Based on the computational fluid dynamics(CFD) software(CONVERGE)

the simulation model of a large low-speed two-stroke marine engine in which pilot diesel and natural gas are injected directly in cylinder was established. Validated by the experimental data from the literature

the model was then applied to studying the suppression effect of the Miller cycle and natural gas two-stage injection strategy on the pressure oscillation in high pressure direct injection mode. The calculation results indicated that when the exhaust valve was closed later

the overall combustion pressure was lower

both the combustion phase and the moment of severe pressure oscillation were delayed. The intensity of pressure oscillation was reduced with smaller delay of exhaust valve closing

while it was increased instead with an excessive delay. This was primarily attributed to the competitive relation between in-cylinder thermodynamic state and auto-ignition characteristics of fuel. Adjusting the pre-injection quantity of natural gas would reduce the formation of premixed combustible mixture before combustion

thus reducing the proportion of premixed combustion in the combustion phase

which could effectively suppress pressure oscillation

but could also reduce the output power and increase the fuel consumption. Compared with the pre-injection quantity

varying injection interval has less effect on the pressure oscillation

but the excessive injection interval would affect the combustion phase

resulting in considerable power loss. A reasonable injection strategy can ensure the power output while suppressing intense pressure oscillation.