Effects of injection parameters on the combustion characteristics of ammonia-diesel dual-fuel direct-injection (ADDI) mode and combustion enhancement at high ammonia energy ratio

Ammonia, as a carbon-free fuel, is regarded as an alternative fuel for compression ignition engines. Due to its low reactivity, ammonia is usually used with high-reactive fuels. This study comprehensively investigates the effects of injection interval, injection pressure, injection mass, and injection ratio on the combustion characteristics of ammonia-diesel dual-fuel direct-injection (ADDI) under engine-relevant conditions. The study was carried out on an optical constant-volume bomb with two symmetrically arranged injectors with diesel nozzles of 0.1 mm and liquid ammonia nozzles of 0.3 mm. Simultaneous chemiluminescence and flame luminosity imaging were applied. Results showed that advancing the diesel injection timing appropriately improved ammonia combustion, whereas excessively early injection of either diesel or liquid ammonia deteriorated overall combustion. Lowering the diesel injection pressure extended the diesel injection duration, thereby increasing the interaction time between the liquid ammonia spray and the diesel flame, which improved ammonia combustion. Increasing the ammonia energy ratio helped reduce carbon emissions, but an excessively high ammonia energy ratio led to suboptimal overall combustion. To address the poor combustion under high ammonia energy ratio conditions, two strategies were employed: lowering diesel injection pressure and advancing diesel injection timing. Both strategies improved overall combustion, with reducing diesel injection pressure showing a more pronounced effect. Therefore, the key to improving ammonia combustion in ADDI mode lies in extending the interaction time between the liquid ammonia spray and the diesel flame.