SPARK PLUG USED FOR AN INTERNAL-COMBUSTION ENGINE AND A METHOD FOR MANUFACTURING THE SAME

Abstract

The present invention provides a highly reliable spark plug used for an internal-combustion engine and including an insulator with a high withstand voltage, and a method for manufacturing the same. Namely, it provides a spark plug containing a cylindrical metal shell having an insulator holding hole, a cylindrical insulator including an axial hole therein which extends in an axial direction, and engaging with said insulator holding hole of said metal shell, and a center electrode held in said axial hole of said insulator, wherein said insulator has a texture in which one or more pores exposed in a judgment area with 50 μm in diameter occupy 40% or less of said judgment area at any locations in an observation area, in the case where a predetermined mirror-finishing section of an enclosed portion surrounded by said metal shell is used as said observation area to observe pores exposed in said observation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-sectional view showing the structure of a spark plug according to an embodiment.

FIG. 2 is a partially-enlarged sectional view showing the front portion of a spark plug shown in FIG. 1.

FIG. 3 is an SEM photograph showing an entire observation area (mirror-finishing section) of an insulator in a spark plug according to an embodiment.

FIG. 4 is an SEM photograph showing a measurement view of an observation area of an insulator of a spark plug according to an embodiment.

FIG. 5 is an explanatory view showing a condition where a ceramic surface and a pore portion present in the observation view of FIG. 4 are binarized.

FIG. 6 is an example of SEM photograph showing an observation area (mirror-finishing section) of an insulator of a spark plug according to a comparative form 1.

FIG. 7 is an SEM photograph showing a measurement view of an observation area of an insulator in a spark plug according to a comparative form 1.

FIG. 8 is an explanatory view showing a condition where a ceramic surface and a pore portion present in the observation view of FIG. 7 are binarized.

FIG. 9 is an example of SEM photograph showing an observation area (mirror-finishing section) of an insulator of a spark plug according to a comparative form 2.

FIG. 10 is an SEM photograph showing a measurement view of an observation area of an insulator of a spark plug according to a comparative form 2.

FIG. 11 is an explanatory view showing a condition where a ceramic surface and a pore portion present in the observation view of FIG. 10 are binarized.

FIG. 12 is a flow chart showing a procedure for calculating a pore occupying area rate in a judgment area of an observation area.

FIG. 13 is a flow chart showing manufacturing steps of an insulator in the manufacturing process of a spark plug according to an embodiment.

Claims

1. A spark plug used for an internal-combustion engine, comprising: a cylindrical metal shell comprising an insulator holding hole;a cylindrical insulator comprising an axial hole therein, which extends in an axial direction, and engaging with said insulator holding hole of said metal shell; anda center electrode held in said axial hole of said insulator;wherein said insulator has a texture in which the area of one or more pores exposed in the area of a judgment area with 50 μm in diameter occupies 40% or less of said judgment area at any locations in an observation area, in the case where a predetermined mirror-finishing section of an enclosed portion surrounded by said metal shell is used as said observation area to observe pores exposed in said observation area.

2. The spark plug used for an internal-combustion engine as claimed in claim 1 wherein, when an end of the spark plug to be inserted in an internal-combustion engine is referred to as “front side”, and the other end of the spark plug to be located outside of the internal-combustion engine is referred to as “rear side” in said axial direction,said metal shell further comprises:an engaging convex portion projecting radially inwardly and including a rear engaging face, which is located at a rear side of said engaging convex portion; anda front cylindrical portion located at a front side of said engaging convex portion and having a larger inner diameter than that of said engaging convex portion; andsaid insulator further comprises:a middle trunk portion including an engaging shoulder face located at a front side of said middle trunk portion and engaging with said rear engaging face of said metal shell from a rear side; anda long leg portion located at a front side of said middle trunk portion, having a smaller diameter than that of said middle trunk portion and forming a space with said front cylindrical portion of said metal shell; andin said insulator, a portion of said long leg portion facing said engaging convex portion of said metal shell has a thickness of 1.80 mm or less in a radial direction perpendicular to said axial direction.

3. A method for manufacturing a spark plug used for an internal-combustion engine which comprises: a cylindrical metal shell having an insulator holding hole;a cylindrical insulator including an axial hole therein, which extends in an axial direction, and engaging with said insulator holding hole of said metal shell; anda center electrode held in said axial hole of said insulator;the method comprising the steps of:a slurry formation step in which a raw material powder composed mainly of alumina powder and an organic binder is kneaded with a solvent to form slurry;a defoaming step in which thus-formed slurry is disposed under low pressure environment so as to defoam;a granulation step in which thus-defoamed slurry is formed into a granular body; anda press step in which thus-formed granular body is filled in a mold and compressed to form a press-molded body.

4. The method for manufacturing a spark plug used for an internal-combustion engine as claimed in claim 3, wherein said alumina powder has an average particle size of 1.0 μm or less.

5. The method for manufacturing a spark plug used for an internal-combustion engine as claimed in claim 3, wherein said slurry contains no agent facilitating a release of air bubbles.

6. The method for manufacturing a spark plug used for an internal-combustion engine as claimed in claim 4, wherein said slurry contains no agent facilitating a release of air bubbles.