Large eddy simulation and experimental investigation of liquid ammonia diffusion combustion and emissions formation under diesel-ignited JCCI conditions

Under the framework of powertrain carbon neutrality, the jet-controlled compression ignition (JCCI) strategy, which utilizes diesel-ignited direct injection liquid ammonia diffusion combustion demonstrates strong potential for improving the power output and reducing emissions in ammonia-fueled engines. However, fundamental insights into the liquid ammonia spray processes remain insufficient, and the mechanisms governing ammonia diffusion combustion and pollutant formation require deeper examination. This study combines optical diagnostics and Large Eddy Simulation (LES) to compare diesel prior-injection and simultaneous injection strategies under dual direct‑injection conditions. The results demonstrate that for efficient combustion of the liquid ammonia spray, the ignition source should be positioned within the well-mixed region at the spray leading edge. This enables flame propagation towards the spray periphery and tail, progressively igniting the entire flammable zone and ultimately achieving complete combustion. Conversely, if ignition occurs in the spray central region, the overly lean mixture at the spray head prevents ignition, leading to substantial escape of unburned ammonia. Regarding emissions, under the diesel prior-injection condition, NO distributes throughout the spray following flame expansion, unburned ammonia is minimal, and N 2 O is primarily located at the flame periphery during the late combustion stage. In contrast, the simultaneous injection condition results in incomplete combustion, confining NO to the central region, yielding higher levels of unburned ammonia, and showing negligible N 2 O decomposition. Based on these findings, a conceptual model incorporating the “head-priority ignition” principle is proposed, correlating pollutant distribution with combustion zones to provide a theoretical basis for optimizing engine combustion and reducing emissions.