Crankcase scavenged Diagram of a crankcase scavenged 2-stroke engine in operation Some SI engines are crankcase scavenged and do not use poppet valves.Instead the crankcase and the part of the cylinder below the piston is used as a pump.
The intake port is connected to the crankcase through a reed valve or a rotary disk valve driven by the engine.
For each cylinder a transfer port connects in one end to the crankcase and in the other end to the cylinder wall.
The reed valve opens when the crankcase pressure is slightly below intake pressure, to let it be filled with a new charge; this happens when the piston is moving upwards.
When the piston is moving downwards the pressure in the crankcase increases and the reed valve closes promptly, then the charge in the crankcase is compressed.
When the piston is moving upwards, it uncovers the exhaust port and the transfer port and the higher pressure of the charge in the crankcase makes it enter the cylinder through the transfer port, blowing the exhaust gases.
4-stroke engines have the benefit of forcibly expelling almost all of the combustion gases because during exhaust the combustion chamber is reduced to its minimum volume.In crankcase scavenged 2-stroke engines, exhaust and intake are performed mostly simultaneously and with the combustion chamber at its maximum volume. The main advantage of 2-stroke engines of this type is mechanical simplicity and a higher power-to-weight ratio than their 4-stroke counterparts. Despite having twice as many power strokes per cycle, less than twice the power of a comparable 4-stroke engine is attainable in practice. In the USA two stroke motorcycle and automobile engines were banned due to the pollution, although many thousands of lawn maintenance engines are in use.citation neededŹródło: https://en.wikipedia.org/wiki/Internal_combustion_engine
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