Dynamic finite element analysis in the development

2022-08-17
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Dynamic finite element analysis in engine research and development

Abstract: This paper describes the dynamic and finite element analysis content completed by using AMS and tran software in the development of engine (including powertrain), and combined with the product development of Chang'an company, it details several typical analysis and calculation examples, which are: crankshaft system analysis, cylinder block and head integration analysis, valve train analysis, common accessory structure analysis Powertrain modal analysis and powertrain mounting system analysis. These examples show that CAE analysis technology has gradually become an important technical means in the process of engine development

1 overview

with the development of modern engine technology, the status of "C" statistical AE analysis based on relevant data in the development process of new engines has been continuously improved, and has gradually become a development means parallel to traditional tests. The stiffness, strength, fatigue, vibration, noise and other problems that need to be considered in the development process of the new model can be solved by CAE means in the design stage, which can greatly improve the design quality, shorten the development cycle, save the development cost, and avoid fatal quality problems when the product is put on the market

in the CAE analysis of engine, it can be divided into three research directions: performance analysis, flow field analysis and structural analysis. Structural analysis is to carry out dynamic and finite element analysis for objects at three levels: component level (such as piston, connecting rod, etc.), subsystem level (such as crankshaft system) and assembly level (such as complete machine and powertrain). The main analysis content includes analyzing the stiffness, strength, fatigue Mode, temperature, rigid body motion, elastic vibration, etc. According to the requirements of modern engine development and previous development experience, the basic content of engine structure analysis is shown in Figure 1

ams is good at kinematics and dynamics analysis, which can effectively analyze the moving mechanism in the engine. Ams/engine module provides the analysis modules of several common engine components. Stran plays an important role in the field of engine structure analysis with mature finite element analysis technology. Combined with the product development of Chang'an company, this paper will discuss in detail several examples of engine structure analysis completed with AMS and tran

Figure 1 basic content of engine structure analysis

2 typical analysis calculation example

2.1 crankshaft system analysis

crankshaft is a part with complex structure and relatively large load in engine. In the working process of crankshaft, fatigue failure is often the main reason for its failure. Therefore, it is particularly important to study the fatigue performance of crankshaft. Because the crankshaft is a high-speed rotating component, if the traditional static strength analysis is used, it can not accurately reflect the real stress situation of the crankshaft in the working state. Therefore, the crankshaft strength analysis based on dynamic analysis is adopted in this paper. It is believed that the spray free technology will be more widely used

first, the actual three-dimensional modeling and assembly of the engine crankshaft system (including crankshaft, connecting rod, main bearing seat, etc.) are carried out. Different connectors are used to connect the components to form a multi-body nonlinear model of the engine crankshaft system. The influence of bearing oil film is also considered in the modeling process. The dynamic characteristics of the crankshaft and the oil film characteristics of the main bearing are obtained by solving this model. Then, the results of dynamic analysis are applied to the finite element model of the crankshaft as input conditions, and the transient stress of the crankshaft under one working cycle is obtained by using stran software, and then the fatigue safety factor of the key points of the crankshaft is obtained to investigate the durability of the crankshaft

Figure 2 shows a finite element model of a crankshaft with hydraulic torque converter. The model includes all parts of the crankshaft and is composed of most hexahedral elements and a few tetrahedral elements. The total number of elements is 251768 and the total number of nodes is 290597. This model is analyzed with the dynamic model formed by connecting rod, main bearing seat and other components, and a series of dynamic characteristics and oil film characteristics of the crankshaft system are obtained, including the maximum dynamic torque at the front and rear ends, the maximum dynamic angular displacement at the front and rear ends, the transient forces and moments received by each main journal in the working cycle, the minimum oil film thickness of each main bearing, the maximum oil film pressure and so on. Among them, figure 3 shows the maximum dynamic angular displacement at the front and rear ends, and Figure 4 shows the minimum oil film thickness of each main bearing. Based on the above dynamic results, the transient stress of the crankshaft in a working cycle is obtained through finite element calculation, as shown in Figure 5, and then the fatigue safety factor of the key points of the crankshaft is obtained according to the crankshaft material properties and transient stress results

Figure 2 finite element model of crankshaft

Figure 3 maximum dynamic angular displacement at the front and rear ends

Figure 4 minimum oil film thickness of each main bearing

Figure 5 transient stress of crankshaft at a certain time

2.2 integrated analysis of cylinder head and cylinder block

cylinder head and cylinder block are the most important main structural components, and their durability and reliability directly affect the performance of the whole engine. Therefore, in the early stage of design, it is necessary to evaluate the temperature and strength of the cylinder head cylinder block under various working conditions, cylinder hole deformation, cylinder pad pressure distribution and high cycle fatigue of these structures

the integrated analysis of cylinder head and cylinder block is mainly divided into three parts: jacket CFD analysis, temperature field analysis of cylinder head and cylinder block, and structure analysis of cylinder head and cylinder block. Through the CFD analysis of the water jacket, the flow rate, pressure loss, convective heat transfer coefficient and so on of the cooling liquid in the water jacket are obtained. Then, some CFD analysis results are applied to the finite element model of structural analysis as input conditions to calculate the temperature field distribution of cylinder and cylinder head. Finally, the structural analysis is carried out based on the analysis results of temperature field, mainly including the following working conditions: (1) assembly load; (2) Thermal load; (3) Working load (gas explosion pressure, etc.); (4) Cooling state. Through the analysis of this series of working conditions, the durability of the cylinder head, the deformation of the cylinder bore and the tightness of the cylinder gasket are evaluated

