Chemical and Spectroscopic Characterization of SOF and Soot from a Euro-4 Diesel Engine Fueled by Model Fuels

This work regards the study of the effect of the fuel properties on the diesel engine emissions of particulate separated in soluble organic fraction (SOF) and soot. A Euro-4 engine was used operating at two engine conditions: 1500 rpm speed - 8% of maximum load and 2300 rpm - 13%. Model hydrocarbon fuels containing 100% of n-alkanes and iso-alkanes were used for studying the effect of cetane number. The effect of fuel composition on soot and SOF emissions was studied at a fixed cetane number (52) by using six fuels formulated with 90 vol% of model alkanes and iso-alkanes and 10 vol% of different components as alkylbenzenes, naphthenes (decaline), diaromatics (methylnaphthalene), fatty acid methyl esters (FAME) and highly paraffinic refinery streams (Fischer-Tropsch GtL and high-pressure Hydro cracking). Gas chromatography-Mass spectrometry (GC-MS), UV-visible spectroscopy and Size Exclusion Chromatography (SEC) were used for studying the effect of fuel properties on the composition and structure of particulate (SOF and soot) emissions. It was found that a low cetane number promoted the emission of SOF, in particular at low engine speeds. In this condition a high content of oxygenated species, derived from the partial oxidation of the fuel, was detected in the SOF. On the other hand, high cetane number fuels and a high engine speed produced higher amounts of soot and lower emissions of SOF. The latter was characterized by a high content of aliphatic species mostly derived from unburnt fuel hydrocarbons. Among the different fuel components, naphthalene showed to increase SOF and soot emissions with respect to the paraffinic base fuel; moreover both SOF and soot exhibited a high presence of oxygenated aromatic species. Independently on the cetane number and fuel composition the SEC analysis of soot showed a three-mode particle size distribution with the main mode peaked at 110 nm and minor peaks at 10 nm and 7 nm. The UV-visible spectrum of the 7-nm peak soot revealed the presence of oxygenated aromatic functionalities whereas the 110-nm and 10-nm peak soot have the typical broad spectra of aromatic/graphitic carbon particles.