Auto-ignition propensity of ethanol-, isobutanol-, and 2-methyl-3-buten-2-ol-gasoline blends under premixed prevaporized conditions in a spark-ignition engine

Abstract

The effects of alcohol addition to gasoline on knock-limited operation were studied in an internal combustion engine. Ethanol, isobutanol (2-methyl-1-propanol), and methylbutenol (2-methyl-3-buten-2-ol) were splash-blended with a gasoline blendstock for oxygenate blending (BOB) and supplied to the engine in a well-mixed gaseous state. By operating the engine in this way, the chemical anti-knock properties of the fuel could be studied without including physical effects such as evaporative cooling. Each alcohol was blended with the BOB to make three blends. Ethanol was blended at 10, 20, and 30% by volume (E10, E20, and E30). Isobutanol and methylbutenol blends were matched to ethanol on an oxygen weight percentage basis resulting in blends of 16, 32, and 49% (I16, I32, and I49) and 18, 37, and 56% (M18, M37, and M56) by volume for isobutanol and methylbutenol, respectively. Additional fuels tested for reference included the neat BOB, a Tier 3 E10 emissions certification gasoline as well as primary reference fuels (PRF) with octane ratings of 87 (PRF87) and 100 (PRF100). The engine was operated at a fixed speed and stoichiometric equivalence ratio while knock-limited spark advance was located across a range of loads. At low blend levels, ethanol and methylbutenol blends exhibited similar knock-limited combustion phasing, slightly advanced from isobutanol. At mid-blend levels, the difference in combustion phasing of methylbutenol and ethanol over isobutanol increased further. At the highest blend level, ethanol and isobutanol showed approximately equal performance. Conversely, methylbutenol was able to operate at significantly advanced combustion phasing relative to the other two alcohols. At loads above 900 kPa gross indicated mean effective pressure (GIMEP), M56 exceeded the knock-limited combustion phasing of PRF100. The overall results of this study show that when compared on an oxygen weight percent basis, the order of chemical auto-ignition resistance for the three alcohols is isobutanol < ethanol < methylbutenol, with methylbutenol matching or exceeding ethanol’s knock-limited performance across the load range tested.