The objective of this work was to investigate the relative particulate formation and knock-resistance of equal oxygen weight blends of ethanol, isobutanol, and 2-methyl-3-buten-2-ol with gasoline. Combustion experiments were conducted in a spark-ignition engine operated in premixed prevaporized and direct-injection modes to examine knocking behavior and particulate formation. Spray visualization experiments were performed to characterize the effects of alcohol addition on spray behavior.
For knock-limited combustion experiments, increasing alcohol content generally reduced knock. Overall, 2-methyl-3-buten-2-ol showed significantly improved knock-resistance compared to the other two alcohols. The results between fuel blends were similar for both combustion modes. Direct-injection operation yielded significant increases in knock resistance for higher-level blends. The results of these studies indicate that the order of knock-resistance of these three alcohols, for both operating modes, is isobutanol < ethanol < 2-methyl-3-buten-2-ol when compared on an equal oxygen weight basis.
The particulate experiments for premixed operation indicated that increases in alcohol content consistently decreased particulate formation for all alcohols tested. The decrease in particulate formation for each alcohol on a volume basis can be ordered isobutanol < 2-methyl-3-buten-2-ol < ethanol. On an oxygen weight percentage basis, the order is ethanol < isobutanol < 2-methyl-3-buten-2-ol. Initial particulate results for direct-injection operation were inconsistent. The most likely cause of the inconsistency was determined to be end of injection dribble from the fuel injector. The injector was replaced and, once consistent results could be obtained, the results indicated that 2-methyl-3-buten-2-ol showed the greatest reduction in engine-out particulates, followed by isobutanol and then ethanol on an equal oxygen weight basis.
Fuel sprays from a direct-injector were visualized with a high-speed camera utilizing a diffuse back-illuminated imaging technique. Fuel was injected into an environment representative of engine relevant conditions. Low-level blends of isobutanol and methylbutenol behaved nearly identically to the neat base gasoline due to their similar vapor pressures. Higher blends of isobutanol and 2-methyl-3-buten-2-ol reduced flash-boiling while ethanol blends increased it. It was found that blending isobutanol or 2-methyl-3-buten-2-ol into gasoline in lieu of ethanol can result in significant changes to spray behavior due to the change in vapor pressure of the fuel blend.