LMS enables accelerated NVH engineering based on integrated simulation process
As part of a plan to radically increase new model launches and vehicle redesigns at parent company Fiat Group, Elasis SCpA aims to cut powertrain development time in half with the help of a unified NVH simulation environment and technology transfer from LMS Engineering Services. Overcoming bottlenecks of the previous fragmented system, the new integrated solution is based on LMS Virtual.Lab Motion multibody software seamlessly linked with LMS Virtual.Lab vibro-acoustic simulation software and LMS test technologies. Modeling and simulation that formerly took weeks can now be completed in just days, providing faster throughput in an overall streamlining initiative for the car-maker.
Jumping out of the hole
Fiat Auto is a dominant force in Italian car-making, encompassing popular Fiat passenger cars such as the Punto and the upcoming rework of the FIAT 500 icon, as well as high-end premium luxury brands including Lancia, Alfa Romeo, Ferrari and Maserati. The largest automotive manufacturer in Italy and ranked fifth worldwide, Fiat has won the prestigious European Car of the Year award eleven times in the past 40 years Ò more than any other company.
Yet when Sergio Marchionne was named CEO of the Fiat Auto in 2004, the company faced serious financial issues and had not been profitable since 2000. Since then, Fiat has returned to profitability in what some analysts describe as possibly the automotive industryÌs most dramatic turnaround. In 2006, revenues increased 35% to reach reached $31.3 billion and the company announced it would pay its first dividends in five years. By 2010, plans are to nearly triple net profits with 23 new vehicle models and as many redesigns of current models. In this respect, Fiat is doing more than climbing out of a financial hole, they are jumping out in full force at a time when many major players in the automotive industry are struggling.
Broadly expanding its product portfolio requires stepped-up efforts and greater efficiency in every aspect of vehicle engineering, including the Fiat Powertrain Technologies (FPT) Division where 2.8 million engines and 2.1 million transmissions are produced annually Ò making FPT one of the most significant worldwide suppliers of automotive powertrains. Design of the powertrains is done primarily at FPTÌs Elasis research center in Southern Italy, where the parent company is aiming for a 50% reduction in development time while maintaining the Italian carmakerÌs world-renowned high quality across its range of brands and models.
The sound and feel of vehicle quality
The powertrain is critical in projecting the sound and feel of vehicle quality, especially since the sound is masked less and less by road noise in todayÌs quieter cars, says Francesco Sbarbati, Director of the CAE group in the Elasis powertrain NVH department. Engineers there use computer simulation to predict noise and vibration levels of engines and transmissions in the early design stages to address problems up front in development such as gear whine, valve rattle, driveline boom, or belt slap.
With thousands of high-speed moving parts, fast and accurate simulation is key to ensuring that the entire powertrain system will operate smoothly and efficiently when the first prototype is built, he explains. ÏA state-of-the art modeling and analysis process is needed to accurately represent the powertrain, evaluate its behavior, identify the root cause of problems and enable us to quickly see the effects of possible design modifications. Our goal is to shorten turnaround times by tightly integrating such a simulation approach into product development.
Mr. Sbarbati notes that the CAE group is revamping its simulation process to overcome a major limitation that previously hindered such a tight integration. ÏWith the support of an intense cooperation with LMS Engineering Services, we are moving beyond fragmented procedures used in the past, where considerable manual effort was required in exchanging data between the different incompatible simulation programs for finite element analysis (FEA), multibody dynamics, vibroacoustics and radiated sound prediction, he explains, noting that transferring flexible body data was especially cumbersome.
"Components such as accessory belts and engine mounts are inherently flexible. Also, at frequencies of 2,000 Hz in the audible range, normally rigid components such as crankshafts and valve trains may bend, twist and otherwise deform. Modeling them as flexible bodies is therefore essential in accurately predicting NVH forces. But with our previous system, key flexible-body parameters such as elasticity and stiffness properties had to be manually extracted from the Nastran FEA model and entered into the multibody simulation model. Then additional manual conversion was needed to import force data from the multibody program into the vibroacoustic software for prediction of noise and vibration. The task was tedious, time-consuming and error-prone."
Unified simulation environment
To address these limitations, Elasis worked closely with LMS Engineering Services in deploying LMS Virtual.Lab Motion multibody software as a key element in its move toward a more tightly integrated process. FEA models including data on loads and flexible body elasticities Ò can be imported directly into LMS Virtual.Lab Motion, where Nastran can be run without leaving the LMS Virtual.Lab environment. Hierarchal information trees in LMS Virtual.Lab Motion allow users to conveniently manage flexible body representations created using the highly accurate Craig-Bampton approach. LMS Virtual.Lab Motion flexible body capabilities are extremely valuable in performing efficient and accurate powertrain simulations, says Mr. Sbarbati. All the representations of flexible and rigid components are combined into a single LMS Virtual.Lab Motion multibody model.
