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Flamelet modeling allows the application of comprehensive chemical mechanisms, which. include all relevant chemical combustion processes that occur in a DI Diesel engine during autoignition, the burnout in the partially premixed phase, the transition to diffusive burning and formation of pollutants like NO, and soot. The highly nonlinear dependencies of the chemistry need not to be simplified, and the complete structure of the flame is preserved. Using the Representative Interactive Flamelet (RIF) model the one-dimensional unsteady set of partial differential equations is solved online with the 3-D CFD code. The flamelet solution is coupled to the flow and mixture field by the current boundary conditions (enthalpy, pressure, scalar dissipation rate). In return, the flamelet code yields the species concentrations, which are then used by the 3-D CFD code to compute the temperature field.

The flame radiant emission from internal combustion engines is investigated experimentally by means of a fast spectroscopic measuring technique. This technique enables cycle resolved emission spectroscopy within a single combustion process and was applied to a DI diesel engine. The ‘cycle resolved emission spectroscopy’ (CRES) measuring technique with its optical components: optical probe, light-guide, spectrograph and ICCD-Camera is described in detail. These components were carefully designed for highest spectral transmittance for maximum signal/noise ratio in the UV and VIS wavelength region. The investigations focus on two different aspects: Broadband spectral radiation caused by thermal excited soot particles and narrowband spectral radiation caused by chemoluminescence from electronically excited OH radicals are measured. The soot radiation measurements in the visible enable calculations of the mean soot particle temperature and the mean soot volume fraction.