PRECOMBUSTION OF FATTY-ACIDS AND ESTERS OF BIODIESEL - A POSSIBLE EXPLANATION FOR DIFFERING CETANE NUMBERS

Fatty acids of C-18 chainlength as well as their methyl, ethyl, n-prop yl, and n-butyl esters were injected into a constant-volume combustion apparatus suitable for collecting material from the fuel spray prior to the onset of ignition. The collected material from this precombusti on phase of the injection event was analyzed by gas chromatography-mas s spectrometry. Compounds identified as forming during the precombusti on phase were straight-chain and branched alkanes, alkenes, and cyclic hydrocarbons, as well as aldehydes, ketones, esters, substituted benz enes, and other species, such as furans. Some of the compounds formed during precombustion have low cetane numbers (CN). Low-cetane aromatic compounds were found more prominently for more unsaturated fatty comp ounds. Thus, the low CN of the intermediary precombustion species may constitute a possible partial explanation why some compounds, for exam ple the more unsaturated fatty compounds, have relatively low CN.