Development processes for modern combustion engines already make substantial use of more or less sophisticated simulation approaches. The enhancement of computational resources additionally allows the increasing use of simulation tools in terms of time-consuming three-dimensional CFD approaches. In particular, the preliminary estimation of feasible operating ranges and strategies requires a vast multitude of single simulations. Here, multi-zone simulation approaches incorporate the advantages of comparably short simulation durations. Nevertheless, the combination with more detailed sub-models allows these rather simple modeling approaches to offer considerable insight into relevant engine operation phenomena. In the context of combustion process development, this paper describes a phenomenological model approach for the prediction of operating point characteristics of a dual-fuel pilot injection combustion process. In order to describe the ignition initiated by pilot fuel injection, the present model approach uses the package-based multi-zone approach as presented by Hiroyasu et al. Therefore, physical phenomena such as spray breakup, atomization, and evaporation are considered. The governing entrainment of premixed cylinder charge into the individual package zones is based on the conservation of momentum. In addition, measured pilot fuel injection profiles are implemented. The calculation of the characteristic ignition delay time applies an Arrhenius-based one-step mechanism taking local gas properties as well as the particular composition within the packages into account. Eventually, the identification of the ignition event triggers a transition process from the combustion within the spray cone to the hemispherical flame propagation of the premixed cylinder filling.
Besides the detailed description of the model approach, this paper discusses crucial influences originating from quasi-dimensional discretization. In particular, the radial discretization of the spray jet indicates a considerable influence on the prediction of spray breakup as well as fresh gas mixing into the package zones and thus eventually on the calculation of the ignition delay time. The validation of the phenomenological model approach regarding its ability to predict different operating characteristics uses experimental data. Here, variations of timing and the quantity of the pilot fuel injection as well as variations of injection pressure and global air-fuel equivalence ratio have been taken into account. The model approach indicates plausible prediction with regard to both the basic phenomenology of the combustion process and the characteristic two-stage ignition behavior. In particular, cylinder pressure curves and combustion rates as well as peak pressures and the overall released heat quantities are correctly calculated.