The Structural Strength And Fatigue Analyses Of An Engine Cylinder Head Under Thermo-Mechanical Coupling

Requirement for diesel engine structure has been increased with increasing power density,which makes the strength and fatigue analyses of the cylinder head essential.The temperature is the main index in the evaluation of the thermal load of cylinder head,and the firing would occur when the temperature exceeded the tolerance.Although the thermal and the mechanical loads stay below the limit,cracks can initiate and propagate in some key regions of the cylinder head due to long-time service.This paper studies the structural strength and the fatigue of the cylinder head based on the computational fluid dynamics and the finite element analysis.Effects of combustion,coolant flow and mechanical load on the thermal reliability of the cylinder head are investigated,which can provide guidance on diesel engine design.The heat transfer model of the cylinder-block-water jacket is established by the fluid-structure coupling method.The pressure,the flow velocity and the heat transfer coefficient of the water jacket are analyzed.On top of that,the temperature field is obtained according to the boundary conditions of the cylinder head and validated by tested temperature of the cylinder head.A finite element model of the cylinder head-gasket-block is established.The stress of the cylinder head is calculated under three conditions:(1)only under mechanical load,(2)only under thermal load,(3)under both thermal and mechanical loads.The stress-strain of the dangerous points is analyzed.The fatigue analysis of the cylinder head is conducted according to the stress-strain calculations under various loading cases.The distribution of the high-cycle fatigue safety coefficient is obtained using the loading parameter and the stress-life curve.The low-cycle fatigue life of the cylinder head is computed under the ratings and the idle by strain fatigue method.The optimal Latin hyper cube method and the ellipsoidal basis function neural network are employed to study the hear reliability of the cylinder head under thermomechanical coupling.The correlation between the combustion,the coolant flow,the mechanical load,the peak temperature of the fire face,the peak stress of the fire face,and the fatigue coefficient is fitted.The influences of parameters on the thermal reliability of the cylinder head are studied.