Investigation of the Use of High-Cetane Mono-ether Bioblendstocks to Improve Catalyst Heating and Warmed Up Operation of MCCI Engines

Subramanian, S. Investigation of the Use of High-Cetane Mono-Ether Bioblendstocks to Improve Catalyst Heating and Warmed Up Operation of MCCI Engines. University of Wisconsin-Madison, 2024.

The use of low-carbon intensity second-generation biofuels in applications where conventional diesel is the predominant source of power can potentially mitigate their environmental impact and contribute to sustainable energy solutions. This work focused on one such biofuel, a C8+ mono-ether bioblendstock derived from catalytic upgrading of ethanol. The long chain mono-ether components have very high cetane number, a lower boiling point range, and higher oxygen content than conventional diesel. Based on these properties, three hypothesis were formulated at the start of the work: 1) increasing the cetane number of #2 diesel fuel through blending with mono-ether components will enable increased enthalpy flux (more rapid heating) and/or lower emissions for catalyst thermal management, 2) blending the mono-ether bioblendstock components with #2 diesel fuel would enable reduced soot emissions even with an increase in the fuel cetane number, and 3) blending the mono-ether bioblendstock components with #2 diesel fuel will not negatively impact warmed-up operation efficiency and emissions. To study these hypotheses, the impact of variations in fuel properties on catalyst thermal management and warmed-up engine operation was evaluated in a single-cylinder CIDI metal engine. The effect of boiling point differences, at matched cetane number, was investigated during catalyst thermal management operation using a three-injection strategy. In-cylinder pressure and engine-out emissions data, analyzed at different post-injection timings, showed little sensitivity to changes in the boiling points of the fuels studied. Using the same injection strategy, at matched operating conditions, increased reactivity of the fuel resulted in higher thermal and combustion efficiency when compared to the baseline #2 diesel fuel. However, the more advanced combustion phasing for the higher reactivity fuel resulted in lower exhaust enthalpy flux for a matched operating condition. A simpler two-injection strategy was used to demonstrate that exhaust enthalpy flux could be increased by ∼ 12% for a mono-ether #2 diesel blend with a derived cetane number (DCN) of 60 while matching nitrogen oxide (NOx) emissions and combustion efficiency to that of a #2 diesel fuel with a DCN of 41. The increased ability to delay the injection timing with the higher CN fuel was leveraged to operate at a lower EGR rate while meeting both NOx and combustion efficiency constraints. This resulted in higher exhaust mass flow and exhaust temperatures, relative to #2 diesel. The DCN 60 fuel was also able to be run with similar exhaust enthalpy flux as the #2 diesel fuel but at significantly lower NOx (up to 40% lower). For warmed-up conditions, the diesel-ether fuel blend (CN60) showed reduction in engine-out soot emissions relative to #2 diesel fuel at high EGR rates for three out of the four load-speed conditions (A75, C50, and C75) investigated. A combination of lower sooting tendency (lower YSI) and higher oxygen concentration at the lift-off length for the high CN oxygenated fuel blend is believed to contribute to the ability to tolerate higher EGR flow, without a large soot emissions penalty. Future experiments in an optical engine are recommended to measure the in-cylinder soot formation and lift-off length to determine the exact source of this reduction in engine-out soot emissions. Finally, efficiency and emissions were similar or improved for all for warmed-up load-speed operating points studied. The results of this work demonstrate the potential for improvements in efficiency and emissions enabled by blending with high-cetane number bioblendstocks that can be derived from catalytic upgrading of ethanol.