JP7580356B2 - Spark plug for internal combustion engine and manufacturing method thereof - Google Patents

Description

The present invention relates to a spark plug for an internal combustion engine and a method for manufacturing the same.

For example, as disclosed in Patent Document 1, a spark plug with a secondary combustion chamber at the tip is known. In this spark plug, a plug cover that covers the secondary combustion chamber is formed with multiple nozzle holes. This allows combustion gas to be ejected from the secondary combustion chamber through the nozzle holes into the main combustion chamber, thereby combusting the mixture in the main combustion chamber.

JP 2008-123865 A

However, in the spark plug described in Patent Document 1, part of the inner corner between the inner peripheral surface of the injection hole and the inner wall surface of the plug cover is an acute angle. Therefore, when the combustion gas is ejected from the auxiliary combustion chamber into the main combustion chamber through the injection hole, the combustion gas may peel off from the inner peripheral surface of the injection hole. This may weaken the momentum of the combustion gas ejected into the main combustion chamber. Therefore, there is room for improvement from the viewpoint of improving ignition performance.

The present invention was made in consideration of these problems, and aims to provide a spark plug for an internal combustion engine that can improve ignition performance, and a method for manufacturing the same.

One aspect of the present invention is a cylindrical insulator (3),
a center electrode (4) held on the inner circumferential side of the insulator and protruding from the insulator toward the tip side;
a cylindrical housing (2) that holds the insulator on its inner periphery;
a ground electrode (6) forming a discharge gap (G) between itself and the center electrode;
a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed,
The plug cover is formed with a plurality of nozzle holes (51) that communicate the auxiliary combustion chamber with the outside,
At an inner corner (53) between an inner peripheral surface (511) of the injection hole and an inner wall surface (52) of the plug cover, a corner curved surface (531) is formed which is curved in a convex state toward the injection hole axis (51L),
The spark plug (1) for an internal combustion engine has an inner corner with a curved surface having a different arc length depending on the position of the inner corner.

Another aspect of the present invention is a method for manufacturing a spark plug (1) for an internal combustion engine, the spark plug (1) having a cylindrical insulator (3), a center electrode (4) held on an inner periphery of the insulator and protruding from the insulator toward a tip end thereof, a cylindrical housing (2) holding the insulator on its inner periphery, a ground electrode (6) forming a discharge gap (G) between the center electrode and the ground electrode, and a plug cover (5) provided at a tip end of the housing so as to cover an auxiliary combustion chamber (50) in which the discharge gap is disposed, the plug cover having a plurality of injection holes (51) communicating the auxiliary combustion chamber with the outside, an inner corner (53) between an inner periphery surface (511) of the injection hole and an inner wall surface (52) of the plug cover having a corner curved surface (531) curved in a convex state toward an injection hole axis (51L), and an arc length of the corner curved surface differs depending on the position of the inner corner,
a nozzle hole forming step of opening the nozzle hole in the plug cover before the plug cover is attached to the housing;
a polishing step of polishing the inner corner portion with a polishing fluid (F) containing an abrasive to form the corner portion curved surface after the nozzle hole forming step,
In the polishing step, an ejection jig (11) having an ejection port (111) for ejecting the polishing fluid is attached to a base end opening end of the plug cover, and the polishing fluid is ejected from the ejection jig toward a portion of the inner wall surface of the plug cover that corresponds to the outer peripheral surface of the auxiliary combustion chamber, and the polishing fluid is discharged from an inner space (501) surrounded by the plug cover and the ejection jig through the nozzle to the outside, thereby polishing the inner corner portion.

In the above spark plug, a curved corner surface is formed at the inner corner. Therefore, the combustion gas ejected from the auxiliary combustion chamber to the main combustion chamber through the nozzle hole is less likely to separate from the inner peripheral surface of the nozzle hole. This allows the combustion gas to be ejected forcefully into the main combustion chamber through the nozzle hole. As a result, ignition performance can be improved.

The method for manufacturing the spark plug includes a polishing step in which the inner corners are polished with a polishing fluid to form curved corner surfaces. This makes it possible to efficiently manufacture spark plugs having curved corner surfaces at the inner corners. As a result, it is possible to efficiently manufacture spark plugs for internal combustion engines that can improve ignition performance.

As described above, according to the above aspect, it is possible to provide a spark plug for an internal combustion engine capable of improving ignition performance, and a manufacturing method thereof.
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.

