A comprehensive review of system-level integration for ammonia engines

With the intensifying threat of global warming and the growing demand for sustainable development, the transportation sector, which relies on conventional internal combustion engines, is increasingly pressured to transition to low-carbon and even zero-carbon fuels. Ammonia, as a carbon–neutral energy carrier with high hydrogen content and well-established production and distribution infrastructure, is regarded as a promising alternative to fossil fuels. However, its application in engines is associated with several technical challenges, including pseudo-zero carbon emissions, slow flame propagation, high auto-ignition temperature and material corrosion. To address these challenges, a systematic review is conducted on the life cycle environmental impact, subsystem parameter optimization and technological innovations related to ammonia-fueled engines, with the goal of achieving efficient and clean combustion, minimizing unburned ammonia and harmful emissions, and ensuring long-term system reliability. Additionally, the effects of combustion strategy optimization on engine performance are investigated, encompassing high-reactivity fuel supplementation, combustion chamber design, injection strategy refinement, intake parameter control and artificial intelligence combustion prediction. These strategies have been found to improve combustion stability and thermal efficiency by enhancing fuel mixing uniformity, strengthening the thermally active atmosphere and promoting flame propagation. This review also emphasizes that current aftertreatment efforts should focus on improving real-time monitoring of ammonia escape and developing N₂O specific catalysts to mitigate high levels of unburned ammonia and N₂O emissions. Through the coordinated optimization of engine subsystems, combustion strategies and aftertreatment systems, ammonia engines are considered to have significant potential for commercial deployment and are capable of supporting the achievement of carbon neutrality.