Modeling turbulent combustion and pollutant formation in stratified charge SI engines

Abstract

A conditional averaging approach is applied to model combustion and pollutant formation in stratified charge spark-ignition engines. A set of diagnostic equations follows the local composition and temperature of the unburned gases during combustion. The Coherent Flame Model is chosen to simulate combustion. It is extended to rich or diluted combustion. NO and CO reaction rates are computed conditionally based on the burned gas temperature with two contributions: a source term function of the flamelet density and unburned gas composition and temperature, and another source term including equilibrium and kinetic mechanisms in the burned gas. The coupled combustion and pollutant models are implemented in the Kiva-2 code and tested on a simple engine geometry for a set of operating conditions including variation of equivalence ratio, dilution by residual gases and fuel stratification. The ability of the model to reproduce observed trends for combustion rate is demonstrated. The influence of the local laminar flame on the burn rate is pointed out. A much larger production rate of NO and CO during combustion resulting in higher computed NO levels for most of the operating conditions, and similar final CO levels after combustion completion is observed for the conditional averaging as compared to the usual ensemble averaging procedures. The predicted levels of emissions are comparable to measured engine values.