Investigating Autoignition Characteristics of Ammonia/Heptamethylnonane Mixtures Over Wide Pressure Ranges: Rapid Compression Machine Measurements and Kinetic Modeling Study

Ammonia (NH₃) blending combustion with high-reactivity fuel has garnered substantial attention in terms of decarbonization potential in internal combustion engines. 2,2,4,4,6,8,8-Heptamethylnonane, denoted as HMN, an important large-molecular weight component for diesel and jet fuel surrogates, was selected to be blended with NH₃ in this study. The ignition delay times (IDTs) of NH₃/HMN mixtures were measured using a heated rapid compression machine (RCM) over an extensive range of conditions (temperature of 680–1025 K, pressure of 20–100 bar, equivalence ratios of 0.5–1.0, and NH₃ energy ratio (NER) of 50–90%). Experimental results show that increasing the pressure, equivalence ratio, and oxygen concentration reduces both the total and first-stage IDTs, while an increase in the NH₃ energy ratio prolongs the IDTs. For the mixture with the lowest NH₃ energy ratio of 50%, non-Arrhenius-type behavior was observed at a pressure of 20 bar, while it transfers to a monotonic decrease of IDTs with increasing temperature at a pressure of 40 bar. An NH₃/HMN blending mechanism was developed by merging individual NH₃ and HMN submechanisms, updating NH₃ submechanism, and adding C–N cross-reaction subset. Simulation results show that under most experimental conditions, the blending mechanism exhibits reasonable prediction on the measured NH₃/HMN IDTs. Kinetic analysis shows that the discrepancy in the first-stage ignition between experiments and simulations may be associated with the inaccurate OH consumption proportion between HMN and NH₃, while at the intermediate-temperature region, it may be related to the core C₀–C₄ mechanism and the NH₃-related reactions. Further experimental or quantum calculations are needed in the future to refine the NH₃/HMN blending mechanism on the basis of this work.