Cylinder Clearance Of Diesel Engine Under Operating Conditions

Compared with gasoline engines,diesel engines have the characteristics of high thermal efficiency,high torque,low fuel consumption and good durability,thus are widely used in various fields.The matching gap between piston and liner directly affects the performance of the cylinder-piston group.In order to improve the matching characteristics of piston and liner,the clearance between piston and liner must be considered in the design process.During the working process of the diesel engine,piston and liner have obvious thermal deformation and mechanical deformation,thus the actual gap and the initial clearance is quite different.In addition,besides,there are certain differences in the clearances at different positions of piston surface affected by the skirt profile,which makes it difficult to determine the clearances.In order to design the clearance rationally,this paper studies the cooperation between piston and liner,and carries out a simulation analysis based on a four-cylinder diesel engine.Firstly,the finite element analysis of the piston is carried out.The coupled mesh model of piston,piston pin and small end of the connecting rod is established.The empirical formula and external flow field simulation are used to obtain the convective heat transfer coefficient of the piston surface,which is used as the boundary condition to obtain the temperature field and the thermo-mechanical coupled deformation field.Adopt the same calculation process to finish the finite element analysis of cylinder.Extract the displacement of piston and cylinder,and use the least square method to fitting the center of piston and cylinder separately to obtain the overall displacement on the horizontal plane,then remove them from displacement to obtian the more accurate deformation.Build a multi-body dynamic model of the whole machine,take the piston-cylinder liner deformation,the initial design of the piston-cylinder liner clearance and the piston profile as single variables,study the minimum working gap between piston and liner,and analyze the lubrication state of the piston skirt and the vibration characteristics of cylinder.The results show that the thermo-mechanical coupling deformation of the cylinder and piston results in a minimum nominal gap height reduction of approximately 37%.The effect of each order of cylinder liner deformation is different.As the order increases,the influence of deformation fluctuation on lubrication and vibration characteristics decreases.Reasonable piston profile increases the minimum nominal clearance height by 5.1 times,while the design of the cylinder clearance has little effect on the results.Five important parameters of piston skirt profile were selected to study based on an orthogonal test plan.It is found that different piston skirt profile design parameters have different influence weights and significant degrees on each evaluation index.It is necessary to optimize the design of each influencing factor according to different design requirements,and comprehensively consider the lubrication and vibration characteristics to determine the optimal profile design scheme.The piston skirt top indentation ΔGt has the greatest weight on the minimum nominal clearance height and minimum oil film thickness,and also has a significant effect on the maximum oil film pressure.The height of the middle bump hm has a significant effect on the peak value of the total friction power loss,while the ellipticity coefficient a has a significant effect on the vibration characteristics of the cylinder liner.