Research on ethanol and toluene's synergistic effects on auto-ignition and pressure dependences of flame speed for gasoline surrogates

Abstract

Spark-assisted compression ignition (SACI) has a promising potential to substantially improve engine's fuel efficiency. To this end, two exothermic stages in SACI combustion, flame propagation and auto-ignition, need to be well organized to increase control authority of bulk ignition timing especially in lean burn. In this study, three gasoline surrogates, namely EPRF, ETPRF and TPRF, formulated through blending ethanol/toluene with primary reference fuel (PRF) and having the same research octane number (RON) and octane sensitivity (S), were used to conduct experiments in a rapid compression machine (RCM) under lean engine-relevant conditions (10–30 bar and 722–862 K). Under different ethanol blending ratios, both ethanol's synergistic effect during auto-ignition and its stronger pressure dependence of flame speed (S_(Flame)) than toluene were observed. The ethanol's synergistic effect is mainly attributed to its more HO₂ production and then faster consumption by benzyl which results in more OH radical production. As for the stronger pressure dependence of S_(Flame) of ethanol, at 722 K, it is primarily determined by the stronger pressure dependence of H radical in EPRF's flame structure rather than the promotion effect from critical reactions on S_(Flame); while at 862 K, these two factors influence the pressure dependence of S_(Flame) simultaneously. Whatever the temperature is, third-body reactions have larger impacts on ethanol's S_(Flame) than on toluene's. In this study, the relative magnitude of S_(Flame)'s pressure dependence between ethanol and toluene shows rationality at lower φ and higher T, which is in line with the pressure exponents extracted from the existing high-p laminar burning velocities of ethanol and toluene. Further verification was made in a spark-ignition engine, which showed that low-carbon alcohols, exhibited stronger pressure dependence of S_(Flame) than monophenyl aromatics in commercial gasoline, represented by toluene. The aforementioned characteristics of ethanol can be utilized under different engine loads and provide a reference in fuel design for lean SACI combustion.

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