Soot formation in ethanol/gasoline fuel blend diffusion flames

The aim of this paper is to examine how adding ethanol to gasoline affects soot formation. This is currently an important question with respect to particulate emissions from gasoline powered motor vehicles, but in this paper the ethanol impact is examined in co-flow diffusion flames to decouple combustion chemistry from the effects of engine operating parameters. Soot size distributions are measured as a function of height above the burner for E0, E20, E50, and E85 blends. For all fuels, the size distributions evolve from a single nucleation mode low in the flame through a bimodal distribution at mid heights and finally a single accumulation mode. The soot agglomerates in the accumulation mode, exhibit a bipolar charge. The nucleation mode initially includes charged particles, but becomes electrically neutral with increasing height in the flame. Thermodesorber measurements reveal significant hydrocarbon condensation on nucleation mode particles. This is more extensive for E0, E20, and E50 fuels as compared to E85. In other respects as well, the flames fall into two classes: (1) E85 versus (2) E0, E20, and E50. These groups of flames are visibly distinct and exhibit quantitatively different trends in terms of the size and quantity of particulate matter. The E85 flame appears similar to an ethylene diffusion flame, whereas those in the second group are more akin to a benzene flame. The results are discussed with respect to their implications regarding the effects of ethanol blends on PM emissions from gasoline engines.