Research for Improving the Accuracy in Numerical Simulations of Gas Flow Behaviors in Internal Combustion Engines

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It is important to reduce the numerical error, that is false diffusion error in order to improve the calculation accuracy of the phenomena of unsteady gas flows in internal combustion engines. The results obtained through the investigation of various methods are as follows :1. In one-dimensional calculation a new scheme of the characteristic method was proposed to eliminate the numerical diffusion. This scheme, in which two kinds of meshes are prepared corresponding to the trajectories of preassure waves and gas particle movement, respectively, provides good agreements between calculated and experimental results, and much less numerical diffusion has been achieved in comparison with the conventional scheme. Consequently, this scheme is seemed to be well suited for the analysis of gas flows in intake and exhaust systems.2. With the purpose to predict quantitatively the noise level and the pressure loss of silencers in intake and exhaust systems, the gas flow with pressure and temperature fluctuations has been calculated by using the CIP method. The comparison between calculated results and the experimental ones shows that both the pressure wave propagations and the gas particle movements can be predicted with high accuracy. The two- and three-dimensional calculations of the gas flow in expansion chamber type silencers have been carried out under actual engine conditions.3. Through the investigation to analyze accurately the turbulence and concentration diffusion in three-dimensional gas flow fields, it has ben shown that the newly developed GTT method with the k-epsilon turbulence model can predict the turbulent flow fields in engine cylinders properly, and that the CIP method enables the accurate analysis of concentration because its false diffusion is very small. Then the process of the stratification of mixture concentration distribution in the cylinder of an engine with two intake valves has been analyzed.