Issue 07 | Dec 2025
Among the candidates, ammonia has been in the spotlight. It is carbon-free, easily liquefied for transport and can be produced on a large scale through wellestablished industrial routes. It also carries a high concentration of hydrogen by volume — a property that makes it an attractive hydrogen carrier. However, ammonia is difficult to ignite, burns slowly and tends to leave behind unburned fuel and nitrogen oxides that harm efficiency as well as the environment.
Hydrogen, by contrast, burns quickly and cleanly, and blending it with ammonia improves both performance and emissions. Its storage requirements, however, are its Achilles’ heel. Hydrogen must be chilled to -253°C or compressed at high pressures, requiring bulky, costly tanks — impractical for long voyages at sea.
This long-standing dilemma — how to capture the best of both fuels without their drawbacks — is what Associate Professor Yang Wenming and Senior Research Fellow Dr Zhou Xinyi from the Department of Mechanical Engineering at the College of Design and Engineering, National University of Singapore set out to address.
Their study, published in Joule, introduces a new concept for an ammonia–hydrogen engine with a single ammonia fuel supply. In essence, the engine makes its own hydrogen as it runs, avoiding the need to carry a separate supply altogether.
Turning fuel into its own catalyst
Most ammonia-hydrogen engine concepts today rely on external reformers: separate reactors that heat ammonia to around 550°C and use catalysts such as ruthenium to break it down into hydrogen and nitrogen. These systems consume additional energy for heating, add mechanical complexity and occupy valuable space. They also face trade-offs between cost, conversion rate and durability, all of which are critical for ship engines expected to run continuously for decades.
Issue 07 | Dec 2025
“Instead of processing ammonia outside the engine, we thought that we could produce hydrogen inside the engine cylinder itself,” says Assoc Prof Yang.
In the team’s concept, one cylinder in a multi-cylinder engine operates on a fuelrich ammonia mixture. Under the intense temperature and pressure of combustion, part of the ammonia decomposes into hydrogen. This hydrogen-rich exhaust is then recirculated to the other cylinders, enriching their combustion and improving efficiency — all using the same fuel.
In-cylinder reforming gas recirculation (IRGR) concept coupled with active pre-chamber.
Forging New Frontiers
“Instead of processing ammonia outside the engine, we thought that we could produce hydrogen inside the engine cylinder itself.”
To keep this process stable, the researchers introduce an active prechamber ignition system. It ignites a small, easily combustible mixture in a prechamber, sending hightemperature turbulent jets into the main chamber to ignite the ammonia-rich fuel. This ensures reliable ignition without relying on pilot diesel, thereby averting carbon dioxide emissions from fossil-based ignition fuels.
“More importantly, integrating active pre-chamber technology can extend the ammonia-rich limit of the main chamber, thereby increasing the total hydrogen production,” says Dr Zhou.
Initial experiments and simulations suggest that this in-cylinder reforming approach could improve thermal efficiency, cut unburned ammonia and significantly reduce
nitrous oxide emissions, which is one of the key environmental challenges for ammonia engines as the nitrous oxides are approximately 273 times more potent than carbon dioxide in terms of warming the planet.
“The concept also simplifies the system. No bulky reformers, no expensive catalysts and fewer energy losses,” adds Assoc Prof Yang.