Figure 6 shows an integrated finite element lattice model of an engine cylinder head and cylinder block, which also includes cylinder gasket, valve, bolt and other structures. Figure 7 shows the calculation results of the temperature field of the cylinder head and cylinder block. Figure 8 shows the pressure distribution of the cylinder gasket under certain working conditions. If the pressure is too low or even zero, it indicates that there are some problems in the sealing of the cylinder gasket, which need to be improved in optimization

Figure 6 finite element model for integrated analysis of cylinder head and cylinder block

Figure 7 temperature field distribution of cylinder head and cylinder block

figure 8 pressure distribution of cylinder gasket under certain working conditions

2.3 analysis of valve train

valve train is an important component of engine, and factors such as valve lift curve and part mass stiffness will greatly affect the target performance of engine power, fuel consumption, emissions, idle speed stability, etc. Therefore, the system simulation of virtual prototype in the design stage of valve mechanism can verify and optimize the design of valve mechanism, reduce the development cost and shorten the development cycle

the dynamic analysis of the valve train is mainly divided into the following two steps:

(1) modeling is based on the valve train module provided by ams/engine, as shown in Figure 9. In the modeling, all components except the valve spring are considered as rigid bodies. The calculation inputs mainly include valve lift curve, camshaft profile, shape, mass and stiffness of valve spring, etc. Through dynamic calculation, the valve seating speed, the Hertz contact force between the cam and tappet (Figure 10 shows the Hertz contact force of an engine at different speeds), the valve spring force and the camshaft driving torque are obtained to test whether the valve train can work normally at different engine speeds

(2) in order to more accurately simulate the dynamic behavior of the valve train, the elasticity of the camshaft itself is considered in further calculation. Through the flexible component interface (MNF file) of AMS, the elastic finite element model of the camshaft is introduced into the dynamic analysis model to calculate the dynamic characteristics of the valve train. Moreover, based on the results of dynamic analysis, the dynamic stress of camshaft at different speeds and angles can be investigated through the software of tran. Figure 11 shows the dynamic stress nephogram of a camshaft at 6000rpm

Figure 9 dynamic analysis model of valve mechanism

Figure 10 Hertz contact force at different speeds

Figure 11 dynamic stress of camshaft at 6000rpm

2.4 structural analysis of common accessories

there are many accessories in the engine, such as compressor bracket, intake pipe bracket, generator bracket, etc., which are used to connect various component assemblies in the engine and fix them on the main structure of the engine. If these accessories fail during operation (such as fracture, etc.), the engine may not work normally

in the engine design stage, the strength and modal analysis of the engine accessory structure are generally carried out. In the strength analysis, according to the design load or the actual working load provided by the design department, the finite element calculation of the structure is carried out by using the stran software, and the deformation, stress distribution, support reaction force and so on of the parts under different working conditions are obtained. The accuracy of strength analysis mainly depends on: (1) the quality of finite element lattice, because the distorted lattice will cause false high stress, which will affect the judgment of the result; (2) For the determination of load, the external load of accessories must be provided by the design department or calculated by the actual working conditions of the engine. Modal analysis is to prevent the natural frequency of accessories from being excited by the excitation of the normal working state of the engine, which leads to dangerous resonance damage. The difficulties of modal analysis mainly lie in: (1) the determination of excitation frequency. For general in-line four cylinder engine, the second-order excitation or fourth-order excitation is mainly considered; (2) For the accurate simplification of boundary conditions, because different boundary conditions will make the calculated natural frequencies different, the components connected with them must be accurately simplified and simulated, such as the simulation of connecting bolts

Figure 12 shows the modal analysis results of four engine accessories of an engine generator bracket, compressor bracket, intake manifold bracket and generator compressor combination bracket

Figure 12 modal analysis of engine accessories

2.5 powertrain modal analysis

the bending vibration of the powertrain has a very important impact on the NVH performance of the vehicle, and in serious cases, it will also cause the early resonance damage and fatigue damage damage of vehicle parts. In the process of engine development, it is necessary to calculate the modal of powertrain and accessory system, get their modal frequencies and vibration modes, and analyze their dynamic natural vibration characteristics

the focus of powertrain modal analysis is reasonable simplified modeling, rather than the calculation itself. Since the powertrain includes engine and transmission, in order to fully reflect the vibration characteristics of the powertrain and accessory system, all parts and components in the system must be taken into account in the analysis and calculation; At the same time, in order to control the scale of the finite element model and save calculation time and resources, when carrying out the finite element modeling of the whole powertrain, appropriate simplification treatment is made for the role of different objects in modal calculation that interpret the strong strength of the company. Generally speaking, there are two modeling methods for each part of the powertrain. One is to use solid element and shell element modeling, including the main parts: cylinder block, cylinder head, oil pan, transmission housing, engine accessories, bearing cover; The other is to use centralized mass to simulate, including the main parts: internal assembly of transmission housing and valve distributor

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