Spark Plug Insert Portion Design for Under-Load Life
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Solution Overview
Problem
Conventional spark plugs with reduced insulator size suffer from deterioration in under-load life due to increased resistance and adhesion issues between the resistor and seal layers, center electrode, and metal terminal member.
Innovation Solution
A spark plug design with a specific insert portion configuration, including a depression formation zone and smooth surface zone on the outer circumferential surface, and a stepped portion in the axial hole, which allows effective transmission of pressing force to the resistor composition, enhancing adhesion and reducing resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the insulator size is reduced to achieve smaller spark plug dimensions, then the spark plug size is reduced, but the under-load life deteriorates due to increased resistance and adhesion issues
Solution Approach 1:
The insert portion is designed with different surface characteristics at different locations: a depression formation zone with reduced diameter and increased surface area for adhesion, and a smooth surface zone for proper fitting. This local differentiation allows the small spark plug to maintain reliable connections despite overall size reduction.
Solution Approach 2:
Depressions are pre-formed on the outer circumferential surface of the insert portion before assembly. These pre-formed depressions ensure that when the metal terminal member is pressed, the pressing force is effectively transmitted to the resistor composition, preventing adhesion issues before they occur.
2Reliability
If the insert portion is made longer to improve pressing force transmission, then adhesion improves, but the insert portion is more likely to bend during assembly
Solution Approach 1:
The insert portion has different diameters at different locations: a smaller diameter in the depression formation zone (improving adhesion) and a larger diameter in the smooth surface zone (improving bending resistance). This diameter variation allows the insert to simultaneously achieve good adhesion and resist bending during assembly.
Solution Approach 2:
The stepped portion creates a curved transition between zones of different diameters. This gradual curvature prevents stress concentration that would occur with sharp transitions, reducing the likelihood of bending while maintaining the adhesion benefits of the reduced diameter section.
3Volume of moving object
If the outer diameter of the insert portion is reduced to fit smaller insulators, then spark plug size is reduced, but pressing force transmission deteriorates
Solution Approach 1:
The insert portion has a smaller overall diameter to fit small insulators, but the depression formation zone has an even smaller diameter that creates increased surface area and mechanical interlocking. This local reduction in diameter at the right location improves pressing force transmission without requiring the entire insert to be larger.
Solution Approach 2:
The depressions are pre-formed on the insert surface before assembly, creating a surface topology that enhances mechanical interlocking with the resistor composition. This preliminary surface preparation ensures effective force transmission even with the reduced overall diameter of the insert portion.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design results in a spark plug with improved under-load life by ensuring proper adhesion and resistance characteristics, even with a reduced insulator size, by effectively transmitting pressing force and preventing bending issues during assembly.
Implementation Method 1
when the insert portion is pressed into the axial hole, the insert portion can sufficiently transmit a pressing force to the resistor composition, so that a resistor having high density can be formed
Implementation Method 2
The depression formation zone (31) has a plurality of depressions formed on an outer circumferential surface of the insert portion (19)
Data Source
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AI summary
An object of the present invention is to provide a spark plug having excellent under-load life. The spark plug includes an insulator having an axial hole extending along an axial line, a metal terminal member having an insert portion accommodated in the axial hole and a depression formation zone existing on an outer circumferential surface of the insert portion and having a plurality of depressions, and a metallic shell accommodating a forward portion of the insulator therein to thereby hold the insulator. The spark plug satisfies the following conditions: (1) the insert portion has a length H of 35 mm or more along the axial line; (2) the depression formation zone has a length F of 13 mm or more along the axial line; (3) the insert portion has a smooth surface zone on its outer circumferential surface; (4) the ratio (A/B) between diameter A of a forward end of the insert portion and inside diameter B of the insulator measured at the forward end satisfies the relational expression 0.9 ≤ A/B ≤ 0.98; and (5) Vickers hardness of the insert portion measured at the center of a cross section of the insert portion cut along a direction orthogonal to the axial line is 150 Hv or more to 350 Hv or less.