Engine Decompression Weight Asymmetry for Startability
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Solution Overview
Problem
The decompression mechanism in existing engines fails to effectively improve startability due to the weight being in an open state during engine starting, attributed to the weakness of the return spring's force, which is reduced by the weight's smaller size when positioned between the cam shaft ends, leading to insufficient centrifugal force and increased gravitational moment.
Innovation Solution
The engine design includes a weight with strategically positioned protruding portions to increase centrifugal force and reduce gravitational moment, maintaining the weight in a closed state during engine start, enhancing startability without increasing the set rotation speed or causing noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If the decompression mechanism is disposed between both ends of the cam shaft to make the cam shaft more compact, then the axial length is reduced, but the weight becomes smaller leading to insufficient centrifugal force and the weight remains in open state during starting
Solution Approach 1:
The weight is designed with an asymmetric structure where the center of gravity is intentionally offset from the rotational axis. The first portion of the weight extends farther from the axis than the second portion, creating an asymmetric mass distribution that increases the moment of inertia and centrifugal force effect without increasing the overall size of the weight component
Solution Approach 2:
The weight is designed to extend in the radial direction from the cam shaft axis, utilizing the radial dimension to increase the effective lever arm for centrifugal force. The first portion extends farther radially than the second portion, maximizing the centrifugal effect within the constrained axial space
2Volume of moving object
If the weight is made smaller to fit between cam shaft ends, then the device becomes more compact, but the return spring force is reduced and gravitational moment increases causing weight to open during start
Solution Approach 1:
The asymmetric design concentrates mass in the first portion that extends farther from the rotational axis, increasing the moment of inertia and centrifugal force effect. This allows a smaller overall volume while maintaining sufficient centrifugal force to keep the weight closed during starting operations
Solution Approach 2:
The weight has different structural characteristics in different regions: the first portion has greater mass and extends farther radially to generate centrifugal force, while the second portion is smaller and positioned closer to the axis. This local differentiation optimizes the force generation while minimizing overall volume
3Length of stationary object
If the weight is positioned between cam shaft ends, then space is saved axially, but the gravitational moment increases causing insufficient spring force to maintain closed state
Solution Approach 1:
The asymmetric weight design with offset center of gravity creates a larger moment arm for centrifugal force generation. The first portion extending farther from the axis increases the effective radius for centrifugal force, counteracting the increased gravitational moment resulting from the compact axial positioning
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 improved weight design enhances engine startability by maintaining the weight in a closed state during engine start, reducing the likelihood of the weight opening due to gravitational force, thus improving engine startability without noise or increased rotation speed.
Implementation Method 1
When the cam shaft rotates, the weight rotates around the support shaft due to centrifugal force. When the rotation speed of the cam shaft meets or exceeds a set rotation speed, the centrifugal force exceeds the spring force of the return spring and the weight enters an open state.
Implementation Method 2
The weight in the decompression mechanism in Japanese Laid-Open Patent Publication No. 2008-64083 is held in a closed state due to the action of a return spring when the cam shaft is not rotating.
Implementation Method 3
The location of the center of gravity of the weight changes in accordance with the phase of the cam shaft rotation. The cam shaft may stop when the opening direction of the weight and the gravitational force direction are in correspondence. In this case, the inventors of the present application came to the conclusion that the moment due to gravitational force acting on the weight becomes greater than the spring force and consequently the weight enters the open state.
Data Source
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AI summary
A straight line that passes through the center of rotation of a cam shaft and the center of rotation of a weight is assumed to be a vertical axis as seen from the axial direction of the cam shaft. A straight line that is orthogonal to the vertical axis and that passes through the center of rotation of the cam shaft is assumed to be a horizontal axis. A direction from the center of rotation of the cam shaft toward the center of rotation of the weight among directions parallel to the vertical axis is assumed to be a first vertical direction. One direction among the directions parallel to the horizontal axis is assumed to be a first horizontal direction. The center of gravity of the weight is disposed in a first region as seen from the axial direction of the cam shaft. The first region is located in the first vertical direction from the horizontal axis and in the first horizontal direction from the vertical axis. The weight includes a first portion that is disposed in the first region as seen from the axial direction of the cam shaft. In a closed state, the first portion includes a first protruding portion that protrudes to the outside of an external peripheral surface of the bearing as seen from the axial direction of the cam shaft.