Experimental study on the effects of pilot injection strategy on combustion and emission characteristics of ammonia/diesel dual fuel engine under low load

With the rise of the concept of carbon neutrality, carbon-free fuel ammonia has attracted widespread attention from internal combustion engine researchers. Ammonia/diesel dual fuel mode can effectively overcome the ignition difficulty of ammonia in the engine and is an effective measure to promote ammonia combustion. Based on a single-cylinder engine, diesel-ignited ammonia combustion experiments were conducted under conditions of 1200 rpm and 30 % load to systematically study the impact of ammonia energy ratio (AER), pilot injection ratio/timing on the combustion and emission characteristics of the ammonia/diesel dual fuel engine. This study finds that the pilot injection improves the upper limit of AER, reduces the cyclic coefficient of variation (COV). The rapid combustion fraction has been defined to understand the dual fuel combustion process. With the AER increases, the rapid combustion fraction of the single injection increases while the trends of the pilot injection strategy are the opposite. Both NOx emissions of the pilot injection strategies decrease while CO emissions increase. The reduction of NOx emissions may be caused by a decline in in-cylinder temperature. At a 35 % AER,Medium-low pilot injection ratios can increase the indicated mean effective pressure (IMEP), which is because as the pilot injection ratio exceeds 35 %, the growth rate in the effective expansion efficiency (EEE) is lower than that of heat transfer loss. When the pilot injection ratio is 35 %, the highest IMEP is achieved, CO and NOx emissions are cut down by 23 % and 20 %, respectively, compared to the single injection.