Ammonia-fueled internal combustion engines: a review of research trends

This study conducts a qualitative and quantitative bibliometric analysis of 978 scholarly records retrieved from the Scopus database from 1943 to 2025. The study analyses publication trends, citations, collaborations, bursts, keywords and clusters using VOSviewer (v1.6.19), CiteSpace (v6.3.R1), and Microsoft Excel. The Fuel and International Journal of Hydrogen Energy dominate in terms of both document output and citations. China, the US, and the UK lead in both publication number and citations. The challenges of ammonia combustion, including high ignition energy, low flame speed, and slow reactivity, are currently being addressed using ignition enhancement techniques and dual fuel combustion, which reflect the research hotspots identified through temporal keyword co-occurrence network visualisation. Burst detection analysis of authors indicates that ammonia–diesel dual-fuel engines have emerged as a major research focus in ammonia-fueled internal combustion engines (ICEs), driven by the growing demand for heavy-duty vehicles. Pure ammonia in SI engines needs enhanced ignition including multi-spark, plasma, or laser, high compression ratios, optimised spark timing, and intake boosting to offset poor combustibility. Studies indicate that stable ammonia combustion can be achieved with co-fuel energy shares as low as 5–30%, using a range of fuels including hydrogen, methane, diesel, gasoline, methanol, ethanol, ethers, and biodiesel. Future work on ammonia-fueled engines should focus on advanced control strategies using artificial intelligence (AI), simultaneous emissions reduction, the use of ternary blends to leverage the strengths of individual fuels, optimisation of ammonia energy ratio, ammonia-fed fuel cells for hybrid electric vehicles, the use of nonadditives for enhanced combustion.