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

为了提高内燃机气体非定常流动现象的计算精度，必须减小数值误差，即伪扩散误差。通过对各种方法的研究，得到以下结果：1.在一维计算中，提出了一种新的消除数值扩散的特征线法方案。该格式分别根据压力波的运动轨迹和气体颗粒的运动轨迹建立了两种网格，计算结果与实验结果吻合较好，数值扩散比传统格式少得多。因此，该方案似乎非常适合于进气和排气系统中气体流动的分析。为了定量预测进排气系统中消声器的噪声水平和压力损失，采用CIP方法计算了具有压力和温度波动的气体流动。计算结果与实验结果的对比表明，压力波传播和气粒运动均能得到较高精度的预测。在发动机实际工况下，对膨胀室式消声器内气体流动进行了二维和三维计算。通过对三维气体流动中的湍流和浓度扩散进行精确分析的研究表明，新发展的基于k-epsilon湍流模型的GTT方法能够较好地预测发动机缸内的湍流流场，而CIP方法由于其虚假扩散很小而能够准确地分析浓度。分析了双进气门发动机缸内混合气浓度分布分层的过程。

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.