Impact of lower and higher alcohol additions to diesel on the combustion and emissions of a direct-injection diesel engine

The present investigation evaluated the combustion, performance, and emissions of four alcohol diesel blends with the same oxygen content, i.e., 13% ethanol (E13), 20% n-butanol (NB20), 20% iso-butanol (IB20), and 25% n-pentanol (P25) by volume on a 4-cylinder direct-injection diesel engine under three different engine loads, respectively. Compared with diesel, higher peak heat release rate, longer ignition delay, and shorter combustion duration have been observed for the alcohol blends; the variations are more evident for higher alcohol blends (i.e., NB20, IB20, and P25) compared with lower alcohol blend (E13), and the most evident one is IB20. Higher premixed combustion fraction and higher displacement of the diesel fuel for the higher alcohol blends suppressing the formation of the soot precursors result in a lower peak of particle size distribution, and therefore, a lower total particle number emission than that of lower alcohol blend. For the three higher alcohol blends, IB20 presents the lowest particle number emission corresponding to its longest ignition delay and highest premixed combustion fraction which inhibits soot formation. The sequence of elemental carbon emission for the alcohol blends is (from lowest to highest): IB20 < NB20 < P25 < E13, which is in line with that of peak of particle size distribution and the total particle number emission. The organic carbon emissions with different alcohol additions show similar levels because of the factors' conflict. Compared with diesel, all blends show a slight variation or no significant change in regulated gaseous emissions (CO, HC, NOx) at different loads.