Ammonia and hydrogen blending effects on combustion stabilities in optical SI engines

Ammonia is a promising energy carrier and carbon-free fuel, which is getting more attention today when fossil energy is increasingly exhausted. However, ammonia combustion in the IC engine suffers from combustion in-stabilities and misfiring issues. Hydrogen is also carbon-free energy and can be directly obtained by ammonia catalytic reforming. The addition of a small amount of hydrogen can effectively alleviate the weakness of ammonia combustion, while ammonia and hydrogen blending effects on combustion stabilities and their critical conditions remain unclear. In this work, the effects of hydrogen addition on ammonia combustion characteristics were experimentally investigated in an optical SI engine with high compression ratios. The results show that the indicated thermal efficiency first increases and then decreases with the elevation of hydrogen-ammonia energy ratios (x), with an optimal value at x approximate to 7.5 %. Further hydrogen addition can aggravate indicated thermal effi-ciency due to the large heat transfer loss caused by high-temperature hydrogen combustion. Combustion visu-alizations show that there is a critical threshold of hydrogen-ammonia energy ratio (x = 10.0-12.5 %), around which engine combustion characteristics are changed significantly. When x < 10 %, the effect of hydrogen addition on the ammonia flame speed becomes more pronounced since the hydrogen substantially improves the stability of the early-stage combustion. When x > 12.5 %, hydrogen shows an obvious influence on the initial flame formation of ammonia, which is attributed to the low flame stretch sensitivity, facilitating early flame development. In addition, flame propagation when x < 10 % is more sensitive to temperature than flame propagation with x > 12.5 %. The current research demonstrates that proper ammonia-hydrogen energy ratios are important for stable combustion and thermal efficiency improvement.