode 6. As a result, pre-ignition can be further suppressed.

[0044] Furthermore, the spark plug 1 satisfies the above formulas (4) and (5) in addition to the formulas (1) to (3). Therefore, the joining area of ​​the ground base end surface 611 with respect to the housing tip side surface 21 can be more reliably ensured. Therefore, overheating of the ground electrode 6 can be more reliably suppressed. As a result, pre-ignition can be more reliably suppressed.

[0045] Moreover, the entirety of the standing portion 62 is disposed inside the cover recess 53. This makes it easier to further increase the bonding area of ​​the ground base end surface 611 with respect to the housing tip side surface 21. As a result, overheating of the ground electrode 6 can be further suppressed.

[0046] The ground electrode 6 has the standing portion 62 and the extending portion 63. Therefore, the strength of the joint between the ground electrode 6 and the housing 2 can be reliably ensured, and overheating of the ground electrode 6 can be further suppressed.

[0047] That is, in this embodiment, the base end surface of the rod-shaped electrode material and the housing front end side surface 21 are brought into contact with each other in the Z direction and welded, thereby fixing the ground electrode 6 to the housing 2. Therefore, the ground electrode 6 can be firmly joined to the housing 2. In addition, the heat of the ground electrode 6 can be more effectively dissipated to the outside via the joint between the housing 2 and the ground electrode 6. As a result, overheating of the ground electrode 6 can be further suppressed.

[0048] Furthermore, since the ground electrode 6 has the erected portion 62 and the extended portion 63, it is possible to suppress a load from being applied to the joint portion between the ground electrode 6 and the housing 2 when adjusting the discharge gap G. In other words, when the extended portion 63 is slightly deformed as necessary to fine-tune the discharge gap G to an appropriate distance, a load is unlikely to be applied to the joint portion between the ground electrode 6 and the housing 2. Therefore, the strength of the joint portion between the ground electrode 6 and the housing 2 can be reliably ensured. In addition, the same effects as those of the first embodiment are achieved.

[0049] (Experimental Example 2) In this example, as shown in the graphs of Figures 9 and 10, a plurality of spark plugs having the same basic structure as in embodiment 2 but different A / C and B / C values ​​were used to analyze the relationship between the A / C or B / C value and the advance amount at which pre-ignition occurs. The graph of Figure 9 is a graph analyzing the relationship between the A / C value and the advance amount at which pre-ignition occurs. The graph of Figure 10 is a graph analyzing the relationship between the B / C value and the advance amount at which pre-ignition occurs.

[0050] In this example, five spark plugs with different values ​​of A / C and B / C were prepared, as shown in Table 1 below. Also, as shown in the "Judgment" section of Table 1, for each spark plug, cases in which the advance angle at which pre-ignition occurred was BTDC 29° CA or more were indicated with "○", and cases in which it was less than BTDC 29° CA were indicated with "×". The other test conditions were the same as in Experimental Example 1.

[0051] [Table 1]

[0052] In the graphs of Figures 9 and 10, the circles indicate plots of the analysis results of each spark plug in Table 1. In addition, the graphs of Figures 9 and 10 show approximation lines for these plots.

[0053] From the graph in FIG. 9, it can be seen that the larger the value of A / C, the smaller the advance amount. Also, from the graph in FIG. 10, it can be seen that the larger the value of B / C, the larger the advance amount. Here, in each spark plug, as shown in Table 1, the value of A / C is greater than 0.9, and it is considered that the heat dissipation of the plug cover is sufficiently ensured. In other words, the cover base end face is joined to a wide range of the housing tip side. Therefore, the larger the value of A / C, the smaller the area where the ground base end face can be joined on the housing tip side. Therefore, it is considered that the larger the value of A / C, the smaller the value of B / C, and pre-ignition caused by the ground electrode is more likely to occur. In other words, the smaller the value of A / C, the larger the value of B / C can be. Therefore, it is considered that the smaller the value of A / C, the better the heat dissipation of the ground electrode and the larger the advance amount.

[0054] Also, from the approximation line shown in the graph of Fig. 9, the value of A / C at which the advance amount at which pre-ignition occurs is equal to or greater than 29° CA BTDC was estimated to be A / C≦0.94. Also, from the approximation line shown in the graph of Fig. 10, the value of B / C at which the advance amount at which pre-ignition occurs is equal to or greater than 29° CA BTDC was estimated to be 0.042≦B / C. From these results, it is considered that a spark plug having the same basic structure as that of embodiment 2 can suppress pre-ignition when used as a spark plug for a general automobile engine by satisfying A / C≦0.94 and 0.042≦B / C.

[0055] (Embodiment 3) In this embodiment, as shown in Figs. 11 and 12, the thickness of the peripheral wall portion 54 is substantially uniform.

[0056] 11, the plug cover 5 has a peripheral wall portion 54. The thickness T1 of the peripheral wall portion 54 is substantially uniform. That is, the thickness of the peripheral wall portion 54 is substantially uniform over the entire circumference in the plug circumferential direction.

