Exploring the Interaction Chemistry of Ammonia with n-Hexadecane over Wide Pressure Ranges: An Experimental and Kinetic Modeling Study

Before applying ammonia (NH₃)/hydrocarbon fuels in compression ignition engines, it is important to comprehensively understand the autoignition characteristics. n-Hexadecane (nC₁₆H₃₄) is an ideal component for characterizing the linear n-alkane class in diesel surrogates. In this investigation, the ignition delay times (IDTs) of NH₃/nC₁₆H₃₄ mixtures were measured in a rapid compression machine at different NH₃ energy ratios (50%, 70%, and 90%), temperatures of 693–1047 K, pressures of 20–60 bar, and equivalence ratios of 0.5–1.0. Experiments reveal that, under the investigated conditions, no distinguishable negative temperature coefficient (NTC) characteristic was observed, and the IDTs decrease monotonically with the increase of temperature. With the increase of pressure, N₂/O₂ ratio, and equivalence ratio, the IDTs are shortened. The nC₁₆H₃₄ and NH₃ submechanisms with important updates were merged, and C-N cross-reaction subsets were added to the blending mechanism. Modeling results demonstrate that the NH₃/nC₁₆H₃₄ mechanism predicts the IDTs well under most conditions. Sensitivity analysis shows that the overestimation of IDTs for stoichiometric conditions may be due to the errors of rate rules for reaction QOOH = cyclic ether + OH and a lack of kinetic understanding under the high fuel and oxygen content conditions. The interaction chemistry between NH₃ and nC₁₆H₃₄ cannot be neglected, in which the NH₂/RH cross-reaction subsets play a crucial role in modeling IDTs. Further quantum calculations are required to study the influence of carbon chain length on the rate rules and gain a comprehensive kinetic understanding of high fuel and oxygen contents and oxygen-rich conditions in the future.