Info: 2 Stroke

The two stroke cycle spark ignition engine is a reciprocating engine in which the piston takes over any valve functions in order to obtain a power stroke each revolution of the crankshaft. This involves the use of ports in the cylinder walls which are covered and uncovered by the movements of the piston. As the piston moves down, it clears these ports so that the exhaust gasses can exit and fresh charge of mixture can enter at the same time.

In a typical two-stroke engine the air-fuel mixture enters the crankcase through a reed valve. When the piston is near the bottom of the cylinder a port is uncovered. As prior movement of the piston has compressed the mixture in the crankcase, it flows into the cylinder. Further compression in the cylinder starts as soon as the piston reverses and covers the ports. At the same time compression is occurring in the cylinder, movement of the piston has created a vacuum in the crankcase which draws a fresh charge of mixture from the carburetor into the crankcase. The compressed charge is fired as the piston reaches top dead center. As the expansion of the burning charge forces the piston downward, the reed valve in the crankcase closes and the mixture in the crankcase is compressed. As the piston uncovers the ports at the bottom of the stroke, compressed mixture from the crankcase enters the cylinder again. This incoming fresh mixture then assists in pushing the burned gasses out of the cylinder and the cycle is repeated.

For comparision an animation from a four stroke engine:

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