However, the two-stroke cycle can be difficult for some to visualize at first because certain phases of the cycle occur simultaneously, causing it to be hard to tell when one part of the cycle ends and another begins. Several different varieties of two-strokes have been developed over the years, and each type has its own set of advantages and disadvantages. This discussion has already prompted many of you to find a more interesting web-site, so I won't go into the sordid details of each type. The easiest way to visualize two-stroke operation is to follow the flow of gases through the engine starting at the air inlet. As the piston moves upward, a vacuum is created beneath the piston in the enclosed volume of the crankcase.
Air flows through the reed valve and carburetor to fill the vacuum created in the crankcase.
The mixture in the crankcase is compressed until the piston uncovers the transfer port openings, at which point the mixture flows up into the cylinder. Let's fast-forward about 25 degrees of crank rotation to the point where the exhaust port is covered by the piston. This event is timed such that the burning mixture reaches peak pressure slightly after top dead center. The expanding mixture drives the piston downward until it begins to uncover the exhaust port. That completes the chain of events for the basic two-stroke cycle.