A cylinder is a circle extended in a third dimension perpendicular to its surface. In the case of engines, it is a void and creates the space in which a piston travels. The motion of the piston within the walls of the cylinder creates changes in the volume of the combustion chamber. The changing of volume allows for the compression of a combustible fuel/air mixture. When ignited, the compressed charge expands in an exothermic reaction. As the charge is contained by the cylinder walls and cylinder head, it transfers all of its power to the piston through the connecting rod and creates rotational motion in the crankshaft.
Piston Port Two Stroke Cylinders
Intake/Compression (Up) Stroke
After the piston and crankshaft reach BDC, the piston begins moving upward in the cylinder. The piston closes off the intake port, then the transfer ports, and then the exhaust port. As the bottom of the piston skirt passes the intake port, charge is taken into the engine cases for transport through the transfer ports on the down stroke. When the piston reaches a designated point in the cylinder, the spark plug fires, the piston reaches TDC, and begins moving downward on the combustion/exhaust stroke.
Combustion/Exhaust (Down) Stroke
As the piston moves downward in the cylinder, it first opens the exhaust port which allows the spent gasses to escape into the exhaust pipe. The downward motion of the piston also forces the fresh charge (which is below the piston) up through the engine cases and through the transfer ports as the piston uncovers these ports. Additionally, an intake port located opposite the exhaust is opened by the piston, and another fresh charge of combustible mixture is added to the cylinder.