An experimental and detailed kinetic modeling study of the auto-ignition of NH3/diesel mixtures: Part 1- NH3 substitution ratio from 20% to 90%

Abstract

Ammonia (NH3) is considered as a good carbon-free hydrogen-carrier fuel and has gained extensive attention as a promising alternative fuel for internal combustion engine in recent years. To explore the application potential of the NH3 burned with diesel fuel in internal combustion engine, the auto-ignition delay times of NH3/diesel fuel blends were measured in a rapid compression machine at a wide NH3 energy fraction of 20%, 40%, 60%, 70%, 80%, and 90%, temperature range of 675–995 K, pressures of 20–50 bar, and equivalence ratios of 0.5–1.5. According to the variation of ignition delay times with temperature, three different combustion regimes for NH3/diesel mixtures can be determined. It is found that the typical NTC behavior and two-stage ignition process were only observed at the regime where diesel chemistry dominates. Both the first-stage and the total ignition delay times increase with rising NH3 energy fraction, but decrease with the increase of equivalence ratio. Then, an updated NH3/diesel kinetic mechanism was proposed by adding new cross reactions between diesel and NH3. Results show that the current mechanism exhibits an obvious improvement for ignition delay time prediction compared to the original mechanism, though it still fails to reproduce the ignition delay times of NTC range for the mixture containing 20% NH3. Further sensitivity analysis and the OH radical rate of production analysis indicate that the overestimation of the reactivity for the NH3/diesel fuel blends at NTC range is closely related to reaction NO+HO2NO2+OH. The promoting effect of NO species on the conversion of inactive HO2 radical into active OH radical at low temperature greatly accelerates the autoignition. In addition, the current mechanism does not include the cross reaction between the large NTC-related species (QOOH, OOQOOH, OQOOH, ROO) and the N-containing species (NO, NO2, NH2), especially for the NO related reactions, which may have a great impact on the IDT simulation of the NH3/diesel mixtures. Consequently, this work presents new ignition delay time measurements and mechanism optimization for NH3/diesel mixtures. Continuous works should be emphasized on these unclear reactions to further understand the combustion behavior of NH3/diesel fuel blends.