Facing to the background of global efforts to reduce carbon emissions and transition toward low- and zero-carbon energy systems in response to climate change, ammonia-hydrogen internal combustion engines have emerged as a promising and increasingly studied solution in the transportation sector due to their potential for zero carbon emissions. Ammonia offers several advantages as a fuel, including high hydrogen energy density, ease of storage and transport, and excellent anti-knock properties. However, its inherently slow combustion characteristics and nitrogen-containing nature bring significant challenges, particularly in terms of high nitrogen oxide (NOx), unburned ammonia (NH3), and nitrous oxide (N2O) emissions. Optimizing the combustion process of ammonia-hydrogen fuels and achieving near-zero emissions in internal combustion engines (ICEs) are relatively new research areas that presents formidable technical challenges.
This paper reviews the latest research developments in the emission characteristics and near-zero emission control strategies for ammonia-hydrogen fueled ICEs. First, in terms of emission mechanisms, NOx formation during ammonia combustion is governed by complex pathways and is highly sensitive to equivalence ratio, pressure, and temperature. Earlier mechanistic studies focused primarily on low-pressure and medium-to-high temperature conditions, which differ significantly from the high-pressure, high-temperature environments of modern engines, highlighting a current gap in research. Second, in-cylinder pollutant formation and control remain key to emission reduction. In-cylinder control techniques, including optimization of fuel injection strategies, ignition timing, and intake conditions can effectively balance the relationship between emissions and thermal efficiency. Studies have shown that hydrogen enrichment can improve combustion efficiency and reduce NH3 and N2O emissions, though it may increase NOx formation. Lastly, aftertreatment technologies are critical to achieving near-zero emissions. Due to the unique characteristic of emissions from ammonia-hydrogen combustion, new dedicated aftertreatment systems are required. These include selective catalytic reduction (SCR) for NOx, ammonia slip catalysts (ASC), and strategies for addressing high global warming potential gases such as N2O. Additionally, hydrogen-selective catalytic reduction (H2-SCR) offers a novel pathway for mitigating hydrogen-related emissions in such engines. Future researches should focus on the synergistic optimization of in-cylinder combustion and specific aftertreatment systems, the development of low-temperature, high-efficiency catalysts, and the exploration of integrated aftertreatment solutions to meet increasingly stringent emission regulations and approach near-zero emissions. While ammonia-hydrogen dual-fuel ICEs hold significant promise in achieving carbon neutrality, their widespread adoption will require overcoming several technical challenges, particularly in emission control.
