SPARK PLUG FOR USE IN AN INTERNAL-COMBUSTION ENGINE

Abstract

A spark plug 100 comprised of a metal shell 1, an insulator 2, a center electrode 3 and a ground electrode 4. A rear-end face of the ground electrode 4 is welded to a front-end face of the metal shell 1, and a bent portion 5 located at the intermediated position in the longitudinal direction is bent toward the center of the spark plug 100. The ground electrode 4 assumes a circular-shape with a diameter of 2 mm or less whereby an inflow of an air-fuel mixture is not disturbed even when the air-fuel mixture directly flows into a back face of the ground electrode 4. The ground electrode 4 is comprised of an outer layer 4A made of a nickel alloy and an inner layer 4B made of pure copper with an excellent thermal conductivity, in which a ratio of a cross-sectional area of the inner layer 4B to the entire cross-sectional area of the ground electrode 4 is 10% or more to 35% or less. Thus, the spark plug 100 which is excellent in heat sinking ability and can prevent a spring back phenomenon due to a difference in a coefficient of thermal expansion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is a partially sectional front view showing an entire composition of a spark plug according to an embodiment of the invention;

FIG. 2 is a partially sectional front view showing a composition of a main portion of a spark plug according to the embodiment;

FIG. 3 is a side view showing a spark plug viewed from the direction perpendicular to the direction of FIG. 2;

FIG. 4 is a top view showing a spark plug viewed from a front-end side;

FIG. 5A is a diagram showing a ground electrode and a center electrode which are projected from a front-end face side of the ground electrode along the central axis of the center electrode;

FIG. 5B is an explanatory view showing an enlarged main portion of FIG. 2;

FIG. 6 is a graph showing a relationship between an increment of gap and a cross-sectional area ratio of an inner layer according to a first embodiment;

FIG. 7 is a graph showing a relationship between an increment of gap and a cross-sectional area ratio of an inner layer according to a second embodiment;

FIGS. 8A, 8B, 8C are explanatory views according to another embodiment and showing a diagram of a ground electrode and a center electrode which are projected from a front-end face side of the ground electrode along the central axis of the center electrode;

FIGS. 9A and 9B are an explanatory view according to another embodiment and showing a diagram of a ground electrode and a center electrode which are projected from a front-end face side of the ground electrode along the central axis of the center electrode;

FIG. 10 shows a side shape of a ground electrode according to another embodiment; and

FIG. 11 is a partial cross-section showing a ground electrode according to another embodiment.

Claims

1. A spark plug for use in an internal-combustion engine, comprising: a center electrode;an insulator at least partially surrounding the center electrode;a cylindrical metal shell at least partially surrounding the insulator;a ground electrode so disposed that one end thereof is joined to a front-end face of the metal shell and the other end thereof faces the front-end face of the center electrode; anda spark discharge gap formed between the front-end face of the center electrode and a front-end portion of the ground electrode,wherein at least a portion of the ground electrode where sparks are discharged has an outer layer made of a nickel alloy and an inner layer made of pure copper or a copper alloy having a better thermal conductivity than that of the outer layer, andwherein a ratio of a cross-sectional area of the inner layer to the entire cross-sectional area of the ground electrode is 10% or more to 35% or less.

2. A spark plug as defined in claim 1, wherein the ground electrode is dimensioned to have a reduced width toward a back face thereof, which back face is furthest from the center electrode, at least in a front-end portion of the ground electrode located forward of a center of the spark discharge gap.

3. A spark plug for use in an internal-combustion engine, comprising: a center electrode;an insulator at least partially surrounding the center electrode;a cylindrical metal shell at least partially surrounding the insulator;a ground electrode so disposed that one end thereof is joined to a front-end face of the metal shell and the other end thereof faces the front-end face of the center electrode; anda spark discharge gap formed between the front-end face of the center electrode and a front-end portion of the ground electrode,wherein the ground electrode has a width of 2 mm or less at least in the front-end portion thereof located forward of a center of the spark discharge gap and a convex-shaped curved face at a back face thereof located furthest from the center electrode side,wherein at least a portion of the ground electrode where sparks are discharged has an outer layer made of a nickel alloy and an inner layer made of pure copper or a copper alloy having a better thermal conductivity than that of the outer layer, andwherein a ratio of a cross-sectional area of the inner layer to the entire cross-sectional area of the ground electrode is 10% or more to 35% or less.

