Strength Calculation and Fatigue Reliability Analysis of Typical Components for CNG Engine

Abstract: Valve actuating mechanism and cylinder head cover have become an important part of new CNG(Compressed Natural Gas) engine.The valve and valve seat of CNG engine work under high mechanical load,thermal load and erosion of corrosive gases.Due to the poor working conditions,the sinking of the valve,the increasing of harmful gas in the combustion chamber and the fracture of the valve head are often happen.As a result,the engine performance is deteriorating and valve-valve seat can not work.Moreover,all of them affect the normal operation of the engine.It can be seen that the structural performances of the valve-valve seat have a significant effect on the safety and reliability of the whole vehicle.Today, magnesium alloys have been widely used in the automotive industry.For example,the cylinder head cover has an improvement on mechanical seal in high-temperature cylinder and the application of magnesium alloy cylinder head cover significantly reduce the weight of the engine although its strength is little lower than that of steel parts.Because the cylinder head cover is under the condition of high temperature,its strength must meet the requirements of design,and the study of the strength and fatigue reliability for the cylinder head cover is also particularly important.In this paper,strength and reliability analysis was conducted for two typical parts such as the valve-valve seat and magnesium alloy cylinder head cover in the 6110 medium-sized CNG engine.The relationship between the collision contact stress,reliability and fatigue life for the valve-valve seat was first analyzed systematically.In order to calculate the dynamic contact stress between the valve and valve seat,finite element(FE) software LS-DYNA was used for the simulation of three complete cycles.In addition,the solid model of the cylinder head cover was established and meshed by HYPERMESH.Thermal stress of the cylinder head cover is then studied by the FE software ABAQUS.According to the result of calculation,under the condition of rated speed,the valve lift and velocity curves are consistent with the corresponding theoretical values,but the acceleration curve is little bigger.The region of the highest stresses is in the head of the valve rod because of stress concentration,while the stress is less than the maximum allowable stress. The valve crash speed is 0.2 m/s,lower than the design requirements(0.5 m/s).The maximum normal stress of the valve head is 600 MPa,lower than the the maximum tensile strength 1164 MPa.The maximum radial contact stress of the valve seat is 80 MPa,lower than the fatigue strength.The results of finite element simulation for cylinder head cover show that the maximum thermal stress at dangerous area is 149.7MPa and it is lower than the material allowable stress(280MPa).The above mentioned research results will provide theoretical basis for structure optimization,the service life and fatigue life analysis of cylinder head cover.The number of full cycle can be obtained by rain-flow counting method and the fatigue load spectrum for random load can be calculated.Fatigue life of the valve and valve seat can be then calculated by local stress-strain method,the reliability of the valve and valve seat can be calculated by stress-intensity interference model at the same time.The calculation results show that the service life of the friction pair is about one year and the reliability is 98.63%.The study of the reliability for the cylinder head cover is based on the nominal stress method,that is to say, the fatigue life and reliability of the cylinder head cover is obtained by the modified S-N curve and p-S-N curve.The life of the cylinder head cover is 5×10~4 cycle,and the value of reliability is about 90%.Life and reliability of the two are in line with the design requirement.Finally,the magnesium alloy cylinder head cover model is simplified and optimized for structural design in the present paper.The main design variable is the minimum edge thickness of the magnesium alloy cylinder head cover under the conditions of the minimum quality and the permitted strength range.The optimization is conducted by using ANSYS APDL parametric design language.The optimized results show that the thickness,quality of the cylinder head cover can be reduced by24.2%,23.6%respectively while the maximum stress value is less than the allowable magnesium alloy stress.The optimizing results will provide an important reference for the optimization of the practical magnesium alloy cylinder head cover…
Key words: Valve and Valve Seat; Transient Dynamic Analysis; Reliability; Cylinder Head Cover; FE Simulation; optimization design

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