Application of FEM-Tools in the Engine Development Process

Transcription

1 Application of FEM-Tools in the Engine Development Process H. Petrin, B. Wiesler AVL List GmbH Graz, Austria Abstract The requirements for the development of a new engine are the considerable reduction of development time and costs as well as the reduction of production costs, considering a higher number of product variants and higher product quality, are boundary conditions for the actual engine development. To achieve this goal the automotive industry worldwide tries to organize their development activities more efficiently in "Development Processes" and searches for optimization potential. AVL is working together with a various number of automotive companies all over the world trying to adjust its services (e.g. FEM-calculations) for best fitting into the "Engine Development Process" of each of the customers. AVL also takes over to some extend working parts of the customer s processes and is even asked to optimize the processes. The paper discusses the effective use of FEM-tools in engine development processes, as derived in co-operation with many automotive companies and in-house engine development, under special consideration of latest tools for mesh generation and calculation performance. The optimized correlation between the different calculation tasks and the development process, the right starting time and extend (time frame) for the use of the simulation and the application of fast and validated simulation software assures reliable results in time. The proper application of the FEM-tools in the design phase and the best support during the testing phase reduces the risk and the testing effort in the prototype and pre-production phase and is the essential basis for the reduction of development time and costs. 1

2 1 Introduction The competitive situation in the industry is characterized by the need of improving the product quality, the reduction of development costs and the reduction of the Time to Market with respect to the fulfillment of more and more legal regulations. The requirements of the automotive industry concerning - considerable reduction of the development time and - considerable reduction of the development costs and the goal to reduce the production costs considering - a higher number of product variants and - higher product quality are boundary conditions for the actual engine development /1,2/. To achieve these goals the automotive industry worldwide tries to organize their development activities more efficiently in "Development Processes" and searches for optimization potential. AVL is working together with a various number of automotive companies all over the world trying to adjust its services (e.g. FEM-calculations) for best fitting into the "Engine Development Process" of each of the customers. AVL also takes over to some extend working parts of the customer s processes and is even asked to optimize the processes. 2 Engine Development Process The following rough plan of a general engine development process under special consideration of the FEM analysis support can be derived as a synthesis of already existing or planned development processes of many automotive companies in co-operation with in house engine development. Fig.1: Integration of FEM-Analysis in the Engine Development Process The complete process can be divided into four main parts, the Concept Study, the Prototype Development (Engine Generation 1), the Pre- Production Development (Engine Generation 2) and the Production Validation. In the Concept Study the main engine parameters based on the engine specifications (development targets and boundary conditions) will be defined. The total engine concept to be developed will be specified. The Prototype Development deals with the design and development of prototype engines (Engine Generation 1). In this period the baseline engine development will be performed at test beds and in vehicle testing performance, emissions, mechanics and acoustics. The aim of this phase is to proof that all development targets are met with prototype components. 2

3 In the Pre-Production Development the engine design will be optimized on basis of Gen. 1 development results. For the manufacturing of the Engine Generation 2 volume production methods were used. The target of this phase is to proof the engine design under volume production constraints. Then the engine development is completed. In the last phase the Production Validation the validation tests according to the requirements of the engine (vehicle) manufacturer will be performed. For an advanced design the support by the calculation has to start as soon as possible to reduce time and costs in the experimental phase at the test beds. So after the first design data are available in the prototype phase all standard calculations start immediately. The main target of the simulation in this phase is to give input in the design before the first prototype engine is manufactured to provide the best possible prototype engine for the test bed work. During the prototype tests the simulation should support the optimization at the test bed and give support if problems occur to assure the necessary input for the Engine Generation 2. During the Pre-Production Development all necessary design modifications resulting from the Prototype Development will influence the design. The changes that influence the simulation results should be checked with recalculations. 3 Analysis Tasks in the Engine Development Process The effective use of FEM-tools in the engine development process - reduce the risk of part failure and testing effort and therefore - the time and costs in the experimental phase at the test beds, - assure the fulfillment of legal regulations and internal specifications and provides the possibility of the optimization of the parts in respect to - manufacturing costs - material costs and - weight. The following figure shows the calculation tasks within the development process under special consideration of latest tools for mesh generation and calculation performance. Fig.2: Calculation Tasks in the Engine Development Process 3

