A quasi-dimensional charge motion and turbulence model for Diesel engines with a fully variable valve train

Abstract

With the increasingly strict emission regulations and economic demands, variable valve trains are gaining in importance in Diesel engines. A valve control strategy has a great impact on the in-cylinder charge motions, turbulence level, thus also on the combustion and emission formation. In order to predict in-cylinder charge motions and turbulence properties for a working process calculation, a zero-/quasi-dimensional flow model is developed for the Diesel engines with a fully variable valve train. For the purpose of better understanding the in-cylinder flow phenomena, detailed 3D CFD simulations of intake and compression strokes are performed at different operating conditions with various piston configurations. In the course of model development, global in-cylinder charge motions including swirl, squish and axial flows are assigned to idealized flow fields. Among them, swirl formation during intake is estimated dependent on the stationary swirl number route that is determined by the valve actuation. Swirl losses during compression and expansion are ascribed to wall friction, turbulent conversion as well as piston motion. In conjunction with the charge motion model, a quasi-dimensional turbulence model is developed based on the k-e turbulence model. Turbulence production and dissipation are determined through submodels. The results from the performed validations demonstrate, the proposed flow model accurately predicts the temporal change of in-cylinder flow quantities and also responds correctly to the variations of piston configuration as well as operating conditions such as engine speed, charging pressure and valve actuation. Further application of the model in other Diesel engines is feasible by tuning certain model parameters.