Research progress of higher alcohols as alternative fuels for compression ignition engines

Higher alcohols are a new generation of biofuels with advantages of renewability and carbon neutrality. This review comprehensively summarized the research progress of higher linear alcohols and aromatic alcohols as alternative fuels for IC engines, mainly including: the effect of binary or ternary mixtures of higher alcohols mixed with diesel, biodiesel, waste oil, biodiesel-diesel, waste oil-diesel, and lower alcohols-diesel on the performance, combustion and emissions of CI engines; the progress of RCCI and PCCI engine using higher alcohols in dual-fuel mode; and the improvement of higher-alcohol engines by adding cetane number improvers, adjusting EGR rate and injection strategy. Higher alcohols addition can significantly reduce PM emissions, improve NOxPM trade-off, and maintain BTE equivalent to diesel, but usually increases BSFC. The effect of higher alcohols addition on CO, HC, and NOx emissions is relatively complex, and the effect of adding higher alcohols to biodiesel and waste oil on CI engine is different from that of adding them to diesel. Higher alcohols are generally inferior to lower alcohols in improving diesel emissions, but they exhibit more significant advantages in improving biodiesel performance and emissions compared to lower alcohols. In the dual-fuel mode of higher alcohols, RCCI engines can simultaneously reduce PM and NOx emissions, and improve engine performance to a certain extent, but the advantage of improving efficiency is related to the injection ratio of higher alcohols. Increasing injection advance angle, introducing ERG, adopting multiple injection and hydrogen addition can be used to further improve performance or emissions of higher-alcohol RCCI engines.