Numerical study on combustion and emission characteristics of a spark-ignition ammonia engine added with hydrogen-rich gas from exhaust-fuel reforming

A numerical study was performed for the combustion process of a spark-ignition engine fueled with ammonia to investigate the effects of hydrogen-rich reformate addition on the combustion and emission characteristics of the engine. For the combustion characteristics, the results show that the in-cylinder pressure is increased and the combustion duration is shortened with the addition of hydrogen-rich reformate. Moreover, when blending 10.0% of the reformate by volume, the combustion efficiency and thermal efficiency of the engine at stoichiometric conditions are improved to 96.3% and 43.6%, respectively. However, increasing the reformate content in the mixture to 12.5% leads to negative work during the compression stroke and a reduction in the power. For the emission characteristics, it is found that the NO_x emissions can meet the IMO Tier III limit at rich-burn conditions for all the reformate blending ratios (R_re). The NH₃ and N₂O emissions decrease monotonically with the increase of R_re. In addition, the chemical kinetic analyses demonstrate that the concentration of H and OH radicals increase with the addition of hydrogen-rich reformate thus accelerating the elementary reaction rates in the NH₃ consumption pathway. Consequently, the combustion phase is advanced and the unburned NH₃ emissions are reduced with the increase of R_re. The present study concludes that 7.5% – 10.0% is the recommended blending ratio of hydrogen-rich reformate for enhanced combustion with low NO_x and NH₃ emissions in ammonia-fueled engines.