Development of a predictive tool for in-cylinder gas motion in engines

A method is described of calculating the flow, temperature, and turbulence fields in cylinder configurations typical of a direct-injection diesel engine. The method operates by solving numerically the Navier Stokes equations that govern the flow, together with additional equations representing the effects of turbulence. A general curvilinear-orthogonal grid that translates with the piston motion is used for the calculations in the complex-shaped piston bowl, while an expanding/contracting grid is used elsewhere. Predictions are presented showing the evolution of the velocity and turbulence fields during the compression and expansion phases of a motored engine cycle, for various shapes of axisymmetric piston bowl and various initial swirl levels. The method is capable of solving the conservation equations governing in-cylinder flow and heat transfer in typical diesel and stratified-charge configurations with swirl under monitoring conditions, but the accuracy of the solutions remains to be assessed. The presence of a bowl provokes, even in the absence of swirl, a complex flow pattern containing at least two strong vortices, one being formed during the approach to top dead center by the squish phenomenon and the other in the clearance gap by reverse squish during the initial descent of the piston. The evolution of the turbulence intensity distribution can be explained largely in terms of the flow behavior. Changes in piston bowl shape and swirl level may profoundly alter the flow structures.