Tech Briefs
NVH and lighter stopping
Major strides are being achieved in reducing NVH in cars, which is central to the global motor industry's on-going quality drive. However, new models bring fresh problems, and the likely trend to smaller, lighter cars from 2005 through 2008 to meet tougher fuel consumption criteria may exacerbate the difficulties in meeting higher end-user expectations.
To reduce brake-related NVH problems within body and chassis, Bosch has initiated a systematic process for optimization of brake noise performance into the brake-design cycle. The concept is based on a structured Quality Assurance Process (NVH-QAP) with well-defined modules or work-packages, ensuring a comprehensive investigation of the underlying physical mechanisms of brake noise in the specific application condition, according to the company. It is applied for friction material noise projects as well as for actuation and other NVH problems, and has been developed and harmonized on a global basis. Special attention is paid to predictive methods, which enable noise optimization in early design phases. Direct links and continuous know-how transfer between NVH specialists and design engineers have been set up. Input from corporate and university research projects on brake noise has proven highly valuable.
For the frictional-element analysis of brakes, the modular work-packages can be divided into four areas of brake NVH work: simulation, laboratory testing, dynamometer testing, and vehicle testing. The simulation work-packages consist of brake component finite-element investigations and complex modal analysis calculations of brake systems. Much attention goes to the integration of friction material properties into these models. Equally, numerical investigations are done using flexible multibody systems.
For laboratory testing, equipment includes laser vibrometers used with laser holography to determine component dynamics together with the dynamic behavior of brakes, suspensions, ABS units, and entire axles. For dynamometer testing, Bosch has invested in inertia and chassis dynamometers. At the NVH centers in Europe, Japan, and the U.S., 15 dedicated squeal rigs and chassis dynamometers are available for NVH work. Intensive research work has been carried out to design dynamometer matrices that reflect real driving conditions as closely as possible, including participation with the new SAE J 2521 noise matrix.
For its vehicle testing, Bosch works on an integral approach for correlation between bench NVH and road NVH. High-range vehicle noise measurement systems are used during the standard long-term brake noise testing, which is conducted at the company's Mojacar facility in Spain. Further work on measurement-based evaluation of brake noise in NVH vehicle testing is an issue that will continue to be of considerable importance, says Bosch.
In parallel with this work, Bosch is also developing braking systems that offer improved packaging and reduced weight. Electronically controlled braking systems, offering both added safety and comfort, are already achieving significant penetration into mass market car sectors. The development of this technology, which includes ABS, traction control, ESP, and adaptive cruise control (ACC), has already led to the technology becoming cheaper for manufacturers.
Bosch recently started production of its Generation 8 braking module. It enables upgradeable configuration levels for ABS, TCS, and ESP systems based on a single modular design and offers scope for the development of the world's smallest modular brake control system, claims the company. Bosch states that its mass is 1.6 kg (3.5 lb), compared to the 6.9-kg (15.2-lb) original ABS in 1978. Even compared to the current version, the development staff saved 0.9 kg (2 lb) of mass. The volume was also reduced by almost 40% compared to the preceding model.
Bosch states that Generation 8 contains all additional braking functions currently available, including electronic braking-force distribution. Brake pedal vibrations are said to be less noticeable during ABS control. By means of an additional software module, a hydraulic brake booster can be integrated in the ESP, increasing braking pressure via the hydraulic pump up to the ABS control limit in case of emergency. Systems such as ACC are capable of braking even more precisely in combination with the control systems of Generation 8, claims Bosch.
- Stuart Birch
Dana goes after a Viper
 Dana is supplying a new center section and differential for the 2003 Viper SRT-10. The Hydra-Lok limited-slip differential is an intelligent axle component that enhances traction without the use of a traction-control system. |
DaimlerChrysler's Viper engineering group worked side-by-side with engineers from Dana Corp. to develop the new center section and hydra-mechanical Hydra-Lok differential Dana is supplying for the 2003 Dodge Viper SRT-10 being launched this month. The SRT-10 debuted at the 2001 North American International Auto Show.
According to Dana, the Hydra-Lok differential—which first appeared on the 1999 Jeep Grand Cherokee—was the first limited-slip differential to be successfully applied to both front and rear axles. Traditional limited-slip differentials are applied only to the rear axle because they typically cause the front wheels to lock together. Also, traditional limited-slip differentials are torque-sensing, in that the traction of the wheel on stable ground is determined by the torque of the spinning wheel. The Hydra-Lok, however, senses speed, so that the torque transferred to the wheel with traction is based on the speed of the spinning wheel.
Hydra-Lok enables vehicles to negotiate challenging road conditions with any two wheels on the ground. The differential provides torque to any wheel that has good traction. The Hydra-Lok uses the same components for all applications, though it can be tuned to maximize a specific vehicle operation such as off-roading or high-performance handling. It can also be customized to operate above a predetermined rpm according to vehicle weight, wheelbase, and design parameters.
When a Hydra-Lok-fitted Viper begins to negotiate a curve, the vehicle reacts as if it had an open differential to reduce the potential of oversteer, according to Dana. As the vehicle continues through the corner, it takes on the characteristics of a limited-slip differential, transferring torque to the wheel with traction. Coming out of the curve, however, the Hydra-Lok allows the Viper to deliver the maximum amount of torque to the ground for improved speed while exiting.
