Improving Ethanol Life Cycle Energy Efficiency by Direct Utilization of Wet Ethanol in HCCI Engines

Homogenous Charge Compression Ignition (HCCI) is a new engine technology with fundamental differences over conventional engines. HCCI engines are intrinsically fuel flexible and can run on low-grade fuels as long as the fuel can be heated to the point of ignition. In particular, HCCI engines can run on “wet ethanol:” ethanol-in-water mixtures with high concentration of water, such as the high water content ethanol-in-water mixture that results from fermentation of corn mash. Considering that much of the energy required for processing fermented ethanol is spent in distillation and dehydration, direct use of wet ethanol in HCCI engines considerably shifts the energy balance in favor of ethanol.

The results of the paper show that an HCCI engine with efficient heat recovery can operate on a 35-65% volumetric mixture of ethanol and water while achieving a high brake thermal efficiency (38.7%) and very low NOx (1.6 parts per million, clean enough to meet any existing or oncoming emissions standards). Direct utilization of ethanol at a 35% volume fraction reduces water separation cost to only 3% of the energy of ethanol and coproducts (vs. 37% for producing pure ethanol), and improves the net energy gain from 21% to 55% of the energy of ethanol and coproducts. Wet ethanol utilization is a promising concept that merits more detailed analysis and experimental evaluation.