Analysis of Loop Scavenging Process in a Small Power SI Two-Stroke Engine

The paper concerns to the filling efficiency in spark ignition two-stroke engines of small power with loop-scavenged system. Amount of lost fresh charge to exhaust system has been presented. By changing of geometrical dimensions of transfer and exhaust ports scavenge efficiency, delivery ratio and exhaust loss of fresh charge can be changed. Full analysis of mass flow rate of air-fuel mixture and exhaust gases was conducted. Comparison of velocity vectors in an opened cylinder by means of finite element method in a CFD-program and during measurement is shown in graphs. There was obtained a big conformity of results from experimental research and calculation, which allowed carrying out the spatial analysis of fresh charge distribution in the cylinder in dependence on shape of combustion chamber. Principally geometry of ports and combustion chamber influence on emission of unburned hydrocarbons in exhaust system. For simulation test zero- and one-dimensional model of thermodynamic processes in engine, inlet and outlet systems were used. In the paper dependences of different scavenge factors were shown as a function of geometrical parameters of the engine ports. In conclusions there were given some indications, how to decrease exhaust losses of fresh charge by change of engine ports.