Low-temperature auto-ignition characteristics of NH3/diesel binary fuel: Ignition delay time measurement and kinetic analysis

Ammonia (NH3) is gaining increasing interest as a carbon-free alternative fuel in engine systems. Co-firing NH3 with diesel can overcome the high auto-ignition temperature and narrow flammability limits of pure NH3, while exploit its advantage. This study presents the auto-ignition properties of NH3/diesel binary fuel, at various NH3 blending ratios (10%, 30% and 50%) in a rapid compression machine. Ignition delay times (IDTs) were measured spanning a temperature range of 670-910 K, pressures of 10-20 bar, and equivalence ratios of 0.5-1.5. Typical two-stage ignition and negative temperature coefficient (NTC) response were identified for the blends. Both the first-stage and the total IDTs increase with the increasing NH3 blending ratio, and there exists a non-linear inhibiting effect of NH3 fraction on IDT. A blending mechanism was then constructed based on the existing diesel mechanism and NH3 mechanism. The mechanism can predict the inhibiting effect of NH3 addition, but fails to well capture the IDTs over the whole temperature range, especially for the first-stage IDT and the NTC response. Further kinetic analysis, including species mole fraction history, brute force sensitivity analysis and reaction pathway analysis, were conducted to gain deeper insight into the auto-ignition chemistry of the blending fuel. These analyses suggest the rate parameters of reactions NH3 + OH double right arrow NH2 + H2O and NH2 + NO double right arrow N-2 + H2O are critical to accurately predict IDTs, and the competition role of NH3 for OH radical inhibits the diesel low-T chain-branching sequence, which eventually leads to restraining the reactivity of the whole reaction system.