Downsized Spark Plug Insulator Leakage Current
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Solution Overview
Problem
Downsized spark plugs face challenges in maintaining ignitability due to reduced insulator wall thickness, leading to leakage current and deteriorated emission performance, especially under high-load and high-temperature conditions.
Innovation Solution
A spark plug design featuring a cylindrical insulator with a small-diameter leg portion and an alumina-based sintered body containing Si, rare earth elements, and Group 2 elements, with specific mass ratios and an aluminate crystal phase, to prevent leakage current and ensure good ignitability, along with a manufacturing method involving press-forming and sintering of raw material powders to achieve high-yield production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the wall thickness of the insulator is decreased to downsize the spark plug, then the size of the spark plug is reduced, but the withstand voltage characteristics deteriorate and leakage current increases
Solution Approach 1:
The patent changes the chemical composition parameters of the insulator material by adding specific amounts of B2O3 (0.01-5 mass%), Al2O3 (90-97 mass%), and SiO2 (1-10 mass%), which modifies the electrical and thermal properties to reduce leakage current while maintaining insulation performance in a thinner wall structure
Solution Approach 2:
The patent creates a composite ceramic material system combining B2O3, Al2O3, and SiO2 in specific ratios, where B2O3 forms a glassy phase that fills grain boundaries and reduces electron mobility, while Al2O3 provides structural integrity and SiO2 enhances chemical stability, achieving both downsizing and maintained reliability
2Volume of moving object
If the wall thickness of the insulator is decreased to downsize the spark plug, then the size of the spark plug is reduced, but leakage current increases and ignitability deteriorates
Solution Approach 1:
The patent optimizes the compositional parameters within the B2O3-Al2O3-SiO2 system, specifically controlling B2O3 content at 0.01-5 mass% to form an optimal glassy phase concentration that effectively blocks electron flow paths without compromising the mechanical strength of the reduced-thickness insulator wall
Solution Approach 2:
The patent utilizes the glassy phase formed by B2O3 to fill and seal porous structures and grain boundaries in the ceramic insulator, creating a denser microstructure that reduces electron hopping paths and minimizes leakage current through the thin insulator wall
3Volume of moving object
If the clearance between the insulator and the metal shell is narrowed to downsize the spark plug, then the size of the spark plug is reduced, but lateral spark occurs and ignitability deteriorates
Solution Approach 1:
The patent applies a protective coating layer with specific electrical properties to the outer surface of the insulator in the clearance region, creating a localized barrier that prevents lateral spark discharge while maintaining the overall compact dimensions of the spark plug
4Temperature
If the insulator is subjected to high temperature during high-load operation, then the temperature increases, but leakage current increases and ignitability deteriorates
Solution Approach 1:
The patent employs a composite ceramic-glass material system where the B2O3-based glassy phase maintains its structural integrity and electrical insulation properties at high temperatures, preventing thermal runaway and maintaining low leakage current even when the insulator temperature rises during high-load engine operation
Solution Approach 2:
The patent selects materials with matched thermal expansion coefficients in the B2O3-Al2O3-SiO2 system, ensuring that the insulator maintains its dimensional stability and structural integrity under thermal cycling and high-temperature operation, preventing microcrack formation that would increase leakage current
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 solution effectively prevents leakage current and maintains good combustible-gas ignitability even in downsized spark plugs, ensuring reliable operation under extreme conditions and high temperatures, thus addressing the ignitability and emission performance issues.
Implementation Method 1
the alumina-based sintered body includes, in a grain boundary phase thereof, an aluminate crystal containing the rare earth element component
Implementation Method 2
a region of the insulator extending from a front end of the insulator to at least 2 mm rear from the imaginary plane is formed of an alumina-based sintered body
Data Source
Figure 1
Figure 2~3
Figure 4(a)~4(b)
AI summary
The present invention relates to a spark plug having good combustible-gas ignitability even when the spark plug is downsized and to a method for high-yield manufacturing of a spark plug with good combustible-gas ignitability. There is provided according to one aspect of the present invention a spark plug 1 having a center electrode 2, an insulator 3 and a metal shell 4 and satisfying the following conditions (1) to (5). There is provided according to another aspect of the present invention a manufacturing method of a spark plug including the step of producing an insulator by preparing a raw material powder in such a manner that the particle size distribution ratio (90% volume diameter/10% volume diameter) between particles of 10% volume diameter and particles of 90% volume diameter in the raw material powder is 3.6 to 5.2, and then, press-forming and sintering the prepared raw material powder. Condition (1): The insulator has a wall thickness T of 0.3 to 1.1 mm at an imaginary plane including a front end face of the metal shell. Condition (2): A region of the insulator extending from a front end of the insulator to at least 2 mm rear from the imaginary plane is formed of an alumina-based sintered body containing a Si component, a rare earth element component and at least two kinds of Group 2 element components and being substantially free from a B component. Condition (3): The mass ratio RRE of a mass of the rare earth element component to a total mass of the Si component, the Group 2 element components and the rare earth element component in the alumina-based sintered body is 0.15 to 0.45. Condition (4): The mass ratio R2 of a total mass of the components of the Group 2 elements to a mass of the Si component in the alumina-based sintered body is 0.25 or greater. Condition (5): The alumina-based sintered body includes, in a grain boundary phase thereof, an aluminate crystal containing the rare earth element component.