The role of alcohol biofuels in advanced combustion: An analysis

The production of alcohol fuels from bioderived feedstocks and the performance of next generation stratified low temperature combustion (LTC) modes for internal combustion engines are two research areas that have recently undergone rapid growth independently. Now, there is a need to bridge these two fields and identify the optimal combustion strategy for these biosynthesized alcohol fuels. This work addresses this need by using a metric called the normalized phi-sensitivity to analyze how the local ignition delays of regions in the combustion chamber respond to stratification induced by the compression stroke injection of fuel, thereby identifying the optimal next generation stratified LTC strategy for a number of biosynthesized alcohol fuels. The large set of next generation stratified LTC modes are generalized into two groups based on how the heat release process proceeds in the compositionally stratified combustion chamber: lean-to-rich burn stratified combustion, where the lean regions ignite first followed sequentially by progressively richer regions, or rich-to-lean burn stratified combustion, the inverse of lean-to-rich. The C1-C4 alcohols are prime candidates to enable lean-to-rich burn stratified combustion based on their high cooling potentials and lack of negative temperature coefficient (NTC) behavior. Specifically, methanol or water-alcohol blends were found to be the best candidates. The C5 + alcohols were found to be prime candidates to enable rich-to-lean burn combustion based on their emergent NTC-behavior and low cooling potentials that approach their alkane counterparts. Specifically, n-pentanol, n-hexanol, and hexanol isomers were found to be the best candidates based on balancing their normalized phi-sensitivity and their autoignition resistance.