| With the wide application of marine diesel engines and the implementation of increasingly strict emission regulations,reducing pollution emissions of marine diesel engines is attracting more and more attention.The factors that affecting the combustion and emissions of engines can be summarized into three aspects:intake system parameters,fuel injection system parameters and combustion system parameters.In this paper,the effect of fuel injection system parameters(pilot injection timing,spray included angle,post injection ratio and post injection duration angle)and intake system parameters(swirl ratio and EGR rate)with multiple fuel injection strategy on combustion and emissions are studied.This research is carried out on a six-cylinder four-stroke supercharged and intercooled marine diesel engine by using simulated method.Pilot injection preheats the combustion chamber by a small amount of heat release,which affects the combustion process and emission characteristic.Earlier pilot injection timing moves forward the instantaneous heat release rate(IHRR)curve generated by early main injection fuel,shortens the ignition delay and advances combustion center.Advancing pilot injection timing at 145° spray included angle,increases the soot generation in the early stage of combustion,but this also promotes the soot oxidation in the later stage of combustion.Therefore,soot emission firstly increases and then decreases.Spray included angle changes the in-cylinder fuel distribution,and hence affecting the uniformity of air-fuel mixing.Larger spray included angle increases the peak value of in-cylinder pressure and the IHRR generated during main combustion.Thus,the increase of spray included angle advances combustion center,and significantly reduces soot emission,as well as the indicated specific fuel consumption.150° spray included angle produces the least local high temperature region and achieves the lowest NOx emissions.Post injection promotes turbulent mixing and combustion in the later stages of combustion.The increase of post injection ratio decreases the IHRR peak generated by main combustion,increases the IHRR peak generated by the combustion of post injection fuel.This also decreases peak in-cylinder pressure and mean temperature.Therefore,larger post injection ratio extends combustion duration and reduces NOx emissions.Coupling a large post injection ratio when using a small post injection duration angle,or coupling a small post injection ratio when using a large post injection duration angle is beneficial to reduce soot emission.Larger post injection duration angle smooths the combustion of post injection fuel and hampers the local high temperature region,resulting in reduction of NOx emissions.This also firstly increases and then decreases the combustion duration.However,increasing post injection duration angle results in more post injection fuel being burned in a low-temperature and oxygendeficient environment,which inhibits soot oxidation and then deteriorates soot emission.Specifically,the increase in post injection ratio at 4℃A or 6℃A post injection duration angle simultaneously decreases NOx and soot emissions.The dilution effect of EGR reduces the oxygen content of air-fuel mixing,and the thermal effect of EGR enhances the heat absorption capability of the in-cylinder mixing.The increase of EGR rate slows down combustion rate and decreases the local high temperature region.Therefore,this prolongs ignition delay and postpones combustion center.Besides,larger EGR rate significantly decreases NOx emissions,but it also increases soot emission.Increasing swirl ratio effectively promotes initial fuel evaporation and mixing with air,which accelerates the initial combustion and shortens ignition delay.Meanwhile,larger swirl ratio adversely affects the combustion in the middle and late combustion process,resulting in prolonged combustion duration.Decreasing swirl ratio within a certain range can reduce both NOx and soot emissions,and this range is determined by EGR rate. |
PDF Full Text Request