3 is a cross-sectional view taken along the axial direction of the spark plug in the vicinity of the tip portion of the spark plug according to the first embodiment, and corresponds to a cross-sectional view taken along line II in FIG. 2 . FIG. FIG. 2 is a cross-sectional view taken along line II-II of FIG. FIG. 4 is an enlarged cross-sectional view of the vicinity of the nozzle hole, showing the angle of an inner corner portion in the first embodiment. FIG. 4 is an enlarged cross-sectional view of the vicinity of the nozzle hole, showing the arc length of the curved surface at the corner in the first embodiment. 7 is a cross-sectional view showing how the polishing fluid is ejected from the ejection jig in the first embodiment, and corresponds to a cross-sectional view taken along line VV in FIG. 6. FIG. 6 is a cross-sectional view taken along line VI-VI in FIG. 5 . FIG. 7 is a cross-sectional view taken along line VII-VII in FIG. 5 . 1 is a cross-sectional view of an internal combustion engine to which a spark plug is attached according to a first embodiment. 11 is a cross-sectional view showing how the polishing fluid is ejected from the ejection jig in Comparative Example 1, and corresponds to a cross-sectional view taken along line IX-IX in FIG. FIG. 10 is a cross-sectional view taken along line XX in FIG. 9 . 13 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 according to the second embodiment, and corresponds to a cross-sectional view taken along line XI-XI in FIG. 12. FIG. 12 is a cross-sectional view taken along line XII-XII in FIG. FIG. 11 is an enlarged cross-sectional view of the vicinity of the nozzle hole, showing lengths D1 and D2 in a second embodiment. FIG. 11 is an enlarged cross-sectional view of the vicinity of the nozzle hole, showing the length of each portion in the second embodiment. FIG. 11 is an explanatory diagram illustrating the direction of a swirl flow formed in the auxiliary combustion chamber during the compression stroke in the second embodiment, as viewed from the plug axis direction. 16 is a cross-sectional view showing the state in which the polishing fluid is ejected from the ejection jig in embodiment 2, and corresponds to a cross-sectional view taken along line XVI-XVI in FIG. 18. FIG. 17 is a cross-sectional view taken along line XVII-XVII in FIG. 18 is a cross-sectional explanatory view illustrating the flow direction of a polishing fluid in embodiment 2, as viewed from the plug axial direction, and corresponds to a cross-sectional view taken along line XVIII-XVIII in FIG. 16. 13 is a graph showing the relationship between the value of D3/D4 and the flow coefficient in Experimental Example 1. FIG. 22 is a cross-sectional view taken along the axial direction of the spark plug in the vicinity of the tip portion of the spark plug in Comparative Example 2, and corresponds to a cross-sectional view taken along the line XX-XX in FIG. 21 . FIG. 21 is a cross-sectional view taken along line XXI-XXI in FIG. 20. FIG. 11 is an analytical diagram showing the state when combustion gas is ejected from the nozzle hole of the spark plug of Comparative Example 2 in Experimental Example 2. FIG. 11 is an analytical diagram showing a state where combustion gas is ejected from the nozzle hole of the spark plug of the second embodiment in the experimental example 2. 13 is a cross-sectional view showing how abrasive fluid is ejected from an ejection jig in the third embodiment. FIG. 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. 13 is a cross-sectional view showing how a polishing fluid is ejected from an ejection jig in the fourth embodiment. FIG. FIG. 13 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 according to a fifth embodiment.

(Embodiment 1)
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A spark plug for an internal combustion engine and a method for manufacturing the same will be described with reference to FIGS.
1 and 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 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 provided at the tip of the housing 2 so as to cover the auxiliary combustion chamber 50 in which the discharge gap G is located.

The plug cover 5 has multiple injection holes 51 that connect the auxiliary combustion chamber 50 to the outside. At the inner corner 53 between the inner circumferential surface 511 of the injection hole 51 and the inner wall surface 52 of the plug cover 5, a corner curved surface 531 that is curved in a convex state toward the injection hole axis 51L is formed. The arc length of the corner curved surface 531 varies depending on the position of the inner corner 53.

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. 8, the threaded portion 21 formed on the outer circumferential surface of the housing 2 is screwed into the female threaded portion of the plug hole 711 of the cylinder head 71, and the spark plug 1 is attached to the internal combustion engine 10.

The internal combustion engine 10 has 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 also has an intake port 721 and an exhaust port 731, each of which is provided with an intake valve 72 or an exhaust valve 73.

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 the Z direction, as appropriate. The plug central axis C refers to the central axis C of the spark plug 1. As shown in Figures 1 and 2, the plug central axis C is also the central axis of the central electrode 4 in this embodiment. 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.

As shown in FIG. 8, when the spark plug 1 is attached to the internal combustion engine 10, the plug cover 5 separates the auxiliary combustion chamber 50 from the main combustion chamber 101. The injection hole 51 connects the auxiliary combustion chamber 50 and the main combustion chamber 101.

