Engine, Two-Stroke

An internal combustion engine (which is also called a two-stroke cycle engine) that is air cooled, without an oil sump, and in which an engine block houses a cylinder with a piston, which is attached to a crankshaft by a connecting rod, and an inlet (intake) port, outlet (exhaust) port, and a transfer port, which connects the cylinder space above the piston with the space below the piston, i.e. the crankcase. The piston moves once down and once up the cylinder, making two separate strokes, with each stroke being 180 degrees of movement, therefore producing one complete revolution of the crankshaft.

There can be some different features within two-stroke engines, but the following is a typical example. At the start of the first stroke the piston is near top dead centre (TDC) and the mixture of fuel, oil and air that is present has been compressed and is ignited. The piston is forced downwards, and a further fuel, oil and air mixture is pulled (induction) into the cylinder space through the inlet port, being drawn through the transfer port (with a valve open) as the piston moves downwards. At the same time waste gases from the combustion are also started to be expelled through the exhaust port.

The second stroke starts at bottom dead centre (BDC), with the fuel, oil air mixture being channelled through the transfer port. The piston then moves upwards compressing the mixture of fuel, oil and air in the cylinder space, with the inlet port valve being closed during this compression stage. With the upward movement any remaining waste gases are expelled through the exhaust port, with the top of a piston typically having a deflector feature which helps force the waste gases outwards. Just before the piston reaches TDC the compressed mixture is ignited by a spark plug and the cycle starts again.