To mitigate global warming, many countries are pursuing carbon neutrality. Reducing CO₂ emissions from vehicles requires the adoption of electrification technologies in hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), as well as improving overall system efficiency by improving the thermal efficiency of internal combustion engines (ICEs). One approach to improving the thermal efficiency of ICEs is lean-burn combustion. Additionally, biofuels and synthetic fuels are expected to contribute to CO₂ reduction, even for existing vehicles, and various studies are underway to promote their adoption. Ethanol, a bio-derived fuel, is widely used in varying contents worldwide, and its further utilization is anticipated.
Therefore, it is essential to clarify combustion characteristics at different ethanol contents; however, research under lean-burn conditions remains limited. In this study, a super-lean-burn engine was used to evaluate the effects of ethanol contents on emissions, thermal efficiency, knocking, and combustion speed.
Experimental results showed that higher ethanol contents reduced NOx emissions due to lower adiabatic flame temperatures. For THC emissions, a decreasing trend was observed in FID measurements; however, after correcting for reduced detection sensitivity to ethanol and aldehydes, no significant differences were found. Thermal efficiency improved with higher ethanol content owing to reduced cooling losses. Regarding combustion speed, at λ = 2.5 and peak in-cylinder pressures above 9 MPa, the 50–90% combustion duration was extended. This was attributed to inhibited ethyl radical formation, as well as the increased influence of third body reactions under high-pressure and high-λ conditions, which enhanced the influence of the reaction pathway in which methyl radical recombination consuming hydrogen radicals to generate methane. Knocking was mitigated at λ = 2.5 due to higher RON with increased ethanol content; however, at λ = 1, knocking was more likely to occur. These mechanisms are discussed in detail.