The Effect of Low Temperature EGR and Low Compression Ratio on NOx Reduction for EU6 Diesel Engine

The NOx reduction by low temperature EGR and low compression ratio were investigated to develop EU6 diesel engine for heavy weighted SUV vehicle. A low temperature EGR is to reduce EGR gas temperature by cold EGR coolant, thus it can decrease NOx at same PM emission. A low compression ratio can improve NOx due to low combustion temperature. These two methods have advantages in practical application, because it could be easily adapted to conventional diesel engine. On the other hand, some possible problem such as EGR fouling and poor cold start-ability might be caused when EGR coolant temperature and compression ratio are reduced too much. It is necessary to find the NOx reduction effect without these abnormal phenomenons.

The results showed that low temperature EGR made more remarkable NOx reduction rate at low load condition where had low EGR flow. It could decrease NOx by around 4% at NEDC 14 modal points. Considering cold starting duration, COV_idle and production deviation, the low compression ratio was determined as 15.2 in this study. Low compression ratio of 15.2 could reduce NOx by about 5%, when EGR rate and main timing were adjusted to keep same PM emission.