Evaluation of Modeling Approaches for NOx Formation in a Common-Rail DI Diesel Engine within the Framework of RepresentativeInteractive Flamelets (RIF)

Representative Interactive Flamelets (RIF) have proven successful in predicting diesel engine combustion. The RIF concept is based on the assumption that chemistry is fast compared to the smallest turbulent time scales, associated with the turnover time of a Kolmogorov eddy. The assumption of fast chemistry may become questionable with respect to the prediction of pollutant formation; the formation of NOx, for example, is a rather slow process. For this reason, three different approaches to account for NOx emissions within the flamelet approach are presented and discussed in this study. This includes taking the pollutant mass fractions directly from the flamelet equations, a technique based on a three-dimensional transport equation as well as the extended Zeldovich mechanism. Combustion and pollutant emissions in a Common-Rail DI diesel engine are numerically investigated using the RIF concept. Special emphasis is put on NOx emissions. A surrogate fuel for diesel consisting of a mixture of n-decane (70% liquid volume fraction) and alpha-methylnaphthalene (30% liquid volume fraction) is applied in the simulations. One engine operating point is considered with a variation of start of injection. The simulation results are discussed and compared to experimental data.

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