Optical study on ammonia-diesel dual fuel combustion at low engine loads

Abstract

Ammonia is an excellent hydrogen carrier that can be produced by renewable energy sources, showing high potential to reduce carbon emissions. Ammonia utilization in internal combustion engines has received extensive attention. One practical pathway is directly injecting diesel into the combustion chamber to ignite the premixed ammonia. However, it encounters significant challenges of unstable and incomplete combustion and low ammonia energy ratios under low engine loads. The ammonia-diesel combustion mechanism at very low engine loads is not well understood. Experimental data combining ammonia flame development, heat release, and emissions at low engine loads remains limited. In this study, cylinder pressure and natural flame luminosity (NFL) were simultaneously measured in an ammonia-diesel dual-fuel optical engine fired continuously. The combustion, flame development, and NO_x emissions at low loads (net indicated mean effective pressure, IMEP <4 bar) were analyzed under varied diesel injection pressures and ammonia energy ratios. The results show that the diesel mass and distribution range are the key determining factors of the ammonia-diesel combustion efficiency under low engine loads. The diesel premixing rates are enhanced when the diesel injection pressure increases from 600 bar to 1000 bar, which forms more ignitable mixture in a larger region. This facilitates the earlier formation of ignition nuclei, enlarged flame zone, and improved ammonia combustion efficiency. Increasing the ammonia energy ratio from 60 % to 70 % slows the combustion process, reducing the IMEP and engine combustion stability. The NFL imaging indicates that the unburned ammonia resides in the center of the combustion chamber where the diesel fuel fails to cover. The ammonia-diesel combustion's NO_x emissions are closely related to the combustion temperature at low engine loads.