Impact of biodiesel fuel on engine emissions and Aftertreatment System operation

The effect of biofuel use on the operation of Diesel Exhaust Aftertreatment System is investigated both numerically and experimentally, by focusing on the contribution of three main factors: raw PM–NOx emissions trade-off, NO–NO2 conversion efficiency of the Diesel Oxidation Catalyst (DOC) and PM reactivity toward oxidation. The possibility of limited interventions on assessed engine technologies is key toward the deployment of the potential related to fuel based greenhouse reduction policies, for both road and non-road markets. To verify its impact on the Aftertreatment System, a Diesel engine for non-road applications has been tested with Waste Cooking Oil (WCO) biodiesel blended with commercial fossil fuel at 6% and 30% v/v. Six engine operating modes have been selected as the most representative of the reference standard cycle (NRTC) for non-road Diesel engines and have been run to evaluate the biodiesel impact on engine emissions. Experimental results indicate a significant reduction of soot emissions, in line with literature trends, especially at high loads, as fuel oxygen enhances oxidation in the fuel rich regions of the combustion chamber. On the other side, only a slight increase in NOx emissions has been observed, along with a similar trend of the equivalence ratio due to both the lower heating value and stoichiometric air/fuel ratio of biodiesel in comparison with fossil fueling. The study has been also focused on the analysis of PM/NO2 ratio, in presence of the Diesel Oxidation Catalyst, when biodiesel is used. Engine tests have in fact demonstrated that, although the NO2/NOx ratio on raw exhaust is almost unaffected, a slight reduction of the NO2/NOx light-off temperature of the DOC is observed. This gives, along with a greater PM reactivity oxidation, more favorable conditions for the Diesel Particulate Filter passive regeneration process, with the final aim of a higher engine conversion efficiency.