In this embodiment, the plug cover 5 has a peripheral wall portion 54, a bottom wall portion 55, and a corner portion 56, as shown in FIG. 1. The peripheral wall portion 54 is a generally cylindrical portion that covers part of the outer periphery of the auxiliary combustion chamber 50. The bottom wall portion 55 is a portion that covers the tip side of the auxiliary combustion chamber 50. The corner portion 56 is a portion that connects the tip of the peripheral wall portion 54 and the outer periphery of the bottom wall portion 55 in a curved shape. The base end portion of the peripheral wall portion 54 is joined to the tip portion of the housing 2. The injection hole 51 is formed in the corner portion 56.

The nozzle hole 51 opens at an angle with respect to the Z direction so that it approaches the outer side in the plug radial direction as it approaches the tip side. The nozzle hole 51 is formed so that the cross section perpendicular to the nozzle hole axis 51L is substantially circular. In this embodiment, the nozzle hole 51 is formed so that the nozzle hole axis 51L substantially passes through the plug center axis C, as shown in Figures 1 and 2.

Around the inner opening 512 of the injection hole 51, an inner corner 53 is formed adjacent to the inner opening 512. In this embodiment, the angle of the inner corner 53 located closer to the base end around the inner opening 512 is smaller. Here, the angle of the inner corner 53 means the angle at which the extension line 511L of the inner circumferential surface 511 of the injection hole 51 and the extension line 52L of the inner wall surface 52 of the plug cover 5 intersect with each other in a cross section including the injection hole axis 51L, as shown in FIG. 3.

The minimum angle of the inner corner 53 is angle θ1, and the maximum angle of the inner corner 53 is angle θ2. In this embodiment, as shown in FIG. 3, in a cross section including the nozzle hole axis 51L and taken along the Z direction, the angle of the inner corner 53 on the base end side of the nozzle hole axis 51L is angle θ1, and the angle of the inner corner 53 on the tip end side of the nozzle hole axis 51L is angle θ2. In this embodiment, angle θ1 is an acute angle.

The inner corner 53 has a curved corner surface 531 formed thereon. The curved corner surface 531 is formed so as to be adjacent to the inner opening 512. In this embodiment, the curved corner surface 531 is formed so as to be adjacent to the inner opening 512 around the entire periphery of the inner opening 512.

The inner corner 53 with the smallest angle has a longer arc length of the corner curved surface 531 than the inner corner 53 with the largest angle. Here, the arc length of the corner curved surface 531 means the length of the contour line of the corner curved surface 531 in a cross section that includes the nozzle hole axis 51L.

In other words, in this embodiment, as shown in FIG. 4, in a cross section including the injection hole axis 51L and along the Z direction, the length D1 of the contour line of the corner curved surface 531 formed at the inner corner 53 with the smallest angle is longer than the length D2 of the contour line of the corner curved surface 531 formed at the inner corner 53 with the largest angle.

In this embodiment, the smaller the angle of the inner corner 53, the longer the arc length of the corner curved surface 531.

In this embodiment, the ground electrode 6 is fixed to the plug cover 5 as shown in Figs. 1 and 2. As shown in Fig. 1, the ground electrode 6 protrudes from the fixed end fixed to the plug cover 5 toward the base end into the auxiliary combustion chamber 50.

A tip 41 is joined to the tip of the center electrode 4. A discharge gap G is formed between the tip 41 and the ground electrode 6. In this embodiment, the discharge gap G is formed by the center electrode 4 and the ground electrode 6 facing each other in the plug radial direction. The tip 41 can be made of a precious metal such as iridium or platinum, or an alloy containing these as its main component.

Next, a method for manufacturing the spark plug 1 of this embodiment will be described.
The manufacturing method of the spark plug 1 for an internal combustion engine according to this embodiment includes a nozzle hole forming step and a polishing step. In the nozzle hole forming step, the nozzle hole 51 is opened in the plug cover 5 before it is attached to the housing 2. In the polishing step, after the nozzle hole forming step, the inner corner 53 is polished with a polishing fluid F containing an abrasive to form a corner curved surface 531.

In the polishing process, as shown in FIG. 5, an ejection jig 11 equipped with an ejection port 111 for ejecting the polishing fluid F is attached to the base end opening end of the plug cover 5. Then, the polishing fluid F is ejected from the ejection jig 11 toward the portion of the inner wall surface 52 of the plug cover 5 that corresponds to the outer circumferential surface of the auxiliary combustion chamber 50. Then, the polishing fluid F is discharged from the inner space 501 surrounded by the plug cover 5 and the ejection jig 11 to the outside through the nozzle hole 51, thereby polishing the inner corner 53.

In the nozzle hole forming process, the nozzle hole 51 is opened in the plug cover 5 before it is attached to the housing 2, for example, by pressing using a punch or cutting using a drill. Then, by opening the nozzle hole 51 in the plug cover 5, the inner corner 53 is formed.

