Towards characterizing the effect of sustainable gasoline additives on the low-T reactivity of n-heptane using CO speciation in a shock tube

The pursuit of global sustainability goals requires development of engines capable of operating with sustainable fuels, including alcohols. To facilitate this advancement, it is essential to comprehensively understand the combustion characteristics of these alternative fuels, particularly in the low-temperature (low-T) regime. In this study, we investigate the influence of three sustainable alcohols (ethanol, isobutanol and prenol), blended with n-heptane to varying extents, on the overall combustion behavior of the fuel during low-T oxidation (700–1000 K) under engine-relevant pressures of 25–27 atm. To assess the overall reactivity of the fuel blends, we perform high-fidelity, time-resolved measurements of CO using laser absorption spectroscopy (LAS) in a shock tube. We report the first- (τ 1 ) and second-stage (τ 2 ) ignition delay times along with key features of CO time histories, such as the maximum slope during first-stage ignition, plateau CO concentrations post first-stage ignition, and the slope during the dwell phase after first-stage ignition for the fuel blends. Notably, a robust correlation between the overall reactivity of fuels having diverse molecular compositions and their CO features is observed. The objective of this work is to establish a framework for characterizing the low-T combustion of gasoline-alcohol blends by discerning trends in their CO time histories and to offer a physical basis for the connection between overall reactivity and key CO features. Comparisons between measured CO time histories and predictions from three recently developed, detailed chemical kinetic models reveal the models' limitations in replicating the observed data, underscoring the ongoing need for improvements in modeling low-T combustion chemistry. We suggest that time-resolved CO speciation in targeted shock tube experiments can be instrumental in characterizing the low-T oxidation of novel fuels, including those containing oxygenates.