4. A spark plug for use in an internal-combustion engine, comprising: a center electrode;an insulator at least partially surrounding the center electrode;a cylindrical metal shell at least partially surrounding the insulator;a ground electrode so disposed that one end thereof is joined to a front-end face of the metal shell and the other end thereof faces the front-end face of the center electrode; anda spark discharge gap formed between the front-end face of the center electrode and a front-end portion of the ground electrode,wherein the ground electrode has a width of 2 mm or less and wherein at least a portion of the ground electrode where sparks are discharged has an outer layer made of a nickel alloy and an inner layer made of pure copper or a copper alloy having a better thermal conductivity than that of the outer layer,wherein a ratio of a cross-sectional area of the inner layer to the entire cross-sectional area of the ground electrode is 20% or more, andwherein the outer layer has a thickness of 0.2 mm or more.

5. A spark plug as defined in claim 4, wherein the ground electrode has a width of 2 mm or less and is dimensioned such that its width decreases toward a back face thereof, which back face is furthest from the center electrode side of the ground electrode, at least in the front-end portion of the ground electrode located forward of a center of the spark discharge gap.

6. A spark plug for use in an internal-combustion engine, comprising: a center electrode;an insulator at least partially surrounding the center electrode;a cylindrical metal shell at least partially surrounding the insulator;a ground electrode disposed so that one end thereof is joined to a front-end face of the metal shell and the other end thereof faces the front-end face of the center electrode; anda spark discharge gap formed between the front-end face of the center electrode and a front-end portion of the ground electrode,wherein the ground electrode has a width of 2 mm or less at least in the front-end portion thereof located forward of a center of the spark discharge gap and a convex-shaped curved face at a back face thereof located furthest from the center electrode side of the ground electrode,wherein at least a portion of the ground electrode where sparks are discharged has an outer layer made of a nickel alloy and an inner layer made of a nickel alloy or pure nickel having a higher purity and a better thermal conductivity than that of the outer layer,wherein a ratio of a cross-sectional area of the inner layer to the entire cross-sectional area of the ground electrode is 20% or more, andwherein a thickness of the outer layer is 0.2 mm or more.

7. A spark plug for use in an internal-combustion engine as claimed in any one of claims 1 to 6, wherein when a distance of the spark discharge gap is taken as G, a diameter of the front-end portion of the center electrode is taken as D, and a distance between a point nearest to the center electrode in the outer layer and a point nearest to the center electrode in the inner layer is taken as T0 when the ground electrode is projected from a front-end face side thereof along a central axis of the center electrode,then the distance T0 satisfies the expression 0.2 mm≦T0≦0.5 mm within a range of ±[(D/2)+G] from the central axis of the center electrode.

8. A spark plug for use in an internal-combustion engine as claimed in any one of claims 1 to 6, wherein when the ground electrode and the center electrode are projected from the front-end face side of the ground electrode along the central axis of the center electrode, tangent lines are drawn from two outer rims of the front-end of the center electrode, respectively, so as not to intersect with each other with respect to a periphery line of the ground electrode, and the periphery line is divided in two parts by contact points to define one part as a center electrode side and the other part as a back face side,then the inner layer is eccentric so that a thinnest portion of the outer layer is located at the back face side of the ground electrode.

9. A spark plug according to any one of claims 1 to 6, wherein when the ground electrode and the center electrode are projected from the front-end face side of the ground electrode along the central axis of the center electrode, tangent lines are drawn from two outer rims of the front-end of the center electrode, respectively, so as not to intersect with each other with respect to a periphery line of the ground electrode, and the front-end portion of the ground electrode are divided by, including a segment which connects both contact points of the tangent lines, a planar face perpendicular to the front-end face of the ground electrode into two portions: an inner side portion serving as a center electrode side; and an outer side portion serving as an opposed side to the center electrode,then, in the front-end portion of the ground electrode, a volume Vo of an outer side inner layer with respect to the outer side portion is larger than a volume Vi of an inner side inner layer with respect to the inner side portion.