In addition, during the polishing process, a pressure of, for example, several megapascals is applied to the polishing fluid F, causing the polishing fluid F to be sprayed into the inner space 501 and to be discharged to the outside through the nozzle hole 51.

The abrasive contained in the polishing fluid F is, for example, silicon oxide. The polishing fluid F can be, for example, oil as a fluid medium to which an abrasive such as silicon oxide has been added.

The ejection jig 11 is attached to the plug cover 5 so as to cover the entire base end opening of the plug cover 5. In other words, the ejection jig 11 is attached so as to block the base end opening of the plug cover 5.

In this embodiment, the ejection jig 11 ejects the polishing fluid F toward the base end side of the nozzle holes 51 in the inner wall surface 52. In other words, the ejection jig 11 is formed with four ejection ports 111 corresponding to the number of nozzle holes 51, as shown in FIG. 7. Then, the four ejection ports 111 eject the polishing fluid F toward the base end side of each of the nozzle holes 51, as shown in FIG. 5 and FIG. 6.

In addition, the ejection jig 11 is attached to the plug cover 5 so that the opening of the ejection port 111 and the inner opening 512 of the nozzle hole 51 overlap each other when viewed from the Z direction (not shown).

The ejection jig 11 also has a protruding portion 112 protruding from the tip side. In the polishing process, an inner space 501 is formed on the outer periphery side of the protruding portion 112 and on the inner periphery side of the plug cover 5. The injection hole 51 also opens into the inner space 501.

In this embodiment, as shown in FIG. 5, the protrusion 112 protrudes from the position of the opening of the ejection port 111 in the Z direction toward the tip side. The protrusion 112 also protrudes from the position of the base end of the plug cover 5 in the Z direction toward the tip side. The protrusion 112 is formed so that its diameter decreases toward the tip side.

In this embodiment, the tip surface of the protrusion 112 abuts against the inner wall surface 52 of the bottom wall portion 55. In other words, the inner space 501 is formed in an annular shape.

In addition, the opening of the ejection port 111 is formed outside the protrusion 112 in the plug radial direction.

Next, the effects of this embodiment will be described.
In the above spark plug 1, a corner curved surface 531 is formed at the inner corner 53. Therefore, the combustion gas ejected from the auxiliary combustion chamber 50 to the main combustion chamber through the injection hole 51 is less likely to separate from the inner circumferential surface 511 of the injection hole 51. Therefore, the combustion gas can be ejected forcefully into the main combustion chamber through the injection hole 51. As a result, the ignition performance can be improved.

The spark plug 1 of this embodiment ignites the mixture in the auxiliary combustion chamber 50 by generating a discharge in the discharge gap G, generating combustion gas. The combustion gas generated in the auxiliary combustion chamber 50 is then ejected into the main combustion chamber through the nozzle hole 51. This causes a flame to propagate to the main combustion chamber and burn the mixture. Here, the combustion gas tends to have a higher viscosity than air at room temperature. Therefore, if the inner corner does not have a curved corner surface, the combustion gas may peel off from the inner peripheral surface of the nozzle hole starting from the inner corner with an acute angle. Therefore, the momentum of the combustion gas ejected into the main combustion chamber tends to be weak, and the ignition range in the main combustion chamber tends to be narrow. On the other hand, in this embodiment, the inner corner 53 has a curved corner surface 531 formed. Therefore, the combustion gas is less likely to peel off from the inner peripheral surface 511 of the nozzle hole 51. Therefore, the combustion gas can be ejected vigorously into the main combustion chamber. Therefore, the ignition range in the main combustion chamber is expanded, which makes it easier for the combustion speed to increase. As a result, ignition performance can be improved and fuel efficiency can also be improved.

The arc length of the curved corner surface 531 varies depending on the position of the inner corner 53. This makes it easier to reduce the amount of polishing and the polishing time required compared to when the arc length of the curved corner surface 531 is the same throughout the entire inner corner 53. In other words, by varying the arc length of the curved corner surface 531 at the inner corner 53 depending on the direction of combustion gas ejection, it is easier to reduce the amount of polishing using the polishing fluid F while maintaining the momentum of the combustion gas. This makes it possible to efficiently manufacture a spark plug 1 that can improve ignition performance.

In addition, the inner corner 53 with the smallest angle has a longer arc length of the corner curved surface 531 than the inner corner 53 with the largest angle. This ensures that the combustion gas is less likely to separate from the inner circumferential surface 511 of the nozzle hole 51. As a result, the combustion gas can be reliably and forcefully ejected into the main combustion chamber through the nozzle hole 51.

In addition, in this embodiment, the smaller the angle of the inner corner 53, the longer the arc length of the corner curved surface 531. This makes it more difficult for the combustion gas to separate from the inner circumferential surface 511 of the nozzle hole 51. As a result, ignition performance can be improved more reliably.

