Low temperature oxidation of biodiesel surrogates in a motored engine and the oxidation behavior of soot generated from the combustion of a biodiesel surrogate in a diffusion flame

As biodiesel becomes a viable alternative to petroleum-derived diesel fuels, there is a necessity to fundamentally understand the combustion and emissions characteristics of biodiesel. This study concerns the low temperature oxidation chemistry of biodiesel-relevant compounds and the impact of fuel-bound oxygen in fatty acid esters on soot oxidation behavior.;Low temperature oxidation of biodiesel surrogates (i.e., C7, C8, C10 and C11 esters with both saturated and unsaturated aliphatic chains) was studied in a motored cooperative fuels research (CFR) engine under fuel-lean conditions. Heat release results indicated that not only the presence of unsaturation in the aliphatic chain of fatty acid esters suppresses cool flame behavior, but also the suppression effect becomes more pronounced as the position of the double bond gets closer to the center of the aliphatic chain. Also, it was observed that cool flame behavior of fatty acid esters becomes stronger with the increase of the alkyl chain length.;Analyses of the intermediate species produced from low temperature oxidation of both saturated and unsaturated esters provided valuable insight into the major oxidation pathways of fatty acid esters at low to intermediate temperatures. It is observed that the abstraction of H-atoms on the α-carbon of the ester carbonyl group plays an important role in low temperature oxidation of saturated fatty acid esters. For unsaturated esters, under the current test conditions, the prominent feature of oxidation is the attack of radical species on the olefinic double bond. In addition, it was observed that a unique six-centered unimolecular elimination reaction channel may exist during the low temperature oxidation of fatty acid ethyl esters, yielding ethylene and alkyl acids.;In this study, soots generated from the combustion of methyl crotonate (MC) and n-pentane (PEN) in laminar co-flow diffusion flames were analyzed via various analytical techniques to determine the impact of fuel-bound oxygen in fatty acid esters on soot oxidation behavior.;Thermogravimetric analysis of the soot samples collected from the diffusion flames showed that MC soot exhibits only slightly higher oxidative reactivity than PEN soot, which indicates that the fuel-bound oxygen in MC may not be able to help to enhance soot oxidative reactivity to any significant extent under the well-defined diffusion flame test conditions with the test fuels having the same number of carbon atoms. Therefore, the remarkable difference in reactivity observed between biodiesel-derived soot and Fischer Tropsch (FT) diesel-derived soot in diesel engines is likely due to the difference in combustion characteristics and soot formation process between biodiesel and FT diesel under diesel combustion conditions.;Furthermore, structural analysis was performed on both MC soot and PEN soot. Based on the analysis of the XRD patterns of the two soots at different stages of oxidation, there is no notable difference in crystallite structural parameters between MC soot and PEN soot. Also, the analysis of the first-order Raman spectra of the two soots revealed that there is no statistically significant difference in the Id/Ig (i.e., the ratio of the integrated intensity of the D band relative to that of the G band) ratios between MC soot and PEN soot, indicating that MC soot is no more disordered than PEN soot. In addition, HRTEM images of the primary particles in unreacted MC and PEN soots showed that both PEN soot particles and MC soot particles exhibit a certain extent of graphitic structural arrangement. Moreover, there is no distinct difference in initial nanostructure observed between the two soots. Overall, structural analysis of the two soots via different analytical techniques consistently showed that there is no significant difference in structural arrangement between MC soot and PEN soot, which agrees with the lack of a notable difference in reactivity between the MC soot and PEN soot observed from the temperature-programmed oxidation experiments. (Abstract shortened by UMI.).