Exploring the efficacy of ether-based ignition enhancers for implementation of (M) ethanol mixing-controlled compression ignition combustion

Abstract

Methanol and ethanol are a promising alternative to fossil fuels for the decarbonization of the off-road sectors. Although (m)ethanol are typically seen as fuels for spark ignition engines, mixing-controlled compression ignition (MCCI) combustion is typically preferred in most off-road and medium-and heavy-duty. However, low ignitability presents a major challenge. Blending reactive ignition enhancers, such as 2-ethylhexyl nitrate (EHN), has enabled stable MCCI operation at 1200 rpm & 3.91 bar IMEPn and 1600 rpm & 8.55 bar IMEPn. However, the nitrate group in EHN adversely affects NOx emissions, leading to an 80.9% increase in engine-out NOx compared to non-nitrate enhancers. Conversely, ether-based molecules show significant potential as ignition enhancers due to their high reactivity, and they can be directly produced from renewable sources. To evaluate their effectiveness, experiments and chemical kinetic simulations were performed using ANSYS CHEMKIN-PRO; and results were compared against previous data obtained with EHN. Compared to case with 7%_vol EHN, achieving similar ignition characteristics required significantly higher concentrations of ether-based enhancers up to 55%_vol DME and 35%_vol DEE. This is attributed to the limited effectiveness of the methyl, ethyl, and H radicals produced by ethers in promoting alcohol combustion, compared to the highly reactive OH radicals generated by EHN. Moreover, ethers formed radicals late during the ignition delay time whereas EHN decomposed rapidly, thereby limiting the effect of ethers on (m)ethanol reactivity. Results suggested that it may be more advantageous to introduce ether-based ignition enhancers separately into the cylinder using a dual-fuel (m)ethanol/ether combustion strategy. This approach required only less than 10% energy-equivalent ethers addition to achieve stable ignition, which is even lower than the 11.6% energy-equivalent EHN addition required in the blend-fuel mode. Compression-ignition of port-injected ethers led to high-reactivity in-cylinder conditions to enable ignition of direct-injected (m)ethanol, providing a viable approach for (m)ethanol MCCI combustion.