The manufacturing method of the spark plug 1 includes a polishing step in which the inner corner 53 is polished with polishing fluid F to form a curved corner surface 531. Therefore, it is possible to efficiently manufacture a spark plug 1 having a curved corner surface 531 at the inner corner 53. As a result, it is possible to efficiently manufacture a spark plug 1 for an internal combustion engine that can improve ignition performance.

9 and 10, if the polishing fluid F is sprayed from the spray jig 119 toward the center of the plug cover 5, the polishing fluid F will collide with the inner wall surface 52 of the bottom wall portion 55 and then head toward the nozzle hole 51. Therefore, the polishing fluid F tends to head toward the nozzle hole 51 along the inner wall surface 52 of the bottom wall portion 55 and is likely to be discharged to the outside from the nozzle hole 51 mainly passing near the inner corner 53 with a large angle. Therefore, the speed of the polishing fluid F passing near the inner corner 53 with a small angle tends to be slower than the speed of the polishing fluid F passing near the inner corner 53 with a large angle. Therefore, the processing time required to sufficiently polish the inner corner 53 with a small angle tends to be long, making it difficult to efficiently manufacture the spark plug 1. On the other hand, in the polishing process of this embodiment, as shown in FIG. 5, the polishing fluid F is sprayed from the spray jig 11 toward the part of the inner wall surface 52 of the plug cover 5 that corresponds to the outer circumferential surface of the auxiliary combustion chamber 50. Therefore, the speed of the polishing fluid F passing near the inner corner 53 with a small angle can be increased compared to the speed of the polishing fluid F passing near the inner corner 53 with a large angle. Therefore, the inner corner 53 with a small angle can be polished efficiently. As a result, the spark plug 1 with improved ignition performance can be efficiently manufactured.

In the polishing process, an inner space 501 is formed on the outer periphery side of the protrusion 112 and on the inner periphery side of the plug cover 5, and the nozzle hole 51 opens into the inner space 501. In other words, the protrusion 112 is disposed inside the space enclosed by the plug cover 5. This makes it possible to reduce the space through which the polishing fluid F does not need to pass. This makes it easier for the polishing fluid F to be efficiently discharged to the outside from the nozzle hole 51, and makes it easier for the polishing fluid F to pass through the inner corner 53 at a faster speed. This makes it easier to polish the inner corner 53 more efficiently. As a result, the spark plug 1 can be manufactured more efficiently.

The opening of the ejection port 111 is formed further outward than the protrusion 112 in the plug radial direction. This allows the polishing fluid F to be ejected from near the small-angled inner corner 53. This makes it easier for the speed of the polishing fluid F passing near the small-angled inner corner 53 to be even faster. As a result, the spark plug 1 can be manufactured even more efficiently.

As described above, this embodiment provides a spark plug 1 for an internal combustion engine that can improve ignition performance, and a method for manufacturing the same.

(Embodiment 2)
As shown in FIGS. 11 to 18, this embodiment is an embodiment in which the opening direction of the injection hole 51 is changed from that of the first embodiment.
That is, the nozzle hole 51 is formed so that an air flow is introduced into the auxiliary combustion chamber 50 through the nozzle hole 51 to generate a swirl flow in the auxiliary combustion chamber 50 .

In a spark plug 1 installed in an internal combustion engine, a nozzle hole 51 formed in a plug cover 5 connects the auxiliary combustion chamber 50 to the main combustion chamber. During the compression stroke of the internal combustion engine, airflow is introduced from the main combustion chamber to the auxiliary combustion chamber 50 through the nozzle hole 51. The airflow introduced into the auxiliary combustion chamber 50 through the nozzle hole 51 creates a swirl flow (see dashed arrow A in Figure 15) in the auxiliary combustion chamber 50.

Specifically, as shown in FIG. 12, when viewed from the plug axial direction Z, the injection hole axis 51L of the injection hole 51 is inclined with respect to the plug radial direction. That is, in this embodiment, when viewed from the Z direction, the injection hole axis 51L of the injection hole 51 is inclined at an acute angle with respect to an imaginary line VL1 that extends in the plug radial direction and passes through the injection hole 51 and the plug central axis C. The injection holes 51 are arranged such that the injection hole axis 51L is inclined with respect to the imaginary line VL1 of each injection hole 51 on the same side in the plug circumferential direction. The plug circumferential direction is the direction along the circumference centered on the plug central axis C.

By forming the nozzle hole 51 in this manner, as shown in FIG. 15, the airflow introduced into the auxiliary combustion chamber 50 through the nozzle hole 51 creates a swirl flow A in the auxiliary combustion chamber 50. In this embodiment, the swirl flow A is generated in a counterclockwise spiral shape in FIG. 15 around the plug center axis C.