4 Some of these calculations supply the FEM calculations at the moment only with input data, e.g. thermodynamic calculation. Others need input from FEM calculations and provide also data for other FEM calculations; e.g. valve train analysis with stiffness as input and reaction forces as output. Others are coupled, e.g. the CFD cooling jacket analysis with the FEM temperature analysis of the Head/Block-Compound. Also the classical analysis of parts as crankshaft, conrod, piston and bulkhead are nowadays improved by coupling with the dynamic crank train analysis in the total engine to get the correct results. This dynamic calculation of the whole power unit is also the basis of the acoustic evaluation of the engine. Nevertheless all now separately calculated effects are somehow interacting in the real engine and so future software developments will bring an increased coupling of all calculations to improve the results of all single calculations. The requirements for all of the software-tools are - accuracy of the calculation process - short application time - reliable results and - easy to use. The following figure shows some of the calculation tools used in connection with the FEM solver. Fig.3: Calculation Tools for the Engine Development Process The software AVL TYCON is used not only for the lay out of the cam design, but also analyses and optimizes the valve train dynamics, timing drive dynamics and the transmission gear dynamics. The calculated reaction forces are an input to the strength and acoustic calculations. With the software AVL GLIDE the piston dynamics is calculated. The results are the piston movement and deformation, the piston impact, the ring dynamics, the blow by and the lube oil consumption. The piston impact forces are mainly an input for the acoustic calculation. The dynamics of the total engine is analysed with the software AVL EXCITE. In this program the dynamics of the rotating crankshaft inclusive conrods and piston inside the power unit is calculated. The coupling inside the bearings is done with an elastohydrodynamic module to guaranty the correct oil film simulation. The results of the program are realistic load distribution in the bearings, detailed bearing analysis, crankshaft dynamics, vibration behaviour of auxiliaries, 4

5 mounts and manifolds and the structure borne noise as an outcome of the non-linear forced vibrations calculation. The basis of the effective use of FEM is a quick mesh generation in time and quality. AVL has therefore developed an automatic hexahedron mesh generator AVL FAME (= Flexible Automatic Meshing Environment). Starting with the geometry information (3D-CAD surface model) the mesh generator creates a fully hexahedron FEM-mesh fitting to the geometry. - the mesh can be as coarse or as fine it is needed in different regions and in different directions. These possibilities guaranty good element quality and a reasonable low number of degrees of freedom. The fig. 5 shows one of the possible approaches for creating a FEM-mesh of a crankshaft. Fig.5: FAME FEM-model of a crankshaft Fig.4: AVL Tool for the Hexahedron Mesh Generation Additional to the fully cartesian cube approach of the automatic hexahedron mesh generators already on the market, AVL FAME provides the possibility to use any kind of hexahedron start topology created with a FEM pre-processor like MSC/PATRAN for the meshing process. This gives the advantages that - the generated mesh can be adjusted to the geometry contour and In this approach the FAME process with start topology covers only one crank. After the mesh of one crank is finished, the rest of the crankshaft is assembled afterwards using a standard FEM preprocessor like MSC/PATRAN. This approach results in the most efficient use of degrees of freedom, but is only reasonable if all cranks are equal. Another approach for creating a crankshaft model is to generate the start topology for the whole crankshaft. This saves effort by avoiding the afterwards assembling, but increases a little bit the generated number of elements. 5

6 A second example for a quick mesh generation with AVL FAME shows fig. 6. The fig. 7 shows the classical use of the FEM in the strength and temperature calculations. Fig.6: FAME FEM-model of a piston Beginning with 2D-meshes of the two important views an extrusion in two directions creates the start topology. After the meshing procedure with FAME the result is a designed automatically created hexahedron mesh with a reasonable number of elements. The additional benefits of this model are the cylindrical mesh in the cylinder axis direction at the piston outside contour in predefined sections (needed for coupling to the cylinder liner) and also a cylindrical mesh in predefined sections in the piston pin area (needed for high accuracy coupling with the piston pin). Once created start topologies can be used again especially by variant studies which reduces the effort for model creation. For similar parts (e.g. pistons) it is also possible to create libraries with different start topologies for a reuse. Fig.7: Strength and Temperature Calculations To increase the reliability of the results the step from the linear to the nonlinear calculations has to be made. Elasto-plastic material, temperature dependent and non-linear contact with friction (e.g. cylinder head / gasket / cylinder block compound) are needed applications in up to date calculations. Also the quasi-static approach in time domain for moving parts like crankshaft, conrod and piston and bulk head calculation has to be substituted by a transient calculation of the load with programs calculating the engine dynamic like AVL EXCITE. 6

7 The acoustic and engine dynamic calculation is shown in fig. 8. Fig.8: Acoustic and Engine Dynamic Calculations Dynamic loads as gas pressure, piston slap and valve train excitation provided by other programs are applied to the complete power unit including the full assembled crank train. Using the static and dynamic reduction feature GDR of MSC/NASTRAN the total structure is reduced to a reasonable number of degrees of freedom to perform the nonlinear forced vibration calculation in time domain with AVL EXCITE /3/. The results of the EXCITE calculation are the forces between the acting parts (e.g. bearing forces), the deformations, the velocities and the accelerations of the total structure. These results are evaluated in view of - radiated noise (integral velocity levels, octave and terz band and displacements and strain for critical frequencies), - interior noise (displacements, velocities and accelerations at the engine mounts), - durability of auxiliaries, mounts and manifolds (accelerations at the surfaces), - durability of crankshaft, conrod, piston and bulk head (dynamic forces, moments, stresses and displacements in time and frequency domain) and - bearing parameters (inclination of pins, journal displacement, oil film thickness, oil film pressure, pressure distribution, friction losses and in detailed bearing analyses heat flow and temperature distribution. This evaluation leads to changes in the different design parameters to optimize the design. 4 Examples This chapter gives a sample of examples for the use of FEM-tools in the engine development process. Fig. 9 shows the temperature results on a piston created by AVL FAME. Fig.9: Temperatures on a piston created by AVL FAME 7

