Optical study of combustion stability in dual fuel approach using ammonia and high reactivity fuel

Ammonia, as a zero-carbon fuel, is considered to be an ideal alternative fuel for a reduction of carbon dioxide emissions. Owing to low laminar flame speed and high ignition energy, the utilization of pure ammonia in powerplant system still presents severe challenges. To solve these issues, the dual fuel combustion of high reactivity fuel and ammonia is a promising solution. However, the dual fuel combustion stability of ammonia and high reactivity fuel has not been clearly understood. In present study, the misfire reasons are investigated using various optical diagnostic methods. Results demonstrate that the misfire reasons are divided into two aspects. One is that the addition of ammonia increases the temperature and pressure required for direction injection fuel auto-ignition, which makes it difficult to generate auto-ignition site, resulting in misfire. The other is that the low flame development speed and degradation of the in-cylinder temperature and pressure causes the difficulty in the further flame development, which results in misfire. A collaborative regulation approach of engine operating condition and direction injection fuel reactivity is proposed to improve combustion stability, which achieves 93% ammonia energy ratio. At 93% ammonia energy ratio, increasing direction injection pressure from 600 bar to 1000 bar decrease combustion stability. The local equivalence ratio of direction injection fuel that can ignite ammonia stably is mainly concentrated between 0.56 and 0.86 in the conditions of 93% ammonia energy ratio and 22 bar in-cylinder pressure. Compared with the in-cylinder temperature, the main factor in determining combustion stability is local equivalence ratio of direction injection fuel. The addition of ammonia prolongs the low temperature reaction and constrains the high temperature reaction of direction injection fuel. In brief, the combustion stability and ammonia energy ratio can be improved simultaneously using the collaborative regulation.