Visualization study on ammonia/diesel dual direct injection combustion characteristics and interaction between sprays

Ammonia is regarded as a highly promising green fuel. Nevertheless, its auto-ignition temperature is exceptionally elevated, posing challenges for its direct utilization in compression ignition engines. Using highly active fuel as the ignition source seems a solution. In this paper, based on the RCEM experimental platform, the diesel/ammonia diffusion combustion mode was studied in detail. The experimental parameters included the ammonia injection pressure, ammonia and diesel injection interval (including ammonia in advance of diesel and diesel in advance of ammonia), diesel injection pressure, and ammonia nozzle diameter. The results indicated that injecting ammonia on diesel flame contributed to the completeness of ammonia combustion. Injecting ammonia before diesel achieved a higher peak heat release rate but aggravated the incomplete combustion. The injection pressure of diesel had no significant impact on the combustion effect of ammonia. Increasing the ammonia injection pressure resulted in enhanced air entrainment, consequently enhancing the effective combination of fuel with air, which improved the peak heat release rate more effectively. Increasing the diameter of the ammonia nozzle was a necessary way to achieve a high ammonia energy substitution ratio, but a large amount of ammonia sprayed simultaneously caused a portion of the ammonia to not burn in time, resulting in aggravation of incomplete combustion. In addition, the interaction between diesel spray and ammonia spray was also studied. The results showed that, due to the higher quantity of ammonia compared to diesel, the two sprays would gather and develop toward the direction of ammonia injection roughly, and the final development direction of the spray was also related to the fuel injection pressure.