Experimental study of high compression ratio spark ignition with ethanol, ethanol-water blends, and methanol

Low carbon alternative fuels like ethanol and methanol are ideal candidates for spark ignition engines due to their fast laminar flame speeds, high volatility, and high auto-ignition resistance partially derived from their high cooling potentials. Previous work performed in a compression ignition engine found that wet ethanol 80 (80% ethanol, 20% water) and methanol behaved similarly in both kinetically-controlled and mixing-controlled compression ignition strategies due to their nearly identical cooling potential and combustion chemistry. In this work, the interchangeability of wet ethanol 80 and methanol was evaluated in a spark ignition engine with a compression ratio of 14.8. In addition, neat ethanol and hydrous ethanol (92% ethanol, 8% water by mass) were also studied to provide a complete comparison between ethanol-water blends and methanol. Experiments demonstrated that methanol's flame speed was faster than ethanol's and that increasing the water concentration in wet ethanol further slowed its flame speed. Using EGR dilution to slow the flame speed of methanol and ethanol to match the spark timing to CA10 duration of wet ethanol 80 required 9.6 % and 7.6 % EGR, respectively. Using air dilution required a 28 % and 24 % increase in air, respectively. The intake air temperature was then adjusted to match the knock-limited CA50 of methanol, hydrous ethanol, and wet ethanol with neat ethanol. Wet ethanol 80 required a 25 K increase in intake temperature compared to neat ethanol, which is much less than the 47 K difference in cooling potential. Contrarily, methanol required an increase of nearly 30 K beyond wet ethanol 80 ' s intake temperature despite having nearly the same cooling potential. These results highlight that methanol, which has a higher volatility (i.e., lower Reid vapor pressure) than either ethanol or wet ethanol, achieves a higher amount of charge cooling from intake stroke injections than the ethanol-based fuels. Thus, methanol was able to achieve significantly more spark advance than ethanol or wet ethanol. Methanol and ethanol or wet ethanol 80 do not behave interchangeably in high compression ratio spark ignition.