Force data from the multibody simulation is imported directly into the LMS Virtual.Lab software for prediction of noise and vibration. Subsequently, LMS Virtual.Lab Acoustics is used to predict radiated sound pressure levels. All three applications use the same model and operate in an integrated manner, thus saving time and eliminating the risk of errors in manually transferring data between applications. Throughout the process, measurements from LMS Test.Lab on existing prototypes are used to calibrate models and validate results using a correlation postprocessing feature that allows easy comparison with simulation predictions.
From the start, the technology transfer provided by LMS Engineering Services has been indispensable in establishing this process,Ó explains Mr. Sbarbati. ÏThe advice, guidance and support provided by LMS personnel enabled us to recognize how best to leverage the software and continues to be the foundation for extending our simulation capabilities into the entire powertrain. LMS Engineering Services was a key success factor enabling Elasis to implement the integrated NVH solution for accurately predicting vibration levels of a full powertrain to design it right the first time.
He notes that technology transfer from LMS spanned a broad range of simulating powertrain NVH behavior in a unified manner including engine noise and vibration, bearing load prediction, crank train dynamics, timing belt behavior and gear train performance. Furthermore, a major strength of LMS Virtual.Lab Motion is that multibody models are parameterized so users can quickly change values and re-run the simulation to explore various what-if scenarios. ÏThe LMS system not only allows our engineers to create models and run simulations faster, but also to complete more analysis iterations in refining and improving the design.
Quieting gear rattle
One of the initial projects where Elasis teamed up with LMS Engineering Services used LMS multibody simulation technology in establishing a procedure for up-front prediction of gear rattle under varying operating conditions. Elasis and LMS Engineering Services worked together to create a multibody model of the transmission representing mechanical parts including flywheel, clutch, shafts, gear pairs, bearings and the cranktrain including the flexible body representation of the crankshaft. In this implementation, Elasis and LMS successfully collaborated on an approach analyzing in particular the way the transmission housing vibrates from bearing forces and how this noise is radiated and transmitted to the vehicle interior.
Rigid bodies were represented in the model as concentrated masses and inertias that included data such as gear stiffness, gear backlash, bearing characteristics and hub and sleeve connection properties. Flexibility of components such as the engine block, input/output shafts and gearbox housing was represented using modal parameters obtained from FEA models. Experimental modal analysis of the transmission was performed and correlated with FEA results to update material properties and validate the model to correctly predict the gear rattle frequency band.
Input to the multibody simulation were the combustion pulsations represented by the engine speed fluctuations and the resistance torque representing the vehicle load as seen by the transmission. The simulation computed modal participation factors as a function of time, with these forces transferred to the frequency domain and then applied to the vibroacoustic model of the gearbox, which yielded the noise radiation prediction as well as to determine forces on the engine bearings.
These predicted results agreed well with measurements made on the Elasis Virtual Engine Simulation (VES) test rig that reproduces speed and torque fluctuations similar to those generated by a physical internal combustion engine. This eliminates the need to have an actual engine for the test and eliminates the mask effects of engine noise, thus ensuring that the gearbox is the sole source of recorded sound.
With LMS Engineering Services having a central role, this initial project served as a benchmark in comparing the efficiency and accuracy of the LMS software against the older technology used at Elasis. It also established a proven method Elasis engineers could then use in modeling other major subsystems of the transmission.
Accessory drive and timing belt studies
With the experience gained on this initial project, Elasis engineers used the approach in studying the accessory drive at the front of the engine for powering the alternator and other equipment. Of particular concern was the serpentine belt system, including the shafts, couplings, pulleys, tensioners and idlers for controlling belt tension and power transmission. NVH is a highly important consideration in design of this system, since the belt is susceptible to flap and other troublesome noises while tensioners also can exhibit oscillations called flutter. The study focused on the systemÌs second-order vibration/acceleration: the dominant vibration order in four-cylinder, two-stroke engines.
Components represented as flexible bodies included the shafts, tensioners and belt, with data on belt damping and stiffness readily transferred from the FE model to the multibody model. As before, experimental data from LMS Test.Lab was used to calibrate the multibody models and also validate results. Likewise, engine combustion torque fluctuations served as input for the multibody model, with results from that simulation then applied to the vibroacoustic model of the accessory drive to predict noise radiation.
When potential problems such as belt flap were identified, engineers readily modified the sizing and positioning of the tensioners and pulleys on the multibody model to arrive at a quieter design before a hardware prototype was built. Similarly, the timing belt was modeled in LMS Virtual.Lab Motion to assess the ability of the flexible component in regulating engine timing to ensure that no valve clatter was generated in the engine. Engineers were able to modify the design, change key parameters on the multibody model and run another simulation in multiple iterations to arrive at an optimal design before going to the time and expense of building and testing a hardware prototype.
One major advantage of using such a CAE model is its promising capability in reducing physical prototypes by virtually testing different design modifications,Ó says Mr. Sbarbati. ÏSuch reduction in physical prototypes will lead to a substantial reduction in cost and time. The gearbox, timing belt and accessory drive projects are important steps in reaching the eventual goal of simulating the complete powertrain for noise and vibration predictions. LMS Virtual.Lab software is a cornerstone of this initiative, and the technology transfer provided by LMS Engineering Services was indispensable.
