Analysis of stratified EGR and air on combustion and NO formation in a spark-ignition engine

Using a phenomenological model (Brunel Engine Emissions, Performance and Autoignition Simulation, BEEPAS) developed at Brunel University, the influence of stratified exhaust gas recycled (EGR) and stratified air (lean burn) on spark-ignition (SI) engine performance and NO emission were analysed. The mixture in the cylinder was divided spherically into three parts: a central core with a stoichiometric air-fuel charge, a dilution region without any combustible charge and a mixing region lying between the core and the dilution region. Three mixture stratification parameters were investigated: the amount of mixing in the cylinder, the extent of dilution in the mixing region and the gradient of stratification in the mixing region. In addition, the difference in the effects on combustion and NO formation of stratified EGR and air was analysed. The results indicate that stratified air operation is better for improving engine efficiency than stratified EGR operation. It was found that the longer combustion duration, as well as the increased heat capacity of EGR, contributes to the superiority of stratified EGR over stratified air in the suppression of in-cylinder NO formation. For the same initial charge temperature, stratified EGR has a much lower dilution limit than strati-tied air owing to retardation of flame propagation. As long as the degree of dilution in the mixing region is within the dilution limit of the combustible charge, the gradient of dilution has little effect on combustion and NO formation.