Potential of in-cylinder exhaust gas recirculation stratification for combustion and emissions in diesel engines

Abstract

It is difficult to decrease the emissions of nitrogen oxides (NO _x ) and soot simultaneously in conventional diesel engines. Low-temperature combustion concepts have been studied in an effort to overcome this problem. Low-temperature combustion has the potential to reduce NO _x and soot emissions, but it has many limitations, including narrow operating ranges, high carbon monoxide and hydrocarbon emissions, and difficulties with ignition control. Exhaust gas recirculation (EGR) stratification is another combustion concept used to reduce NO _x and soot emissions simultaneously using the local non-uniformity of EGR gas instead of increasing the overall EGR rate.

In this study, the EGR stratification concept was improved using computational fluid dynamics. First, a two-step piston was developed to maximize the stratified EGR effects by obtaining a favourable EGR distribution pattern and injecting fuel into the high-EGR region. Then, the possibility of combustion and emission control using stratified EGR was estimated. The ideally distributed EGR in the cylinder results showed that the region of locally high EGR effectively influences the combustion characteristics and, thus, horizontally and centrally stratified EGR has the potential to reduce the nitric oxide (NO _x ) and soot emissions at the same time. Engine simulation results also showed simultaneous reductions in the NO _x and the soot emissions.