On Non-Equilibrium Turbulence Corrections in Multidimensional HSDI Diesel Engine Computations

The introduction of high-pressure injection systems in D.I. diesel engines has highlighted already known drawbacks of in-cylinder turbulence modeling. In particular, the well known equilibrium hypothesis is far from being valid even during the compression stroke and moreover during the spray injection and combustion processes when turbulence energy transfer between scales occurs under non-equilibrium conditions. The present paper focuses on modeling in-cylinder engine turbulent flows. Turbulence is accounted for by using the RNG k-ε model which is based on equilibrium turbulence assumptions.

By using a modified version of the Kiva-3 code, different mathematically based corrections to the computed macro length scale are proposed in order to account for non-equilibrium effects. These new approaches are applied to a simulation of a recent generation HSDI Diesel engine at both full load and partial load conditions representative of the emission EUDC cycle.

The numerical results show that the proposed corrections improve the physical behavior of the combustion model by a self-scaling of the eddy-turnover time depending on the engine operating conditions. The overall achievement is the extension of modeling reliability over a wider range of operating conditions and in particular over those that are of interest in the European emission test cycle.