Actually the two-stroke engine should perform twice the performance of a four-stroke engine with the same cubic capacity. Though it is just possible to gain a performance that is about 50% better. The reasons are obvious: The cylinder can't be filled up with the same amount of fuel as in the four-stroke engine, because the individual strokes are seperated not so clearly. If more fuel is induced, it leaves the combustion chamber through the ejection pipe without being burnt. Many concepts were developed to provide a better expulsion of the exhaust in way that the fresh gas doesn't leave the combustion chamber (as for example the "nosepiston" you can see in the animation above, which causes turbulences of a certain type). Though all these inventions, the filling of the two-stroke engine is always worse than in the four-stroke engine, which loses fresh fuel only because of the "overlap" of the valve times (both valves are open for an instant). Beside these performance-technical problems, there are also increasing difficulties with the environment. The fuel mixture of the two-stroke engine often gets shifted with a certain quantity of oil because of the necessary lubrication. Unfortunately the oil gets burnt partly, too, and harmful gases are expulsed by the engine.
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