Optical diagnostics in impact of ammonia energy ratio and diesel split ratio on combustion process and flame propagation in an Ammonia-Diesel Dual-Fuel engine

Abstract

In the context of ammonia-diesel dual-fuel (ADDF) engines, challenges such as ignition difficulties and misfires emerge prominently at elevated ammonia energy ratios. To alleviate these challenges, this study aims to elucidate the effects of ammonia energy ratios (AER) and diesel split ratios (DSR) on the combustion process and flame propagation in an optical ADDF engine configuration. Utilizing optical diagnostics, including flame luminescence and high-speed photography, the study provided insights into combustion boundary conditions specific to ADDF engine operation. The results highlight that an increase in AER to 60 % results in a marked decrease in peak cylinder pressure, peak apparent heat release rate (AHRR), and the in-cylinder pressure rise rate. This is accompanied by a reduction in IMEP from 4.16 bar to 3.24 bar, indicating a decline in combustion efficiency upon greater ammonia addition. Flame luminosity exhibits a shift from a diesel-like blue hue to an ammonia-tinted pale yellow. This change coincides with a significant reduction in soot formation and restrictions in flame area and propagation speed, largely due to ammonia's intrinsically slower burning velocity and its chemical interplay with diesel. Implementing a dual injection approach further accentuates the peak in-cylinder pressure and AHRR, with the second injection playing a crucial role in ignition. Optimizing the DSR to 40 % improves combustion efficiency, attributed to enhanced fuel-oxygen interactions. Conversely, DSR values of 60 % and 80 % pose challenges. A reduction in diesel content in the subsequent injection leads to a leaner blend, subsequently affecting combustion efficiency, as indicated by reduced heat release rate and diminished IMEP. This study emphasizes understanding the ammonia-diesel interaction and the importance of choosing the appropriate DSR to exploit the advantages of ADDF engines.