A Comprehensive Vehicle NVH Performance Correlation Study

One of main NVH (Noise, Vibration and Harshness) deliverables for Chrysler Development System (CDS) is a fully correlated finite element model to be used for the referencing and the target setting process for the future program. To satisfy CDS requirements, a comprehensive feasibility study using the finite element modeling techniques along with the experimental methods to validate the CAE (Computer Aided Engineering) model has been launched by NVH CAE and NVH Development and Engineering in DaimlerChrysler. The objective of this study was to evaluate the fidelity of current CAE processes based on the best practice approach and to simulate a wide range of vehicle NVH performances with minimal model compensation. Parallel to this effort, a full battery of experimental tests including modal testing, bushing rate and engine mount measurement, and standard load-case testing were performed on a vehicle in the component, sub-system, and full vehicle level. A correlation was established and proposed, and the correlation results based upon one production vehicle were analyzed. A set of recommendations to improve testing and simulation process was developed and presented in this paper. INTRODUCTION Reliable NVH Simulation: An Important Tool in Gaining Competitive Advantage In the last decade, automotive industry has experienced a tremendous increase in the level and intensity of competition. OEMs faced with global over-capacity, reduced margins and increased customer demands, are constantly looking for innovative ways of differentiating their products in an increasingly competitive market. Automakers believe that “overall quality perception” is a major decision factor when it comes to customers’ perception of a vehicle and corresponding buying decision. Vehicle noise and vibration is considered to be an important part of quality perception. However, as a result of increased competition, automakers want to develop vehicles with competitive NVH performance in shorter design cycles and with reduced development and vehicle costs. Number of hardware prototypes has reduced dramatically and today it is more important than ever before to provide reliable NVH simulation for all NVH vehicle performance attributes. However, due to the complexity of the NVH simulations, subjective nature of the NVH operating conditions and inherent randomness especially in high frequencies, large-scale system-level full-vehicle NVH modeling suffers from a quality perception problem. Without addressing this “perception” problem, CAE engineer’s contribution will be limited. Chrysler Design System and NVH Simulation To address the correlation issue with NVH models, NVH team has imbedded a special provision in the CDS [1]. As part of the CDS process, before the start of any new program, NVH community should deliver a fully correlated baseline model to the product team simulation group. This certified model should be used in all program simulations as a reference point. All the design directions and trade-offs made based on NVH simulation should be referenced back to this model. The reference vehicle should be selected from a production vehicle with stable manufacturing process. All the subjective ratings for NVH attributes should be performed and recorded for that vehicle. In doing these a full set of subjective and objective baseline information will be available. Since the model is correlated or at least the relationship between test and simulation data is known, the results from the simulation of the next level vehicle can be easily interpreted and credible recommendations to guide the design and project the performance of the next level vehicle can be made. To make sure the correlated model is ready for the next generation vehicle program, work on the new model starts when the last program ends. Lead time is very crucial. Minivan Correlation Project A 2001 model year vehicle was the Chrysler current production minivan and this vehicle was used as the reference vehicle for the future minivan program. Since the NVH model of the reference vehicle is required for the prediction of the new vehicle model in the CDS process, a comprehensive vehicle NVH performance correlation study between the finite element model and the experimental validation testing was launched to develop a representative minivan vehicle model. A current production minivan was secured and dedicated to this correlation project, and a full battery of experimental tests including all the NVH operating and non-operating tests were performed on the full vehicle, sub-system, and component levels. Test Plan and Corresponding CAE Loadcases The operating tests include the objective measurements for each of the NVH load cases in a controlled NVH laboratory environment. The coarse road noise, rough road shake, impact harshness, and powertrain noise and vibration are the standard NVH load cases used for the benchmark of vehicle level NVH. Additional transfer path analysis (TPA), panel mobility, and running mode analysis were also considered for this correlation study. The following are brief descriptions of each standard NVH loadcase: The coarse road noise test is a NVH load case to simulate the noise in the vehicle on coarse road surfaces. The noise often has qualities of boom, roar, and sometimes ringing. This test was conducted in a semianechoic chamber equipped with a 10-foot dynamometer, where a pre-determined coarse road surface was built-in to the chassis roll. A frequency spectrum, an equivalent to the coarse road surface, was established as an excitation for the NVH CAE simulation. The rough road shake test is a NVH load case to simulate the vibration in the vehicle in low frequencies. The shake is felt mostly in the steering wheel, seat, and floor of the vehicle. This test was conducted in a semianechoic chamber with a 6-foot dynamometer, where a pre-determined cleat was used on the smooth chassis roll surface. No simulation was performed for this test. The impact harshness test is a NVH load case to simulate the ride harshness. Not only the harshness noise but also the harshness feel are captured in this test. Again, this test was conducted in a semi-anechoic chamber with a 6-foot dynamometer, where a pre-determined cleat was used on the smooth chassis roll surface. A frequency spectrum, an equivalent to the cleat profile, was established as an excitation for the NVH CAE simulation The powertrain noise and vibration is a NVH load case to quantify the overall engine noise, intake noise, exhaust noise, and powertrain vibration, which comes through not only the steering wheel, seat, and floor, but also the throttle acceleration pedal, clutch pedal, shifter, and door. This test can be conducted on the road or in an engine dynamometer, and the typical engine operating conditions are the wide-open throttle (WOT), part throttle (PT), and idle. The engine mount excursions and the engine gas pressure data were also acquired to correlate with engine excitation loads for the powertrain CAE simulation. Gas pressure loads combined with the inertial loads were used as a source of excitation for a detailed CAE model of the P/T and driveline. CAE model simulated the structure-borne portion of P/T noise. Air-borne portion was not accounted for. The non-operating tests include the vehicle level modal tests for the full vehicle, trimmed body, and body-in-white (BIW), the component level modal tests for the power train, exhaust system, steering column system, seat, and suspension and chassis components, and the body and acoustic sensitivity (A/F & P/F) measurements. All these loads were closely replicated by CAE loads. Furthermore, all the engine mounts, transmission mounts and bushing rates were measured under different engine operating conditions in order to provide right properties for different CAE simulations.