Spark Plug Insulator Isolation Portion Radial Clearance Design
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Solution Overview
Problem
Conventional spark plugs face challenges in preventing creeping discharge while maintaining heat value requirements as engine combustion pressures increase, leading to potential failures in ignition performance.
Innovation Solution
A spark plug design with a specific ratio of surface area to volume for the isolation portion of the insulator, combined with a cylindrical shape and controlled dimensions, ensures sufficient insulation distance and heat transfer, preventing creeping discharge and maintaining heat value requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the axial length of the isolation portion is increased to prevent creeping discharge, then insulation distance is improved, but thermal capacity increases and heat transfer performance deteriorates
Solution Approach 1:
The patent transitions from increasing insulation distance in the axial direction (length dimension) to increasing it in the radial direction (diameter dimension). The isolation portion's outer diameter is made equal to or less than the insulator's outer diameter at the holding portion contact position, creating radial clearance that prevents creeping discharge without extending axial length, thereby maintaining heat transfer performance.
Solution Approach 2:
The patent applies different dimensional strategies to different portions of the isolation portion. The axial length is controlled to maintain heat transfer, while the outer diameter is specifically constrained to prevent creeping discharge. This localized dimensional control allows simultaneous optimization of both insulation and thermal properties.
2Reliability
If the gap between the holding portion and the isolation portion is reduced to prevent unburnt gas entry, then contamination resistance is improved, but the size of the spark plug increases
Solution Approach 1:
The patent changes the critical parameter from axial gap distance to radial outer diameter of the isolation portion. By controlling the outer diameter to be equal to or less than the insulator diameter at the contact position, the patent achieves contamination prevention through radial clearance rather than axial gap, thereby reducing overall spark plug size while maintaining reliability.
3Reliability
If the outer diameter of the isolation portion is increased to prevent creeping discharge, then insulation distance is improved, but the approach to the metallic shell wall surface increases causing aerial discharge
Solution Approach 1:
The patent changes the approach from increasing outer diameter to controlling it to be equal to or less than the insulator diameter at the contact position. This parameter constraint creates radial clearance that prevents both creeping discharge and aerial discharge to the metallic shell, achieving insulation without introducing new harmful discharge paths.
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 effectively prevents creeping discharge and ensures reliable spark generation across the regular spark discharge gap, even at increased combustion pressures, while maintaining the spark plug's heat value and reducing size.
Implementation Method 1
a portion of the insulator located frontward of a position where the holding portion of the metallic shell is in direct or indirect contact with the insulator, thereby being insulated from each other
Implementation Method 2
an insulator for holding the center electrode in an axial hole thereof
Implementation Method 3
a metallic shell for holding the insulator in a cylindrical hole thereof; the holding portion provided in the cylindrical hole thereof is brought in direct or indirect contact with an outer surface of the insulator
Implementation Method 4
A spark discharge (initiated through dielectric breakdown of gas and also called an aerial discharge for distinguishing from a creeping discharge to be described later) is generated across the spark discharge gap, thereby igniting an air-fuel mixture
Data Source
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AI summary
An isolation portion (P) of an insulator (10) of a spark plug (100) electrically insulatively isolates a front end portion (22) of a center electrode (20) and a holding portion (56) of a metallic shell (50) from each other and has an intermediate portion (P2) which extends while its outside diameter varies, thereby ensuring an insulation distance between the two portions. The ratio (S/V) of the surface area (S) of the outer surface (14) of the isolation portion (P) to the volume (V) of the isolation portion (P) satisfies the relation 1.26 mm-1 ≤ S/V, whereby the insulation distance between the front end portion (22) and the holding portion (56) is sufficiently ensured while existing dimensional conditions are held unchanged. Through satisfaction of the relation S/V ≤ 1.40 mm-1, an increase in temperature of the center electrode (20) that accompanies an increase in the amount of heat received from a combustion chamber owing to an increase in the surface area (S) of the outer surface (14) is restrained, thereby maintaining a required heat value.