Spark Plug Melting Portion Geometry for Crack Resistance
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Solution Overview
Problem
The existing spark plugs with a noble metal discharge portion and pedestal portion are prone to cracking or separation due to thermal stress, as the melting portion formed during laser welding can restrain deformation, leading to increased internal stress at the interfaces.
Innovation Solution
The spark plug design features a melting portion formed throughout the circumference of the ignition portion, with a specific exterior angle and forming proportion that connects the side surfaces of the discharge and pedestal portions, providing resistance to radial expansion and reducing internal stress, thereby minimizing the risk of cracking or separation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a pedestal portion is introduced as an intermediate member between the discharge portion and ground electrode, then thermal stress is relaxed and crack/separation resistance is improved, but the melting portion structure may restrain thermal expansion deformation and increase internal stress at interfaces
Solution Approach 1:
The patent changes the geometric parameters of the melting portion, specifically setting the exterior angle θ between 135° and 175° and the forming proportion T/S to 0.5 or more. These parameter optimizations allow the melting portion to provide structural support while accommodating thermal expansion deformation, reducing internal stress concentration at the interfaces between the discharge portion, pedestal portion, and ground electrode.
2Strength
If the melting portion is formed to unite the side surface of discharge portion and surface of protrusion top end side of pedestal portion, then structural strength is improved, but the melting portion holds the discharge portion inwards and restrains radial expansion, increasing internal stress
Solution Approach 1:
The patent optimizes the geometric parameters of the melting portion configuration. By controlling the exterior angle θ to be between 135° and 175° and the forming proportion T/S to be 0.5 or more, the melting portion maintains adequate joint strength while providing sufficient space for thermal expansion deformation, thereby reducing internal stress buildup.
3Stress or pressure
If the exterior angle θ of the melting portion is reduced to enhance restraint on radial expansion, then resistance to thermal expansion is improved, but internal stress increases and crack risk rises
Solution Approach 1:
The patent identifies an optimal range for the exterior angle θ (135° to 175°) that balances two competing requirements: providing sufficient restraint on radial expansion to maintain structural integrity, and allowing adequate deformation space to prevent internal stress concentration and crack formation. This parameter optimization resolves the contradiction between expansion resistance and crack prevention.
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
This design effectively suppresses the occurrence of cracks and separations by controlling thermal expansion and internal stress, ensuring the spark plug's structural integrity and longevity.
Implementation Method 1
by laser welding which can easily concentrate heat onto the junction and set a melting depth to be deep also reduces a tendency for internal stress to remain after the welding
Implementation Method 2
although each of the discharge portion and the pedestal portion expands when subjected to the thermal load by combustion of the engine then deforms
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
The present invention provides a spark plug which is capable of suppressing a occurrence of crack and separation by determining a structural configuration of a melting portion formed in a junction portion between a discharge portion and a pedestal portion which form an ignition portion that protrudes from a ground electrode. In a profile line shape of a cross section including a center axis P of an ignition portion 80, an exposure surface 88 of a melting portion 83 connects a side surface 82 of a discharge portion 81 and a side surface 85 of a pedestal portion 84. Further, an exterior angle θ formed between an imaginary line Q, which passes through a boundary position X1 between the melting portion 83 and the pedestal portion 84 and a boundary position X2 between the melting portion 83 and the discharge portion 81, and the center axis P at a node C, satisfies 135° ≦ θ ≦ 175°. Furthermore, a proportion T/S of a forming depth T of the melting portion 83 to an outside diameter S of the discharge portion 81 satisfies T/S ≧ 0.5.