Two-Stroke Engine Scavenging Passage Design for Blow-by Reduction
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Solution Overview
Problem
Two-stroke internal combustion engines face the challenge of blow-by, where the air-fuel mixture is discharged outside the combustion chamber, leading to deteriorated fuel consumption and increased unburned components in exhaust gas, despite existing solutions like stratified scavenging and scavenging passage configurations.
Innovation Solution
A two-stroke internal combustion engine design featuring first and second scavenging ports oriented away from the exhaust port, with branch scavenging passages extending aslant and having a smaller cross-sectional area than main scavenging passages, which slows down scavenging airflows and enhances directional control, preventing short-cuts and elongating the travel distance of airflows away from the exhaust port.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional scavenging passages are used, then the engine structure is simple, but blow-by occurs and fuel consumption deteriorates
Solution Approach 1:
The scavenging passage is divided into a main body and a branch portion that extends in a direction away from the exhaust port. This segmentation creates distinct flow paths that prevent short-circuiting of the air-fuel mixture, reducing blow-by and improving fuel consumption without significantly complicating the overall structure.
Solution Approach 2:
The branch scavenging passage is designed with specific local characteristics (extending away from the exhaust port) to control the flow direction of the air-fuel mixture. This local modification optimizes the scavenging effect and prevents blow-by in the critical region near the exhaust port while maintaining simplicity elsewhere in the engine structure.
2Volume of moving object
If scavenging ports are positioned closer to the exhaust port, then the engine is more compact, but air-fuel mixture short-circuits to the exhaust port
Solution Approach 1:
The scavenging passage is segmented into main and branch portions with the branch extending away from the exhaust port. This segmentation allows the scavenging ports to be positioned closer to the exhaust port for compactness while the branch portion prevents short-circuiting, effectively resolving the contradiction between compactness and blow-by prevention.
3Power
If scavenging airflow velocity is high, then the engine has high power output, but air-fuel mixture flows directly to the exhaust port
Solution Approach 1:
The scavenging passage is divided into main and branch portions, where the branch extends away from the exhaust port. This segmentation forces the high-velocity air-fuel mixture to follow a longer, more controlled path, preventing direct short-circuiting to the exhaust port while maintaining the high power output capability.
Solution Approach 2:
The branch scavenging passage acts as an intermediary structure that mediates between the high-velocity air-fuel mixture from the main passage and the exhaust port. It redirects the flow away from the exhaust port, preventing harmful short-circuiting while allowing the engine to maintain high power output.
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 effectively reduces blow-by in both halves of the exhaust stroke by modifying the conventional four-flow scavenging type engine, achieving a 30% reduction in HC emissions and improving fuel efficiency.
Implementation Method 1
branch scavenging passages branched from the main scavenging passages and extending aslant away from the exhaust port up to the second scavenging ports, wherein the branch scavenging passages have a mean cross-sectional area smaller than a mean cross-sectional area of the main scavenging passages
Data Source
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AI summary
It is intended to effectively prevent blow-by with no need for large changes in typical structures of two-cycle internal combustion engines. A main scavenging passage (24) for supplying air-fuel mixture from a crankcase to a combustion chamber for scavenging purposes has a branch scavenging passage (26) that extends upward aslant toward an intake port (14). The main scavenging passage (24) communicates with a first scavenging port (20) located nearer to an exhaust port (16). The branch scavenging passage (26) communicates with a second scavenging port (22). A mean cross-sectional area of the branch scavenging passage (26) is smaller than that of the main scavenging passage (24). Cross-sectional area of a portion (24b) next to an inlet port (24a) of the main scavenging passage (24) opening to the crankcase is smaller than the sum of cross-sectional areas of the first and second scavenging ports (20, 22).