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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.

Strict regulations on fuel economy are driving modern gasoline engines to adopt more advanced technologies to improve thermal efficiency. Some of the technologies, for example ultra-high compression ratio and spark assisted compression ignition, will elevate the thermodynamic condition near top dead center (TDC) to a considerably high level, even up to or beyond the negative temperature coefficient (NTC) region. This will definitely increase knock tendency when the combustion is not well controlled. Previous knock-related research mainly focused on temperature ranges in/below the NTC region, while the knock combustion beyond the NTC region has rarely been studied. To understand the knock behavior beyond the NTC region, in this study the flame propagation process and end-gas auto-ignition of iso-octane under wide thermodynamic conditions across the NTC region were optically studied using dual-camera photography. The results showed that the flame propagation speed increased with increasing initial temperature and decreasing initial pressure, exhibiting no NTC characteristic. With the intervention of flame propagation, the residence time of the end-gas was shortened as the initial thermodynamic conditions were promoted, indicating no NTC behavior in the overall ignition delay time of the end-gas. Two kinds of detonation initiation processes were identified. In the cases strongly affected by low temperature chemistry (LTC), the auto-ignition showed a two-stage characteristic during which a widespread but relatively weak auto-ignition (first-stage) was observed prior to the final detonation initiation. In contrast, when the LTC was absent, the detonation was initiated directly in a single auto-ignition event. Lower initial energy densities were needed to initiate detonation in the cases less affected by LTC. Thermodynamic analyses based on Bradley's e-ξ diagram showed that, for the LTC-affected cases, the pressure rise which resulted from the widespread weak first-stage auto-ignition had vital impacts on the final detonation initiation by shifting the e-ξ location into or away from the detonation region. Finally, thermal diffusivity was demonstrated to be capable of distinguishing detonation from other combustion modes as detonation tended to occur with lower thermal diffusivities of the mixture.