JPH09260018A - Spark plug - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate normal spark discharge in a mixture in a wide range of condition of use while preventing flash-over at the sparking point. SOLUTION: This plug comprises a center electrode 3 of which the tip extends into the combustion chamber, and a body attachment 8 holding insulator which is mounted on a cylinder. The center electrode 3 is comprised of a cylindrical portion 30 inserted into the insulator and the flange which is larger diameter than the cylinder and connected to the back end of the cylinder 30, engaged in the insulator. The tip 31 of the cylinder 30 is comprised of a step 33 of approximately cone-shape tapered toward the tip, and a precious metal tip 34 of anti-spark wearing member in the form of disk of the same diameter of the top surface 39, which is coupled to the top surface 39 of the step 33. The boundary 36 of the cylinder 30 and the step 33 is placed inside of the top surface 63 of the leg 62 of the insulator and provides a radius R in the circumferential direction of the boundary 36.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spark plug which is improved so as to suppress the occurrence of flashover from the tip end surface of an insulator to the inside during ignition discharge.

[0002]

2. Description of the Related Art Conventionally, a spark plug for generating a spark discharge in an air-fuel mixture has a rod-shaped center electrode inserted into a cylindrical insulator, and the tip of the center electrode protrudes from the tip surface of the insulator to form an outer electrode. The metal shell to which is attached has a structure for fixing the insulator. Then, spark discharge is generated in the discharge gap between the tip of the center electrode and the outer electrode. In the spark plug having this structure, as shown in FIG.
It is known to have a stepped portion 101 which is connected to the tip end side of the columnar portion 100 and is tapered toward the tip end, and a small columnar portion 102 which is connected to the tip end surface of the stepped portion 101 and reaches the tip end. . Further, it is known that such a small cylindrical portion is made of a noble metal material such as Pt-Ir alloy.

[0003]

When the boundary 103 between the cylindrical portion 100 and the step portion 101 of the center electrode is projected in a convex acute angle, the boundary 103 forms the tip surface 10 of the insulator.
Especially when it is outside 4, the distribution density of the isoelectric field curve at the boundary 103 becomes high, and the electric field strength at the position of the momentum boundary 103 becomes large. Therefore, flashover is more likely to occur from the boundary 103 to the back of the insulator than the spark discharge in the regular discharge gap between the tip of the center electrode and the outer electrode, as indicated by the symbol FO in FIG. .
In particular, a spark plug for a gas engine that burns a gaseous fuel such as natural gas, city gas, or LPG gas has a short leg length of the insulator, a high compression ratio of the air-fuel mixture, and is a gaseous fuel. The discharge voltage was likely to be high, and flashover was likely to occur.

An object of the present invention is to provide a spark plug in which such a flashover hardly occurs and a regular spark discharge can be generated in a mixture under a wide range of use conditions.

[0005]

According to the present invention, a rod-shaped center electrode, an insulator holding the center electrode inside, a metal shell holding the insulator inside, and a tip portion of the center electrode. In a spark plug having an outer electrode that forms a discharge gap in the air between the center electrode, the center electrode has a tapered step portion, and a spark consumable member is provided on the tip surface of the tapered step portion, the center electrode The boundary between the columnar portion and the tapered stepped portion is configured to have a rounded shape in the circumferential direction.

Further, in the spark plug of the present invention, the boundary between the cylindrical portion of the center electrode and the tapered step portion is located outside the tip surface of the insulator.

The spark plug of the present invention is characterized in that the roundness has a radius R of 0.1 mm or more.

Further, in the spark plug of the present invention, the tip end portion of the center electrode is inscribed in a conical surface coaxial with the center electrode and having a central angle of 110 ° or less, and the outer periphery of the tip end of the insulator is It is characterized by being inscribed.

Further, in the present invention, a rod-shaped center electrode,
An insulator holding the center electrode inside, a metal shell holding the insulator inside, and a spark plug having an outer electrode forming a discharge gap in the air between the tip of the center electrode, The center electrode has a tapered step portion, and the tip surface of the tapered step portion includes a small cylindrical portion having the same diameter as the tip surface of the tapered step portion and reaching the tip of the center electrode. Is opposed to the side surface of the small cylindrical portion, and the boundary between the cylindrical portion of the center electrode and the tapered step portion is rounded in the circumferential direction.

