The Influence of Charge-Gas Dilution and Temperature on DI Diesel Combustion Processes Using a Short-Ignition-Delay, Oxygenated Fuel

The influence of nitrogen dilution and charge-gas temperature on in-cylinder combustion processes and engine-out NO_x and smoke emissions was investigated in an optically accessible heavy-duty DI diesel engine using a high-cetane-number, oxygenated fuel. Engine-out measurements of NO_x and smoke emissions and in-cylinder images of natural luminosity were obtained for charge-gas oxygen concentrations from 9% to 21% and TDC charge-gas temperatures of 680 and 880 K. Charge-gas temperature was found to have a significant influence on engine-out NO_x emissions, but NO_x emissions levels less than 0.2 g/ihp-hr were achieved at both the 680 and 880 K charge-gas temperatures within the investigated range of oxygen concentrations. An indicated engine-out NO_x emissions level of 0.09 g/ihp-hr at 18 bar IMEP was achieved using charge-gas dilution and 3.0 bar intake boost pressure. The proportion of NO₂ to NO emissions increased with decreasing oxygen concentration, with NO₂ reaching 81% of NO_x emissions at an oxygen concentration of 12%. The increasing fraction of NO₂ with decreasing oxygen concentration is attributed to increased quenching of NO₂-to-NO reactions due to decreasing flame temperatures. Flame lift-off lengths were measured using in-cylinder images of natural luminosity. The measured flame lift-off lengths and estimated charge-gas conditions were used to determine the local mixture stoichiometry at the flame lift-off length. The results show that soot incandescence can be negligible for fuel-rich local mixture stoichiometries that would result in intense soot incandescence under undiluted operating conditions. It is hypothesized that flame temperatures and/or residence times are too small for soot inception under highly dilute charge-gas conditions. Reduced flame temperatures also potentially explain the low measured NO_x emissions levels.