[0057] Furthermore, the spark plug 1 satisfies the following formula (6) and formula (7) in addition to the formulas (1) to (3). 0.358≦A / C≦0.366 (6) 0.036≦B / C≦0.056 (7)

[0058] In this embodiment, the plug cover 5 has a substantially uniform thickness. That is, the thickness T1, the thickness T2 (see FIG. 7), and the thickness of the corner portion 57 are substantially equal to each other.

[0059] In this embodiment, the tip of the housing 2 has a step portion 23 formed in a stepped shape. Two housing tip side surfaces 21 are formed on the step portion 23. Each housing tip side surface 21 is formed in an annular shape. The cover base end surface 51 is joined to one of the housing tip side surfaces 21, and the ground base end surface 611 is joined to the other housing tip side surface 21. The housing tip side surface 21 to which the cover base end surface 51 is joined is located closer to the base end than the housing tip side surface 21 to which the ground base end surface 611 is joined.

[0060] Further, the step portion 23 and the base end portion of the plug cover 5 face each other in the plug radial direction.

[0061] 12 , when the spark plug 1 of this embodiment is attached to the internal combustion engine 10, the housing front end side surface 21 is located on the base end side of the front end 712 of the spark plug hole 711. In other words, the joint between the housing front end side surface 21 and the cover base end surface 51, and the joint between the housing front end side surface 21 and the ground base end surface 611 are located inside the spark plug hole 711. The rest is the same as in the first embodiment.

[0062] The thickness T1 is substantially uniform. Moreover, the spark plug 1 satisfies the above formulas (6) and (7) in addition to the formulas (1) to (3). Therefore, the plug cover 5 can be easily assembled to the housing 2, and overheating of the plug cover 5 and the ground electrode 6 can be reliably suppressed. As a result, productivity can be improved, and pre-ignition can be reliably suppressed.

[0063] In other words, by making the thickness T1 substantially uniform, the plug cover 5 can be joined to the housing 2 without determining the circumferential position of the plug cover 5 relative to the ground electrode 6. This makes it possible to easily assemble the plug cover 5 to the housing 2. As a result, productivity can be improved.

[0064] As described above, the thickness T1 is substantially uniform. Therefore, the larger the value of A / C, the more the heat dissipation of the plug cover 5 can be improved, but the joint area of ​​the ground base end surface 611 with respect to the housing tip side surface 21 tends to be small. On the other hand, the larger the value of B / C, the more the heat dissipation of the ground electrode 6 can be improved, but the joint area of ​​the cover base end surface 51 with respect to the housing tip side surface 21 tends to be small. Therefore, the spark plug 1 of this embodiment satisfies the above formula (6) and formula (7) as described above. Therefore, it is possible to reliably ensure both the joint area of ​​the cover base end surface 51 with respect to the housing tip side surface 21 and the joint area of ​​the ground base end surface 611 with respect to the housing tip side surface 21. Therefore, it is possible to reliably suppress overheating of the plug cover 5 and the ground electrode 6. As a result, it is possible to reliably suppress pre-ignition.

[0065] The plug cover 5 is joined to the step portion 23. Therefore, when joining the plug cover 5 to the housing 2, it is easy to determine the position of the plug cover 5 in the plug radial direction relative to the housing 2. As a result, the spark plug 1 can be manufactured more efficiently.

[0066] The plug cover 5 has a substantially uniform thickness. Therefore, the plug cover before the injection hole 55 is opened can be formed by plastic processing a plate-shaped member having a uniform thickness. Therefore, the plug cover 5 can be manufactured efficiently. As a result, productivity can be improved.

[0067] The joint between the housing front end side surface 21 and the cover base end surface 51, and the joint between the housing front end side surface 21 and the ground base end surface 611 are disposed inside the spark plug hole 711. This makes it possible to reduce the amount of protrusion of the front end of the spark plug 1 into the main combustion chamber 101. This makes it possible to reduce the amount of heat received by the plug cover 5 and the ground electrode 6. This makes it possible to further reduce overheating of the plug cover 5 and the ground electrode 6. As a result, pre-ignition can be further suppressed. In addition, the same effects as those of the first embodiment are achieved.

[0068] (Experimental Example 3) In this example, as shown in the graphs of Figures 13 and 14, a plurality of spark plugs having the same basic structure as in embodiment 3 but different thicknesses T1 (see Figure 11), A / C values, and B / C values ​​were used to analyze the relationship between the A / C or B / C value and the advance amount at which pre-ignition occurs. The graph of Figure 13 is a graph analyzing the relationship between the A / C value and the advance amount at which pre-ignition occurs. The graph of Figure 14 is a graph analyzing the relationship between the B / C value and the advance amount at which pre-ignition occurs.