In this embodiment, as shown in FIG. 12, when viewed from the Z direction, the inner corner 53 located upstream of the swirl flow relative to the nozzle hole axis 51L has a wider range of acute angles than the inner corner 53 located downstream of the swirl flow relative to the nozzle hole axis 51L.

In addition, in this embodiment, the inner corner 53 with the smallest angle is located between the inner corner 53 adjacent to the base end of the inner opening 512 and the inner corner 53 located most upstream of the swirl flow when viewed from the Z direction. Also, the inner corner 53 with the largest angle is located between the inner corner 53 adjacent to the tip of the inner opening 512 and the inner corner 53 located most downstream of the swirl flow when viewed from the Z direction.

As shown in FIG. 13, in a cross section that includes the injection hole axis 51L and passes through the inner corner 53 with the smallest angle and the inner corner 53 with the largest angle, the arc length D1 is the arc length of the longest corner curved surface 531.

Next, a method for manufacturing the spark plug 1 of this embodiment will be described.
18, in the polishing process, the ejection jig 11 ejects the polishing fluid F in the inner space 501 so that the polishing fluid F flows in the circumferential direction of the plug. When viewed from the plug axial direction Z, the flow direction of the polishing fluid F is along the swirl flow.

In this embodiment, as shown in FIG. 17, the ejection ports 111 formed in the ejection jig 11 have central axes 111L that are inclined relative to the plug radial direction when viewed from the Z direction. In other words, when viewed from the Z direction, the ejection ports 111 have central axes 111L that are inclined at an acute angle relative to an imaginary line VL2 that passes through the ejection ports 111 and the plug central axis C and extends in the plug radial direction. The ejection ports 111 have central axes 111L that are inclined relative to the imaginary line VL2 on the same side in the plug circumferential direction.

In addition, the inclination direction of the central axis 111L of the ejection port 111 with respect to the imaginary straight line VL2 when viewed from the base end side (not shown) is opposite to the inclination direction of the injection hole axis 51L with respect to the imaginary straight line VL1 when viewed from the base end side, as shown in FIG. 12. This allows the flow direction of the polishing fluid F to be aligned with the swirl flow when viewed from the Z direction.

16, a gap is formed between the tip surface of the protrusion 112 and the inner wall surface 52 of the bottom wall portion 55. In other words, the tip surface of the protrusion 112 and the inner wall surface 52 of the bottom wall portion 55 do not abut against each other.
The rest is the same as in embodiment 1. Note that, among the reference symbols used in embodiment 2 and onwards, the reference symbols that are the same as those used in the above-mentioned embodiments represent the same components, etc. as those in the above-mentioned embodiments, unless otherwise specified.

When viewed from the plug axial direction Z, the injection hole 51 has an injection hole axis 51L inclined relative to the plug radial direction. This allows the airflow introduced into the auxiliary combustion chamber 50 through the injection hole 51 to form a swirl flow within the auxiliary combustion chamber 50. Therefore, the initial flame formed by the discharge is likely to spread within the auxiliary combustion chamber 50 due to the swirl flow. This makes it easy to promote combustion within the auxiliary combustion chamber 50. As a result, ignition performance can be improved.

In the polishing process, the ejection jig 11 ejects the polishing fluid F in the inner space 501 so that the polishing fluid F flows in the plug circumferential direction. In addition, the flow direction of the polishing fluid F is along the swirl flow A as viewed from the plug axial direction Z. This makes it possible to increase the flow speed of the polishing fluid F near the inner corner 53 located upstream of the swirl flow A as viewed from the Z direction. This makes it possible to efficiently polish the inner corner 53, which has an acute angle. As a result, it is possible to efficiently manufacture the spark plug 1 with improved ignition performance.
In addition, the second embodiment has the same effects as the first embodiment.

(Experimental Example 1)
In this example, a number of spark plugs were prepared that had the same basic structure as in the second embodiment but differed in the arc length of the curved corner surface, and the relationship between the arc length of the curved corner surface and the flow coefficient in the flow rate calculation formula for the orifice was analyzed, as shown in the graph in Fig. 19. Specifically, the relationship between the value obtained by dividing the arc length D3 (see Fig. 14) of the curved corner surface formed at the acute inner corner 53 in a cross section that includes the nozzle hole axis 51L and is along the Z direction by the diameter D4 (see Fig. 14) of the nozzle hole, and the flow coefficient when the nozzle hole is regarded as an orifice, was analyzed.

