Ceramic Glow Plug Electrode Design for Thermal Stress Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Ceramic glow plugs face breakage issues at electrode portions due to thermal stress caused by differing coefficients of thermal expansion between the resistor and substrate, leading to compressive and tensile stresses that compromise the heater's strength, especially during the lubricant removal process.
Innovation Solution
The ceramic glow plug design features electrode taking-out portions with axial dimensions smaller than circumferential dimensions and without corner portions, ensuring the axial and circumferential dimensions of the exposed surface fall within 1.0 mm to 1.8 mm, which reduces thermal stress and enhances the substrate's strength around the exposed surface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the electrode taking-out portion is exposed on the outer circumferential surface of the ceramic heater, then electrical connection is enabled, but thermal stress causes breakage at the electrode portions
Solution Approach 1:
The patent applies local quality by creating a recessed structure at the electrode taking-out portion, where the exposed surface has different dimensions (axial dimension smaller than circumferential dimension) compared to the main body. This localized geometric modification reduces thermal stress concentration at the electrode portions while maintaining electrical connection functionality.
Solution Approach 2:
The patent employs curvature by designing the exposed surface of the electrode taking-out portion without corner portions and with rounded edges. This curved geometry distributes thermal stress more evenly compared to sharp corners, preventing breakage at the electrode portions during thermal cycling.
2Strength
If the axial dimension of the exposed surface is reduced, then thermal stress is reduced, but electrical connection area is limited
Solution Approach 1:
The patent applies asymmetry by making the axial dimension of the exposed surface smaller than the circumferential dimension. This asymmetric geometry optimizes the balance between reducing thermal stress (achieved by smaller axial dimension) and maintaining adequate electrical connection area (achieved by sufficient circumferential dimension).
3Ease of manufacture
If corner portions are present on the exposed surface, then manufacturing is easier, but stress concentration occurs leading to breakage
Solution Approach 1:
The patent eliminates corner portions from the exposed surface of the electrode taking-out portion, replacing them with curved or rounded edges. This curvature prevents stress concentration that would occur at sharp corners during thermal cycling, significantly reducing breakage risk while the rounding process remains compatible with standard manufacturing methods.
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 significantly improves the resistance to breakage of the ceramic heater without affecting the electrical connection, maintaining the strength of the substrate and preventing breakage, especially when the difference in thermal expansion coefficients is 0.3 ppm/K or greater.
Implementation Method 1
the resistor has a coefficient of thermal expansion greater than that of the substrate. Since the resistor and the substrate have different coefficients of thermal expansion, in a cooling step of a process of firing the ceramic heater, the resistor shrinks more than does the substrate.
Implementation Method 2
a radially inward force from the outer sleeve acts on the heater, whereby the heater is firmly constricted and held.
Data Source
Figure 1(a)~1(b)
Figure 2
Figure 3
AI summary
A ceramic glow pug has a structure in which a portion of an embedded resistor is exposed on a portion of a substrate and forms an exposed surface of an electrode taking-out portion. The breakage resistance in the vicinity of the electrode taking-out portion is improved without causing a resistor failure at the electrode taking-out portion. The resistor formed of an electrically conductive ceramic is embedded in the substrate formed of an electrically insulating ceramic, and a portion of the resistor is exposed on the surface of the substrate and has an exposed surface. The exposed surface and the embedded portion differ in stress acting on the substrate, which may decrease the breakage resistance in the vicinity of the electrode taking-out portion. The present invention solves that problem by determining the shape of the exposed surface such that each of the dimension in the axial direction and the dimension in the circumferential direction falls within a range of 1.0 mm to 1.8 mm.