A simulation model is presented for open chamber direct injection diesel engines. This model is an extension of phenomenological multi-zone thermodynamic models with emphasis on stochastic description of combustion processes and thermodynamic properties of the charge. This combination of stochastic treatment coupled with deterministic description of flow and spray development gives significant improvements in the predictions of thermochemical sensitive processes such as ignition and pollutant formations.The equilibrium composition and thermodynamic property model is extended to allow partially burned elements and to cover the full fuel-air ratio range as present in diesel combustion. A new mixing model is proposed to provide a more realistic description of the initial phase of mixing which strongly affects the burn rate. This model is compared with several existing models from the literature with respect to burnrate and PDF predictions. The overall model is demonstrated to correlate well with experiments in terms of engine performance. Representative examples of NOx and soot emission predictions are presented as functions of inlet pressure, over-all equivalence ratio, EGR and fuel injection timing.