Combustion and efficiency improvement of ammonia-hydrogen SI engine through hydrogen fraction and spark timing optimization

Internal combustion engines (ICEs) are heavily used in the transport systems, largely relying on fossil fuels and contributing significantly to carbon emissions. As a carbon-free fuel, ammonia shows strong potential for energy applications. Nevertheless, its inherently low reactivity and slow flame propagation restrict its standalone application in ICEs. To address these limitations, hydrogen is introduced as a promoter to enhance the performance of ammonia-fueled engines. Experimental investigations reveal that hydrogen addition at a 5 % energy ratio significantly promotes flame kernel formation and accelerates the combustion duration, especially during the early flame development stage. With the hydrogen energy ratio rising from 5 % to 25 %, combustion efficiency improves markedly, while unburned ammonia emissions are reduced by approximately 30.5 %. This reduction lowers incomplete combustion losses and improves BTE to 36.9 %. Meanwhile, NOx emissions remain relatively stable at around 1600 ppm. Optimizing spark timing and increasing engine load are both effective strategies for enhancing BTE and combustion stability. However, at a high load of 0.9 MPa, crevice effects and flame quenching intensify, resulting in increased unburned ammonia emissions. In contrast, N2O emissions decrease by approximately 38.5 % with increasing load, primarily due to the suppression of low-temperature ammonia oxidation and enhanced thermal decomposition of N2O. Additionally, heat transfer losses exhibit an inverse correlation with BTE and are recognized as a key factor influencing the thermal efficiency of ammonia-hydrogen engines.