Modeling the Effects of In-Cylinder Flows on HSDI Diesel Engine Performance and Emissions

In the present work the three-dimensional KIVA CFD code was used to simulate the combustion process in a HSDI diesel engine. State-of-the-art models, including the KH-RT spray breakup model, the RNG k-ε turbulence model, and a n-heptane reduced chemistry including reduced GRI NOx mechanism were used. The performances of two combustion models, KIVA-CHEMKIN and GAMUT (KIVA-CHEMKIN-G), coupled with 2-step and multi-step phenomenological soot models were compared. The numerical results were compared with available experimental data obtained from an optically accessible HSDI engine and good agreement was obtained. To assess the effects of the in-cylinder flow field on combustion and emissions, off-centered swirl flows were also considered. In these studies, the swirl center was initialized at different positions in the chamber for different cases to simulate the effects of different intake flow arrangements. To aid the comparisons, the angular momentum referred to the swirl center was kept constant in each case. Numerical results of off-centered swirl cases were compared with the centered swirl case, as well as with available experimental images obtained using laser induced fluorescence (LIF), laser induced incandescence (LII), and particle image velocimetry (PIV). Eccentric phenomena observed in the experiments were well predicted by the present method. The off-centered swirl was found to have higher turbulence intensities and effective diffusivities, which enhance the mixing process. Soot formation is strongly correlated with the turbulent viscosity. Off-centered swirl flow results in an asymmetric distribution of turbulent viscosity and consequently an asymmetric distribution of soot emissions. Analysis of the mass distributions of temperature showed that the resulting mixture is a little more homogeneous with larger off-centered swirl flows, which leads to a slight reduction in NOx emissions. The off-centered swirl delays the formation of acetylene, which changes the phasing of soot formation.