Influence of Iso-Butanol Blending with a Reference Gasoline and Its Surrogate on Spark-Ignition Engine Performance

This study investigated the ability of a five-component gasoline surrogate (iso-octane, toluene, n-heptane, 1-hexene, and ethanol) to replicate the combustion and knocking behavior of a reference gasoline (PR5801 - RON 95.4, MON 86.6), under pressure boosted spark-ignition engine conditions at various levels of blending with iso-butanol. The ability of the neat surrogate was first evaluated for stoichiometric air/fuel mixtures, at an intake temperature and pressure of 320 K and 1.6 bar, respectively, and an end of compression pressure of 30 bar, over a range of spark discharge timings. Throughout this regime, the surrogate was found to produce a good representation of the gasoline, particularly in terms of mean engine cycle properties, knock onsets, and knock intensities. This high degree of similarity between the surrogate and gasoline has also seen previous rapid compression machine work, at comparable end-gas temperatures (Michelbach and Tomlin Int. J. Chem. Kinet. 2021, 53 (6), 787-808, DOI: 10.1002/kin. 21483). However, significant differences were observed between the cyclic variability of surrogate and gasoline results, which was attributed to compositional differences between the two fuels. This study also investigated the impact of iso-butanol blending (at ratios of 5-70% iso-butanol by volume) on the performance of the gasoline at knocking and nonknocking conditions, as well as the ability of the surrogate to replicate the observed blending behavior, at the same experimental conditions. In general, increasing the iso-butanol volume was shown to decrease the knocking propensity of the fuel, except for a nonlinear crossover behavior witnessed for 5% and 10% iso-butanol blends, wherein the 5% blend became less reactive than the 10% blend due to the heavy suppression of NTC behavior in the 10% blend. Even at such low concentrations, iso-butanol appears to act as a strong radical sink, as identified by brute-force sensitivity analysis of predicted knock onsets. This is consistent with the findings of the aforementioned rapid compression machine study. Blends of 20-50% iso-butanol were found to be optimal for use in SI engines, providing considerable antiknock benefits and comparable indicated power to gasoline, with blends of 20-30% being the most viable due to the lower quantities of biofuel required. Under blending with iso-butanol, the surrogate continued to perform well, but blends were observably less reactive than the corresponding gasoline blends at spark advance timings <8 degrees CA before top dead center. The consistency found between trends within the literature sourced rapid compression machine measurements, and engine data presented in this study highlight the proficiency of fundamental measurements in predicting combustion behavior within an engine at similar thermodynamic conditions.