A Quasi-Dimensional NO _x Emission Model for Spark Ignition Direct Injection (SIDI) Gasoline Engines

A fundamentally based quasi-dimensional NO_x emission model for spark ignition direct injection (SIDI) gasoline engines was developed. The NO_x model consists of a chemical mechanism and three sub-models. The classical extended Zeldovich mechanism and N₂O pathway for NO_x formation mechanism were employed as the chemical mechanism in the model. A characteristic time model for the radical species H, O and OH was incorporated to account for non-equilibrium of radical species during combustion. A model of homogeneity which correlates fundamental dimensionless numbers and mixing time was developed to model the air-fuel mixing and inhomogeneity of the charge. Since temperature has a dominant effect on NO_x emission, a flame temperature correlation was developed to model the flame temperature during the combustion for NO_x calculation.

Measured NO_x emission data from a single-cylinder SIDI research engine at different operating conditions was used to validate the NO_x model. The effects of fuel injection timing, injection pressure, spark timing, overall engine AFR, and intake temperature on NO_x emission were examined and well captured by the model. Comparison of all the NO_x emission data with the model indicates that the NO_x model is a good predictive tool for NO_x emissions in SIDI engines.