Contact Stress Calculation Of Valve Train Of 4L20 Diesel Engine

As an important part of diesel engine,the design of valve train is directly related to the power performance,economic performance,emission performance and working reliability of diesel engine.When the diesel engine is running,the valve mechanism is required to ensure the quality of air exchange in the cylinder,as well as the stability and safety of operation.As a mechanical mechanism composed of several parts,the movement of the parts should be coordinated during operation,and the contact stress of the contact surface should be within a reasonable range.Too high contact stress will make the contact surface fatigue early,which will lead to the failure of the mechanism.This paper takes Wartsila 4L20 diesel engine as the research object,establishes the kinematics and dynamics model of valve mechanism,and uses AVL elite timing drive software to simulate and analyze the valve mechanism.The specific work is as follows:(1)Through the mapping of 4120 diesel engine valve mechanism in the manual,three-dimensional parameters are obtained and substituted into the established model for simulation,and the motion law and working condition of each component are calculated.The results show that the contact stress of cam and tappet is large in some working sections,and there is a large vibration phenomenon.(2)Based on the measured timing and maximum lift parameters of valve train,two new cam profiles are designed by using polynomial function and subsection function through the cam design module.These two lines are replaced by kinematic and dynamic models for evaluation and comparison.The results show that the improved profiles meet the requirements of kinematics and dynamics,and the polynomial function cam is better than the piecewise function cam.(3)The contact stresses of cam and tappet,push rod and rocker arm,and air valve bridge and rocker arm are calculated respectively.The results are as follows:the contact stresses of intake mechanism are 752Mpa,185Mpa and 319Mpa;the contact stresses of exhaust mechanism are 758Mpa,187Mpa and 329Mpa.(4)The contact force of the valve mechanism is affected by many factors,such as the spring stiffness and preload of the valve.Change the three parameters of air valve spring stiffness,preload and air valve clearance to observe the effect on the valve mechanism.The best combination of the three parameters is selected to reduce the contact stress.