Based on a three-dimensional simulation model established using CONVERGE

the effects of exhaust gas recirculation and the Miller cycle on the combustion characteristics and emissions were studied to explore the feasibility of the Miller cycle coupled with EGR to meet Tier Ⅲ emission regulations for low-speed high-pressure direct injection natural gas marine engines. The results show that using 30% of EGR rate alone can meet Tier Ⅲ emission standards

but with a significant increase in the indicated fuel consumption and soot emissions; the Miller cycle is not as effective as EGR in the potential to reduce NOx emissions; excessive exhaust valve closing delay will cause increase in compressor workload

and lead to a considerable increase in fuel consumption when the same amount of NOx emissions is reduced. Coupling mild Miller cycle(delaying exhaust valve closing by 5° in crank angle) with 25% of EGR rate and the proper natural gas injection timing(advancing 2° in crank angle) can reduce NOx emissions by 77% with only a slight increase in the fuel consumption by 1.58%

which is a feasible way to meet Tier Ⅲ emission regulations.