Validation of Advanced Combustion Models Applied to Two-Stage Combustion in a Heavy Duty Diesel Engine

Two advanced combustion models have been validated with the KIVA-3V Release 2 code in the context of two-stage combustion in a heavy duty diesel engine. The first model uses CHEMKIN to directly integrate chemistry in each computational cell. The second model accounts for flame propagation with the G-equation, and CHEMKIN predicts autoignition and handles chemistry ahead of and behind the flame front. A Damköhler number criterion was used in flame containing cells to characterize the local mixing status and determine whether heat release and species change should be a result of flame propagation or volumetric heat release. The purpose of this criterion is to make use of physical and chemical time scales to determine the most appropriate chemistry model, depending on the mixture composition and thermodynamic properties of the gas in each computational cell.

Recently developed spray models have been included in the KIVA code to reduce the dependency of the mesh size on the spray processes. In order to test the models over a range of conditions, several engine operating parameters were varied, including pilot start-of-injection timing, pilot injection duration, main start-of-injection timing, boost pressure, and EGR percent. The in-cylinder pressure and predicted NOx and soot emissions from the simulations were compared with available experimental data. Differences between the CHEMKIN model and G-equation model in the context of two-stage combustion are assessed. The results show that flame propagation is not significant in two-stage combustion under the current operating conditions.