Numerical Analysis of Unsteady Exhaust Gas Flow and Its Application for Lambda Control Improvement

A multidimensional computational fluid dynamics (CFD) tool has been applied to analyze the exhaust system of a gasoline engine. Since gas flow in the exhaust manifold is affected by exhaust pulsations, prediction methods based on steady flow are not able to predict gas flow precisely enough. Therefore, a new multidimensional calculation method, called pulsation flow calculation, has been developed. A one-dimensional gas exchange simulation and a three-dimensional exhaust gas flow calculation are combined to simulate gas flow pulsations caused by the gas exchange process. Predicted gas flow in the exhaust manifold agreed with the experimental data. With the aim of reducing emissions, the pulsation flow calculation method has been applied to improve lambda feedback control using an oxygen sensor. The factors governing sensor sensitivity to the exhaust gas from each cylinder were clarified. The possibility of selecting the oxygen sensor location in the exhaust manifold on the basis of calculations was proved. The effect of an exhaust manifold with equal-length cylinder runners on achieving uniform sensor sensitivities was made clear. In addition, a new lambda feedback control method for an exhaust manifold with different-length cylinder runners is proposed.