An experimental and kinetic modeling study of the impact of nitric oxide and hydrogen on iso-Octane combustion

The knowledge of the combined effects of exhaust recirculation and hydrogen enrichment on hydrocarbon fuels combustion is limited to the complexity of the ternary interaction. In this work, experimental and kinetic modeling techniques were combined to investigate the chemical effects of high-level NO, H2, or NO/H2 additions on hydrocarbon fuels. A series of jet-stirred reactor experiments were conducted on the oxidation of iso-Octane with NO/H2 additions at atmospheric pressure and over a wide range of equivalence ratios and temperatures. Experimental results reveal that high concentrations of NO improves iso-Octane oxidation in the temperature region of 800-1000 K and promotes its consumption more efficiently under the ultra-lean condition than stoichiometric and rich conditions. The addition of H2 slightly enhances iso-Octane oxidation by facilitating hydrogen abstraction reactions under the ultra-lean condition, but this effect is insignificant in the stoichiometric condition. The addition of combined H2 and NO demonstrates a more significant promoting effect on iso-Octane oxidation under both ultra-lean and stoichiometric conditions, indicating additive effect in the global reactivity. Existing kinetic models from the literature were incapable to capture these effects. We developed a detailed model which better accounts for the competing chain promotion/termination effects involved in NO sensitization on fuel oxidation, by modifying important reaction rate constants and introducing key nitrogen- containing C3 species. NO and combined NO/H2 additions both enhance the fuel initiation process, primarily through the NO+HO2=NO2+OH reaction, which promotes hydrogen abstraction pathways by converting HO2 to more active OH radical. The NO/H2 combination leads to intricate effects on intermediate species, driven by the competition between NO+HO2 and H+NO2 reactions for OH evolution, as well as the competition between H+O2 and H+NO2 for H consumption. This work contributes to the understanding of ternary hydrocarbon/NO/H2 interactions, providing insights into the design of improved fuel systems to optimize engine performance and mitigate emissions.Novelty and significance statement: This study investigates the effects of high-level NO, H2, or NO/H2 additions on iso-Octane oxidation through experiments and kinetic modeling. New JSR experiments were performed on the oxidation of iso-Octane with and without NO, H2, or NO/H2 additions at various equivalence ratios, 1 atm, and temperatures ranging from 600-1100 K. Experimental results reveal that high-level NO enhances iso-Octane oxidation under the ultra-lean condition in 800-1000 K. H2 has a moderate impact under the ultra-lean condition, but its effect is insignificant in the stoichiometric condition. Combined H2 and NO significantly enhance iso-Octane oxidation in ultra-lean and stoichiometric conditions. The developed detailed model outperforms existing ones in predicting these effects on fuel oxidation. This research contributes to understanding hydrocarbon/NO/H2 interactions and optimizing fuel systems.