Two advanced combustion models have been validated with the KIVA-3V Release 2 code in the context of multiple injections in heavy duty diesel engines. The first model uses CHEMKIN to directly integrate chemical rate equations in each computational cell. The second model accounts for flame propagation with the G-equation, and CHEMKIN predicts autoignition and chemistry ahead of and behind the flame front. In the G-equation model, a Damköhler number criterion is used in flame containing cells to characterize the local mixing status and determine whether heat release and species change should result from flame propagation or volumetric heat release. The purpose of this criterion is to make use of physical and chemical time scales to determine which chemistry model is more appropriate in each computational cell, depending on the composition and thermodynamic properties of the fuel-air mixture.
Recently developed spray models have been implemented in the KIVA-3V Release 2 code with the purpose of reducing the dependency of the spray processes on the mesh size and computational time step. The models used in the current validation work include a steady-state gas jet model, an unsteady gas jet model, an improved Kelvin-Helmholtz / Rayleigh-Taylor (KHRT) droplet breakup model, and an improved Radius of Influence (ROI) droplet collision model.
Validation of the models has been performed by comparing simulation results with available data from heavy-duty diesel engine experiments. In particular, the steady-state gas jet model, ROI collision model, CHEMKIN combustion model, and G-equation combustion model are applied to two-stage combustion in a Caterpillar heavy-duty diesel engine. Several operating parameters relating to the injection events and initial thermodynamic conditions are varied in order to test the models over a range of conditions. The gas-jet model is found to improve the consistency of the predicted heat release and soot and NOx emissions as the mesh size is reduced. Little difference is seen between the CHEMKIN and G-equation combustion models; this suggests that flame propagation is not a significant contributor to heat release in the two-stage combustion regime studied in the experiment.
Next, the mesh and time step independent spray models listed above are applied to a Mitsubishi Heavy Industries heavy-duty diesel engine operated with single and multiple injections over a range of speeds and loads. Application of the spray models improves the fuel vapor distribution leading up to the combustion event. Similar to the calculations of two-stage combustion in the Caterpillar engine, the CHEMKIN and G-equation combustion models are found to give similar predictions in terms of heat release and emissions formation.