THE DUAL-FUEL ETHANOL-DIESEL COMBUSTION: A STUDY ON SIZE, NUMBER, NANOSTRUCTURE AND CHEMICAL FEATURES OF THE EMITTED SOOT

In the field of second generation biofuels, ethanol is receiving a growing attention as a valid alternative to conventional fuels in internal combustion engines thanks to its storage facility, availability and handling. The most common use of ethanol is for gasoline engine applications, but the interest in burning ethanol in diesel engine is increasing. Several methods and systems have been examined in order to evaluate the applicability of ethanol in compression ignition engines, including dual fuel (DF) injection. In DF configuration, the port fuel injection (fumigation) of ethanol is coupled with the direct injection of the conventional diesel fuel. Besides the well-known effects of DF ethanol-diesel system on regulated engine raw pollutant emissions (HCs, CO, NOx and PM), ethanol fumigation is very effective also in the reduction of the number of the emitted particles. This study aimed to characterize the impact of the dual-fuel ethanol-diesel combustion system on size, number, nanostructure, reactivity and chemical features of the emitted carbonaceous particles. The tests were conducted on a single cylinder research engine provided with a modern architecture and properly modified in a DF configuration. The selected test points, critical in terms of soot emissions, were performed using engine calibration parameters (injection, boost, swirl, etc.) and exhaust recirculation gases (EGR) values derived from the real Euro 5 compliant four-cylinder engine of equal unit displacement. Overall, the chemico-physical characterization of soot particles clearly indicates that soot features, including the oxygen incorporation and morphology, slightly vary with the increasing amount of injected ethanol, even at high ethanol loading. The thermal stability of soot occurs in the range of 500-550 °C, as typically estimated for other diesel soot. UV-Visible investigation indicates that diesel soot appears as high-graphitized soot with a specific absorption (6-8 m2/g in the UV and 3-4 m2/g in the visible) comparable to those of furnace carbon black and soot from a benzene laminar flames.