DaimlerChrysler is currently the only OEM using Hydra-Lok, though Dana is working with other OEMs to develop the differential for both front-wheel-drive and high-performance-vehicle applications.
- Jean L. Broge
Acoustic treatment from FAIST
 The electrically driven BCA modules in front of the observation window at the Audi wind tunnel can be lifted and lowered. |
The increasing importance of noise control and sound design in the automotive industry requires suitable measuring conditions in wind tunnels, engine test rigs, and unit- and axle-test facilities. Progressive design and innovative acoustic lining of free-field chambers represent important modules when creating such test environments. Especially in the low frequency range, the Fraunhofer-Institut für Bauphysik in Stuttgart has made substantial progress in the above areas, which has prompted the development of a new product by FAIST Anlagenbau GmbH in Krumbach, Germany.
To achieve optimal acoustic conditions for sound rating vehicles and units, a number of factors have to be considered. For example, an accurate analysis between 63 Hz and 8 kHz requires that background noise levels caused by exterior noise sources are at least 10 dB lower in that frequency range than the sound levels under investigation. In wind-tunnel systems, ventilators installed in the tunnels and the resulting air turbulence are the main sources of noise. Free-field conditions in the test chamber are required for the location of sound sources at vehicles. In accordance with the relevant standards, this condition is achieved by lining the chamber walls with absorbers in such a way that reflection of sound is avoided.
   FAIST uses simulation software that incorporates the size of the room to determine modal sound fields and thus the design of appropriate BCAs. Click to enlarge |
Wedge- or pyramid-shaped porous absorbers with a length of at least a quarter wavelength of the lowest frequency are used when free-field conditions have to be met. If the design frequency has to be 50 Hz, then a thickness of approximately 1.7 m (5.6 ft) is required, which is still ineffective at lower frequencies. A disadvantage of mineral fiber wedges is that the chambers may be polluted by fibers that come off the wedges.
With a thickness of only 250 mm (9.8 in), BCA modules developed by FAIST and the Fraunhofer-Institut offer economical, technical, and optical advantages for acoustic engineers in the automotive industry, claims FAIST. BCA modules are sandwiched panels with two rectangular absorber plates joined with an intermediate layer of metal sheet acting as resonator and integrated into a flat, perforated sheet-metal casing with a size of approximately 1.0 x 1.5 m (3.3 x 4.9 ft). The acoustic performance is based on an innovative combination of baffle-type resonators for low frequencies and porous absorbers for medium and high frequencies. The absorption of the BCA at low frequencies is "significantly" improved due to the embedded resonator, allowing engineers to create free-field conditions even below 50 Hz.
In hard-walled rooms with dimensions that are very large compared to the acoustic wave length, the sound waves radiated by a source travel in all directions and are reflected many times from the boundary walls. Thus, a local sound source produces a steady "diffuse" sound field. In small rooms and at frequencies lower than 125 Hz, the sound field becomes unsteady since the wavelengths of the sound fit to the room dimensions. This phenomenon leads to "standing waves" in the room.
The resonator part of these new absorbers is adjusted to the chamber dimensions and the related modal sound field of the room. Therefore, the panels contain carefully engineered different resonators to achieve the anechoic or semi-anechoic conditions and the specified lowest frequency that has to be met.
 The use of BCA modules allows for the reduction of the thickness of absorbing baffles, resulting in reduced flow speed and pressure loss, as well as energy savings. |
The BCA modules consist of completely non-fibrous material and creates an ecologically friendly atmosphere. The perforated metal sheet casings protect the absorbers and, at the same time, allow an individual color design. BCA modules were employed for the first time in aero-acoustic wind tunnels at Audi in Ingolstadt, Germany, which opened in 1999. Audi claims it is the quietest aero-acoustic full-scale wind tunnel for automotive testing to be found anywhere in the world. The maximum wind speed is over 300 km/h (186 mph).
DaimlerChrysler used BCA technology when building its research and development center, EVZ, in Sindelfingen with six anechoic test rooms as well as its wind tunnel in Auburn Hills, MI. In the EVZ in Sindelfingen, three chambers for interior noise testing as well as one chamber for structure-borne noise testing and another for structure analysis were planned as semi-free-field rooms. One facility for component testing was planned as a totally anechoic room.
 The BCAs can be integrated with lighting, crane facilities, an energy supply, and fire detectors.  |
As proven by ulterior noise records, free-field conditions down to 63 Hz have been realized at the facility. The application of BCAs compared to conventional wedge absorbers made it possible to reduce the building footage of the EVZ by 1500 m3 (53,000 ft3), allowing for a cost reduction of about €500,000 ($457,500).
Even at wind speeds of 250 km/h (155 mph), there are no fibers coming off that could pollute the test area of the 21- x 15- x 10-m (69- x 49- x 33-ft) wind tunnel at Auburn Hills, claims FAIST.
In the last four years, 40 test rooms have been built with BCA technology for automotive OEMs. Another aero-acoustic wind tunnel will be completed in France this year.
- Jean L. Broge
