Optical diagnostic study of the synergistic improvement in ammonia combustion process by pilot fuel volatility and two-stage injection strategies

For the ammonia/highly active fuel dual-fuel mode (AHDM), the physicochemical characteristics of pilot fuel directly determine the injection, ignition, and combustion characteristics of the engine, but at present, studies on the differences in physicochemical characteristics, especially in terms of fuel volatility, have not yet been carried out. This study thoroughly investigates the influence of pilot fuel volatility on the combustion characteristics of AHDM through optical diagnostics. On this basis, a combustion control method is proposed that synergistically improves ammonia combustion process through the pilot fuel volatility and the two-stage injection strategies (TIS). The results show that using high volatility fuel to ignite ammonia can mitigate the negative effects of increased ammonia ratio (AR) on the combustion process, thereby improving the ammonia combustion process. The higher the AR, the more significant the improvement effect. Optical test results show that, at the 70 % AR, the combustion flame of high volatility fuel igniting ammonia can still propagate to the central area of the combustion chamber. Compared with the single-stage injection strategy, the introduction of the TIS in the ammonia/DCTL dual-fuel mode (ADCDM) further leverages the advantages of the high volatility fuel and synergistically improves the ammonia combustion process. However, no similar synergistic improvement is observed in the low volatility fuel D0 ignition ammonia mode. Setting the pre-injection timing to −30°CA ATDC or appropriately increasing the pre-injection ratio to 70 % can effectively improve the IMEP of the ADCDM. Optical test results show that the optimization of the above two-stage injection parameters can further promote the propagation of flame jets toward the center of the combustion chamber, providing more ignition areas and energy input for the ammonia.