Automotive NVH Technology

The optimal styling of the exterior surface of a vehicle and its suspension system has a direct impact on interior wind noise. Both are determined in early project phases when typically no hardware prototype is available. Turbulent flows produce both external pressure fluctuations at the vehicle shell, known as hydrodynamic excitation, and sound waves, known as acoustic excitation. Hydrodynamic and acoustic sound sources are evaluated separately and relative to each other in the frequency domain in order to perform evaluations of different body shapes. The technical aim of the presented work is to investigate how acoustic quantities measured either directly in the exterior flow or as characteristic values of surface subsystems at the outside of a vehicle can be used to assess the influence of styling modifications to interior sound pressure level. The methodology is required to be capable of being integrated into the serial development process and therefore be quickly applicable. MAGNA STEYR Engineering has conducted extensive research to develop a method to ensure the best option is selected in early project stages.

Today, the number of downsized engines with two or three cylinders is increasing due to an increase in fuel efficiency. However, downsized engines exhibit unbalanced interior sound in the range of their optimal engine speed, largely because of their dominant engine orders. In particular, the sound of two-cylinder engines yields half the perceived engine speed of an equivalent four-cylinder engine at the same engine speed. As a result when driving, the two-cylinder engine would be shifted to higher gears much later, diminishing the expected fuel savings. This chapter presents an active in-car sound generation system that makes a two-cylinder engine sound like the more familiar four-cylinder engine. This is done by active, load-dependent playback of signals extracted from the engine vibration through a shaker mounted on the firewall. A blind test with audio experts indicates a significant reduction of the engine speed when shifting to a higher gear. In the blind test, experts favored the interior sound of the proposed sound generation system and perceived better interaction with the vehicle.

The noise performance of fully electric vehicles is essential to ensure that they gain market acceptance. This can be a challenge for several reasons. Firstly, there is no masking from the internal combustion engine. Next, there is pressure to move to cost-efficient motor designs such as Switched Reluctance Motors, which have worse vibro-acoustic behaviour than their Permanent Magnet counterparts. Finally, power-dense, higher speed motors run closer fundamental frequency to the structural resonances of the system [1]. Experience has shown that this challenge is frequently not met. Reputable suppliers have designed and developed their “quiet” sub-systems to state of the art levels, only to discover that the assembled E-powertrain is unacceptably noisy. The paper describes the process and arising results for the noise simulation of the complete powertrain. The dynamic properties are efficiently modelled as a complete system and subjected to motor excitation (torque ripple, electro-magnetic forces and rotor imbalance). Innovation in this project comes from the speed of the modelling and analysis, so that analysis and data interpretation comes early enough in a project to be effective in reducing the noise problems. This contrasts with the approach of simulating problems that have already occurred in testing. Actions to reduce the motor noise are explained and identified. System dynamic response identifies the operating points in which different excitation mechanisms are most problematic and steps are taken to reduce the dynamic response. Also, problematic conditions can be identified where innovative motor control algorithms are necessary.

This chapter discusses Nearfield Acoustical Holography (NAH) for the characterization of cylindrical sources. Cylindrical NAH is an experimental airborne characterization technique, and it is suited for any type of cylindrical source. NAH allows to evaluate sound intensity, pressure level and particle velocity. Practical aspects of Nearfield Acoustical Holography such as positioning error, measurement noise, hologram distance and measurement aperture are investigated and discussed with the aid of numerical examples. Moreover, a technique referred to as compressive sampling (CS) is discussed, aiming to reduce the number of sensors required by the classical NAH in the high frequency range.

The main source of excitation in gearboxes is generated by the meshing process. It is usually assumed that static transmission error (STE) and gear mesh stiffness fluctuations are responsible of noise radiated by the gearbox. They generate dynamic mesh forces which are transmitted to the housing through wheel bodies, shafts and bearings. Housing vibratory state is directly related to the noise radiated from the gearbox (whining noise). This work presents an efficient method to reduce the whining noise The two main strategies are to reduce the excitation source and to play on the solid-borne transfer of the generated vibration. STE results from both tooth deflection (depending of the teeth compliance) and tooth micro-geometries (voluntary profile modifications and manufacturing errors). Teeth compliance matrices are computed from a previous finite elements modeling of each toothed wheel. Then, the static equilibrium of the gear pair is computed for a set of successive positions of the driving wheel, in order to estimate static transmission error fluctuations. Finally, gear mesh stiffness fluctuations is deduced from STE obtained for different applied loads. The micro-geometry is a lever to diminish the excitation. Thus, a robust optimization of the tooth profile modifications is presented in order to reduce the STE fluctuations. The dynamic response is obtained by solving the parametric equations of motion in the frequency domain using a spectral iterative scheme, which reduces considerably the computation time. Indeed, the proposed method is efficient enough to allow a dispersion analysis or parametric studies. The inputs are the excitation sources previously computed and the modal basis of the whole gearbox, obtained by a finite element method and including gears, shafts, bearings and housing. All the different parts of this global approach have been validated with comparison to experimental data, and lead to a satisfactory correlation.

In recent years the automotive industry has been using an increasing number of high powered engines with fewer cylinders, with the goal to reduce weight and fuel consumption and hence to achieve lower CO2 emissions. Following, an overview about the currently existing methods and products within the exhaust development is given which follow automotive lightweight trend. Continuous innovations in new materials, structural design and manufacturing process as well as mastering the integration of the components and modules within the system with a thorough understanding and optimization of the system behavior is enabling the reduction of weight in exhaust system. Another possibility to reduce the weight is the use of additional components such as valves. In the following, a discussion about the different types of valves is presented. These valves can be implemented within the exhaust system in order to bring a constraint in the system and consequently additional acoustic damping. Due to engine downsizing, many premium vehicles lost their class-representing sound signature. An active system can be used in order to enhance the sound according to the customer demands. In addition to that, an active system can help reducing muffler volume.

Driven by both the ever-increasing tightening of legal regulations and the growing customers’ expectations, the noise, vibration and harshness (NVH) is becoming a crucial aspect in the vehicle development process. To achieve the NVH targets set for modern vehicles, sound insulation materials became an indispensable instrument to improve the vibro-acoustic behaviour. Typically, the sound insulation materials take advantage of so-called porous materials, which exhibit favourable properties when it comes to structural damping as well as transmission and absorption of sound. However, due to the highly complex material micro-structure and the sound propagation mechanisms involved the computational modelling of porous materials is a fairly challenging topic. An efficient yet accurate prediction of the NVH attributes of sound insulation materials therefore remains an unresolved issue. This chapter reports on recent developments based on so-called Patch Transfer Function (PTF) approach. Here the PTF approach is adopted for the analysis of coupled vibro-acoustic problems involving porous domains. The PTF is a sub-structuring technique that allows for coupling different sub-systems via impedance relations determined at their common interfaces. The coupling surfaces are discretised into elementary areas called patches. Since the impedance relations can be determined in either numerical or experimental manner, the PTF approach offers very high degree of versatility and is hence well-suited for combining test and simulation data into one workflow. Efficiency of the methodology proposed has been demonstrated by means of a validation example consisting of a rigid cavity backed by a dynamic plate with porous treatment. The full-system measurements are compared with the PTF predictions based on component measurements and/or simulations.