Influence of Carbonate Ester Molecular Structure on Compression Ignition Combustion and Emissions

Abstract

A primary motivation for the development of future fuels is to halt the increasing concentration of atmospheric carbon dioxide. As such, a fuel that can potentially be produced from a feedstock of alcohols and atmospheric carbon dioxide is an attractive proposition. Symmetrical carbonate esters may thus offer a sustainable means of converting alcohols to larger molecules that are potentially better suited to use in compression ignition engines. This paper presents experimental investigations carried out on a compression ignition engine fuelled with a range of binary fuel mixtures to determine the effect of carbonate molecular structure on combustion and exhaust emissions. Six symmetrical carbonate esters, dimethyl carbonate, diethyl carbonate, di-n-butyl carbonate, di-1-methyl-propyl carbonate, di-2-methyl-propyl carbonate, and ditert-butyl carbonate, one ester, butyl valerate, and one ketone, 5-nonanone, were investigated so as to ascertain the way in which the alkyl chain length, branching in the alkyl chain, and molar oxygen content of similar molecules affects compression ignition combustion and emissions. The engine tests of the fuels were carried out as binary mixtures with 30% (w/w) n-decane, as they were found not to combust in a steady manner as single component fuels; dimethyl and diethyl carbonate were also tested as binary mixtures with n-decane at a range of proportions. All fuels were initially tested at constant injection and constant ignition timings and repeated at two constant ignition delay timings, which were achieved through the addition of small quantities of ignition improver (2-ethylhexyl nitrate). Relative to di-n-butyl carbonate, decreasing the alkyl chain length and also introducing methyl branches to the alkyl moiety increased the duration of ignition delay, while decreasing the molar oxygen content to form butyl valerate and 5-nonone decreased the ignition delay. The respective changes in ignition delay with molecular structure were attributed to the importance of the alkyl moieties of the fuels tested in determining low temperature reactivity. When the dimethyl carbonate or diethyl carbonate content of blends with n-decane was increased, levels of peak heat released increased linearly, suggesting an influence of ignition delay and the carbonate physical properties on the extent of fuel and air premixing. At constant ignition delay timings, an effect of decreasing particulate matter and CO emissions with an increasing level of fuel bound oxygen was observed, while at all conditions NOx emissions were most significantly influenced by in-cylinder thermal conditions.