Layered Ground Electrode for Spark Plug Heat and Fatigue Management
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Solution Overview
Problem
The existing spark plugs with needle-like electrode tips face issues of reduced spark wear resistance due to high temperature heating, increased internal stress in the bent portion, and potential breakage from metal fatigue, especially when subjected to thermal and vibrational loads during engine operation.
Innovation Solution
A spark plug design featuring a ground electrode with a layered structure composed of multiple structural members, including a first structural member for strength, a second structural member for heat radiation, and a third structural member for corrosion resistance, with a total thermal conductivity of 35 W/(m·K) or higher, and a curvature radius of the bent portion minimized to 2.3 mm or smaller to enhance heat radiation and reduce internal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the ground electrode is made longer to accommodate the needle-like electrode tip, then the electrode tip can be positioned to face the center electrode, but the weight of the ground electrode increases and internal stress from vibrations increases
Solution Approach 1:
The ground electrode is divided into multiple sections with different functions: a bent portion for positioning, a straight portion for heat radiation, and a connection portion for structural support. This segmentation allows each section to be optimized independently, reducing overall weight while maintaining necessary length for electrode tip positioning.
Solution Approach 2:
Different portions of the ground electrode are given different material properties or structural characteristics. The bent portion has optimized curvature radius for stress reduction, while the straight portion has dimensions optimized for heat radiation. This local differentiation allows the electrode to perform multiple functions without requiring uniform increases in all dimensions.
2Length of moving object
If the curvature radius of the bent portion is decreased to position the ground electrode end facing the center electrode, then the radial distance is reduced, but internal stress in the bent portion increases
Solution Approach 1:
The curvature radius of the bent portion is precisely controlled within a specific range (0.5-2.0 mm) to achieve optimal balance between positioning the ground electrode end facing the center electrode and minimizing internal stress. This parameter optimization allows the electrode to maintain necessary geometry without excessive stress concentration.
3Temperature
If the ground electrode has high thermal conductivity to rapidly radiate heat from the electrode tip, then heat radiation ability improves, but the electrode may become more susceptible to thermal fatigue and breakage
Solution Approach 1:
The ground electrode is constructed as a composite structure combining materials with different properties. A high thermal conductivity material (such as copper or copper alloy) forms the core for heat radiation, while an outer layer of nickel-based alloy provides corrosion resistance and fatigue strength. This composite structure allows simultaneous achievement of high heat radiation ability and improved breakage resistance.
4Volume of moving object
If the spark plug diameter is reduced for engine downsizing, then the size and diameter decrease, but the radial distance between the ground electrode and spark gap becomes smaller
Solution Approach 1:
The ground electrode is designed with a bent portion that utilizes axial space rather than relying solely on radial distance. By bending the electrode along the axial direction, the electrode tip can be positioned to face the center electrode while maintaining adequate radial clearance. This dimensional approach allows compact spark plug design without compromising the necessary spacing for proper electrode function.
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 significantly improves the breakage resistance and life of the ground electrode by effectively managing thermal and vibrational loads, ensuring the internal stress does not exceed the fatigue limit, thereby enhancing the overall performance and durability of the spark plug.
Implementation Method 1
the heat radiation ability of the ground electrode decreases as the heat radiation passage of the ground electrode (i.e. the passage of heat radiation from the other end to the one end of the ground electrode and then to the metal shell) increases with the length of the ground electrode
Data Source
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AI summary
A spark plug of the present invention is designed as a small-diameter spark plug, which includes: a metal shell having a mounting thread formed with a nominal diameter of M12 or smaller based on JIS standard; a ground electrode consisting a first structural member extending from one end toward the other end thereof and at least one ith structural member (i = 2, 3, 4, 5) laminated to cover an outer surface of the first structural member, the length of protrusion of the other end of the ground electrode from a front end face of the metal shell being 4.5 mm or larger, the ground electrode having a bent portion formed between the one end to the other end thereof with a curvature radius of 2.3 mm or smaller; and an electrode tip joined to the other end of the ground electrode at a position facing the front end of the center electrode and having a protrusion length of 0.5 mm or larger and a cross sectional area of 0.20 to 1.13 mm2. It is possible to increase the heat radiation ability of the ground electrode and prevent the ground electrode from decreasing in metal fatigue strength by selecting the materials of the structural members of the ground electrode in such a manner as to control the total thermal conductivity X of the ground electrode as expressed by the formula [1] to 35 W/(m·K) or higher at 20°C. Formula6X=volume of first structual membervolume of ground electrode×thermal conductivity of first structural member+∑i=2nvolume of ith structural membervolume of ground electrode×thermal conductivity of ith structural member