Unravelling the unique kinetic interactions between N₂O and unsaturated hydrocarbons

Abstract

The interaction between unsaturated hydrocarbons and N2O has attracted considerable attention in recent years due to their important role as potential propellants for advanced propulsion systems (e.g. Nitrous oxide fuel blend (NOFBX)) and key combustion intermediates in exhaust gas recirculation systems. Although experimental studies and kinetic models have been developed to investigate its fuel chemistry, discrepancies remain between modeled and measured ignition delay times at low temperatures. In this work, we characterize previously unreported direct interaction pathways between N2O and unsaturated hydrocarbons (C2H4, C3H6, C2H2, C3H4-A, and C3H4-P) through quantum chemistry calculations, comprehensive kinetic modeling, and experimental validation. These reactions proceed via O-atom addition from N2O to unsaturated hydrocarbons, forming five-membered ring intermediates that decompose into N2 and hydrocarbon-specific products. Distinct differences are identified between alkenes and dienes and alkynes, arising from the disparity in N–C bond lengths within the intermediates (∼1.480 Å for alkenes and 1.429 Å for dienes vs. ∼1.381 Å for alkynes), which governs their decomposition pathways. The corresponding rate coefficients are determined and implemented into multiple kinetic models, with autoignition simulations showing a pronounced promoting effect on model reactivity and improved agreement with experiments, especially at low temperatures. Comprehensive uncertainty analyses of the potential energy surfaces, rate coefficients, and ignition delay times are conducted to ensure the robustness and reliability of the findings. Flux analysis further reveals that the new pathways suppress conventional inhibiting channels while enabling aldehyde- and ketone-forming pathways that enhance overall reactivity, with JSR simulations further confirming the feasibility of validating these pathways through experiments. This work provides a more complete description of N2O–hydrocarbon interactions and reveals other important N2O–hydrocarbon interaction chemistries that need to be further studied via both theoretical and experimental investigations.