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-propyl, 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 precombustion phase of the injection event was analyzed by gas chromatography-mass spectrometry. Compounds identified as forming during the precombustion phase were straight-chain and branched alkanes, alkenes, and cyclic hydrocarbons, as well as aldehydes, ketones, esters, substituted benzenes, 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 compounds. Thus, the low CN of the intermediary precombustion species may constitute a possible partial explanation why some compounds, for example the more unsaturated fatty compounds, have relatively low CN.