Numerical Study On The Combustion And Emission Characteristics Of High-pressure Direct Injection Natural Gas Marine Engine

Seagoing vessels,known for their efficient means of transportation,cause substantial emissions into the environment.Nowadays,tougher emissions regulations for seagoing ships,increasing fuel prices and low-quality residual fuels are the major challenges facing the marine industry.This requires the development of novel emission reduction technologies.Meanwhile,alternative fuel,such as natural gas,is also conceivable.Hence,this demands the development of clean and efficient gas engines.Firstly,the 3D model of the marine engine was established using CONVERGE and sufficient validations were processed by comparing simulation results with experimental data from literatures.Based on that,the effect of EGR and gas fuel injection rate shapes on the performance and emissions of HPDI natural gas marine engines were investigated.The results show that,compared with traditional marine engines,engines mainly fueled with natural gas can fulfill requirements of the emission regulation with lower EGR.And,the further compromise between engine performance and emission can be achieved if EGR is combined with fuel injection strategies.However,comparisons among different gas injection rate shape reveal that cases with triangle injection rate shape have a higher peak value of in-cylinder pressure and heat release rate as well as shorter combustion duration,which result in higher combustion temperature and NOx emissions.Cases with rectangle and slope injection rate shapes have lower combustion temperature as well as lower NOx emissions.But soot and UHC emissions are higher due to the initially sharp fuel injection rate and lower in-cylinder temperature in the late combustion stage.A preferable balance between fuel consumption and emissions can be achieved in cases with wedge and trapezoid gas injection rate shape.On this base,investigations were adopted to analyze the effect of the start of fuel injection and gas twice injection strategies on combustion and emission of HPDI natural gas marine engines.The results show that the variations of pilot injection timing have less influence than that of gas fuel injection timing on the combustion process.However,it also reveals that minor improvements,both fuel consumptions and NOx emissions,can be achieved by adjusting the fuel injection timing of these two fuels.Either the advancement of gas fuel injection or the retard of pilot fuel injection can increase the ratio of premixed combustion.However,the combustion phase is delayed due to the retard of gas fuel to be ignited in the latter case.Meanwhile,either retard of the gas fuel injection or advancement of pilot fuel injection can enlarge the injection interval of two fuels.In the former case,combustion phase is delayed and NOx emissions can be improved slightly.And,in the latter case,there are no obvious changes in the in-cylinder pressure and temperature,but the apparent drop in UHC and soot emissions can be observed.For gas pre-injection strategies,premixing level of gas fuel with fresh air can be influenced as well as the ratio of gas pre-mixed combustion after ignited by the flame of pilot fuel.Reasonable gas pre-injection strategies can improve both the fuel consumption and NOx emission,while higher soot emission is inevitable compared with the single injection strategy.For gas late-injection strategies,gas combustion in late-injection part can be interfered by main-injection part with longer combustion duration and higher fuel consumption.However,NOx emissions can be improved in a certain extent as larger late-injection ratio of gas fuel is adopted,and,in smaller late-injection ratio,soot emissions are better than the single injection case.To summarize,the present work investigates the effect of EGR and fuel injection strategies on the combustion process and emissions for the HPDI natural gas marine engine.It has practical value for the development of new natural gas marine engines.