The two-stroke engine is simple in construction, but complex dynamics are employed in its operation. The simplest two stroke has no valves, the piston acts as a valve by opening and closing ports (holes) in the cylinder and the crankcase.
The rising piston creates a partial vacuum in the sealed crankcase, a port between the crankcase and the carburettor (inlet port) is uncovered by the piston as it rises, and the air-fuel mixture enters the crankcase. As the piston descends, it closes the inlet port and pressurizes the crankcase. The air fuel mixture is forced into passageways that connect the crankcase to the cylinder. Holes connecting these passages to the upper cylinder (tranfer ports) are uncovered by the descending piston and air-fuel mixture is forced into the upper cylinder. As the piston reaches the bottom and then starts to rise again, the transfer ports are closed by the piston and the air/fuel mixture is compressed.
When the piston reaches the top of its stroke, the mixture is ignited, and the piston is forced down by the rapidly expanding gases of combustion. As the piston descends a hole in the side of the cylinder connected to the exhaust pipe (exhaust port) is opened, and the burned gases can escape. The transfer ports are just a bit lower than the top of the exhaust port, so there is a period of time when fresh air-fuel mixture is coming in while exhaust is leaving. The major components of two-stroke engines are tuned so that optimum airflow results. Intake and exhaust pipes are tuned so that resonances in airflow give better flow. The cylinder ports and piston top are shaped so that the intake and exhaust flows do not mix. The incoming fresh charge assists in forcing the exhaust gas out, and a tuned pipe provides back pressure at just the right time to push fresh air-fuel mixture sneaking out the exhaust back in again. A major problem with the two-stroke engine is however the short-circuiting of fresh charge from intake to exhaust which increases fuel consumption and emissions of unburned hydrocarbons.
Two strokes can develop a great deal more power for size than a four stroke, but do so at the price of greater fuel consumption, 'dirty' exhaust and what most people consider to be a rather unpleasant sounding exhaust noise. Jurisdictions with stiff exhaust emmission standards have made two strokes impractical. Two stokes can be cleaned up, but at the cost of their advantages in light weight, superior power and simplicity. Many large manufacturers, including Ford and Honda are still actively researching ways to build practical and clean two strokes for automotive use.
The very large two stroke diesels do not use the crankcase as an air pump. A supercharger or a turbocharger is used to pump air into the cylinder through ports in the side of the cylinder. The exhaust exits through the top of the cylinder head via conventional four stroke style poppet valves.