Comparison of the Characteristic Time (CTC), Representative Interactive Flamelet (RIF), and Direct Integration with Detailed Chemistry Combustion Models against Optical Diagnostic Data for Multi-Mode Combustion in a Heavy-Duty DI Diesel Engine

Three different approaches for modeling diesel engine combustion are compared against cylinder pressure, NOx emissions, high-speed soot luminosity imaging, and 2-color thermometry data from a heavy-duty DI diesel engine. A characteristic time combustion (KIVA-CTC) model, a representative interactive flamelet (KIVA-RIF) model, and direct integration using detailed chemistry (KIVA-CHEMKIN) were integrated into the same version of the KIVA-3v computer code. In this way, the computer code provides a common platform for comparing various combustion models. Five different engine operating strategies that are representative of several different combustion regimes were explored in the experiments and model simulations. Two of the strategies produce high-temperature combustion with different ignition delays, while the other three use dilution to achieve low-temperature combustion (LTC), with early, late, or multiple injections.

Comparison of simulated cylinder pressure and heat-release rates with the experimental results shows that all the models predict the cylinder pressure and heat release rate reasonably well. The KIVA-CTC model under-predicts ignition delay and over-predicts rates of combustion for the late-injection conditions. All the models predict NOx emissions trends very well but the absolute magnitudes are different from the experimental measurements. The KIVA-CHEMKIN model better predicted the emissions for LTC conditions. For the KIVA-RIF model, the NOx emissions predictions were only slightly sensitive to the number of flamelets used. With a single flamelet assumed in the entire computational domain, the emissions are under-predicted but the predictions are little better with the use of multiple flamelets. Detailed comparisons of the spatial and temporal evolution of modeled in-cylinder soot with the experimental data are also presented using the direct integration and the KIVA-RIF combustion models, and good qualitative agreement is found with both the models.