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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.

In Common-Rail DI Diesel Engines, a low combustion temperature process is considered as one of the most important possibilities to achieve very small emissions and optimum performance. To reduce NOx and Soot strongly, it is necessary to achieve a homogenization of the mixture in order to avoid the higher local temperatures which are responsible for the NOx formation [1]. Through the homogenization it is also possible to obtain a stoichiometric air-fuel ratio in order to significantly reduce the Soot emissions. One way to achieve this homogeneous condition is to start injection very early together with the use of higher EGR rates. The direct effect of these conditions cause a longer ignition delay (this is the time between start of the injection and auto-ignition during physical and chemical sub processes such as fuel atomization, evaporation, fuel air mixing and chemical pre-reactions take place) so that the mixture formation has more time to achieve a homogeneous state.

In this work, surrogate fuels composed of n-decane and alpha-methylnaphthalene (AMNL) with different compositions according to the reference cetane numbers 53, 45, 38, and 23 are investigated. In addition to the two-component mixtures, we examine a three-component mixture composed of n-decane, AMNL, and di-n-butyl ether (DNBE) corresponding to a reference cetane number of 53. Spray characteristics of liquid and fuel vapor phase and the relationship between ignition quality and lift-off length are investigated. The experimental results show, first of all, that for these mixtures, the cetane number is a good indicator for the ignition delay. Diesel and surrogate fuels have different liquid penetration lengths, which depend on the evaporation rate, and hence vapor pressure and boiling point of the fuels.