This intake system is called a piston - port system because the piston acts as a valve to cover and uncover the ports.
This intake system uses a rotary valve. The crankcase has a hole open to the crankcase chamber, and a rotary valve cover attached to the carburetor. The rotary valve is mounted on the crankshaft, between the crankcase and the valve cover. As the crankshaft turns, the rotary valve rotates with it, opening and closing the inlet port as it does so.
Some 2-stroke engines such as outboard motors use an intake system with a reed valve. It is a small flexible metal plate that covers the inlet port, and it can be attached to the crankcase, or to the inlet port. It opens and closes automatically, according to changes in pressure in the crankcase.
The 2-stroke engine makes only two strokes, one up and one down for each time it fires. That’s only one revolution of the crankshaft for each cycle.
Compare this with a 4-stroke that needs two revolutions of the crankshaft for each engine cycle and only one power stroke for each four engine strokes.
