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Ethanol, n-propanol, isopropanol, n-butanol, isobutanol, and sec-butanol are six potential carbon-neutral fuels of the future. One application of these carbon-neutral fuels is in high-dilution spark ignition. To understand the potential of these fuels in high-dilution spark ignition, this work experimentally determines the spark ignition lean flammability limit of each fuel with no external, cooled exhaust gas recirculation (EGR), and with 20% external, EGR. The spark ignition lean flammability limit in this work is defined as the excess-air ratio that results in a coefficient of variance of gross indicated mean effective pressure greater than 5%. This is done on an engine with a compression ratio of 12.5, using an intake pressure of 1 bar and an intake temperature of 320 K. It was found that ethanol had the leanest lean limit, due to its high flame speed, followed by n-propanol, isopropanol, and sec-butanol, which all had similar lean limits. n-Butanol and isobutanol had the richest lean limits due their high knock propensity and low flame speed, respectively. The lean limit of each fuel decreased with external, cooled EGR addition, with ethanol as the least sensitive and isopropanol as the most sensitive to EGR addition. Overall, using a high dilution strategy increased the cycle efficiency for each fuel. Ethanol, n-propanol, isopropanol, and sec-butanol all showed promising performance and are great candidates to be combined with an advanced high-dilution SI strategy to enable high-dilution SI with a carbon-neutral fuel.

Wet ethanol is a biofuel with unique fuel properties: a high latent heat of vaporization and low equivalence ratio sensitivity. These two fuel properties enable control over the heat release process in low temperature combustion through the enhancement of in-cylinder thermal stratification. In this work, it is shown that the direct injection of wet ethanol 80 (80% ethanol, 20% water, by mass) during the intake stroke on a mid-compression ratio engine with a side-mounted gasoline direct injector does not fully premix with incoming air, leaving stratification in the cylinder which elongates the combustion process, enabling high-load low temperature combustion without engine-damaging knock. As a result, the load under naturally aspirated conditions was only limited by oxygen content as the global equivalence ratio approached stoichiometry. The results show that swirl can not only modulate the induced stratification effectively by increasing the amount of bulk motion in the cylinder but also cause a thermal efficiency penalty, since swirl increases heat transfer losses. Modulation over the induced stratification can be achieved without an efficiency penalty by employing either a dual injector system where a fraction of the total fuel is injected into the port or a split injection strategy where a fraction of the total fuel is injected during the compression stroke. This strategy enables high-load low temperature combustion, while reducing the number of regions that form NOx and avoiding excessively rich regions that contribute to combustion inefficiency.