Effects Of Intake Port Water Injection On Knock,Combustion And Emissions Of GDI Engine

Gasoline direct injection(GDI)engine has been gradually studied and popularized for its precise control of fuel injection strategy,as well as the high efficiency,low fuel consumption and emissions.However,due to its high compression ratio,the heat load and pressure in the cylinder of GDI engine increasing greatly is easy to cause knock,which greatly restricts the improvement of engine performance.Water has a large latent heat of vaporization and can participate in a series of combustion chemical reactions,so injecting it into GDI engine can effectively inhibit the occurrence of knock.In this paper,the numerical simulation method is used to study the influence of intake port water injection on combustion.Firstly,the knock of a GDI engine is induced by adjusting the clearance volume to increase the compression ratio and advancing ignition angle;Then,the layout of different port water injectors and the influence of injection pressure on the distribution of water droplets,the combustion and emission of the engine are explored;Finally,under the given layout of the water injector and the condition of the water injection pressure,the influence of different water injection proportion on the knock combustion and emission of the GDI engine are explored,as well as the possibility of increasing the engine cycle power under the water injection condition.Firstly,the model of the engine is established by three-dimensional software,and the corresponding sub models are selected according to the mechanism of each model.The start and end time of simulation calculation are 360°CA and 960°CA,respectively.Through the verification of grid independence,the basic size of grid is chosen to be4 mm.Under the condition of low speed and high load with the speed of 2000 r/min,the accuracy of the model is verified by comparing the simulated average pressure curve with the experimental data.In order to achieve the knock condition of the engine,which is realized by increasing the ignition advance angle to 704°CA and increasing the compression ratio of the original engine model from 9.6 to 10.2.Then,according to the port structure of the engine model,two kinds of water injector layout schemes,high inclined and low aclinal,are selected to explore the influence of the position of the water injector on the movement of water droplets,turbulent kinetic energy and equivalent ratio distribution before ignition.The results show that the lower the arrangement of water injector without angle,the water droplets are less likely to collide with the port and can be injected into the cylinder earlier when adopt the low aclinal way,which can also effectively improve the turbulence intensity and the equivalent ratio distribution before ignition at the same time;Then,under the arrangement scheme of low aclinal water injector,three kinds of injection pressures,2.5bar,5bar and 7.5bar,are selected to simulate the influence of different injection pressures on combustion and emission of GDI engine.The results show that the inhibition effect of the high inclined arrangement is less than that of the low aclinal;And that of the low aclinal,the inhibition effect of the 2.5bar injection pressure is the weakest under the arrangement scheme of low aclinal.Finally,in the third chapter,under the simulated knock condition,choose the low aclinal layout scheme,set the injection pressure to 5bar,simulate and study the influence of the port injection on the combustion,cycle capacity and emissions of the GDI engine under the six conditions of the injection ratio of 0,10%,20%,30%,40%and 50%.The results show that the auto-ignition trend of the mixture at the end of the inlet can’t be inhibited by the water injection at low water injection ratio,but when the water injection ratio reaches 30%,the knock intensity and pressure amplitude are obviously reduced,and the knock combustion trend is effectively inhibited;The cumulative heat release decreases with the increase of the injection proportion,and the cycle work amount reaches the maximum when the injection proportion is 30%;The emission of Soot and CO increased with the increase of water injection proportion,but the emission of NO_x decreased significantly with the increase of it,and the emission of NOx decreased 91.97% under 30% water injection proportion compared with that without water.