two-stroke engines

The two-stroke engineCouldn't an engine be constructed, that needs less than four strokes and that performs the same power as the Otto engine? Couldn't you reduce or even replace the complicated valve mechanics? These questions led to the development of the two-stroke otto engine. It needs only one revolution of the crankshaft to indicate a new ignition. Additionally, the piston has at the same time the function of a valve, which saves many mobile parts: The two-stroke engine consists of only three mobile parts: Piston, connecting rod and crankshaft. Let's have a look at the mode of operation of this engine:

Mode of operation of the two-stroke engine1st stroke: The piston is at the bottom of the cylinder. A pipe at the left side is opened and lets the fuel mixture, which is already compressed a bit, flow from the lower to the upper part of the cylinder. The fresh gases expulse now the exhaust through an ejection pipe, which is not closed by the piston at this moment.2nd stroke: After being hurried upward, the piston now covers the pipe on the left side and the ejection pipe. Because there is no way out any more, the upper, fresh gas mixture gets compressed now. At the same time in the part below fresh gas is taken in by the piston driving upward through the open suction pipe. At the upper dead-center, the compressed fuel mixture is ignited by the sparking plug, the piston is pressed downward while he compresses at the same time the fresh gas below. The process begins again as soon as the piston arrives at its lowest point.
Animation: two-stroke engineenlarge (21 K, 850 * 650), slow animation (21 K, 260 * 510) DevelopmentThe idea to build a two-stroke engine goes back to the year 1879. But this engine became a qualitatively good product only after many years, when the German DKW company accelerated its development. Because of its disadvantages compared with the four-stroke engine, the two-stroke engine is used practically just in a small range of capacity, e.g. in small motorcycles. Formerly the engine was even used to power tiny cars.Problems of the two-stroke engineActually 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.