The purpose of this work is to perform an analysis on the modifications necessary to convert a four-stroke engine into a non-conventional two-stroke engine. The first aim of this work is to reach the theoretical advantages of the two stroke engine (high torque values at lower rpm and working regularity) and, at the same time, to avoid the usual problems of the two stroke cycle (short-circuit of fresh air-fuel mixture and consequently pollutant emissions and high specific fuel consumption). The target is to develop a small engine with innovative solutions that allows to obtain high performance coupled with good mechanical and thermodynamic efficiency. The starting base engine is a 125cc four-stroke two-valves scooter engine equipped with a volumetric compressor. The idea is to convert it from four to two stroke cycle, using head valves and adding scavenge ports in the cylinder. Two different cylinder layouts were studied and evaluated: in the first, the cylinder ports are used for fresh mixture intake and the valves are used for exhaust gases; in the second configuration the scavenging process was inverted, with fresh mixture entering trough the valves and exhaust gases going out trough the cylinder ports.
An engine 1D model was built using a commercial code, and after some preliminary simulations, the solution with head valves for air and air/fuel mixture and cylinder ports for exhaust gases was chosen. After the 1D fluid-dynamic analysis of the whole engine, a 3D fluid-dynamic simulation was performed to investigate the gas exchange process and in particular the in-cylinder charge motion and the flow patterns during the scavenging process obtained with the chosen cylinder geometry. The 3D simulation also gave a reliable estimate of the fresh fuel amount that flows directly through the exhaust ports (short-circuit) and allowed to study in detail the fuel injection and vaporization process in the intake duct. All the analyses results were used to manufacture the first engine prototype that, at the present time, is under construction.
