Auto Choke Bimetal Drive with Heat Shield for Engine Maintenance
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Solution Overview
Problem
Existing auto choke devices in general purpose engines are complex in structure and difficult to maintain due to the use of temperature sensors and stepping motors, and bimetal drives that are prone to overheating and require removal of muffler coverings for access.
Innovation Solution
A simplified auto choke device that uses a bimetal drive source fitted to the muffler covering, non-contact with the muffler, and connected to the choke valve via a link member, with a heat shield to block cooling wind and maintain stable operation, allowing easy access and maintenance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a temperature sensor and stepping motor are used to control the choke valve, then the air-fuel mixing ratio can be controlled, but the number of component parts increases and the structure becomes complicated
Solution Approach 1:
The patent replaces the electronic control system (temperature sensor and stepping motor) with a mechanical drive source (bimetal) that directly responds to temperature changes. The bimetallic strip automatically adjusts the choke valve opening based on thermal expansion and contraction, eliminating the need for electronic sensors and motors while maintaining reliable air-fuel mixing ratio control.
Solution Approach 2:
The bimetallic strip serves as both the temperature sensing element and the actuating mechanism. It automatically detects temperature changes and converts them into mechanical motion to adjust the choke valve, making the system self-regulating without requiring external control systems or power sources.
2Device complexity
If the bimetal is fitted to the muffler as a drive source, then the structure is simplified, but the temperature of the drive source increases and maintenance becomes difficult
Solution Approach 1:
The patent introduces a heat-insulating bracket as an intermediary between the hot muffler and the bimetallic strip. This bracket provides thermal isolation, allowing the bimetal to respond to engine temperature changes without being directly exposed to the excessive heat of the muffler, thus preventing overheating while maintaining structural simplicity.
Solution Approach 2:
The heat-insulating bracket is strategically positioned only where needed - between the muffler and the bimetal - providing localized thermal protection. This allows other parts of the system to remain thermally coupled for proper operation, achieving temperature management without compromising the overall simplified structure.
3Volume of moving object
If the bimetal is covered by a muffler covering, then the structure is compact, but access to the drive source requires removal of the muffler covering and maintenance is not good
Solution Approach 1:
The patent segments the assembly by making the muffler covering removable or separable from the main structure. This allows the covering to be taken off easily for maintenance access to the bimetal and link member, then reattached to maintain the compact appearance and space efficiency during normal operation.
Solution Approach 2:
The system transitions from a static, permanently enclosed design to a dynamic configuration where the muffler covering can be easily removed and reattached. This dynamic accessibility feature allows the compact structure to be maintained during operation while providing easy maintenance access when needed.
4Temperature
If cooling wind flows towards the bimetal, then the bimetal temperature decreases, but the operation of the bimetal becomes unstable
Solution Approach 1:
The heat-insulating bracket serves as a thermal mediator that selectively blocks cooling wind from reaching the bimetallic strip. It allows the bimetal to maintain a stable temperature by preventing direct exposure to cooling airflow, ensuring consistent thermal response and stable operation of the choke valve control mechanism.
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 results in a simpler structure, reduced temperature rise of the drive source, improved assemblability, and easier maintenance by exposing the drive source and link member for access, while stabilizing the operation of the bimetal and cooling the connection point between the muffler and cylinder head.
Implementation Method 1
a bimetal fitted inside the bracket
Implementation Method 2
the bracket includes a heat shield extending towards an inner side of the muffler covering and configured to block out a cooling wind then flowing towards the bimetal
Implementation Method 3
a cooling fan fixed to an engine rotary shaft
Data Source
AI summary
A general purpose engine includes: a cooling fan fixed to an engine rotary shaft; a muffler to silence exhaust gases; a muffler covering to cover the muffler; and an auto choke device to control an opening of a choke valve in an air intake passage by the utilization of heat evolved from the muffler. The auto choke device includes: a drive source, which is fitted to the muffler covering in a non-contact fashion to the muffler and exposed to an outer surface of the muffler covering; and a link member to connect between the drive source and the choke valve. The drive source includes a bracket, which is fitted to the muffler covering, and a bimetal fitted inside the bracket. The bracket includes a heat shield extending towards an inner side of the muffler covering, and the heat shield blocks out a cooling wind then flowing towards the bimetal.