8 1st Worldwide MSC Automotive Conference An example for the use of coupled calculations is shown at fig. 10. The fluid-structure coupling between CFD (AVL FIRE) and FEM (MSC/NASTRAN) increases the accuracy of both types of calculations. The figure shows the improved results between the first and the second iteration and also a comparison to the measurement /4/. Fig.11: Non-linear Analysis of a Cylinder Head Block Compound Fig.10: CFD-FEM Coupling between AVL FIRE and MSC/NASTRAN Especially in cylinder head block compound calculations different types of non-linearity had to be considered. Geometrical non-linearity in the gasket contact area as well as material nonlinearity (elasto-plastic material, temperature depended) can be handled in FEM codes like MSC/NASTRAN. With this calculation an optimization of the bore distortion, the stresses in the cylinder head and block, the deformation of the cylinder head and the pressure distribution at the gasket can be performed. The fig. 11 shows the von Mises stresses at a deformed cylinder head and block. Fig. 12 shows the dynamic stresses in a 5 cylinder crankshaft at a particular time step. The load is calculated by a dynamic analysis of the total engine over a full engine cycle within AVL EXCITE. For the stress calculation of the crankshaft of the different time steps MSC/NASTRAN is used. Fig.12: Dynamic Stresses at the Crankshaft of 5 Cylinder Engine The quasi-static approach for dynamical loaded parts is no longer acceptable for up to date calculations. Fig. 13 shows a comparison of the results between the quasi-static 8

9 approach and dynamic stresses calculated by AVL EXCITE. The last example at fig. 15 shows a comparison of the structure borne noise between different load conditions at a 4-cylinder Diesel engine. The left picture shows the integral velocity levels at the surface of the power unit with only the gas forces as external load. The right picture shows the same results with additional excitation by the piston slap and valve train forces. Fig.13: Comparison of Quasi-Static and Dynamical Calculated Stresses The dynamic calculation of a full power unit allows also a detailed analysis of the bearing loads under real operating conditions. AVL EXCITE with its EHD (=elasto-hydrodynamic) bearing module provides also information of the oil film behaviour in the bearings. Fig. 14 shows the dynamic oil film pressure in the main bearings at a particular time step. Fig.15: Influence of Different Load Conditions to the Structure Borne Noise To increase the quality of the calculated results the simulation models have to be more and more extended in the future by additional parts and more and more effects (loads) have to be considered. Fig.14: Oil Film Pressure in the Main Bearings at a Particular Time Step 9

10 5 Future Engine Development To fulfill the requirements of future engine development in time and costs a considerable reduction of the time frame for the engine development is needed as well as additional and more detailed analyses to satisfy higher quality requirements. Additional to the time saving by quicker simulation application also an increase of the quality of the results should lead to a reduction of the time and effort needed at the test bed. So the future trend will bring together the engine development on two different engine generations to a development at only one engine generation before SOP. Fig. 15 shows the comparison between the time frame of the present engine development and the estimated time frame of the future process. Fig.16: Future Engine Development 6 Conclusions The optimized correlation between the different calculation tasks and the development process, the right starting time and extend (time frame) for the use of the simulation and the application of fast and validated simulation software assures reliable results in time. A reduction of development time can be obtained by quicker simulation application but also an increase of the quality of the results should lead to a reduction of the time and effort needed at the test bed. The proper application of the FEM-tools in the design phase and the best support during the prototype phase reduces the risk and the testing effort in the prototype and pre-production phase and is one essential basis for competitive engine development. References: /1/ Rainer, Gotthard Ph.: Computersimulation as Part of the Development Process, International Symposium Engineering Centre Graz, Austria, Juli 1998 /2/ Rainer, Gotthard Ph.: Verkürzung des Entwicklungsprozesses durch Computer-simulation, 3. Handelsblatt- Jahrestagung, Mai 1999 /3/ Loibnegger, B.; Pramberger, H.; Rainer, G.Ph.: Zusammenwirken von MSC/NASTRAN mit dem Mehrkörpersystem AVL EXCITE für akustische Berechnungen von Motoren, MSC Anwenderkonferenz München, September 1996 /4/ Petrin, H.; Ennemoser, A.; Rainer, G.Ph.: Gekoppelte Fluid-Struktur Berechnung mit MSC/NASTRAN (FEM) und AVL FIRE (CFD), MSC Anwenderkonferenz Bad Neuenahr, Juni