In 2-stroke motors, the operating cycle is carried out in two races. Active fluid intake must occur during a fraction of the compression stroke, and exhaust must occur during a fraction of the working stroke.
For this to be verified, it is necessary that the active fluid be previously compressed to be able to enter the cylinder and that the exhaust of the combustion gases is carried out by its own pressure.
The 2-stroke cycle was conceived to simplify the distribution system, eliminating and reducing the number of valves, and to obtain greater power at the same engine dimensions.
With the 2-stroke engine, there is a useful stroke for each turn of the crankshaft. In this way, the frequency of the useful stroke and, consequently, the power obtained, is theoretically twice that obtained from a 4-stroke engine of the same displacement.
However, increasing the frequency of the useful stroke tends to cause excessive heating. This heating tends to break the lubricating oil film with the danger of piston and cylinder damage. For this reason, the speed of the 2-stroke engine should, in general, be a little lower than that necessary to perform twice the power.
1. First Stage - Combustion - Expansion - Exhaust
The working stroke begins with the ignition and combustion of the fuel. Subsequently, the stroke continues with expansion until the piston opens the exhaust ports.
The combustion gases begin to escape at this point due to their still high pressure. During the exit of the gases from the cylinder, a direct current is created in the fluid mass towards the exit.
Immediately afterwards the intake ports are opened. At this moment the active fluid, pushed by the pressure reached in the crankcase and dragged, also by the current of the combustion gases that leave, enters the engine cylinder.
In this way, the sweep and admission phase begins, which comprises the rest of the degree.
2. Second Stage - Transfer - Aspiration - Compression
The second time of the two-stroke engine operating cycle corresponds to the return stroke of the piston to the upper maximum point (TDC).
The first part of this second cycle time is still dedicated to the sweep and intake phase. The second part is dedicated to the compression phase.
Before the stroke of the piston in the cylinder is complete, the lower edge of the piston clears the fluid inlet port in the crankcase. This penetrates as a result of the depression created by the movement of the piston and is then compressed during the next stroke.