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.

Automated summary

This study identifies the optimal next generation stratified low temperature combustion (LTC) strategy for various biosynthesized alcohol fuels by analyzing how local ignition delays respond to stratification induced by fuel injection. Two groups of next generation stratified LTC modes are generalized based on the heat release process, with C1-C4 alcohols suitable for lean-to-rich burn stratified combustion and C5+ alcohols suitable for rich-to-lean burn stratified combustion. Methanol or water-alcohol blends are found to be the best candidates for lean-to-rich burn stratified combustion, while n-pentanol, n-hexanol, and hexanol isomers are identified as the best candidates for rich-to-lean burn stratified combustion.