3-D Simulation On Marine Low-speed Two-stroke Diesel Engine In-cylinder NO_x Control Measures

With the advantage of high capability in power,thermal efficiency and reliability,two-stroke marine diesel engines have become the major source of propulsion in maritime transportation.Heavy oil is mainly used as the fuel of low-speed two-stroke marine diesel engines.The economy is greatly improved whilst pollution issues become very serious.The International Maritime Organization(IMO)regulation in NO_x emissions is gradually stricter.Tier ? emission regulation,which came into force in the year of 2016,requires a decrease of NO_x limit by 74%?76%compared to its earlier level Tier II.Hence,to satisfy the Tier ? regulation on NO_xemissions of two-stroke marine diesel engines,developing NO_xcontrol technologies for marine diesel engines is of great urgency.The present paper uses three-dimensional(3-D)simulation on a two-stroke marine diesel engine by the use of CONVERGE software.Firstly,a 3-D model is built according to the engines configuration.Secondly,model is calibrated and validated to be accurate with test data.Thirdly,analysis under baseline condition is carried out on distributions of equivalence ratio,oxygen concentration,temperature and NO_x,as a basis for later works.The parameter settings of the injector in the original model are modified to analyze the influence of the nozzle hole diameter,the number of nozzle holes and the angle of the injection on the combustion and emission.The results show that the nozzle hole diameter has a great influence on the NO_xemissions,which is almost linear within the range of 0.90?1.05 mm.The NO_xemissions decrease by about 16.7%for every 0.05mm increase of the diameter.The number of nozzles and the angle of injection affect the oil spray distribution in the horizontal and vertical directions,respectively,which in turn affect the combustion process.The EGR and Miller cycle are applied in the original model,and various EGR rates and Miller timings are schemed.The influences of EGR and Miller cycle on combustion and emissions are analyzed by observing the distributions of the in-cylinder parameters,temperature,NO_xemissions and fuel consumption.The results show that EGR can significantly reduce NO_xemissions.With the increase of EGR rate,the peak pressure and cylinder temperature decrease significantly.The overall NO_xemissions decrease in a“reversed S”shape.The Tier ? level is satisfied at 31%EGR rate whilst the fuel consumption is 9.6%higher than that of the baseline engine.The Miller cycle has a great influence on cylinder pressure.As the Miller cycle is deepened,the NO_x emissions have a nearly linear but limited decrease.For further study in NO_xreduction potential by injector parameters,the nozzle hole diameter is respectively coupled with the number of nozzle holes and the angle of injection.Three optimization schemes,S1,S2 and S6,are picked out to play a role in improving the NO_x-BSFC trade-off after analysis.In order to decrease the impact of paticulate matters and sulphides in EGR exhaust gas on engine and equipment,the use of EGR rate should be lowered,thus the coupling of EGR with Miller cycle is implemented.Two of the coupling schemes,EM3 and EM8,reach the Tier ? level,but the fuel consumption increases.In order to control the fuel consumption increase of the above Scheme EM3 and EM8,the optimized injector parameter schemes,S1,S2 and S6,are added to study the combined effect of EGR,Miller cycle and injector parameter settings on NO_xand fuel consumption.The results show that the combination of the three can effectively reduce the fuel consumption while keeping the NO_xemissions basically unchanged.Two schemes that meet the Tier ? level are picked out,namely Scheme EM3S2 and Scheme EM8S2,whose fuel consumption decreased by 5.67%and 4.95%that of the Scheme EM3 and Scheme EM8,which can be used as a feasible solution for Tier ? emission regulation for two-stroke marine diesel engines.