A lean–rich coupled combustion strategy for high efficiency and low emission in large low-speed dual-fuel marine engines

Decarbonizing large two-stroke marine engines remains a central challenge for meeting forthcoming IMO carbon regulations. Although ammonia and other low-carbon fuels have attracted growing interest, existing combustion strategies still struggle to simultaneously achieve high thermal efficiency and low emissions under engine-relevant conditions. This work proposes a Lean–Rich Coupled Combustion (LRCC) strategy that reorganizes staged heat release by combining a globally lean premixed background with a directly injected fuel-rich stage. A numerical study is conducted on a low-speed two-stroke natural gas–diesel dual-fuel engine, with direct ammonia injection introduced to investigate the LRCC concept in conjunction with the Miller cycle to assess the feasibility of high compression ratio operation. The results demonstrate that LRCC enables knock-free operation at an elevated compression ratio of 18, increasing indicated thermal efficiency to 52.8%, corresponding to a 5.8% improvement, while maintaining baseline power output. Analysis of unburned- and burned-region ammonia injection modes reveals that unburned-region injection induces strong evaporative cooling and forms a low-temperature dilution layer ahead of the flame front, reducing wall heat-transfer losses and suppressing thermal-NO formation. Across a wide control space of ammonia injection timing, injection pressure, and pilot-fuel phasing, the LRCC strategy shows the potential to approach the 3.4 g/kWh IMO Tier III NOx limit without aftertreatment, while maintaining very low NH3 and N2O emissions. By partially substituting natural gas with zero-carbon ammonia and achieving higher thermal efficiency, the combined Miller–LRCC strategy reduces CO2 emissions by about 56% relative to a diesel baseline on a tank-to-wake basis, supporting alignment with IMO mid-term decarbonization objectives.