17 Two Stroke Engine

Pagina 1

Two Stroke Engine Two Typical Features 1) In two stroke engines the working fluid of the previous cycle is replaced when the piston moves around the bottom dead centre (BDC), so reducing the cycle duration to only two strokes: compression and expansion. Since there is no piston suction stroke, some form of external compressor is required to force the fresh charge into the cylinder. Since the two stroke engine is nor self exhausting, the fresh charge, driven by the compressor, is used to force out the residual exhaust gases in the cylinder and to fill it with the working fluid for the new cycle. The whole operation of cleaning the cylinder of burned gases and filling it with a new charge is called scavenging. 2) Indeed the basic idea of a two stroke is to separate the two functions exploited in a four stroke engine by the same group cylinder-piston: the engine and the compressor. Separated the two functions, the pump of a two-stroke engine can be designed to satisfy the lower requirements (from the point of view of stresses and thermal loads) of the gas exchange process. Positive Points A comparison of two-stroke versus four-stroke engine points out that the two stroke: 1) generated an higher power per unit displaced volume. In theory it should be double because each engine cycle, with one power stoke, is completed in only one, instead of two crankshaft revolution. In practice, it is difficult to completely fill the displaced volume with the fresh charge. So the actual gain is lower (50-60%), but still very high. 2) presents a simpler structure since the frequency of the thermo-dynamic cycle and of the crankshaft are now the same (poppet valves can be replaced by ports controlled by the piston movement, auxiliary device, i.e. cooling system, are generally simplified) 3)…