Further, in the spark plug of the present invention, the boundary between the cylindrical portion of the center electrode and the tapered step portion is located outside the tip surface of the insulator.

Further, in the spark plug of the present invention, the roundness is characterized in that R is 0.1 mm or more.

Further, in the spark plug of the present invention, the tip portion of the center electrode is inscribed in a conical surface coaxial with the center electrode and having a central angle of 110 ° or less, and the outer periphery of the tip of the insulator is It is characterized by being inscribed.

Further, in the spark plug of the present invention, the tip portion of the center electrode is made of a noble metal material as a spark consumable member at a portion facing the tip of the outer electrode. To do.

In addition, in the spark plug of the present invention, the noble metal material as the spark-resistant member is
Pt, Ir, Pt alloy, Ir alloy or Y
It is characterized by comprising at least one of alloys containing 2O3.

[0015]

According to the present invention, the boundary between the cylindrical portion of the center electrode and the tapered step portion is located outside the tip surface of the insulator, and a roundness having a predetermined R is provided around this boundary. As a result, the distribution of the isoelectric field curves at this boundary becomes gentle and relatively rough, and as a result,
Since the potential from the boundary of the stepped portion in the center electrode to the insulator step is always higher than the potential in the regular discharge gap, a regular spark discharge is generated even during ignition with a high voltage applied, It is possible to effectively suppress flashover from the tip end surface of the insulator to the inside. The tip of the center electrode is coaxial with the center electrode and has a center angle of 11
The tip of the center electrode is inscribed in the conical surface of 0 ° or less, and the outer periphery of the tip of the insulator is inscribed. The reason for this configuration is that it has the following advantages. In order to suppress flashover, it is necessary to reduce the discharge voltage, and as one method therefor, the central angle of the conical surface inscribed between the tip of the center electrode and the outer circumference of the tip of the insulator is set to 110 ° or less, and the insulator is It has been found that it is effective to reduce the diameter of the outer circumference of the tip. With the configuration based on this finding, the isoelectric field curve on the outside of the insulator becomes rough at the time of ignition with high voltage application, and therefore flashover can be effectively suppressed.

Further, Pt, I as a spark-depleting member is provided at a portion of the tip portion of the center electrode facing the tip of the outer electrode.
Since at least one of r, Pt alloy, Ir alloy, or alloy containing Y2O3 in these is provided, the consumption of the electrode can be suppressed and the life can be extended.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Next, a spark plug of the present invention will be described based on a first embodiment shown in FIGS. The spark plug of this embodiment is a parallel electrode type and is used by being attached to each cylinder for an internal combustion engine of an automobile. As shown in FIG. 1, the spark plug 1 has a center electrode 3 that is electrically connected to a secondary coil of an ignition coil and has a tip inside a combustion chamber (not shown), and an insulator 6 that holds the center electrode 3. Insulator 6 attached to cylinder
And a metal shell 8 for holding the inside.

Metal shell 8 which is a housing made of steel
The outer electrode 2 that faces the tip of the center electrode 3 and forms the discharge gap G is joined to the tip surface of the electrode using a joining means such as resistance welding. On the outer periphery of the metal shell 8, a male screw portion 82 for attaching to each cylinder head of the internal combustion engine and a hexagonal portion 83 for engaging a tool such as a tightening wrench are formed.

The center electrode 3 is integrally formed by cold extrusion molding by encapsulating a core material made of a conductive metal having excellent thermal conductivity in an electrode base material made of a nickel alloy having excellent heat resistance and corrosion resistance. The center electrode 3 is connected to the cylindrical portion 30 inserted into the shaft hole 60 of the insulator 6 and the rear end portion of the cylinder portion 30 and has an outer diameter larger than that of the cylinder portion 30 and is disposed in the shaft hole 60 of the insulator 6. It is composed of a collar portion 35 to be locked.

The tip portion 31 is integrally molded with the columnar portion 30 and is joined to the tip surface 39 of the tapered step portion 33 and the step portion 33, which is formed in, for example, a substantially truncated cone shape toward the tip portion. And a noble metal tip 34 as a disk-shaped spark-consumable member having the same diameter. The precious metal tip 34
Is made of at least one of Pt, Ir, a Pt alloy, an Ir alloy, or an alloy containing an oxide such as Y2 O3, and is formed on the tip surface 39 of the tapered step portion 33 by laser welding, resistance welding, or the like. It is joined. Further, instead of the noble metal tip 34, a high chromium tip having spark wear resistance or a chromium alloy in which ceramic powder is dispersed in chromium may be used. As a result, the consumption of the electrodes can be suppressed and the service life can be extended.

