I love simplicity. And a 2-stroke engine is that. It’s lighter and more powerful than its equivalent 4-stroke engine. Alas, it’s also not as efficient, not as reliable, and pollutes more than the 4-stroke. Hence, nowadays it’s rare in the world of road bikes.
Two-stroke engines operate in two strokes, instead of four, and this animation illustrates the two-stroke engine operation in conjunction with its expansion chamber showing the reflected pressure wave on the fuel charge.
1) Power/exhaust: This stroke occurs immediately after the detonation of the charge. The piston is forced down. After a certain point, the top of the piston passes the exhaust port, and most of the pressurized exhaust gases escape. As the piston continues down, it compresses the air/fuel/oil mixture in the crankcase. Once the top of the piston passes the intake port, the compressed charge enters the cylinder from the crankcase and any remaining exhaust is forced out.
2) Compression/intake: The air/fuel/oil mixture has entered the cylinder, and it begins to move up. This compresses the charge in the cylinder and decompresses the crankcase, pulling in more air, fuel, and oil from the carburetor. The compressed charge is detonated by the spark plug, and the cycle begins again. (Animation and indented material from Wikipedia.org)