Effect of Port Sizes and Timings on the Scavenging Characteristics of a Uniflow Scavenged Engine

Multidimensional prediction of the scavenging characteristics of a homogeneous charge uniflow scavenged two-stroke cycle engine has been carried out. An engine geometry having the same dimensions as the General Motors EMD 710 engine has been analyzed. A multidimensional computer program, CARE, has been developed for this purpose. The fluid flow problem is subdivided into its global and local parts and the two parts are solved simultaneously. Flow is assumed to be axisymmetric. Turbulence is modelled by a zero-equation eddy viscosity model. Combustion is treated as a stoichiometric heat release phenomenon. The simulation is repeated over many cycles of the crank to achieve a periodicity in the results obtained. This computer program is used to study the effects of port/valve sizes and timings on the scavenging characteristics of the engine, given that the pressures at the inlet and exhaust ports are held constant. The results are preseted in the form of flow field plots and efficiency plots as functions of port sizes and timings. It is observed that a larger inlet port area and its early opening-late closing results in a considerable increase in scavenge ratio, hence resulting in a higher scavenging efficiency and lower trapping efficiency. An increase in exhaust port area results in an increase in scavenge ratio but a decrease in trapping efficiency; this gives rise to an optimum exhaust area for a given inlet area at which scavenging efficiency is a maximum. Exhaust valve closure timing has very little effect on the scavenging, as long as the timings of inlet and exhaust closure are not far apart.