The terminal electrode 4 is made of a conductive metal (for example, mild steel).
Are integrally molded by. The terminal electrode 4 is a round bar-shaped shaft portion 4
0, and a collar-shaped locking portion 43 that is formed at one end of the shaft portion 40 and that forms a circular ring having a diameter larger than that of the shaft portion 40 that contacts the rear end portion of the insulator 6.
And a terminal portion 44 formed on the locking portion 43 and connected to the secondary coil. The shaft portion 40 has a tip screw portion 41 in which a screw is formed on the outer circumference on the tip side in order to enhance the close contact with the insulator 6 and the conductive glass sealing material 52.

The center electrode 3 is provided in the shaft hole 60 of the insulator 6, the tip is further arranged in the combustion chamber, and the terminal electrode 4 connected to the secondary coil is located behind the center electrode 3 in the shaft hole 60. It is arranged at the end. Between the center electrode 3 and the terminal electrode 4 in the shaft hole 60, a monolithic resistor powder 51 is sandwiched with a conductive glass sealing material 52 and sealed in the shaft hole 60. The insulator 6 is fixed in an airtight state by the talc powder 7 between the insulator 6 and the metal shell 8. The insulator 6 is mainly composed of aluminum oxide (Al ₂ O ₃ : alumina) or is a ceramic sintered body obtained by adding a sintering aid to aluminum nitride (AlN) and firing the mixture. Further, the insulator 6 is penetrated by an axial hole 60 extending in the axial direction from the front portion to the rear end portion.

The center electrode 3 is inserted into the shaft hole 60, and the collar portion 35 is locked to the step portion 66, so that the conductive glass sealing material 52 and the conductive glass sealing material 52 are provided on the collar portion 35 of the center electrode 3 in the shaft hole 60. The monolithic resistor powder 51 and the conductive glass sealing material 52 are filled, the terminal electrode 4 is inserted and heated, and these are integrally sealed in the shaft hole 60 of the insulator 6. Then, the insulator 6 covers the shaft portion 40 of the terminal electrode 4 inserted in the rear end portion of the shaft hole 60, and the corrugation portion 61.
A leg long portion 62 that covers the center electrode 3 inserted into the shaft hole 60 and is exposed to the air-fuel mixture in the combustion chamber, and a corrugation portion 61.
And a leg-length portion 62 and a large-diameter engagement portion 67 that engages with the metal shell 8.

On the outer periphery of the corrugated portion 61, a step is provided for increasing the flashover generation voltage between the terminal electrode 4 and the metal shell 8. Then, the stepped seat 68 of the insulator 6 is locked to the step portion 84 provided in the inner cavity of the metal shell 8, and the talc powder 7 is placed in the space between the inner cavity and the corrugation portion 61 to dispose the metal shell 8 The insulator 6 is fixed by swaging the rear end portion 85 inward.

As clearly shown in FIG. 2A, the boundary 36 between the cylindrical portion 30 of the center electrode 3 and the tapered step portion 33 has a radius of curvature of 0.1 mm or more. A roundness R is formed in the direction and the boundary 36
Is located outside the tip surface 63 of the insulator 6 by 0.5 mm, for example, outside the insulator 6. A predetermined discharge gap G is formed between the tip portion 37 of the center electrode 3 and the tip 21 of the outer electrode 2. In this case, as the boundary 36, as shown in FIG. 2B, the extension surface L1 along the outer surface of the tapered step portion 33 and the extension surface L2 along the outer surface of the columnar portion 30 are formed to intersect with each other. A circle is used as a reference plane, and the intersection of this reference plane and the outer peripheral surface of the tip portion 31 is a boundary 36.
Is considered. Here, the tapered step portion 3 in the center electrode 3
Since 3 is formed in a substantially truncated cone shape, a large volume can be secured in this step portion 33, and thus heat transfer to the rear end portion of the center electrode 3 can be improved, and the tip portion of the center electrode 3 is excessively heated. It is possible to avoid problems that occur.