As shown in the graph in Figure 19, it can be seen that the larger the value of D3/D4, the larger the flow coefficient. In other words, it is believed that the larger the value of D3/D4, the greater the mass flow rate of the combustion gas passing through the nozzle hole. Therefore, it is believed that the larger the value of D3/D4, the greater the momentum of the combustion gas passing through the nozzle hole, and the more forcefully the combustion gas can be ejected into the main combustion chamber. In other words, it is believed that the longer the arc length of the corner curve formed on the inner corner with an acute angle, the more forcefully the combustion gas can be ejected into the main combustion chamber through the nozzle hole.

(Comparative Example 2)
As shown in Figs. 20 and 21, in this embodiment, unlike the second embodiment, no curved corner surface is formed at the inner corner 53.
The rest is the same as in the second embodiment.

(Experimental Example 2)
In this example, as shown in Fig. 22 and Fig. 23, a CFD analysis (short for "Computational Fluid Dynamics analysis") was performed on the speed of the combustion gas CG ejected from a spark plug installed in an internal combustion engine. Specifically, the speed of the combustion gas CG ejected from the auxiliary combustion chamber 50 to the main combustion chamber 101 through the injection hole 51 was analyzed using the spark plug 9 of the comparative embodiment 2 and the spark plug 1 of the embodiment 2. Fig. 22 shows an analysis diagram of the spark plug 9 of the comparative embodiment 2 installed in an internal combustion engine 90, and Fig. 23 shows an analysis diagram of the spark plug 1 of the embodiment 2 installed in an internal combustion engine 10.

As shown in FIG. 22, in the case of comparative form 2, when the combustion gas CG was ejected from the nozzle hole 51, a separation region SE of the combustion gas CG was confirmed along the inner peripheral surface 511 of the nozzle hole 51. Here, the high-temperature combustion gas CG tends to have a high viscosity compared to air at room temperature. Therefore, in the case of comparative form 2 in which the inner corner 53 does not have a curved corner surface, it is considered that when the combustion gas CG was ejected, the combustion gas CG separated from the inner corner 53, which has an acute angle. Note that the separation region SE refers to a region formed inside the nozzle hole 51 when the combustion gas CG was ejected into the main combustion chamber 101, where the flow velocity is significantly lower or almost nonexistent compared to other regions inside the nozzle hole 51.

On the other hand, as shown in FIG. 23, in the case of embodiment 2, when the combustion gas CG was ejected from the nozzle hole 51, almost no separation of the combustion gas CG was observed. In particular, it is believed that almost no separation of the combustion gas CG occurred because the corner curved surface 531 with a long arc length is formed at the inner corner 53 with an acute angle. Therefore, it is believed that the combustion gas CG can be ejected forcefully into the main combustion chamber 101.

(Embodiment 3)
As shown in FIG. 24, this embodiment is an embodiment in which the position at which the ejection port 111 is formed in the ejection jig 11 is changed from that in the second embodiment.
That is, the ejection port 111 is also formed in the protrusion 112 of the ejection jig 11 .

The ejection port 111 has an axial port 113 formed along the Z direction, and an ejection hole side port 114 opening from the axial port 113 toward the base end side of each ejection hole 51. In the ejection jig 11, four ejection hole side ports 114 are formed to correspond to the number of the ejection holes 51. In addition, the openings of the ejection hole side ports 114 opening into the inner space 501 are formed on the side surface of the protruding portion 112.
The rest is the same as in the second embodiment.

In this embodiment, the opening of the nozzle hole side port 114 that opens into the inner space 501 is formed on the side surface of the convex portion 112. This makes it easy to bring the opening of the nozzle hole side port 114 close to the inner corner 53. This makes it easy to increase the flow rate of the polishing fluid F passing near the inner corner 53. As a result, the inner corner 53 can be polished efficiently.
In addition, the second embodiment has the same effects as the second embodiment.

(Embodiment 4)
In this embodiment, as shown in FIGS. 25 and 26, an axial injection hole 513 is formed in the plug cover 5.

As shown in FIG. 25, the axial injection hole 513 is formed along the Z direction to connect the auxiliary combustion chamber 50 to the outside. The axial injection hole 513 is formed along the plug center axis C. The axial injection hole 513 is formed in the bottom wall portion 55.

The axial injection hole 513 has an enlarged diameter portion on the auxiliary combustion chamber 50 side in the Z direction, which increases in diameter toward the base end. The enlarged diameter portion of the axial injection hole 513 can be formed, for example, by performing cutting on the axial injection hole before the enlarged diameter portion is formed.

Next, a method for manufacturing the spark plug 1 of this embodiment will be described.
26 , in the polishing process, the tip surface of the convex portion 112 abuts against the inner wall surface 52 of the bottom wall portion 55. The tip surface of the convex portion 112 is disposed so as to block the entire opening of the axial injection hole 513 on the auxiliary combustion chamber side. In other words, in the polishing process, the axial injection hole 513 does not open to the inner space 501. Therefore, the polishing fluid F does not pass through the axial injection hole 513.
The rest is the same as in the second embodiment.

