Effects of Ethanol Blending on the Reactivity and Laminar Flame Speeds of Gasoline, Methanol-to-Gasoline, and Ethanol-to-Gasoline Surrogates

Abstract

<div class="section abstract"><div class="htmlview paragraph">Ethanol blending is one method that can be used to reduce knock in spark ignition engines by decreasing the autoignition reactivity of the fuel and modifying its laminar flame speed. In this paper, the effects of ethanol blending on knock propensity and flame speed of petroleum and low-carbon gasoline fuels is analyzed. To do so, surrogate fuels were formulated for methanol-to-gasoline (MTG) and ethanol-to-gasoline (ETG) based on the fuels’ composition, octane number, and select physical properties; and 0-D and 1-D chemical kinetics simulations were performed to investigate reactivity and laminar flame speed, respectively. Results of MTG and ETG were compared against those of PACE-20, a well-characterized surrogate for regular E10 gasoline.</div><div class="htmlview paragraph">Similarly to PACE-20, blending MTG and ETG with ethanol increases the fuel’s research octane number (RON) and sensitivity. The trends of the ethanol blending effects were slightly stronger with PACE-20 and MTG than with ETG, with 13.6% volume of ethanol necessary to reach a RON of 98 for MTG and 18.4% volume necessary for ETG. 1-D modeling of the flame speed showed that while ethanol has a faster flame speed than gasoline at pressures below 2.4 bar, the flame speed decreases at increasing pressure, with regular gasoline having a higher flame speed at pressures representative of combustion. Sensitivity analyses to identify the reactions and species relevant in controlling laminar flame speed showed that for ethanol, the active radicals in the flame decreased as pressure increased due to increasing methyl recombination leading to a decrease of the flame speed. For regular gasoline, the formation of active radicals increased with pressure due to increasing HCO decomposition leading to an increase in the flame speed.</div></div>