Simulation Study On Optimization Of Combustion And Emission For Mixed Naturalgas-Diesel Engine

With the increasing pollution of atmospheric environment and the increasingly strict regulations of IMO,the demand to control the destruction of emissions on atmospheric gases and greenhouse gas is increasingly urgent.The quest to reduce Marine engine emissions has become a priority.In this paper,the research object of the hybrid natural gas-diesel engine modified by the marine medium-speed pressurized 4190 diesel engine was studied.AVL-BOOST software and AVL-FIRE software were used to build a three-dimensional cylinder internal high-pressure cycle model.Through this model,the influence of gas substitution rate and EGR mixing ratio on combustion characteristics,economic characteristics and emission characteristics of engine under different load characteristics were studied and the parameter matching of fuel injection time,EGR mixing ratio and natural gas blending ratio was studied.First of all,AUTO-CAD software was used to draw two-dimensional external contour lines of combustion chamber which was imported into the ESE module of AVL_FIRE software,and the2d and 3d grids were divided.The appropriate chemical reaction mechanism was selected for the natural gas-diesel fuel mixture,which is the coupling of the mechanism file in the FIRE software.Then the appropriate initial conditions,boundary conditions and submodules are set.Finally,the accuracy of the model was verified by the comparison between the simulation value of the cylinder pressure curve and the test value,and the high pressure circulation model of the natural gas-diesel engine was established.Secondly,natural gas mixing ratios,EGR mixing ratios and engine load points were selected as influencing parameters to simulate the combustion characteristics,emission characteristics and economic characteristics of hybrid natural gas diesel engines through the high pressure circulation model of the natural gas-diesel engine.Then influence rule of the combustion,emission and comprehensive fuel consumption of the engine under the interaction of three parameters was obtained.In the rated condition,the mixing ratios of natural gas were 50%,60%,70%and 80%respectively,and the EGR mixing ratio were 0%,10%,15%and 20%respectively.In partial load,the blending ratios of natural gas were 50%,60%and 70%respectively,and the mixing ratios of EGR were 0%and 10%.The parameters were brought into the simulation model for calculation.According to the 2d results and 3d results under rated load and the 2d results under partial load analysis,the effect law of these parameters on pressure,temperature,heat release rate,comprehensive fuel consumption,the mass fraction of NO and CO,as well as the distribution of physical fields in cylinder were analyzed.Lastly,natural gas blending ratios,EGR mixing ratios and oil injection time were selected,and the orthogonal design method was used to arrange test simulation calculation,then fuzzy mathematical method was used to analyze the simulation data.Under 100%load and 50%load,in order to reduce engine NO_X emissions and lower fuel consumption rate,match the selected parameters respectively.Finally,under different working point,a group of optimal parameter which reach a purpose of lowering engine NO_X emission and lowering synthesized fuel consumption was obtained and provided a theoretical basis for the design and transformation of hybrid natural gas-diesel engine.