[Operation and Effect of Embodiment] Since the roundness R is formed at the boundary 36 between the cylindrical portion 30 of the center electrode 3 and the tapered step portion 33, the boundary shown in FIG. 2D has an acute angle. Unlike the conventional case where the electric field curve is
As shown in (c) of FIG. 2, the distribution of the isoelectric field curve Eq at the boundary 36 becomes a relatively gentle coarse state, and the boundary 3
The electric field concentration phenomenon on 6 is relaxed, and the potential between the boundary 36 of the tapered step portion 33 of the center electrode 3 and the step 68 of the insulator 6 is always higher than the potential between the regular discharge gaps. As a result, as shown in the graph of R-FO relationship in FIG. 3, the formation of the roundness R at the boundary 36 can improve the flashover generation voltage by 2 kV or more, and the flashover can be effectively suppressed. According to FIG. 3, boundary 3
It can be seen that the spark plugs A and C having the roundness R at 6 are more advantageous than the conventional spark plug B in suppressing flashover.

[Second Embodiment] Next, a spark plug of the present invention will be described based on a second embodiment shown in FIGS. 4 and 5. The spark plug of this embodiment is different from the spark plug 1 of the first embodiment in the following points.

The cylindrical portion 30 of the center electrode 3 and the tapered step portion 3
Although the boundary 36 with 3 is located outside the tip surface 63 of the insulator 6, the roundness R is formed in the boundary 36, the tip 37 of the center electrode 3 and the tip outer periphery 64 of the insulator 6 are formed. An inscribed conical surface is formed, and as shown in FIG. 4A, the central angle θ of the conical surface is 110 ° or preferably 100 ° or less. In this case, for convenience, the central angle θ adopts the apex angle of a triangular cross section obtained by cutting the conical surface along a plane passing through the central axis. The characteristic of the central angle θ with respect to the discharge voltage is shown in FIG. As described above, in the present embodiment, the central angle θ of the conical surfaces inscribed in the tip 37 of the center electrode 3 and the tip outer circumference 64 of the insulator 6 is 110 ° or less. As a result, the diameter of the outer circumference 64 of the tip of the insulator 6 can be relatively small.
As shown in (b) of FIG. 1, the distribution of the isoelectric field curve Eq is prevented from being extended outward, and thus a regular spark discharge can be generated at a relatively low discharge voltage, so that the insulator It is possible to effectively suppress flashover from the front end surface 63 of 6 to the back.

[Effect of Embodiment] The spark plug of this embodiment can be used up to a high discharge voltage without causing flashover. Therefore, it is possible to use up to a high combustion chamber pressure that requires a high discharge voltage.

[Third Embodiment] Next, a spark plug of the present invention will be described based on a third embodiment shown in FIG.
The spark plug 10 of this embodiment is a multipolar spark plug, and differs from the spark plug 1 of the first embodiment in the following points.

The tip portion 31 of the center electrode 3 has a tapered step portion 33 formed in a substantially truncated cone shape and a small columnar portion 38 having the same diameter as the tip diameter of the step portion 33 formed on the tip side. The metal fitting 8 is provided with a plurality of outer electrodes 14, and the tip portion 15 of the outer electrode 14 is opposed to the tip side surface of the small columnar portion 38. Further, in the small columnar portion 38, a thin disk-shaped noble metal tip 34 of platinum or the like similar to that of the first embodiment is provided at a position where the tip portion 15 of the outer electrode 14 faces by known laser welding and plastic working. . The boundary 36 of the tapered step portion 33 is also located outside the tip surface 63 of the insulator 6 in the same manner. In this case, the geometrical relationship of the central angle θ of the conical surface shown in FIG. Further, contrary to this embodiment, the boundary 36 may be positioned inside the insulator 6 so as to be drawn inward from the tip surface 63 of the insulator 6. This circumstance can be similarly applied to the above-described first and second embodiments.

[Effects of the Embodiment] In addition to the effects of the first embodiment, the spark plug of the present embodiment is particularly used in an internal combustion engine equipped with a distributorless igniter (DLI) type ignition device, and has a high combustion chamber. It can be used up to the discharge voltage without flashover.
Therefore, it is possible to use up to a high combustion chamber pressure that requires a high discharge voltage.

