A Comparison of Methyl Decanoate and Tripropylene Glycol Monomethyl Ether for Soot-Free Combustion in an Optical Direct-Injection Diesel Engine

Oxygenated fuels have been reported to have beneficial effects for leaner lifted-flame combustion (LLFC), a non-sooting mode of mixing-controlled combustion associated with lift-off length equivalence ratios below approximately 2. A single-cylinder heavy-duty optical compression-ignition engine was used to compare two oxygenated fuels: neat methyl decanoate (MD) and T50, a 50/50 blend by volume of tripropylene glycol monomethyl ether (TPGME) and #2 ultra-low sulfur emissions-certification diesel fuel (CF). High-speed, simultaneous imaging of natural luminosity and chemiluminescence were employed to investigate the ignition, combustion, and soot formation/oxidation processes at two injection pressures and three dilution levels. Additional Mie scattering measurements were employed to observe fuel-property effects on the liquid length of the injected spray. Results indicate that both MD and T50 reduced considerably the engine-out smoke emissions by decreasing soot formation and/or increasing soot oxidation during and after the end of fuel injection. MD further reduced soot emissions by 50–90% compared with T50, because TPGME could not completely compensate for the aromatics in the CF. Despite the low engine-out soot emissions, both fuels produced in-cylinder soot because the equivalence ratio at the lift-off length never reached the non-sooting limit. With respect to the other engine-out emissions, T50 had up to 16% higher NO_x emissions compared with MD, but neither fuel showed the traditional soot-NO_x trade-off associated with conventional mixing-controlled combustion. In addition, T50 had up to 15% and 26% lower unburned hydrocarbons (HC) and carbon monoxide (CO) emissions, respectively, compared with MD.