Numerical study for the comparison between direct dual-fuel stratification and reactivity-controlled compression ignition of ammonia-based engines

Abstract

Ammonia (NH3) has attracted significant interest as a promising carbon-free alternative fuel for internal combustion engines. However, its high ignition energy and slow flame speed pose substantial challenges for its widespread application. Reactivity-Controlled Compression Ignition (RCCI) demonstrated superior efficiency and reduced greenhouse gas emissions; nevertheless, the emissions of NOx and unburned NH3 remain significantly high. Alternatively, direct dual-fuel stratification (DDFS) is a novel combustion strategy that integrates the benefits of partially premixed combustion and RCCI by modifying the fuel injection strategy. In this study, comprehensive numerical simulations were conducted to investigate the potential of the DDFS mode for enhancing thermal efficiency and reducing pollutant emissions in ammonia-fueled engines. Specifically, a comparative analysis of RCCI and DDFS combustion modes was performed to assess the effect of high-reactivity fuel injection timing, using n-heptane as a diesel fuel surrogate, on combustion and emission characteristics. The results show that advancing the injection timing increases the peak in-cylinder pressure, optimizes the combustion phasing, and shortens the combustion duration for both modes, thereby enhancing the thermal efficiency. The DDFS mode effectively regulates the combustion rate while maintaining indicated thermal efficiency comparable to that of the RCCI mode. Furthermore, compared to the RCCI mode, the DDFS mode significantly reduces the generation of thermal NOx; however, it still produces substantial soot due to its diffusion-combustion nature. The emissions of NOx, N2O, and unburned NH3 were reduced by 31 %, 84.67 %, and 85 %, respectively. Therefore, the DDFS combustion mode has considerable potential for reducing NOx, N2O, and unburned NH3 emissions while maintaining comparable thermal efficiency.