Researchers from Tianjin University and Lund University have developed a pioneering in-cylinder active reduction strategy to combat the high NO_x emissions that have long hindered the adoption of ammonia-fueled internal combustion engines (ICEs). By utilizing an ammonia post-injection (API) technique, the research team successfully reduced NO_x emissions by 14.4% without compromising combustion efficiency.
Ammonia is widely recognized as a promising carbon-free fuel for the transportation and energy sectors. However, its fuel-bound nitrogen content leads to significant NO_x emissions during combustion, posing a major challenge for meeting increasingly stringent environmental regulations, such as the Euro 7 standard.
In the study published in ENGINEERING Energy, the researchers demonstrated that ammonia can serve a dual purpose: as a fuel and as an agent for Selective Non-Catalytic Reduction (SNCR)—a chemical process that breaks down NO_x into harmless nitrogen.
Key Research Highlights:

• Innovative NO_x Control: The study reveals the NO_x evolution and reduction mechanisms induced by ammonia post-injection, providing a robust theoretical and experimental basis for in-cylinder emission control.
• Mechanistic Insights: Detailed chemical kinetics analysis identifies that NH₂ radicals play the overwhelmingly dominant role in reducing both NO and NO₂ within the combustion chamber.
• Effective Emission Reduction: Engine experiments confirmed that by appropriately delaying the post-injection timing, a 14.4% reduction in NO_x emissions was achieved while maintaining high combustion efficiency.
• Reaction Pathways: The study elucidates the SNCR reaction pathways, showing that NO is primarily reduced to N₂ via three distinct pathways involving NH and NH₂ radicals.

The researchers utilized a high-precision Computational Fluid Dynamics (CFD) model to visualize the combustion process. Their simulations showed that post-injected ammonia efficiently reacts with existing NO_x in the cylinder. While the strategy effectively cuts NO_x, the team also observed trade-offs, including increased N₂O (a potent greenhouse gas) and unburned NH₃ emissions when the post-injection timing was excessively delayed.
"This study provides an innovative method for reducing NO_x emissions from ammonia combustion," the authors noted. "Combined with the inherent low NO_x emission characteristics of ammonia diffusion combustion, this approach holds great promise for achieving effective NO_x emission control in future ammonia-fueled engines".
The team emphasizes that while the API strategy is highly effective, further optimization of combustion chamber geometry and ammonia injection hardware will be critical for practical, large-scale industrial application.
DOI:10.1007/s11708-026-1079-8