Liquid ammonia: A carbon-neutral fuel candidate for engine applications

A comprehensive review published in Engineering has systematically examined liquid-ammonia injection and combustion for engine systems, outlining its potential as a carbon-neutral fuel while mapping technical challenges and recent advances in spray dynamics, combustion behavior, and engine integration.

As a carbon-free energy carrier, liquid ammonia offers high volumetric and gravimetric energy density, manageable storage and transport conditions, and compatibility with existing infrastructure, making it attractive for decarbonizing marine shipping, heavy-duty transport, and stationary power generation. The review synthesizes progress in fundamental fuel properties, injection and spray physics, combustion characteristics, and practical engine performance, noting that liquid ammonia can substantially lower greenhouse gas emissions when used in engines. Key hurdles identified include ammonia’s high latent heat of vaporization, slow laminar flame-propagation speed, narrow flammability range, and elevated NOₓ and N₂O emissions during combustion. The strong evaporative cooling effect during droplet vaporization can suppress flame propagation and reduce reaction rates, often requiring extra energy input or supporting fuels to maintain stable combustion.

Dual-fuel modes, such as ammonia–diesel, have proven effective in overcoming ammonia’s low reactivity, with both low-pressure and high-pressure dual-fuel injection configurations delivering sizable emission reductions. High-pressure direct injection is reported to deliver better thermal efficiency and lower NOₓ emissions compared to low-pressure premixed approaches, partly due to improved atomization and mixing control. Optimized injection strategies and nozzle geometries are needed to enhance flash-boiling atomization, spray penetration, and fuel–air mixing under engine-relevant high-pressure and high-temperature conditions. Recent studies have explored turbulent jet ignition, stratified fuel injection, and hydrogen co-injection to strengthen ignition reliability and combustion stability, helping extend the operable range and reduce cyclic variations.

Engine demonstrations reveal that liquid-ammonia dual-fuel systems can achieve marked CO₂ cuts, yet they face trade-offs among thermal efficiency, combustion stability, and nitrogen-based emissions. Unburned ammonia slip and high NOₓ remain major concerns, while N₂O—with a high global warming potential—can offset greenhouse gas benefits if uncontrolled. The review points to targeted solutions, including exhaust-gas recirculation, selective catalytic reduction systems using ammonia as a reductant, and enhanced combustion-phase control to mitigate these pollutants.

Future work should prioritize integrated solutions that combine advanced combustion concepts, engine hardware tailored for ammonia, and coordinated emission-control strategies, the authors state. They also emphasize the need for improved thermodynamic and fluid-dynamic models that account for ammonia’s unique phase change and turbulence–chemistry interactions, alongside compatible materials, safety systems, and durability validation for long-term operation. Cross-sector collaboration among academia, industry, and policymakers is deemed essential to accelerate the adoption of ammonia as a sustainable low-carbon engine fuel.

The paper “A Review on Liquid-Ammonia Injection and Combustion for Engine Applications,” is authored by Hao Wu, Fahad Almatrafi, Moez Ben Houidi, Tiegang Fang, William L. Roberts. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.09.008. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.