The protruding portion 112 is disposed so that its tip surface covers the entire opening of the axial injection hole 513 on the auxiliary combustion chamber side. Therefore, the abrasive fluid F easily flows out to the outside through the other injection holes 51 without passing through the axial injection hole 513. Therefore, the corner curved surface 531 can be formed only for the desired injection hole 51. As a result, the spark plug 1 can be manufactured efficiently.
In addition, the second embodiment has the same effects as the second embodiment.

(Embodiment 5)
As shown in FIG. 27, this embodiment is an embodiment in which the shape of the ground electrode 6 is modified from that of the second embodiment.

In this embodiment, the ground electrode 6 is fixed to the housing 2. As shown in Fig. 27, the ground electrode 6 protrudes from a fixed end portion fixed to the housing 2 toward the plug central axis C. The ground electrode 6 is inclined with respect to the Z direction so as to move toward the tip side as it approaches the protruding end portion 61. A discharge gap G is formed between the base end surface of the ground electrode 6 and the tip end portion of the center electrode 4.
The other configurations and effects are the same as those of the second embodiment.

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

1...spark plug, 2...housing, 3...insulator, 4...center electrode, 5...plug cover, 50...auxiliary combustion chamber, 51...nozzle hole, 51L...nozzle hole axis, 511...inner circumferential surface, 52...inner wall surface, 53...inner corner, 531...curved corner surface, 6...ground electrode, G...discharge gap

Claims (6)

A cylindrical insulator (3);
a center electrode (4) held on the inner circumferential side of the insulator and protruding from the insulator toward the tip side;
a cylindrical housing (2) that holds the insulator on its inner periphery;
a ground electrode (6) forming a discharge gap (G) between itself and the center electrode;
a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed,
The plug cover is formed with a plurality of nozzle holes (51) that communicate the auxiliary combustion chamber with the outside,
At an inner corner (53) between an inner peripheral surface (511) of the injection hole and an inner wall surface (52) of the plug cover, a corner curved surface (531) is formed which is curved in a convex state toward the injection hole axis (51L),
A spark plug (1) for an internal combustion engine, wherein the arc length of the corner curved surface varies depending on the position of the inner corner. The spark plug for an internal combustion engine according to claim 1, wherein the inner corner with the smallest angle has a longer arc length of the corner curved surface than the inner corner with the largest angle. The spark plug for an internal combustion engine according to claim 1 or 2, wherein the nozzle hole axis is inclined with respect to the plug radial direction when viewed from the plug axial direction (Z). a spark plug cover (5) provided at a tip end of the housing so as to cover an auxiliary combustion chamber (50) in which the discharge gap is disposed, the plug cover having a plurality of injection holes (51) for communicating the auxiliary combustion chamber with the outside, and an inner corner (53) between an inner peripheral surface (511) of the injection hole and an inner wall surface (52) of the plug cover having a corner curved surface (531) curved in a convex state toward an injection hole axis (51L), the inner corner having a length of an arc of the corner curved surface varying depending on a position of the inner corner,
a nozzle hole forming step of opening the nozzle hole in the plug cover before the plug cover is attached to the housing;
a polishing step of polishing the inner corner portion with a polishing fluid (F) containing an abrasive to form the corner portion curved surface after the nozzle hole forming step,
In the polishing step, an ejection jig (11) having an ejection port (111) for ejecting the polishing fluid is attached to a base end opening end of the plug cover, and the polishing fluid is ejected from the ejection jig toward a portion of the inner wall surface of the plug cover that corresponds to the outer peripheral surface of the auxiliary combustion chamber, and the polishing fluid is discharged from an inner space (501) surrounded by the plug cover and the ejection jig through the nozzle to the outside, thereby polishing the inner corner portion. The manufacturing method of a spark plug for an internal combustion engine according to claim 4, wherein the ejection jig has a protruding portion (112) protruding toward the tip side, and in the polishing process, the inner space is formed on the outer periphery side of the protruding portion and the inner periphery side of the plug cover, and the injection hole opens into the inner space. the injection hole has an axis that is inclined with respect to a radial direction of the plug when viewed from a plug axial direction (Z), and the injection hole is formed such that an air flow is introduced into the auxiliary combustion chamber through the injection hole to generate a swirl flow in the auxiliary combustion chamber,
6. The method for manufacturing a spark plug for an internal combustion engine according to claim 4, wherein in the polishing step, the ejection jig ejects the polishing fluid in the inner space so that the polishing fluid flows in a circumferential direction of the plug, and the flow direction of the polishing fluid is a direction along the swirl flow as viewed in an axial direction of the plug.

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