Effect of Hydrocarbon Molecular Structure on Diesel Exhaust Emissions Part 2: Effect of Branched and Ring Structures of Paraffins on Benzene and Soot Formation

The effect of the chemical reactivity of diesel fuel on PM formation was investigated using a flow reactor and a shock tube. Reaction products from the flow-reactor pyrolysis of the three diesel fuels used for the engine tests in Part 1⁽¹⁾ (“Base”, “Improved” and Swedish “Class-1”) were analyzed by gas chromatography. At 850C, Swedish “Class-1” fuel was found to produce the most PM precursors such as benzene and toluene among the three fuels, even though it contains very low amounts of aromatics. The chemical analyses described in Part 1 revealed that “Class-1” contains a large amount of branched and cyclic structures in the saturated hydrocarbon portion of the fuel. These results suggest that the presence of such branched and ring structures can increase exhaust PM emissions. This finding was confirmed by flow reactor experiments at 850 and 1000C with octane and hexane isomers which revealed that iso-and cycloparaffins produce more benzene than n-paraffins, both during pyrolysis and fuel-rich oxidation. With branched hexane isomers, the benzene amount increased with the increasing number of branches in the structure. The shock tube measurements on fuel-rich oxidation near 2000C showed that the soot formation yield increases in the order of n-hexane, 2-methylpentane, 2,2-dimethylbutane and cyclohexane. These results indicate that the specific molecular structures of the paraffinic components need to be considered as one of the diesel fuel properties closely related with PM formation.