[0033]

Since flashover does not occur up to a high discharge voltage, a spark discharge can be generated in the air-fuel mixture in the regular discharge gap between the center electrode and the outer electrode, so that the ignitability of the internal combustion engine is improved. You can In particular, it is effective in a gas engine that burns a gaseous fuel such as natural gas, city gas, or LPG gas that has a high compression ratio and requires a high voltage for ignition. Moreover, since it is only necessary to form the roundness R on the boundary 36, it is advantageous in terms of cost without increasing the number of members with a simple structure.

[Brief description of drawings]

FIG. 1 is a front half sectional view of a spark plug according to a first embodiment of the present invention.

2 (a) is an enlarged cross-sectional view of the main part of the ignition part of the spark plug of FIG. 1, (b) is a schematic diagram for defining a boundary reference plane, and (c) is an isoelectric field curve. FIG. 3D is a schematic diagram for showing the electric field curve distribution in the related art.

FIG. 3 is a graph comparing spark plugs A and C according to the present invention in relation to a flashover generation voltage.

FIG. 4A is an enlarged front view of a tip end portion of an insulator according to a second embodiment of the present invention, and FIG. 4B is a schematic view showing isoelectric field curves.

FIG. 5 is a graph showing the relationship between the discharge voltage and the central angle θ of the conical surface.

FIG. 6 is an enlarged sectional view of an essential part of an ignition part of a spark plug according to a third embodiment of the present invention.

FIG. 7 is an enlarged cross-sectional view of a center electrode of a conventional spark plug and a tip portion of an insulator.

[Explanation of symbols]

 1, 10 Spark plug 2 Outer electrode 3 Center electrode 6 Insulator 8 Metal shell 30 Column part 31 Tip part 33 Step part 34 Precious metal tip (spark consumable member) 36 Boundary 37 Center electrode tip 38 Small column part 39 Step part Tip surface of the leg 62 Leg part (insulator) 63 Tip surface of the leg section 64 Tip outer circumference of the leg section G Discharge gap R Roundness

Claims (10)

[Claims] 1. A discharge gap in the air between a rod-shaped center electrode, an insulator holding the center electrode therein, a metal shell holding the insulator inside, and a tip of the center electrode. In the spark plug having an outer electrode forming a, the center electrode has a tapered step portion, and a spark consumable member is provided on a tip surface of the step portion, and the cylindrical portion of the center electrode and the tapered step portion The spark plug is characterized in that the boundary is rounded in the circumferential direction. 2. The spark plug according to claim 1, wherein the boundary between the cylindrical portion of the center electrode and the tapered step portion is located outside the tip end surface of the insulator. Spark plug. 3. The spark plug according to claim 1 or 2, wherein the roundness has a radius R of 0.1 mm or more. 4. The spark plug according to claim 1, wherein the spark plug is coaxial with the central electrode and has a central angle of 1.
A spark plug, wherein the tip portion of the center electrode is inscribed in a conical surface of 10 ° or less and the tip outer periphery of the insulator is inscribed. 5. A discharge gap in the air between a rod-shaped center electrode, an insulator holding the center electrode therein, a metal shell holding the insulator inside, and a tip portion of the center electrode. In the spark plug having an outer electrode forming a center electrode, the center electrode has a tapered step portion, and a small cylinder reaching the tip of the center electrode with the same diameter as the tip surface of the taper step portion on the tip side of the step portion. A tip portion of the outer electrode faces a side surface of the small columnar portion, and a boundary between the columnar portion of the center electrode and the tapered step portion is rounded in the circumferential direction. And a spark plug. 6. The spark plug according to claim 5, wherein the boundary between the cylindrical portion of the center electrode and the tapered step portion is located outside the tip surface of the insulator. Spark plug. 7. The spark plug according to claim 5, wherein the roundness has a radius R of 0.1 mm or more. 8. The spark plug according to claim 5, wherein the spark plug is coaxial with the center electrode and has a center angle of 1.
A spark plug, wherein the tip portion of the center electrode is inscribed in a conical surface of 10 ° or less and the tip outer periphery of the insulator is inscribed. 9. The spark plug according to any one of claims 1 to 8, wherein the tip portion of the center electrode is a spark consumable member at a portion facing the tip portion of the outer electrode. A spark plug characterized by using a precious metal material. 10. The spark plug according to claim 9, wherein the noble metal material serving as the spark-resistant member is Pt, Ir, Pt alloy, Ir alloy, or an alloy containing Y2O3. A spark plug characterized by comprising at least one.