Examining diesel-spray assisted ignition of ammonia under reactivity-controlled conditions using large-eddy simulations

Abstract

Ammonia (NH3) has gained increasing attention as a promising carbon-free fuel for compression ignition engines. Nonetheless, its poor combustion characteristics and elevated nitrogen oxides (NOx) emissions present substantial obstacles. In the present study, we examine the utility of incorporating NH3 as a low-reactivity fuel (LRF) in diesel-assisted dual-fuel combustion under Reactivity Controlled Compression Ignition (RCCI) conditions. Three large-eddy simulations (LES) are performed to quantify the effect of varying concentrations of NH3 as LRF on the ignition characteristics and flame structure. The computational setup corresponds to the Engine Combustion Network (ECN) Spray A configuration, which provides the baseline for the present analysis. The ignition of the dodecane spray is found to be delayed by the presence of NH3, which increases with increasing NH3 content in the ambient. Local flamelets are extracted to examine the evolution of the flame structure starting from ignition at richer mixtures through low-temperature chemistry of dodecane, to finally stabilizing at the stoichiometric conditions. Near ignition, NH3 oxidation is observed to follow the autoignition behavior of the most reactive mixture fraction, whereas at post-ignition the behavior shifts towards canonical premixed flame propagation. This study shows that using NH3 as LRF under RCCI conditions offers an effective solution for NH3 operation in CI engines to reduce carbon emissions.