Performance Simulation And Optimization Of Marine Diesel Engine Based On Intake Air Humidification

In the face of increasingly serious environmental problems,the IMO has developed more stringent emission regulations for marine diesel engines,particularly in terms of NO_x emission limits.How to effectively reduce NO_x emissions,and compromise soot emissions as well as fuel consumption has become a research focus.Intake air humidification is able to suck water vapor into cylinder,which can optimize combustion process and reduce pollutant emissions.In this paper,based on the intake air humidification,the effects of different intake conditions on the in-cylinder combustion and emissions are researched by means of simulation.A six-cylinder turbocharged and intercooled marine diesel engine was used as the prototype.Firstly,one-dimensional numerical simulation,under the corresponding operating conditions,was carried out by using GT-POWER software and the corresponding initial conditions were provided for the three-dimensional calculation.AVL-FIRE software was used to establish three-dimensional combustion model.On this basis,the research about the effects of intake air humidification and EGR on the combustion and emissions of marine diesel engine were carried out.The similarities and differences between the two methods were compared and analyzed.Finally,the method of intake air humidification coupled with Miller cycle was explored.The results showed that intake air humidification reduced in-clyinder peak pressure and temperature,prolonged ignition delay and combustion duration,which increased the proportion of premixed combustion.The center of combustion and heat release was offset backwards,which brought down combustion efficiency.Due to the reducing of peak combustion temperature in flame area and in-cylinder oxygen concentration,NO_x emissions decreased with the increase of humidifying level.However,the increasing of equivalence ratio and the decreasing of the oxidizing capacity in the later stage of combustion deteriorated soot and CO emissions.Therefore,the humidification ratio must be strictly controlled when the intake air humidification is used to reduce the NO_x emissions.The effect of EGR on the combustion and emissions was similar to that of the intake air humidification,which also resulted in the decrease of the peak pressure and combustion temperature,as well as the prolongation of ignition delay and combustion duration.However,when comparing the two methods with the same reduction ratio of NO_x emissions,the impact of EGR on ignition delay,combustion duration and CA 50was higher than that of intake air humidification,the deterioration of combustion efficiency,soot and CO emissions was also more obvious.Therefore,compared with EGR,intake air humidification has the potential to balance the relationship between NO_x and soot emissions as well as fuel consumption.Delaying the intake valve closing timing was able to reduce intake air charge and in-cylinder temperature at the end of compression,prolong ignition delay and increase instantaneous heat release rate and premixed combustion ratio.Delaying the intake valve closing timing had the equivalent effects to increasing the humidifying level for lowering in-cylinder peak pressure.While the effect of reducing NO_x emissions was less significant than that of increasing humidifying level,but the deterioration of soot and CO emissions was also less.Therefore,a further reduction in NO_x emissions could be achieved by coupling late intake valve closing with intake air humidification,thereby reducing negative effect of intake air humidification at high humidifying level.