Model Parameter Sensitivity of Mixing and UHC/CO Emissions in a PPCI, Low-Load Optical Diesel Engine

The present study attempted to model experimental results obtained on an optical engine at the Sandia National Laboratory. Measurements of in-cylinder unburned hydrocarbon (UHC) distributions were provided using advanced optical diagnostics on a near production type piston. Previous multidimensional modeling provided accurate pressure profiles and heat release rate (HRR) predictions. However, the experimental UHC distribution was not matched, and the model predicted UHC extending from the bowl into the squish region in the expansion stroke. To explore the causes of this discrepancy a parametric study was performed using a variety of initial conditions, boundary conditions and model constants to explore their effects on the UHC distribution.

Of the initial conditions, the swirl ratio was found to have the biggest impact on the UHC distribution. With an increase from the stock value of swirl ratio of 2.2 to 3.0 the UHC plume was vastly reduced in size, while the pressure profile and HRR still matched the experimental data well. The injection timing was the only boundary condition examined that showed any effect on the UHC distribution, though it was still relatively weak compared to the effect the specification of the initial conditions had. Turbulence and wall impingement models also showed an effect on the UHC distribution. In summary, the UHC distribution in the piston bowl was found to be most affected by parameters that influence the mixing of the fuel and air.