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Piston development process based on simulation analysis

this paper first uses finite element analysis technology to carry out finite element lattice division and topology of the CAD model of piston piston pin connecting rod group based on digital design, and then analyzes the temperature field, thermal stress and thermal mechanical coupling of the piston, and obtains the dynamic response and fatigue strength of the piston under typical working conditions of the engine; Under the condition of meeting the fatigue strength, the multi-body dynamics analysis of the cylinder liner piston piston pin connecting rod system is further carried out to obtain the contact stress and clearance between the piston and the cylinder liner, the instantaneous motion characteristics of the piston in the cylinder liner, and finally the best piston contour is designed. "Simulation driven design", from the CAD design of engine piston to the closed-loop iteration of simulation analysis and test, this paper describes in detail the development and design process of engine piston based on simulation analysis technology. Following this development process, the performance of engine piston will be accurately designed

piston is one of the key parts of the engine to convert the heat energy released by fuel combustion into mechanical energy. It also bears the gas explosion pressure generated by engine combustion, the reciprocating inertia force generated by crank rotation, the side thrust of the cylinder wall on its skirt when the piston moves in the cylinder liner, and the thermal stress generated by the temperature change of the piston itself due to the absorption of the heat energy released by gas combustion. Based on the harsh working environment and complex stress conditions, the design and development of piston must consider many factors such as force transmission, heat transfer, guidance, sealing, lightweight, friction reduction and wear resistance at the same time. Based on the modern design method of advanced digital design and virtual prototype simulation analysis, this paper comprehensively expounds the application of CAE analysis technology such as finite element calculation and multi-body dynamics in the whole development and design process of engine piston design, analysis and optimization, and establishes the advanced development process of engine piston with correct design at one time

piston design and development process

the key to piston design and development lies in piston strength analysis and profile optimization. In order to shorten the development cycle, the piston mold is generally manufactured after the piston strength calculation is qualified, and then the casting and heat treatment of the piston blank are started. At this time, the piston profile design has been preliminarily completed. After the piston profile processing is completed, enter the piston temperature field test process, and compare the test results with the calculation results. If there is a large difference between the test results and the calculation results, it is necessary to adjust the design (if the early strength calculation is qualified, it will ensure that there will be no major design changes) and/or re iterate the finite element calculation of the piston temperature field and the comparison of the test and test, Until the judgment conditions (Engineering indicators) are met. Figure 1 shows the general process of engine piston design and development

cad modeling and finite element lattice division

due to the complex shape of the internal cavity of the engine piston, some rounded corners may not be solved normally by the general CAD modeling method. This paper recommends that the piston CAD model should be established from the piston pin seat, and the piston structure should be reasonably divided at the same time, using the Boolean operation function of CAD software to achieve twice the result with half the effort. The CAD model used for finite element analysis needs to be simplified. Generally, piston lock ring groove, piston skirt, ring bank chamfer, oil hole, etc. can be ignored. In addition, the torque generated by the piston pin hole offset rotating around the piston pin mainly affects the calculation of piston dynamics. Grivoryht2vs-hh is used between 230 ℃ and 250 ℃, which can be ignored in the calculation of piston strength

first, we use the pre-processing software of finite element analysis (such as HyperMesh) to divide the initial lattice of each component. The principle is to make the nodes of the initial lattice not fall on the surface of the component as much as possible, especially the nodes of key parts must not fall on the surface of the component, and make the volume of the initial topological model (hereinafter referred to as startmesh) larger than and should contain the actual three-dimensional solid model

generally, the piston head and skirt should be stretched along the centerline of the piston, while the piston pin seat should be stretched along the centerline of the pin hole. For the connecting rod, in order to make the startmesh not on the surface of the connecting rod as much as possible, the unit where the connecting rod small end hole is located should be stretched along the center direction of the hole

on the basis of startmesh, automatic topology operation is also required. In this paper, AVL fame software is used to call in the startmesh and its STL geometric file of parts and components, and judge the elements and nodes of startmesh. If the elements are completely or partially within the boundary plane of STL geometric file, keep their elements, otherwise delete the elements and nodes, or project the nodes outside the geometric boundary plane onto the geometric plane. After the automatic topology is completed, the cell quality needs to be checked, and the checked bad cells and their related layers (the layers are customized by the user) should be optimized separately

for the grid division of the piston pin, the piston pin hole and connecting rod hole can be copied to the outer surface of the piston pin hole, and then the topology is carried out with the CAD drawing of the piston pin. The grid surface nodes obtained will correspond to the nodes of the piston pin hole and connecting rod small head hole one by one

calculation of temperature field

the heat exchange of the piston under stable working conditions has reached equilibrium. At this time, the heat absorbed by the piston is equal to the heat released by it. Therefore, it can be simplified to solve the Laplace equation with stable piston temperature and no internal heat source (formula 1). The analysis of piston temperature field is essentially the analysis of piston heat conduction. Among the three common boundary conditions of heat conduction, in order to simplify the calculation, the third boundary condition is generally used: defining the heat transfer coefficient between the object on the boundary and the surrounding liquid and the temperature T of the surrounding liquid (formula 2). Because the materials of piston pin and connecting rod are not sensitive to temperature, the piston pin and connecting rod can not be considered in the calculation of temperature field. However, most modern piston materials are aluminum alloys, and their heat transfer characteristics are nonlinear, that is, with the increase of temperature, their heat transfer coefficient increases nonlinearly, so we should pay special attention to this factor when calculating the piston temperature field. The nonlinear characteristics of thermal conductivity of piston aluminum alloy material are shown in Figure 2

