Spark Plug Insulator Leg Surface Roughness and Shell Gap Volume
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Solution Overview
Problem
Carbon accumulation on the insulator surface of spark plugs in internal combustion engines leads to current leakage and abnormal spark discharges, hindering normal ignition, particularly in direct-injection engines where fouling is more pronounced.
Innovation Solution
A spark plug design featuring a tubular insulator with a leg portion having a centerline average roughness of 1.8 µm or less and a gas volume of 100-300 mm³ between the insulator and metallic shell, along with a noble metal tip configuration, to prevent carbon adhesion and ensure reliable spark discharge.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the insulator surface is exposed to air-fuel mixture and combustion gas for normal spark plug operation, then the spark plug can perform its ignition function, but carbon accumulates on the insulator surface causing current leakage and abnormal spark discharges
Solution Approach 1:
The patent extracts the problematic region by creating a space between the insulator and metallic shell, effectively removing the insulator surface from direct contact with the harmful combustion environment while maintaining the necessary electrical insulation and spark generation functions
Solution Approach 2:
The metallic shell acts as an intermediary barrier between the insulator and the combustion chamber environment. The space between them allows the metallic shell to protect the insulator from carbon accumulation while still enabling the spark plug to function properly
2Device complexity
If the insulator is positioned close to the metallic shell for compact design, then the device complexity is reduced, but carbon can adhere to the insulator surface more easily causing fouling
Solution Approach 1:
The patent solves the spatial conflict by utilizing the radial dimension - creating a space between the insulator and metallic shell in the radial direction. This dimensional approach allows protection against carbon adhesion without significantly increasing overall device complexity
3Reliability
If the insulator surface is smoothed to prevent carbon adhesion, then resistance to fouling is improved, but the manufacturing precision requirements increase
Solution Approach 1:
The patent applies preliminary protective action by creating the space between insulator and metallic shell before carbon accumulation can occur. This preventive structural measure reduces fouling without requiring extreme surface smoothing, thereby balancing reliability improvement with manufacturing feasibility
Data Source
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AI summary
Resistance to fouling is improved through reliable prevention of adhesion and accumulation of carbon onto an insulator. A spark plug 1 includes a center electrode 5 extending in the direction of an axis CL1, a ceramic insulator 2 having an axial hole 4 which extends in the direction of the axis CL1 and in which the center electrode 5 is provided, a cylindrical metallic shell 3 provided externally of the outer circumference of the insulator 2 and having a support portion 21 formed on the inner circumferential surface thereof, and a ground electrode 27 extending from a front end portion of the metallic shell 3. The insulator 2 has a stepped portion 14 supported by the support portion 21 of the metallic shell 3, and a leg portion 13 formed frontward of the stepped portion 14 along the direction of the axis CL1. A space SP formed between the leg portion 13 of the insulator 2 and the inner circumferential surface 3i of the metallic shell 3 has a volume of 100 mm3 to 300 mm3 inclusive. Further, the surface of the leg portion 13 has a centerline average roughness of 1.8 µm or less.