Simulation Study On Combustion Process Of Diesel Engine For Light Duty Vehicle

The crisis of energy and the population of environment become a focusrecently. Dependent on high thermal efficiency and good economy, thediesel engine is widely used in the world. The key to improve the emissionlevel is to degrade the NOx and particulate. The main method is to improvethe diesel engine combustion to satisfy the strict emission regulations. Thediesel engine combustion is a physical and chemic process which is complexand changeable. Experiment is the main combustion research method. Theresearch method used in this thesis is software simulation with FIRE whichis developed by the Austrian AVL company. Without the experiment restrict,this software can simulate the combustion in the combustion chamber andget the particular information which is difficult to measure. In this thesis the compression stroke, combustion stroke and expansionstroke of direct injection CA498 diesel have been simulated. Because there isno intake stroke, details like intake ports and valve are not considered. Asimplified geometry is used to reduce the effort for mesh generation. Inmesh generation process, triangular combustion chamber from Pro/E wasimported. To get the correct compression ratio in the model withoutchanging the characteristics of the flow field in the bowl (squish flow), acompensation volume is added at the outer part of the *.stl surface model.This volume accounts for geometrical details which cannot be considered inthis segment approach. Because of relatively complex structure, combustionchamber is meshed with hexahedron unstructured grids by automatic meshgenerating order. The ground of the bowl should be meshed with at leastone continuous layer for a proper calculation of the heat transfer through thepiston wall. The cylinder mesh at the different crank angle has beengenerated by moving grid maker in this thesis. Because of movementcharacteristic of the piston, the cylinder is meshed with hexahedron gridwhich short the calculation time more. As the piston moves up and grids ofcylinder are compressed, grids of the cylinder field are meshed by theRedimension order to keep the cell size at about 2.0mm×5.3mm×1.8mm. Themesh density should have approximately ten times the diameter of thenozzle hole. The parameters in the model were adjusted to ensure theindicated diagram resulting from the simulation accorded with the datafrom the testing.The influence has been studied in this thesis which is the injection timing,EGR and the intake temperature on the NOx and particulate in diesel engineemissions. The ignition delay period is controlled by the fuel injectionadvance angle which is an important parameter. If the advance angle ofinjection is adjusted earlier, the density of the combustion chamber is smalland its temperature is low, when the fuel is injected into the combustionchamber. So it needs long time to atomization. SMD of the spray is biggerwhich indicates that the gasification surface of the spray is small. The fuel isatomized worse. And the We of the fuel is large, it dedicates the spray isunstable and has bigger inertia force than the surface tension. It will bebroken up until reach the critical We. In a word, if the advance angle ofinjection is adjusted earlier, the ignition delay period will become long andthere will be more mixture in the premixed period. When the combustionstarts at this condition, the maximum combustion temperature, pressure, themaximum pressure increaser ratio all increased. The increase of NOxemissions is resulted by the high temperature. But the energy released in thediffusion period decreased, soot emissions is less. If the advance angle ofinjection is adjusted too earlier, knock phenomenon will been show in thecombustion process.EGR is a good way to improve NOx emission. If EGR is big, the ignitiondelay period will become long. In spite of the SMD of the spray is small,which indicates that the spray mixed well before the combustion begin, theamount of the mixture don't increase, because of the short of oxygen. Theformation of the NOx and soot is restrained by the lower combustiontemperature and the less intensity of the combustion enormously which iscontrolled by EGR. Though the mount of formation of the soot become small,the soot is not in oxidation resulted by EGR and the particulate increases.Combustion process will be improved by increasing charge air.Combustion temperature will be lower by cooling the intake temperature.The formation of the NOx will be restrained by this method and emissionsare improved, but particulate decreased less.The combustion mechanism of diesel engine CA498 has been studied bythe FIRE software in this thesis. The advanced research later has beenprepared.