Analysis on the mechanism of ammonia-diesel impinging spray combustion regulated by relative injection timing

Ammonia–diesel direct-injection represents a promising low-carbon combustion strategy, yet the spray impingement generated substantially modifies mixing and combustion, with the fundamental mechanisms of flame development remaining unclear. In this study, the mixing and combustion characteristics of impinging ammonia-diesel sprays were investigated within a constant-volume chamber under high-temperature and high-pressure conditions. A combination of optical diagnostics and computational fluid dynamics simulations was employed to analyze the effects of injection sequencing on flame evolution. The findings indicate that diesel injection timing dominates combustion phasing. However, despite comparable turbulence intensities across different injection modes, the diesel first (DA) mode exhibits significantly enhanced flame development relative to the ammonia first (AD) mode. Further investigation reveals that this disparity is not attributable to turbulence intensity itself, but rather to the temporal alignment between turbulence generation and ignition kernel development. In the DA mode, favorable values of Damköhler number (Da) and Karlovitz number (Ka) during early ignition facilitate flame propagation, whereas in the AD mode, reduced Da and elevated Ka shift the flame toward a broken reaction regime, thereby suppressing its development. In addition, spray impingement in the DA mode aids in redistribution of excessively rich fuel into flammable equivalence ratio ranges, further enhancing combustion. Finally, flame sustainability is governed by diesel injection duration rather than total fuel quantity. Continuous diesel supply is required to maintain stable combustion, while ammonia alone fails to sustain flame propagation under the tested conditions. These results offer valuable insight into turbulence-chemistry interaction and provide guidance for optimizing injection strategies in engines.