【作者】 高岚；

【导师】 周霞；

【作者基本信息】 大连理工大学 ， 工程力学， 2009， 硕士

【摘要】 配气机构和气缸盖罩是新型CNG(Compressed Natural Gas)发动机的重要组成部分。CNG发动机中气门和气门座在工作中承受极高的机械负荷、热负荷及腐蚀性气体的冲刷,常造成气门下沉、燃烧室的有害气体容积增大、气门头部断裂,严重时气门-气门座的密封作用失效,发动机性能变坏,影响发动机的正常工作,因此气门-气门座的结构性能对整车的安全性和可靠性有着重要的影响。如今镁合金已经广泛的应用于汽车工业,气缸盖罩置于气缸盖上起到密封的作用,镁合金气缸盖罩的使用使其质量有大幅度的减少,强度比钢结构件有所降低,而其处于高温的工作环境,这就要求其强度必须符合设计要求,因此气缸盖罩的强度可靠性及疲劳寿命的研究亦显得尤为重要。本文主要对6110中型CNG发动机中的气门-气门座及镁合金气缸盖罩两种典型部件进行了强度可靠性分析。文中首先系统地分析了气门-气门座这一对摩擦副碰撞接触应力、可靠性及疲劳寿命之间的关系。应用有限元软件LS-DYNA模拟研究了3个完整周期内气门-气门座的动态接触应力。本文还对汽缸罩盖进行了无简化实体建模,使用HYPERMESH对气缸盖罩进行网格划分,并应用有限元软件ABAQUS模拟研究气缸盖罩的热应力。根据计算结果可以看出,在额速工况下,气门-气门座的升程、速度与理论值吻合,加速度值偏大。气门杆头部由于存在应力集中现象而成为气门最大受力区域,但其值小于气门的最大许用应力。气门落座速度为0.2m/s,小于设计要求(0.5m/s),气门杆头部最大正应力为600MPa,小于气门最大抗拉强度1164MPa。气门座最大径向应力为80MPa,小于材料规定的疲劳极限。镁合金气缸盖罩有限元模拟得出危险点的最大热应力值为149.7MPa,小于材料的许用应力(280MPa)。为气缸盖罩的结构优化、使用维护、疲劳寿命分析等提供了理论依据。采用雨流计数法对气门-气门座危险点应力进行循环计数,得出随机载荷下的疲劳载荷谱,采用局部应力应变法分析气门-气门座这对摩擦副的疲劳寿命,同时运用应力-强度干涉模型得出气门-气门座系统的可靠度。计算结果表明,该摩擦副使用寿命范围为一年左右,可靠度大约是98.63%。镁合金气缸盖罩的可靠性研究采用名义应力法,通过修正零件S-N曲线和p-S-N曲线得到气缸盖罩的寿命和可靠性,得出气缸盖罩使用寿命为5万次,可靠度约为90%。二者寿命与可靠性均符合设计要求。本文最后对镁合金汽缸罩盖进行了模型简化,并对其进行结构优化设计。本文设计变量主要是在质量最小、强度在允许范围内镁合金气缸盖罩边缘厚度的最小值。采用ANSYS APDL参数化设计语言进行优化,得出优化后结果,气缸盖罩厚度减小24.2%,即质量减少23.6%,而最大应力值小于镁合金的许用应力。该优化结果为实际镁合金气缸盖罩优化提供了重要的参考。更多还原

【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.更多还原