Numerical Study On Low-speed Marine Diesel Engine To Meet Tier ? Regulation

With the development of shipping industry and the depravation of environmental pollution,stringent emission regulations have been proposed to control the pollutants emissions from marine diesel engines,with the objective of simultaneously reducing both the NO_x emission and break specific fuel consumption.In the current study,numerical investigations have been conducted to develop combustion system that could meet the Tier?emission regulation on a large bore two stroke marine diesel engine.Firstly,a three-dimensional CFD model of the objective marine diesel engines was established and validated.A micro-genetic algorithm was applied to optimize the injection strategy of both injectors,and a strategy of pilot injection coupled with sequential injection was found to be able to greatly reduce the NO_x emission without significant penalty in fuel consumption.It was found that the interval between sequential injection affects both the NO_x and fuel consumption.The effects of pilot injection parameters on emission and performance are different between the first and second injectors:the former is mainly due to the pilot heat release and shorter ignition delay of main injection,while the latter is mainly due to the pilot fuel heat release.Based on the optimized injection strategy,technical routes to meet the Tier III regulation were further explored with this CFD approach,including miller cycle,high geometric compression ratio,EGR,humid air motor(HAM)and direct water injection(DWI).It is seen that the miller cycle through exhaust valve closing timing retarding,can significantly reduce NO_x emission;however,intake charge flow mass reduces,and consequently higher fuel consumption is observed.Fuel consumption can be dramatically reduced without much penalty in NO_x emission when higher geometric compression ratio is adopted with miller cycle,which enables the possibility to further reduce NO_x with EGR without consideration on fuel consumption.It was found that coupled with miller cycle and high geometric compression ratio,20%EGR ratio is capable of meeting Tier?regulation with a 5.7%increase in fuel consumption.It is also known that HAM can reduce NO_x emission through improving the intake charge specific heat capacity,however,intake humidification highly depends on the intake charge thermodynamic state,which also deteriorates the fuel consumption.DWI shows notable potential in both NO_x and fuel consumption reductions.Combined with appropriate intake humidification,DWI with optimized injection strategy(high injection pressure,medium water injection volume and earlier water injection timing)can meet the Tier?regulation with 1.2%reduction of fuel consumption.In addition,SCR can also meet Tier?,however,penalty in equivalent fuel consumption(urea consumption was taken into consideration)(3.2%)was also observed.It is seen that the current study could provide significant theoretical and application values for the development of combustion technical routines to meet the Tier?emission regulation.