In the 2-stroke motors, the operating cycle is carried out in two races. The admission of the active fluid must be made during a fraction of the compression stroke, and the exhaust must be produced during a fraction of the working stroke.
The 2-cycle cycle was conceived to simplify the distribution system, eliminating and reducing the number of valves, and to obtain a greater power with equal dimensions of the engine.
With the 2-stroke engine there is a useful stroke for each rotation of the crankshaft. In this way, the frequency of the useful stroke and, consequently, the power obtained, is theoretically double that of a 4-stroke engine of equal displacement.
However, the increase in the frequency of the useful stroke tends to produce excessive heating. This heating tends to cause a rupture of the lubricating oil film with the danger of damage to the piston and the cylinder. For this reason, the speed of the 2-stroke engine must, in general, be a little lower than that needed to perform twice the power.
1 - First time - Combustion - Expansion - Exhaust
Immediately after, the intake ports open. At this time the active fluid, pushed by the pressure reached in the crankcase and dragged, also by the current of the combustion gases coming out, enters the cylinder of the engine.
In this way the sweep and admission phase begins, which includes the rest of the race.
2 - Second time - Transfer - Aspiration - Compression
The first part of this second cycle time is still dedicated to the sweep and admission phase. The second part is dedicated to the compression phase.
Before the stroke of the piston inside the cylinder is finished, the lower edge of the piston releases the inlet port of the fluid in the crankcase. It penetrates as a result of the depression created by the movement of the piston and is then compressed during the next stroke.