An active in-cylinder DeNOx strategy for two-stroke low-speed ammonia marine engines

Ammonia is considered one of the most promising alternative fuels for marine transportation in achieving the 2050 net-zero greenhouse gas (GHG) emissions target. However, the combined formation of thermal- and fuel-borne NOx leads to high NOx emissions, making it a major challenge for ammonia engines. Moreover, according to the most recent experimental results, due to the long chemical timescale of low-speed marine engines, the amount of unburned NH3 in the exhaust is usually insufficient for effective NOx reduction in SCR aftertreatment systems. This paper proposes an active in-cylinder DeNOx strategy for high-pressure direct-injection (HPDI) ammonia-biodiesel dual-fuel marine engines. By utilizing liquid ammonia post-injection, the strategy facilitates in-cylinder NOx reduction through selective non-catalytic reduction (SNCR) reactions, thereby mitigating the need for catalysts and aftertreatment systems. The study reveals that intermediate radicals, such as NHi and NNH, play a crucial role in SNCR, with its efficiency highly influenced by in-cylinder temperature and equivalence ratio. Three-dimensional CFD simulations were conducted to investigate the effects of post-injection fraction, timing, and excess air ratio on NOx reduction efficiency and engine performance. Results indicate that properly increasing the post-injection fraction and retarding injection timing enhance NOx reduction. However, excessively delayed injection or overly large fractions significantly decrease ITE and sharply increase N2O and unburned NH3 emissions. Additionally, higher excess air ratios improve ITE by reducing heat transfer losses and effectively suppress NOx emissions but also lead to higher NH3 and N2O emissions. Finally, under the optimized condition, the proposed strategy achieves over 40 % NOx reduction, meeting IMO Tier Ⅲ limits while maintaining high ITE (with only minor ITE loss) and keeping N2O and unburned NH3 at ultralow levels, along with increased exhaust energy for potential waste heat recovery. Moreover, with a high ammonia substitution rate (95 %) and low N2O emissions, GHG emissions are reduced by over 95 % compared to conventional diesel engines, demonstrating strong potential for zero-carbon marine transportation.