Spark Plug Center Electrode Adhesion via Axial Segmentation
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Solution Overview
Problem
Spark plugs experience loosening and play in the axial bore of the insulator due to repeated heating and cooling cycles, leading to deteriorated adhesion between the center electrode and the seal material, which affects the fixing strength and reliability in severe engine environments.
Innovation Solution
A spark plug design featuring an insulator with a ledge connecting two inner circumferential surfaces, a center electrode with a head supported by the ledge and a circular columnar leg, and a seal material charged into the space between the ledge, head, and second inner circumferential surface, with specific axial distances and diameters to enhance adhesion, including an axial distance of 15 mm or more from position P1 to the rear end of the center electrode, and a diameter of the imaginary cylinder surrounding the protrusion being 2 mm to 3.3 mm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the center electrode is fixed in the axial bore of the insulator by means of a seal material, then the center electrode can be held in position, but the fixing strength deteriorates due to repeated heating and cooling cycles
Solution Approach 1:
The center electrode is divided into two functional parts: a head portion that is fixed by the seal material in the axial bore, and a long leg portion (15mm or more) that extends downward to provide additional anchoring. This segmentation allows the fixing function to be distributed between the seal material-adhered head and the mechanically anchored leg, preventing loosening under thermal cycling.
Solution Approach 2:
Instead of relying solely on radial fixing by the seal material, the invention extends the center electrode in the axial dimension (downward by 15mm or more). This axial extension provides a longer engagement path with the insulator bore, transforming a two-dimensional radial fix into a three-dimensional fix that includes axial anchoring, thereby preventing play and loosening.
2Strength
If the axial distance from position P1 to the rear end of the center electrode is increased to improve adhesion, then the fixing strength is enhanced, but the device complexity increases
Solution Approach 1:
The center electrode is segmented into a head portion and a leg portion with a clear boundary at position P1. The head portion (with diameter greater than the leg) provides the adhesion interface with the seal material, while the leg portion extends downward to provide mechanical anchoring. This simple segmentation achieves enhanced adhesion without complex structural modifications.
Solution Approach 2:
The invention specifies a quantitative parameter: the axial distance from position P1 to the rear end of the center electrode must be 15mm or more. This parameter change transforms a qualitative adhesion problem into a quantitative design solution, where meeting the minimum length requirement ensures sufficient adhesion strength without requiring complex structural features.
Data Source
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AI summary
An object of the present invention is to provide a spark plug having good adhesion between a center electrode and a seal material. A spark plug of the invention includes an insulator having an axial bore extending in the direction of an axis, a first inner circumferential surface extending at a forward side of the axial bore, a second inner circumferential surface extending at a rear side of the axial bore and having a diameter greater than that of the first inner circumferential surface, and a ledge connecting the first inner circumferential surface and the second inner circumferential surface; a center electrode having a head supported by the ledge and extending in a space surrounded by the second inner circumferential surface, and a circular columnar leg extending continuously from the forward end of the head in a space surrounded by the first inner circumferential surface; and a seal material for holding the center electrode in the axial bore. The spark plug is characterized in that, assuming that a position P1 on the center electrode is where the outside diameter of the center electrode begins to increase, beyond an average outside diameter H of the leg, from the leg toward the rear end of the center electrode, when the axial distance along the axis from the position P1 to the forward end of the center electrode is 15 mm or more, the axial distance along the axis from the position P1 to the rear end of the center electrode is 3.8 mm or more.