The two-stroke cycle is a mechanical method of operation in an internal combustion engine. By this form of operation, the engine cycle is completed in only two strokes of the piston within the cylinder; or two stages. It was developed to obtain a higher power output, from a given engine size, than the Otto cycle (four-stroke) machine. It also has a simpler valve design. This system can be applied to both spark-ignition and compression ignition engine.
In the two-stroke cycle, ports in the cylinder liner are opened and closed by the piston motion. They control the exhaust and inlet flow while the piston is close to bottom center (BC). The two stages are:
1- A compression stroke. It starts by closing the inlet and exhaust ports and then it compresses the cylinder contents, drawing fresh charge into the crank case. As the piston nears the top center (TC) combustion is initiated.
2- Power or expansion stroke. The piston is pushed back by the expanding hot gases of combustion. When it approaches the bottom center, first the exhaust ports and then the intake ports are uncovered. Most of the gases exit the cylinder in an exhaust blown down process. When the inlet ports are uncovered, the fresh charge, which has been compressed in the crank case, flows into the cylinder.
Below, diagram of a two-stroke cycle system
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