The two stroke petrol engine differs immediately from the four stroke petrol engine; by this I mean that it is obvious that it is different from the two strokes. The two stroke engine uses ports located on the side, along the length of the cylinder instead of valves. As the piston moves up and down the ports are covered and uncovered, depending on where the piston is in the cylinder. There are usually 3 ports, and inlet port, exhaust port, and a transfer port. The transfer port connects the lower chamber beneath the piston to the upper chamber above the piston.

The first stroke of the two stroke petrol engine begins with the piston about halfway down the cylinder heading toward the top of the cylinder, going upwards, because the piston is halfway down the cylinder, all of the ports are covered. Because of the upwards motion of the piston, the bottom chamber (below the piston) falls below atmospheric pressure. As the piston moves closer to the top of the cylinder it uncovers the inlet port, which then, because of the pressure in the lower chamber sucks air and fuel into the lower chamber.

Image Copyright © Nicholas Seal. Do not copy.

The second stroke of the two stroke engine as carrying on from the first stroke, which sucked the air into the lower chamber. The piston has nearly reached the top of the cylinder, just before it does the mixture of air and fuel in the upper chamber (above the piston) is ignited. The high pressure of the burnt gases forces the piston down, on the way down the cylinder the piston re-covers the inlet port, stopping any fuel/air from being sucked into the lower chamber. As the piston moves downwards the mixture of air and fuel in the lower chamber is being compressed. When the piston gets to B.D.C. (Bottom Dead Centre) the exhaust port is fully uncovered, which releases the burnt fuel in the upper chamber, and also draws fuel and air from the lower chamber through the transfer port into the upper chamber ready for the next stroke.

Image Copyright © Nicholas Seal. Do not copy.