Calling an engine 2-stroke means its engine cycle has two strokes. As in a 4-Stroke engine, a stroke is the distance from top dead center to bottom dead center.
The piston moves up the cylinder from its lowest point, bottom dead center, compressing air-fuel mixture already in the combustion chamber. As it rises, the top of the piston covers the exhaust port and the transfer port. This is a third port or passage between crankcase and cylinder. The bottom of the piston uncovers the inlet port. At the same time, the rising piston is increasing the volume of the crankcase below the piston. Pressure in there falls until it is less than outside air pressure. This pressure difference forces more mixture into the crankcase. As the piston reaches top dead center, mixture already there ignites, and expansion of gases starts to drive the piston downward.
The piston moves down. The bottom of the piston covers the inlet port as the top of the piston uncovers the exhaust port. Opening the exhaust port starts the scavenging process. It releases the pressure above the piston and burned gases escape. At the same time, closing the inlet port means the descending piston is compressing the air-fuel mixture in the crankcase. The top of the piston uncovers the transfer port. Increased pressure in the crankcase forces fresh air-fuel mixture up through the transfer port into the cylinder, replacing the burned gases, and the cycle starts again.
