JP3533684B2 - Spark plug - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spark plug used as an ignition device for an internal combustion engine such as a gasoline engine. 2. Description of the Related Art Conventionally, in internal combustion engines used for automobiles and the like, fuel economy has been strongly promoted from the global standpoint of environmental problems or global resource problems. As a specific means, particularly in a gasoline engine, a high-compression, lean-mix leaner mixture is used.
However, the discharge voltage of the spark plug tends to be very high. As the discharge voltage rises, there is a gap between a stepped portion having a sharp corner provided on the inner surface of the housing of the spark plug and an insulator of the spark plug.
A portion having a high electric field intensity is formed. Therefore, in recent years, there is a tendency that corona discharge concentrates on the insulator portion at a position facing the corner portion, so that a pinhole is easily formed due to dielectric breakdown. In many cases, the pinhole is generated at the position of the insulator facing the step. As a disclosed technique applicable to such a situation, there is, for example, Japanese Patent Application Laid-Open No. 60-14781. The publication discloses that, at the step portion provided with the corner portion provided on the inner surface of the housing for fixing the insulator, the corner portion is formed into a smooth shape so that corona concentrated on the corner portion is generated. This discloses a technique for suppressing pinhole formation by suppressing the formation of pinholes. [0005] However, the above-described disclosed technology alone does not
It has become clear that the occurrence of corona cannot be suppressed. SUMMARY OF THE INVENTION In view of the above situation, it is an object of the present invention to provide a spark plug having a structure capable of suppressing the corona discharge. [0007] For this purpose, a screw is provided on the outer periphery.
Having a portion, a hollow pipe-shaped housing having a tapered step inside the screw portion, and held by the step,
Moreover the insulator having a through-hole, and a ground electrode provided on the front end surface of the housing, before intruded into the through-hole
A central electrode held so as to protrude from the front end side of the insulator, and means for transmitting electric power to the central electrode. A spark gap is formed by the ground electrode and the central electrode , and used for automobiles. In the spark plug described above, the insulator has a maximum diameter and is attached to the housing.
The swaged portion to be fixed and the swaged portion are
The step is provided at the tip side of the swaged holding portion with a small diameter.
A held portion held by the difference portion, and a smaller diameter than the held portion;
A leg provided on the tip end side of the held portion, and
The jing is formed between the step portion and the front end surface.
The insulator has an inner surface facing the leg portion, and the inner surface is
The gap between the inner peripheral edge of the step portion and the tip surface
A tapered surface whose diameter increases toward the tip end surface side and the tapered surface.
And a cylindrical surface with a constant diameter connected to the end surface of the surface.
Bending from the tapered surface to the cylindrical surface
The object is solved by setting the axial distance L between the point and the corner to be 6 mm or more . The housing may be configured such that a line connecting the sole corner provided at the step portion and the tip end surface is a straight line or a smooth curve, or a corner provided at the step portion. Equipotential lines are concentrated between the step and the insulator facing the step because of having a cross-sectional shape that is a straight line or a smooth curve having a bending point on a line connecting the tip end surface. Can be avoided. As a result, a thick corona discharge that greatly affects the dielectric breakdown is eliminated. As described above, since the equipotential lines can be prevented from being concentrated, the dielectric breakdown strength is greatly improved, and a spark plug which can sufficiently cope with a future increase in the discharge voltage can be provided. . As a result of various studies to solve the above problems, it was found that the shape of the inner surface of the housing and the presence of burrs had a great influence. Hereinafter, the present invention will be described based on an embodiment shown in the drawings. FIG. 1 shows a half sectional view of a spark plug 1 according to the present invention. The spark plug 1 has a center electrode 5 at the tip of a through hole 4a provided in an insulator 4 made of alumina or the like.
And a stem 8 at the other end of the through hole 4a.
Further, it comprises the insulator 4 enclosing the conductive glass 6 and the resistor 7, and the housing 2 holding the insulator 4 via a ring 9 and a packing 10. The housing 2 is provided with a screw portion 2a, and a ground electrode 3 is provided on the front end surface 2b of the housing. The curved tip portion 3a of the ground electrode 3 is disposed so as to form a spark gap 12 facing the tip surface 5a of the center electrode 5. The packing 11 prevents airtightness and is used by being mounted on the engine with the screw portion 2a. The housing 2 is manufactured and assembled by cold forging a metal such as low carbon steel. FIG. 2 shows a main part according to the present invention. A step 2c for holding the insulator 4 is provided on an inner surface 2d of the housing 2.
Are provided, and the held portion 4d of the insulator 4 is fixed to the ring 9 and the swaged held portion 4e of the insulator 4.
Is interposed with the packing 10 so that the insulator 4 is fixed. Also, the corner 2 of the step 2c
e is cut into a smooth rounded shape. And from the said corner part 2e to the said front-end | tip part 2b, the smooth surface without remarkable unevenness | corrugation and a scratch is formed. FIG. 3 shows a main part of a conventional product. In the conventional product, a step 2c for holding the insulator 4 is provided on the inner surface 2d of the housing 2, and the held portion 4d of the insulator 4 is connected to the ring 9 and the caulked held portion of the insulator 4. The insulator 4 is held firmly by interposing the packing 4e with the packing 10. The corner 2e and the corner 2f of the step 2c are cut into a smooth and rounded shape. However, corner 2f
No significant irregularities and scratches from
It forms a gentle surface. The spark plug 1 according to the present invention described above and the conventional product are manufactured, and no spark is generated between the center electrode 5 and the ground electrode 3 using the housing 2 as an earth. As described above, a voltage was gradually applied, and a voltage of 24 KV was applied to the center electrode 5 to generate a corona. FIG. 4 shows the state of occurrence of the corona. FIG. 4A is a schematic view showing the state of corona generation of a conventional product. A very thick corona is generated at the corners 2e and 2f, and a thin corona is generated at an inner surface 2d near the housing tip 2b. There is a feature to do. FIG. 4A is a schematic diagram showing the corona generation state of the product of the present invention. There is a feature to do. The results of a study on the potential distribution of the conventional product and the product of the present invention using the finite element method are shown in FIGS.
FIG. 5 shows the equipotential distribution of the conventional product, and FIG. 6 shows the equipotential distribution of the product of the present invention. Comparing the two, in the equipotential distribution of the conventional product of FIG. 5, the electric field strength is remarkably large at the position of the insulator 4 facing the step portion 2c, whereas the equipotential of the present product of FIG. In the distribution, the electric field intensity is small at the position of the insulator 4 facing the step 2c. From this, it is expected that the dielectric breakdown strength of the product of the present invention will be improved as compared with the conventional product, so the shape of the inner surface 2d was further examined. That is, FIG. 7 shows a cross section of a main part of the product of the present invention.
c, the linear shape from the corner 2e to the housing tip 2b is actually insulated by forming a bending point 2g between the corner 2e of the step 2c and the housing tip 2b. The breaking strength was measured. The result is shown in FIG. FIG. 8 shows the results of measuring the dielectric breakdown strength with the distance between the corner 2e of the step 2c of the bending point 2g and the corner 2e being Lmm. L> 1mm
In the above, the dielectric breakdown strength gradually increases, and L ≧
At 6 mm, it is improved by about 10% over the conventional product . As described above, from the corner 2e of the step 2c to the inner surface 2d of the housing 2,
By setting L to be greater than at least 1 mm, the straight line shape leading to b is formed as a shape having a bending point 2g between the corner 2e of the step 2c and the front end surface 2b of the housing. Lines can be prevented from concentrating. As a result, the thick corona discharge is eliminated, and the dielectric breakdown strength is greatly improved.
It is possible to provide a spark plug that can sufficiently cope with a future increase in discharge voltage.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a half sectional view of a spark plug according to an embodiment of the present invention. FIG. 2 is an enlarged half section of a main part of a spark plug according to an embodiment of the present invention. FIG. 3 is an enlarged half section of a main part of a conventional spark plug. FIG. 4A is a schematic diagram for explaining a corona generation state of a conventional product. (A) is a schematic diagram for explaining the state of corona generation of the product of the present invention. FIG. 5 is a result of calculating a potential distribution of a conventional product using a finite element method. FIG. 6 is a result of calculating a potential distribution of the product of the present invention using a finite element method. FIG. 7 is a half sectional view for explaining a distance L provided on an inner surface of a housing of a spark plug according to an embodiment of the present invention. FIG. 8 is a graph showing measurement results of dielectric breakdown strength . [Description of Signs] 1 Spark plug 2 Housing 2a Screw 2b Tip 2c Step 2d Inner surface 2e Corner 2f Corner 2g Bending point 3 Ground electrode 4 Insulator 4a Through hole 4b Leg 4c Corrugation 4d Holding Part 4e swaged part 5 center electrode 6 conductive glass 7 resistor 8 stem 9 ring 10 packing 11 packing 12 spark gap

