Chemical kinetic study of gasoline surrogate with ammonia on combustion: Iso-octane modeling

To facilitate the transition towards a carbon–neutral society, it is necessary to undertake comprehensive research on the utilization of ammonia in gasoline engine applications. In the present work, a new chemical kinetic mechanism for iso-octane/ammonia (iC₈H₁₈/NH₃) was proposed and widely validated with the experiment data of ignition delay time (IDT) and laminar burning velocity (S_L). The iC₈H₁₈ model of Cai [Combust. Flame, 162, 2015] and NH₃ model of Zhang [Combust. Flame, 234, 2021] was chosen as the based model, and they connected through the chemical coupling between the iC₈H₁₈ and the NH₂ chemistry. In addition, the influence of C-N interaction on the prediction ability of iC₈H₁₈/NH₃ model was also analyzed and found that the H-abstraction reaction between NH₂ and iC₈H₁₈ is the key reaction affecting the IDT and S_L of mixture. Present work was also focused on the chemistry kinetic of iC₈H₁₈/NH₃ mixture. The reaction pathway analysis shows that the addition of NH₃ slightly enhanced the H-abstraction reaction of iC₈H₁₈ in tertiary site (cC₈H₁₇) and slightly inhibited the H-abstraction reaction of primary site (aC₈H_17, bC₈H_17, and dC₈H₁₇). In the end, the mole fraction of NO in iC₈H₁₈/NH₃ flames is analyzed. The results show that NH₃ addition increased the concentration of NO, but with the gradual increase of equivalence ratio, the increasing trend of NO concentration gradually decreases. A higher equivalence ratio can be selected to effectively control the increase of NOx concentration. Through the analysis of NO formation and reduction reaction pathway, it was found that NH₃ addition increases the mole fraction of NH₂ radical, the NO formation pathway was stronger than NO reduction pathway due to the sufficient OH radical, and finally leading to the increase of NO concentration. However, with the increase of equivalence ratio, the concentration of NH₂ radical exceeds OH, and the number of OH is insufficient in satisfying the NH₃-related NO reaction pathway, resulting in a decrease of the influence of NH₃ addition on NO concentration.