Modeling NO Formation in Spark Ignition Engines with a Layered Adiabatic Core and Combustion Inefficiency Routine 2001-01-1011
A thermodynamic based cycle simulation which uses a thermal boundary layer, either, a fully mixed or layered adiabatic core, and a crevice combustion inefficiency routine has been used to explore the sensitivity of NO concentration predictions to critical physical modeling assumptions. An experimental database, which included measurements of residual gas fraction, was obtained from a 2.0 liter Nissan engine while firing on propane. A model calibration methodology was developed to ensure accurate predictions of in-cylinder pressure and burned gas temperature. Comparisons with experimental NO data then showed that accounting for temperature stratification during combustion with a layered adiabatic core and including a crevice/combustion inefficiency routine, improved the match of modeling predictions to data, in comparison to a fully mixed adiabatic core. With the layered routine, it was concluded that the simple three reaction Zeldovich mechanism, alone, was capable of predicting NO concentration, as a function of spark timing, equivalence ratio, and intake manifold pressure, to better than 15% accuracy. Following the comparison with experimental data, a parametric sensitivity analysis was performed which showed NO predictions to be relatively insensitive to changes in overall heat loss and size of the crevice volume used to model combustion inefficiency. The sensitivity analysis also confirmed that residual fraction and burn rate are the most critical engine variables for making NO predictions. Finally, a set of kinetic calculations was performed that showed model accuracy could be modestly improved by removing the equilibrium burned gas radical pool assumption.