Unraveling cyclohexane combustion mechanisms with ReaxFF_(CHO)-S22 reactive molecular dynamics simulations

Cyclohexane is a typical surrogate model for hydrocarbon fuels whose combustion is responsible for pollutant formation in the engine emission. This study leverages ReaxFF molecular dynamics simulations to delve into the combustion dynamics of cyclohexane across varying temperatures, densities, and equivalence ratios, while also considering the influence of water. The simulations replicate quite well the primary intermediates and reaction pathways observed in experimental studies, underscoring the reliability of our approach. Central to this investigation is the application of the ReaxFF_CHO-S22 force field, meticulously re-parameterized recently against high-level quantum mechanical calculations to capture the detailed behaviors of combustion at a molecular level. Our findings underscore the predominance of homolytic C-C bond cleavage as the initial ring opening step in the combustion sequence, leading to the generation of significant intermediates and products such as C₂H₄, CH₂O, CO, CO₂, and H₂O. Notably, our results reveal that elevated temperatures and large densities facilitate the oxidation process of cyclohexane. Furthermore, the incorporation of a small amount of H₂O into the system promotes the conversion of CO to CO₂, providing nuanced insights into the clean combustion mechanism. These discoveries not only offer a detailed understanding of cyclohexane combustion process at the atomistic level but also validate that the ReaxFF_CHO-S22 force field is a powerful tool for future research into the combustion behaviors of general hydrocarbon fuels including fossil fuels and biofuels, paving the way for more efficient energy utilization and the development of cleaner combustion technologies.