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

Abstract

<div class="htmlview paragraph">The influence of nitrogen dilution and charge-gas temperature on in-cylinder combustion processes and engine-out NO<sub>x</sub> 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<sub>x</sub> 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<sub>x</sub> emissions, but NO<sub>x</sub> 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<sub>x</sub> 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 NO<sub>2</sub> to NO emissions increased with decreasing oxygen concentration, with NO<sub>2</sub> reaching 81% of NO<sub>x</sub> emissions at an oxygen concentration of 12%. The increasing fraction of NO<sub>2</sub> with decreasing oxygen concentration is attributed to increased quenching of NO<sub>2</sub>-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<sub>x</sub> emissions levels.</div>