Effects of oxygenates blended with diesel fuel on particulate matter emissions from a compression-ignition engine

Environmental and human health concerns over emissions from internal combustion engines continue to bring about increasingly stringent emissions standards and drive research into the development of cleaner-burning fuels. For compression-ignition (diesel) engines, the use of oxygen-containing compounds blended with conventional diesel fuel has been shown to dramatically reduce emissions of particulate matter (PM) while maintaining acceptable levels of other regulated emissions. A series of experimental tests and a numerical modeling effort were carried out to investigate the mechanisms governing the effect of oxygenates on diesel exhaust PM.; Engine tests were conducted under steady-state conditions using a Cummins B5.9 diesel engine and test fuels prepared with varying levels of oxygenate addition. The specific oxygenates investigated were dimethoxy methane, diethyl ether, dimethyl ether, monoglyme, diglyme, and ethanol. Experimental results demonstrate that oxygenates are capable of reducing PM emissions to an extent (% reduction) greater than their amount of addition (% of fuel by volume). Observed reductions were well correlated to the overall oxygen content of the blended fuels—PM was reduced by about 3.5% for each 1% of fuel oxygen by mass. In all cases, nitrogen oxide (NO_x) emissions decreased slightly or remained unchanged.; A subset of experiments performed with a scanning mobility particle sizer showed that oxygenated fuels produce a similar particle size distribution to that of conventional diesel fuel, with no significant increase in the number density of ultrafine (<100 nm) particles. Radioisotope tracing conducted with ethanol blends revealed that ethanol carbon participates in PM formation, but is about half as likely to contribute to PM when compared to carbon originating from the diesel portion of the fuel.; Results of numerical modeling indicate that oxygenated fuels suppress the production of soot precursors through several key mechanisms. Pyrolysis and combustion products change as oxygenate level increases and the long carbon chains that make up diesel fuel are displaced. In addition, high radical concentrations generated by oxygenate addition limit aromatic ring growth and promote the oxidation of carbon to CO₂, limiting carbon availability for soot precursor formation. High OH concentrations further serve to limit aromatic ring growth and soot particle inception.