Based on the process of gas exchange, internal combustion engines can be classified into:
Two-stroke Engines
Four-stroke Engines

Two-Stroke Engines
The figure indicates the p-V Diagram as well as the Pressure-to-Crank Angle Diagram of a 2-stroke engine.
In the 2-stroke system, the exhaust gases are displaced by a fresh charge between the working cycles. The piston undertakes the control of gas exchange by blocking or opening the inlet and outlet ports. A working cycle is hence reduced to two strokes compression and expansion (Fig. 3-4). In contrast to the 4-stroke engine, no valve train is required for gas exchange. However, in order to manage a sufficient intake of scavenging air, the 2-stroke engines requires a blower. Either the bottom end of the piston - with the crankcase as compressor space - or a separate mechanical compressor takes over this task.
Due to emission norms which have to be fulfilled in the meanwhile, modern 2-stroke engines also require a considerable mechanical expense (valves, injection systems). Their advantage lies in the fact that work is produced in every rotation and not in every alternate rotation as is the case with 4-stroke engines. As a result, very low weight-to-power ratios can be realized. Currently, the chances for penetrating the market cannot be clearly seen.