Effect of the hydrocarbon molecular structure in diesel fuel on the in-cylinder soot formation and exhaust emissions

Abstract

Evaluations of diesel fuel effects on combustion and exhaust emissions in single-cylinder direct-injection diesel engines led to the unexpected result that a Swedish ‘class 1’ fuel generated more particulate matter (PM) than a fuel denoted ‘improved’, even though ‘class 1’ fuel had much lower distillation temperatures, aromatic concentration, sulphur level, and density than the ‘improved’ fuel. Little differences were observed in the combustion characteristics between these fuels, but detailed compositional analyses showed that ‘class 1’ fuel contains higher levels of cyclic and/or branched paraffins. Subsequent investigations in a laboratory flow reactor showed that ‘class 1’ fuel produces more soot precursors such as benzene and acetylene than the ‘improved’ fuel. In addition, experiments in a low-pressure laminar flame apparatus and shock tube with model (single-molecule) paraffin fuels showed that isoparaffins and cycloparaffins generate more soot precursors and soot than n-paraffins do. These results strongly suggested that the effect of molecular structure on exhaust PM formation should be more carefully examined. Therefore, a new series of investigations were performed to examine exhaust emissions and combustion characteristics in single-cylinder engines, with well-characterized test fuels having carefully controlled molecular composition and conventional distillation characteristics and cetane numbers (CNs). These investigations revealed the following. Firstly, under low and medium loads, cycloparaffins (naphthenes) have a higher PM formation tendency than isoparaffins and n-paraffins. Under high-load conditions, however, the paraffin molecular structure has a very small effect. Secondly, a highly n-paraffinic fuel does not yield PM reductions as high as expected, due to its high CN and corresponding shorter ignition lag, which initiates combustion under a state of insufficient fuel-air mixing. This finding was corroborated by laser-induced incandescence analyses. Thirdly, the lowest PM emissions were observed with a paraffinic fuel containing 55 per cent isoparaffins and 39 per cent n-paraffins. Fourthly, aromatics give higher soot and PM levels than paraffins do at high and medium load conditions. Smaller differences are observed at lower speeds and loads. Fifthly, the best fit to the PM emissions was obtained with an equation containing the regression variables CN, aromatic rings, and naphthene rings. This expression of the fuel effects in chemical terms allows well-to-wheel analyses of refining and vehicle impacts resulting from molecularly based fuel changes.