Two-Stroke Engine Crankcase Pressure Control via Inlet Timing
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Solution Overview
Problem
Two-stroke engines experience high pressures, reduced power, and unstable rotational speeds due to an undesirable combustion pattern caused by the pushing back of exhaust gases into the crankcase, particularly at higher rotational speeds or when cold, leading to component wear and noise.
Innovation Solution
A control element connected to the crankshaft is used to partially close the inlet opening into the crankcase at least 5° after the transfer window opens, preventing exhaust gases from being pushed back into the crankcase and allowing the combustion pressure to be relieved, ensuring a uniform combustion pattern by controlling the inlet opening timing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the inlet opening is kept open when the transfer window opens, then exhaust gases can be pre-stored in the transfer channels, but exhaust gases are pushed back into the crankcase causing high pressures and unstable combustion
Solution Approach 1:
The inlet opening is closed preliminarily at a precisely calculated moment (5° after transfer window opening) to prevent exhaust gases from being pushed back into the crankcase. This timing ensures that the transfer channels are already filled with exhaust gases before the inlet closure prevents any reverse flow, thereby maintaining combustion stability while avoiding crankcase pressure buildup.
Solution Approach 2:
The invention changes the timing parameter of the inlet opening closure from the conventional simultaneous opening with transfer window to a delayed closure (5° after transfer window opening). This parameter change optimizes the scavenging process by ensuring complete exhaust gas storage in transfer channels before preventing reverse flow, thus resolving the contradiction between combustion stability and crankcase pressure control.
2Stress or pressure
If the inlet opening is closed early to prevent exhaust gas pushback, then crankcase pressure increases are avoided, but mixture availability for next combustion may be reduced
Solution Approach 1:
The inlet opening closure is timed to occur preliminarily at the optimal moment (5° after transfer window opening) when the transfer channels are already filled with exhaust gases. This preliminary closure prevents exhaust gas reverse flow into the crankcase while occurring late enough in the cycle to ensure sufficient mixture has already been drawn in during the upward stroke, thus avoiding both crankcase pressure buildup and mixture deficiency.
Data Source
AI summary
A two-stroke engine has a cylinder wherein a piston is journalled for reciprocating movement. The piston delimits a combustion chamber configured in the cylinder. The engine has an outlet from the combustion chamber and a transfer channel which opens at an inlet opening into the crankcase and at a transfer window in the combustion chamber. The piston drives a crankshaft via a connecting rod. The crankshaft is rotatably journalled in the crankcase. At least one control element for the inlet opening is connected to the crankshaft so as to rotate therewith. The control element is so arranged that the inlet opening is at least partially closed thereby in at least one operating state of the engine during the downward stroke of the piston at least 5° crankshaft angle after the opening of the transfer window.