Soot formation in OMEn combustion in an optically accessible CI engine

Abstract

Oxymethylene dimethyl ether (OME_n), if produced from a renewable source, can reduce “well-to-wheel” CO₂ emissions. The literature reports that diesel-blended fuel can overcome the soot-NOx trade-off while neat OME_n combustion in CI engines is smokeless. The in-cylinder optical imaging described in the recent literature revealed that no bright luminous flames are observed for neat OME_n combustion under diesel-based exposure settings. On the other hand, some exhaust emissions tests reported that particulate matter can be detected after neat OME_n combustion. In this study, the conditions under which soot is formed from neat OME_3-5 combustion in a diesel engine were examined. The authors used in-cylinder imaging, 3D-CFD simulation, and 0D detailed chemical reaction calculation. Two cases of pilot fuel injection were compared; a base case (“Base”), with a small amount of pilot fuel, and a second case (“Soot”), in which the pilot fuel amount was quadrupled to raise the in-cylinder average temperature at the beginning of the main injection. The AVL-FIRE with the improved ECFM-3Z+ combustion model was used to correctly determine the trajectory of the fuel in the rich region on the φ-T map. A detailed decomposition reaction model for OME₃ based on molecular dynamics and first-principles calculation was combined with detailed soot formation/oxidation models to determine the PAH formation on the grid points of the φ-T map. In-cylinder imaging showed that luminous flames appeared in the Soot case but did not appear in the Base case. The calculated equivalence ratio and temperature in each cell were plotted on the φ-T map of acepyrene formation determined by the 0D detailed chemical reaction calculation. In the Soot case, PAH is formed from OME_3-5 combustion if the trajectory of the fuel crosses the acepyrene formation region, while the trajectory of the fuel in the Base case does not cross the acepyrene formation region.