Investigation of a tripropylene-glycol monomethyl ether and diesel blend for soot-free combustion in an optical direct-injection diesel engine

•TPGME addition to ultra-low-sulfur diesel fuel diesel effectively eliminated engine-out smoke emissions.•TPGME curtailed soot formation and/or increased soot oxidation.•TPGME addition alone was insufficient to enable leaner lifted-flame combustion. Natural luminosity and chemiluminescence imaging diagnostics were employed to investigate if a 50/50 blend by volume of tripropylene-glycol monomethyl ether (TPGME) and ultra-low sulfur #2 diesel certification fuel (CF) could enable leaner-lifted flame combustion (LLFC), a non-sooting mode of mixing-controlled combustion associated with equivalence ratios below approximately 2. The experiments were performed in a single-cylinder heavy-duty optical compression–ignition engine at three injection pressures and three dilution levels. Results indicate that TPGME addition effectively eliminated engine-out smoke emissions by curtailing soot production and/or increasing soot oxidation during and after the end of fuel injection. TPGME greatly reduced soot luminosity when compared with neat CF, but did not enable LLFC because the equivalence ratios at the lift-off length, ϕ(H), never reached the non-sooting limit and incandescence from hot soot within the combustion chambered remained visible. Concerning other engine-out emissions, injection pressure influenced the effects of TPGME addition on NOx emissions. HC and CO emissions were higher compared to the baseline fuel, likely due to the lower net heat of combustion of TPGME and the need to limit fuel-injection duration for valid optical measurements.