[0069] In this example, five spark plugs were prepared, each having a different thickness T1, a different value of A / C, and a different value of B / C, as shown in the following Table 2. The other test conditions were the same as those in Experimental Example 2.

[0070] [Table 2]

[0071] In addition, in the graphs of Figures 13 and 14, the circles indicate plots of the analysis results of each spark plug in Table 2. In addition, the graphs of Figures 13 and 14 show approximation curves for these plots.

[0072] From the graph in Figure 13, we can see that if the A / C value is too small, the advance angle at which pre-ignition occurs will be smaller than BTDC 29° CA. Also, if the A / C value is too large, we can see that the advance angle at which pre-ignition occurs will be smaller than BTDC 29° CA.

[0073] Furthermore, from the graph in Figure 14, it can be seen that when the value of B / C is too small, the advance angle at which pre-ignition occurs is smaller than BTDC 29° CA. It can also be seen that when the value of B / C is too large, the advance angle at which pre-ignition occurs is also smaller than BTDC 29° CA.

[0074] From these results, it is believed that pre-ignition mainly caused by the plug cover occurred because the A / C value became too small as a result of making the B / C value too large. On the other hand, it is believed that pre-ignition mainly caused by the ground electrode occurred because the B / C value became too small as a result of making the A / C value too large. In other words, it is believed that overheating of the plug cover and ground electrode can be suppressed and pre-ignition can be suppressed by keeping the A / C value and the B / C value within their respective specified ranges.

[0075] From the approximation curve shown in the graph of Fig. 13, the value of A / C at which the advance amount at which pre-ignition occurs is BTDC 29° CA or more is estimated to be 0.358≦A / C≦0.366. Also, from the approximation curve shown in the graph of Fig. 14, the value of B / C at which the advance amount at which pre-ignition occurs is BTDC 29° CA or more is estimated to be 0.036≦B / C≦0.056. Therefore, it is considered that a spark plug having the same basic structure as that of the third embodiment can suppress pre-ignition when used as a spark plug for a general automobile engine by satisfying 0.358≦A / C≦0.366 and 0.036≦B / C≦0.056.

[0076] In the above-mentioned embodiments 1 to 3, the ground electrode 6 is made of a nickel alloy. However, the ground electrode may have a core made of copper, which has a relatively high thermal conductivity, and an outer layer made of a nickel alloy or the like that covers the core.

[0077] Also, a tip can be joined to each of the tip of the center electrode and the ground electrode that form the discharge gap. That is, a discharge gap can be formed between the tip joined to the tip of the center electrode and the tip joined to the ground electrode. The tip can be made of a precious metal such as iridium or platinum, or an alloy containing these as a main component.

[0078] 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]

[0079] Reference Signs List 1...spark plug, 2...housing, 21...housing tip side surface, 3...insulator, 4...center electrode, 5...plug cover, 50...auxiliary combustion chamber, 51...cover base end surface, 6...ground electrode, 61...fixed end portion, 611...ground base end surface, G...discharge gap

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) joined to 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 a fixed end portion (61) fixed to the tip portion of the housing, A cover base end surface (51) of the plug cover and a ground base end surface (611) of the fixed end portion are joined to a housing front end side surface (21) of the front end portion of the housing, A spark plug (1) for an internal combustion engine, which satisfies the following formulas (1) to (3), where A is an area of ​​the cover base end face joined to the housing front end side face, B is an area of ​​the ground base end face joined to the housing front end side face, and C is an area of ​​the housing front end side face. 0.358≦A / C≦0.964 (1) 0.036≦B / C≦0.642 ... (2) A / C+B / C≦1...(3)

2. 2. The spark plug for an internal combustion engine according to claim 1, wherein the ground electrode has the fixed end and has a standing portion (62) standing from a tip side surface of the housing along a plug axial direction (Z) toward a tip side, and an extension portion (63) extending from a tip of the standing portion toward an inner side in a plug radial direction, the extension portion forming the discharge gap between the center electrode and the plug cover, the plug cover has a cover recess (53) formed by a part of an inner circumferential surface (52) receding toward an outer side in the plug radial direction, the cover recess opening toward a base end side, at least a part of the standing portion being disposed inside the cover recess, and the following formula (4) and formula (5) are further satisfied: A / C≦0.94...(4) 0.042≦B / C...(5)

3. 2. The spark plug for an internal combustion engine according to claim 1, wherein the ground electrode has the fixed end, and has a standing portion (62) standing from a tip side surface of the housing along a plug axial direction (Z) toward a tip side, and an extension portion (63) extending from a tip of the standing portion toward an inner side in a plug radial direction, the extension portion forming the discharge gap between the center electrode, the plug cover having a peripheral wall portion (54) covering at least a part of an outer circumferential side of the auxiliary combustion chamber and including the cover base end surface, a thickness (T1) of the peripheral wall portion being substantially uniform, and further satisfying the following formulas (6) and (7): 0.358≦A / C≦0.366 (6) 0.036≦B / C≦0.056 (7)

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