Improved NOx submodel for in-cylinder CFD simulation of low- and medium-speed compression ignition engines

The NOx submodels currently used in the CFD-based simulation of engines hav e been developed for conditions where homogeneous and stoichiometric mixtur es of fuel and air are burned. Such conditions apply well for spark ignitio n engines (SIE) but not for compression ignition engines (CIE), where combu stion takes place in a highly heterogeneous environment. As a consequence, current NOx submodels do not satisfactorily describe the fate of nitrogen o xides in CIE. The aim of this work was to determine, by detailed chemical k inetic investigation, the mechanisms leading to in-cylinder NOx in low- and medium-speed CIE and to develop a more accurate submodel for the predictio n of NOx in CIE by means of CFD. Calculations at constant pressure (1-150 b ar) and constant temperature (1500-2200 degreesC) under ideal plug flow con ditions show that, relative to the proportion of NO, the proportion of NO2 in NOx is negligible and that the formation and destruction of NO occurs ma inly via 10 reactions. These reactions can be organized into three NO mecha nisms: thermal, N2O intermediate, and N2O extension, the latter being a set of five reactions that oxidize N2O to NO via NH and HNO intermediates. To our knowledge, the importance of the N2O-extension mechanism is reported he re for the first time. An improved NOx submodel was developed taking into a ccount all three mechanisms, after introducing approximations relevant to C IE. According to our kinetic investigations, partial equilibrium approximat ion can be applied to O, OH, and H and a quasi-steady-state approximation t o N2O, N, NH, and HNO. The performance of the improved submodel is illustra ted by comparing its NOx predictions with those of a detailed kinetic schem e and a NO submodel currently used in CFD. The comparison shows that the im proved submodel always produces more accurate predictions of NOx than does the current submodel.