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 NOx emissions can meet the IMO Tier III limit at rich-burn conditions for all the reformate blending ratios (Rre). The NH3 and N2O emissions decrease monotonically with the increase of Rre. 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 Rre. The present study concludes that 7.5% - 10.0% is the recommended blending ratio of hydrogen-rich reformate for enhanced combustion with low NOx and NH3 emissions in ammonia-fueled engines.