Working cycle of the two-stroke carburettor engine

The two-stroke engine has no valves. The intake of the air-fuel mixture and the exhaust of the burnt gases are effected through ports cut in the cylinder wall, which are timely opened and closed by the moving piston.

As piston moves upwards, it closes exhaust ports in the cylinder wall, as a result of which final compression of the combustible charge transferred earlier in the stroke from sealed crank chamber to the cylinder takes place above the advancing piston. At the same time, a fresh charge of fuel and air is taken from carburettor into crank chamber through intake ports, as a result of the depression produced below the piston as it retards towards the end of its stroke.

As the piston nears TDC, a spark jumps across the points of spark plug and the combustible charge in the cylinder is ignited. This marks the end of the first (intake) and compression stroke.

Under the pressure of the expanding combustion gases the piston moves downwards on its power stroke which continues until the exhaust ports are opened and the evacuation of the exhaust gases from the cylinder commences. As the piston moves downwards, the previously induced charge trapped in the crank chamber beneath the advancing piston is partially compressed. At the end of the second (power and exhaust) stroke, the piston opens scavenging (transfer) passage (port) and the partially ompressed charge of air-fuel mixture previously trapped in the crank chamber enters the cylinder, expelling the exhaust gases out of it. The cylinder is simultaneously scavenged and filled with a fresh air-fuel mixture, the latter being partially lost together with the exhaust gases leaving the cylinder. Thus, the working cycle of the engine is completed in two strokes of the piston.