Figure 2 thermal conductivity curve of aluminum alloy material

the main equation of the mathematical model for calculating the piston temperature field is:

Where, α Is the heat transfer coefficient between the object and the surrounding liquid, in w/(M2 · K); t? Is the temperature of the surrounding liquid, in K; λ (t) Is the thermal conductivity of the object, in w/(m · K), which changes with temperature; Г To solve the exothermic outer boundary of the region

when using the third kind of boundary conditions, there is no general calculation formula for the heat transfer coefficient between the piston and the environment at present. Empirical and semi empirical formulas are usually used to obtain the thermal boundary conditions of the piston, and the calculation results (Fig. 3) are compared with the temperature values of the test for correction. When the difference is less than 1 ℃, the next calculation can be carried out

Figure 3 piston temperature field calculation results

thermal stress calculation

1. Displacement boundary conditions

when calculating thermal stress, the displacement in the normal direction of the piston should be constrained on the symmetrical plane of the piston. As the piston pin is equivalent to a support in the Z direction for the piston, the thermal deformation of the piston is close to zero in the Z direction at the node at the center of the piston pin hole, which is consistent with taking the center of the piston pin hole as the design basis and considering the shrinkage of the piston blank (the reduction caused by temperature reduction) in the mold design. Therefore, the author suggests that the node at the center of the piston pin hole should be projected onto the plane of z=0 when establishing the piston finite element lattice, To constrain the displacement of these nodes in the cylinder liner direction

2. Temperature field loading

map the calculation results of the piston temperature field to the piston finite element model as the temperature load, and assign the initial temperature t0=20 ℃ to all nodes of the piston at the same time

3. Material nonlinearity

the thermal expansion coefficient and elastic modulus of aluminum alloy piston material have nonlinear characteristics. With the increase of temperature, the thermal expansion coefficient increases nonlinearly and the elastic modulus decreases nonlinearly. These factors should be paid special attention to when calculating thermal stress. The nonlinear characteristics of expansion coefficient of aluminum alloy material are shown in Figure 4, and the nonlinear characteristics of elastic modulus are shown in Figure 5

Figure 4 nonlinear characteristics of expansion coefficient of aluminum alloy

Figure 5 elastic modulus curve of aluminum alloy

4. Calculation results of thermal stress

the purpose of thermal stress calculation is to obtain the thermal deformation of piston skirt (as shown in Figure 6 and Figure 7), and at the same time, it is used as the input condition for force synthesis in fatigue strength calculation

Figure 6 piston radial thermal deformation

Figure 7 piston radial thermal deformation

stress under two working conditions

normal displacement x=0, y=0 of the symmetrical plane of the constraint piston, piston pin and connecting rod, and normal displacement z=0 of the cross section at the lower end of the constraint connecting rod body. We use the method of deformation compatibility equation to establish the contact element to simulate the stress state between piston pin and piston pin connecting rod. In order to reduce the calculation scale and make better use of the characteristics of the symmetry plane to constrain the displacement boundary, the offset of the piston pin hole can be ignored in the strength calculation, and the additional torque caused by the offset of the piston pin hole by the gas explosion pressure can be ignored, so the contact problem between the piston and the cylinder liner can be ignored

under the working condition of gas explosion pressure, load the gas explosion pressure on the top of the piston, the first ring bank and the first ring groove; Under the condition of heat + gas explosion pressure, in addition to the gas explosion pressure, the piston temperature field load should also be loaded. For hot + gas explosion pressure conditions, we also need to pay attention to the nonlinear characteristics of piston aluminum alloy materials. The calculation results are shown in Figure 8 and Figure 9

figure 8 Mises stress, maximum principal stress and minimum principal stress under gas explosion pressure condition

Figure 9 Mises stress, maximum principal stress and minimum principal stress under heat + gas explosion pressure condition

under stable working conditions, after the heat transfer process of the engine reaches equilibrium, the temperature change of the piston is small, especially the high-speed gasoline engine piston, because the heat exchange frequency is almost twice that of the diesel engine piston, Therefore, the temperature of the piston changes less. Based on this, assuming that the temperature field of the piston is constant under stable working conditions, the gas explosion pressure is the main factor causing the stress amplitude of the piston. Because there is a temperature gradient in the three-dimensional space of the piston under stable working conditions, and the piston material has nonlinear characteristics, the stress range of the piston is equal to the stress of "heat + gas explosion pressure working condition" minus the thermal stress, the stress amplitude is equal to the stress range divided by 2, and the average stress is equal to the stress minus the stress amplitude of "heat + gas explosion pressure working condition may return to the market in 3 (4) months to replenish the inventory". The average stress (FIG. 10) and stress amplitude (FIG. 11) of the piston can be obtained by force synthesis

Mises stress of average stress, maximum principal stress and minimum principal stress in Figure 10

Mises stress of stress amplitude, maximum principal stress and minimum principal stress in Figure 11

fatigue strength calculation

the strength of piston under high temperature working conditions will decline rapidly, so the envelope of piston fatigue strength at different temperatures is also different. Due to the large dispersion of piston aluminum alloy material strength, a relatively conservative Goodman straight line is generally used, The fatigue strength is solved by using Haigh diagram. The fatigue life calculation results are shown in Figure 12, meeting the design strength requirements

Figure 12 fatigue life of piston top

dynamic calculation

the dynamic analysis model of piston is shown in Figure 13, including two-dimensional piston, piston pin, connecting rod and crankshaft. In the down stroke, the top of the piston bears the gas explosion pressure, and the connecting rod, as a two force member, is pressurized, and its reaction force on the piston makes it live

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