Soot and chemiluminescence in diesel combustion of bio-derived, oxygenated and reference fuels

Abstract

High-speed imaging, spectroscopy and thermodynamical characterization are applied to an optically accessible, heavy-duty diesel engine in order to compare sooting and chemiluminescence behaviour of bio-derived, oxygenated fuels and various reference fuels. The fuels concerned include the bio-derived fuels Jatropha oil (pure), Jatropha-methylester (JME) and rapeseed-methylester (RME), all containing ≈10% mass oxygen, and fossil oxygenated fuels tripropyleneglycolmonomethylether (TPGME) and cyclohexanone, both mixed with synthetic fuel to have similar total oxygen content. Commercial diesel, model fuel IDEA, a Fischer–Tropsch synthetic fuel (FT) and a blend containing cyclohexane serve as reference fuels. The start of hot combustion is derived from OH ∗ and CH ∗ chemiluminescence as observed through a spectrograph. Both species occur at about the same time, CH ∗ signal being weaker. Soot incandescence is observed two-dimensionally at 0.3° crank angle resolution (200 images/cycle). Ignition delays derived from soot incandescence and chemiluminescence are compared to those determined from heat release. The heat release rate and exhaust NO concentrations are used as indicators of average and peak temperatures, respectively, which are combined with soot incandescence signal to get a relative measure for a fuel’s sooting propensity. Both the fuel molecular structure and its oxygen content are found to be important for soot abatement.