| With the enhancement of diesel engine,the in-cylinder burst pressure increases continuously.At present,the maximum explosive pressure of foreign advanced turbocharged diesel engines has reached more than 25 MPa,and the maximum explosive pressure of domestic turbocharged diesel engines has reached more than 23 MPa,and is moving towards a higher goal of explosive pressure research trend.In the process of engine operation,piston bears high temperature and mechanical alternating load.Through finite element analysis and test,it is found that the stress distribution along the axis of pinhole is unbalanced due to the inconsistency of deformation,and the stress gradient is relatively large.There is a very serious phenomenon of local stress concentration in the pin seat of piston,while the pin seat is bitten by high temperature and excessive pressure.And the excessive mechanical load causes the pin seat cracking and even the whole piston to be destroyed.In order to improve the stress distribution of pinhole and the bearing capacity of piston pin seat,the improvement scheme of special pinhole and copper sleeve at pinhole was put forward.The research object is the piston of high power density diesel engine.The finite element analysis model of three-dimensional piston of diesel engine is established by numerical simulation analysis method.The influence of the structure of special pin hole and pin hole copper sleeve on the stress distribution of piston is studied in order to achieve the design goal of the diesel engine withstanding the maximum explosive pressure of 23 MPa and the maximum working temperature of 420 C.The main research work is as follows:(1)Verification of mesh independence of piston finite element modelThe sensitivity of the results to the number of meshes in axial,circumferential and thickness directions at pin holes is analyzed.Based on the thermo-mechanical coupling stress,the chamfering structure of the oil return hole,inner cavity and the third ring groove of piston is finally determined.The grid structure unit with the size of 1mm is used for the thermal insulation coating,and the 3mm structure unit is used for the skirt of piston where the size is more regular.The shore and inner cavity adopt 2 mm mesh structure unit.The mesh type of piston is tetrahedron element,the number of nodes is 62851,and the number of elements is 290474.(2)Effect of Piston Pinhole Structure Parameters on Piston StrengthIn the process of engine operation,piston bears high temperature and mechanical alternating load.Through finite element analysis and test,it is found that the stress distribution along the axis of pinhole is unbalanced due to the inconsistency of deformation,and the stress gradient is relatively large.There is a very serious phenomenon of local stress concentration in the pin seat of piston,while the pin seat is bitten by high temperature and excessive pressure.The pin seat cracks and even damages the whole piston due to excessive mechanical load.In order to improve the stress distribution of pinhole and the bearing capacity of piston pin seat,the special pinhole of piston and the copper sleeve at pinhole are proposed.The temperature field,mechanical load and thermo-mechanical coupling of the piston are analyzed and calculated.The maximum working temperature of the top surface of the piston is 419.9 C,the maximum thermal stress is 164.416 MPa,the serious stress concentration in the inner and upper edge of the piston pin seat hole occurs,and the maximum thermo-mechanical coupling stress is 209.096 MPa,and the maximum coupling deformation of the head is 0.515 mm.The influence of pinhole copper sleeve on piston structure under operating conditions was studied.The highest working temperature of the piston was 421.9 C,which appeared at the top throat;the maximum mechanical stress was 204.863 MPa,which decreased 401.323 MPa,66.2% compared with the piston without pinhole bushing;the maximum thermal-mechanical coupling stress was 148.48 MPa,which decreased by 60.616 MPa,28.9% compared with the same period of last year,and the phenomenon of stress concentration at pinhole was improved.Based on the research of pinhole copper sleeve,different types of elliptical special pinhole copper sleeve and wedge shaped special pinhole copper sleeve with different sizes are made respectively,and the influence of special pinhole copper sleeve on piston structure strength is studied.The research shows that pistons with outer ellipse inner circular special pinhole copper sleeve structure and inner and outer ellipse special pinhole structure have bad performances.The structure can further improve the bearing capacity of the piston.Comparing four piston schemes with different wedge-shaped pinhole bushing,the piston performance parameters of each scheme are less than the original piston,of which the cone angle of 10 degrees and the cone angle edge is chamfered.The maximum mechanical stress and the maximum mechanical coupling stress are 185.396 MPa and 135.491 MPa,respectively,which are 19.467 MPa and 12.989 MPa lower than the previous year,accounting for 8.7% and 12.9% respectively.Piston pin hole and copper sleeve are interference fit.This paper analyses the influence of interference fit on contact pressure and stress distribution of piston pin hole.It is found that the contact pressure and the maximum stress of the piston pinhole increase linearly with the increase of the interference when the piston pinhole fits with the copper sleeve.The contact pressure at the pinhole is the most sensitive to the interference value.The influence of special pin hole and pin hole bushing on piston thermal insulation coating was studied.On the basis of optimum wedge shaped pin hole structure,a 0.4mm ceramic thermal insulation layer + 0.1mm metal bonding layer was made on the top of piston.The calculation results show that the maximum working temperature of piston decreased by 12.5%,from 421.9 to 367.4%,and the coupled stress and strain of heat engine were 132.708 MPa and 0.412 mm,respectively.It decreased by 10.6% and 20.3%. |
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