1. At The Beginning the cycle, there is a change of petrol-air mixture already in the combustion chamber above the piston. The piston is moving upwards and as it rises it uncovers the inlet port. The rising piston also causes a partial vacuum in the crankcase and in rushes the petrol-air mixture from the carburetor.
2. As the piston continues to move upwards it compresses the change already in the combustion chamber. When it reaches to the top of its stroke a spark at the spark plug ignites the change.
3. As the piston is driven down, it uncovers the exhaust port, and allows the burnt gas to escape to exhaust. At the same time it covers the inlet port and starts to compress the petrol-air mixture in the crankcase. Before the exhaust port has been fully uncovered, however, the piston starts to uncover the transfer port and the compressed mixture in the crankcase rushes up the transfer passage into the combustion chamber. The transfer of the mixture is now completed and the piston is forced upwards again by the momentum of the flywheel.
4. Just as the inlet port is uncovered, the firing cycle starts again. Many engines have twin exhaust ports and two or more transfer ports. They may also have more than one cylinder, in which case each piston drives a common crankshaft.
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