| Diesel engine is the main power source of light-duty vehicle in our country.In order to meet the requirements of state V and state IV emission regulations for the light-duty vehicle,the techniques such as turbocharge,inter-cooling turbocharge and electronically controlled fuel system are widely used to improve the combustion process and decrease PM emission,this lead to cylinder pressure increasing.At present,the maximum explosion pressure of diesel engine satisfying the state V emission regulation has reached to 16 MPa,and even higher.As a result,the mechanical load of crankshaft is increased,and the strength and fatigue reliability of crankshaft are facing challenging.The strength and reliability of the crankshaft depend to a great extent on crankshaft design,physical and chemical properties of the material and the strengthening process.The crankshaft of 480 Q diesel engine satisfying the light-duty vehicle is taken as the research object,the strength simulation model is established,and the research of the crankshaft material,the process,and stress,strain and fatigue reliability of the crankshaft in different conditions is carried out by the methods of simulation and experiments.According to the physical and chemical properties of material and the characteristics of strengthening process are analyzed.To meet requirement light-duty car,the QT800-2 ductile iron has been selected as the crankshaft material.The influence of different material and process to increase crankshaft strength are analyzed.Some strengthening process,such as the undercut fillet rolling,induction quenching of the journal has been selected.Based on requirements of 480 Q diesel engine,some properties of material such as spheroidization level of graphite,graphite ball diameter,and pearlite content of QT800-2 ductile iron material are analyzed by the instrument of MR2000 inverted metallographic microscope.The measurement test for carbon,sulfur,phosphorus,silicon,manganese and magnesium in the material is carried out by CS-744 analyzer and ARL3460 direct-reading spectrometer,to analyze the effect of element content on its properties.The tensile strength,elongation,hardness and other mechanical properties of QT800-2 ductile iron are measured by CMT5305 electronic universal testing machine and Brinell hardness tester.The results show that the spheroidization level of grade 2,the graphite ball diameter of grade 6,the pearlite content of 95%,the average Carbon content of 3.72%,the Sulfur content of 0.01%,the Phosphorus content of 0.029%,the Silicon content of 2.06%,the Magnesium content of 0.044%,the Manganese content of 0.46%,the Cuprum content of 0.77%,and the Cerium content of 0.021%.The average tensile strength is 926 MPa,with the elongation of 4.2% and the hardness of 292 HB,the structure properties of the material after normalizing can meet the requirement of crankshaft.The one-cylinder crankshaft model and the whole crankshaft model of 480 Q diesel engine are established and meshed in full hexahedral format by using the ANSYS Workbench simulation software.The different boundary according to the crankshaft working conditions have been set up respectively.The 4 different positions of crankshaft and the conditions of the journal of connecting rod with different load are selected,the stress and strain of the one-cylinder crankshaft model have been calculated and analyzed respectively.While the stress and strain of the whole crankshaft model in 4 different positions are also explored,and finally,the crankshaft strength is checked.The results show that the maximum stress for the one-cylinder crankshaft model occurs mainly at the transition fillet of the main journal,with the maximum value of 370 MPa,and the maximum strain occurs at the far end of the balance block,with the maximum value of 0.02 mm,while the stress concentration for the whole crankshaft model mainly occurs at the crankshaft fillet of the main journal,and the maximum stress when the fourth cylinder fires appears on the transition fillet of the journal of the right connecting rod of the fourth crank and the transition fillet of the right main shaft,with the maximum stress of 318 MPa in the entire cycle.The maximum stress value of the whole crankshaft model is much less than that of the one-cylinder crankshaft model,the main reason is that the one-cylinder crankshaft model cannot consider the interacting influence of other cylinders on the crankshaft load when they are working.And the strength safety coefficient of the crankshaft is 2.16.It can meet the strength requirement after calculating and checking.The stress of the journal of crankshaft connecting rod of 480 Q diesel engine is studied.The stress and strain of the journal of crankshaft connecting rod are measured by strain gauge,the variation of stress,strain and test load are discussed,and the linear equation of crankshaft bending moment and the stress and strain is deduced.The strain-moment equation of the journal of crankshaft connecting rod is ?(28)-20.00381(10)1.51836 M,and the stress-moment equation is ?(28)-3.76667(10)0.285 M,which provides the basis for prediction of stress and strain of the journal of crankshaft connecting rod.Three fatigue strength test methods for crankshaft are analyzed.In terms of the crankshaft of 480 Q diesel engine,the bending fatigue test is carried out with PDC-2 electrical resonant fatigue test device by using the dual up-and-down method.13 crankshafts are selected to add different load,and the relationships between bending moment fatigue limit and reliability of the crankshaft under different survival rate are explored.The fatigue limit value of the crankshaft under the survival rate of 50% is 1533N·m,with the allowable safety coefficient of 2.08,and the fatigue limit value of the crankshaft under the survival rate of 99.99% is 1373N·m,with the allowable safety coefficient of 1.86.The simulation results and those data of the crankshaft of 480 Q can achieve the allowable safety. |
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