Continuation of the front page (56) References JP-A-50-149832 (JP, A) JP-A-60-240086 (JP, A) JP-A 51-19131 (JP, U) JP-A 50-114524 (JP 49-76125 (JP, U) JP-B 5-44150 (JP, B2) JP-B 48-26687 (JP, B1) JP-B 33-5155 (JP, B1) 18-9117 (JP, Y1) Jiko 18-12012 (JP, Y1) Jiko 33-15307 (JP, Y1) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01T 13/00 -21/06

Claims (1)

(57) Claims: 1. A hollow pipe-shaped housing having a threaded portion on the outer periphery and having a tapered stepped portion inside the threaded portion, and the stepped portion. An insulator provided with a through hole, a ground electrode provided on a front end surface of the housing, and a protrusion protruding from the front end side of the insulator penetrating the through hole.
A center electrode held to output, is composed of a means for transmitting power to the central electrode, Spa by the center electrode and the ground electrode - forming a Kugyappu, use in an automobile
In this spark plug, the insulator has a maximum diameter and is caulked to the housing.
The swaged part to be fixed and a smaller diameter than the swaged part
The caulked portion is provided on the tip side of the held portion, and
The held portion to be held, and the held portion having a smaller diameter than the held portion.
A leg provided on the distal end side of the portion, and the housing is provided between the step portion and the distal end surface.
And an inner surface facing the legs of the insulator.
Is between a corner, which is an inner peripheral edge of the step portion, and the front end surface.
In the above, a tapered surface whose diameter increases toward the tip end surface side and the tapered surface
It is composed of a cylindrical surface with a constant diameter connected to the tip end side of the tapered surface.
Made, bending point switched from the tapered surface on the cylindrical surface
A spark plug having a distance L in the axial direction of 6 mm or more between the spark plug and the corner.