Systematic isolation and extensive insulation
The issue of NVH (Noise, Vibration, Harshness) is particularly important in an electric car. In the absence of the noises normally associated with a car with a combustion engine, any vibrations or irritating noises such as tyre noise become particularly intrusive. On top of this is the high-frequency excitation of the electric motor. In order to meet the high standard of noise and vibration comfort that is such a hallmark of the brand, complex measures have been taken to isolate the electric power train from the chassis and body. Numerous damping measures enhance this effect.
One of the development goals for the EQA was to achieve a balance between noise comfort and ride comfort at a high level. A particular focus of this work was on the noise from the drive system and the integration of the electric powertrain. Components with a relevance to NVH were configured accordingly during the digital development phase, checked in the hardware phase on test rigs and only then integrated into the vehicle. The final refinements were then undertaken during road testing: in the complete vehicle it was then possible to identify transmission paths, eliminate critical resonances and reduce the noise from the drive system to an acceptable level. As when a house is built, efforts were concentrated first on the body-in-white/platform and then on the interior fit-out/insulation, since the isolation or immediate encapsulation of the electric drive unit is demonstrably more effective than damping measures in the interior. The acoustic insulation measures include an enclosed passenger compartment, efficient damping systems on the metal surfaces and acoustically effective trim elements.
Pleasantly unobtrusive noise from the powertrain thanks to isolation of front- and rear-axle drive systems
The single-speed transmission that forms an integral part of the electric powertrain at the front axle (eATS) operates particularly smoothly thanks to the improved microgeometry of the gearing. NVH measures around the electric powertrain were incorporated into the EQA at an early stage of component development. The power electronics unit, for example, was reinforced and secured firmly to the housing.
In an electrically powered vehicle, the low-frequency background noise associated with a combustion engine is missing, which means that high-frequency noises can become more apparent. For this reason, the front- and rear-axle powerpacks on the EQA have been isolated at several points. Components such as the supporting frame at the front axle, the rear-axle subframe and even the rubber bushings were developed in tandem with the digital development phase and continually optimised. These efforts ensure that no irritating noises are perceptible inside the vehicle.
Ancillary components, too, were significantly improved with respect to noise and vibration comfort. The air-conditioning compressor, for example, together with the front powerpack, was isolated from the supporting frame by means of rubber-bonded metal elements. In order to reduce structure-borne sound, the air-conditioning pipes were modified to follow the rigid elements in the bodyshell, and the bearing points isolated as far as possible. In addition, the operating strategy of the air-conditioning compressor avoids engine speeds that could have a detrimental impact on the noise level in the interior. This has no effect on climate comfort.
Low road noise thanks to targeted improvements in rigidity and an ingenious bearing concept
In order to reduce road and tyre noise, the engineers introduced a compact, shear-resistant integral mount which has significantly increased the introductory rigidity at the guide bearing of the front axle. The subframe of the multi-link rear axle is furthermore elastically isolated by rubber bushings. The front subframe connection is integrated into the C-ring structure and therefore has the necessary rigidity for isolation. A cross-member is integrated into the multifunction recess to increase the introductory rigidity of the rear subframe connection.
Low road and tyre noise is an important factor in terms of ride comfort, particularly when driving on country roads. As well as on the configuration of the bodyshell structure and the sound insulation, development work focused on the dynamic transmission characteristics of the various axle components and elastomer bearings. The redesigned front axle subframe allows the connection rigidity for the suspension strut and brake to be significantly increased. Subframe mounts are used in all instances for the rear axle. In conjunction with the reinforcements introduced at the various connection points, this reduces the intrusion of noise. The bearing concept used in the case of the multi-link axle allows greater longitudinal vibration damping, while the dynamic transmission in a vertical direction remains limited.