Ammonia as a sustainable fuel: Review and novel strategies

Ammonia is increasingly recognized as a viable alternative fuel that could significantly reduce greenhouse gas emissions without requiring major modifications to existing engine technologies. However, its high auto-ignition temperature, slow flame speed, and narrow flammability range present significant barriers, particularly under high-speed combustion conditions. This review explores the potential of ammonia as a sustainable fuel for internal combustion engines, focusing on its advantages and challenge. The review draws on a wide range of studies, from NH3 production, application, to the combustion mechanisms, that explore various strategies for enhancing NH3 combustion in both spark ignition and compression ignition engines. Fundamentals and key approaches discussed include using hydrogen and hydrocarbon fuels as combustion promoters, which have been shown to improve ignition and flame propagation. Literature on fuel injection strategies, such as port fuel injection, direct injection, and dual-fuel injection, are examined to highlight their influence on NH3-air mixing and combustion efficiency. Furthermore, the review delves into advanced ignition technologies, such as low- temperature plasma ignition, turbulent jet ignition, and laser ignition, which are explored for the potential to overcome the ignition difficulties associated with NH3. After a comprehensive analysis based on the literature, the intelligent liquid-gas twin-fluid co-injection system (iTFI) emerges as a promising approach, offering improved combustion stability and efficiency through better fuel-air mixture preparation. By synthesizing the existing research, this review outlines the progress made in NH3 combustion and identifies areas where further study is needed to fully realize its potential as a sustainable fuel.