Using CAE to assess whining noise of electric powertrains

  • 29-Jul-2014 02:07 EDT
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The 2014 Mercedes-Benz B-Class Electric Drive (left) and smart electric drive were the subject of extensive NVH evaluation by Daimler engineers.

Engine sound is a characteristic feature of passenger cars, and the well-balanced adaption to vehicle design and driving dynamics is a must for premium cars. The noise and sound of passenger cars have been dominated by the acoustic characteristics of the combustion engine for a long time. But now there is a change: modern cars with electric powertrains.

There is a wide range of activities to implement an electric motor in a car's powertrain: from additional e-motors in hybrids and complete battery electric driving up to fuel cell systems. The research and development department at Daimler is working on tomorrow's powertrains for electrical powertrains, and many basics have been investigated in the past and presented in research cars.

A customer’s decision to buy an electric car depends on many factors, but the expectation concerning the sound is clear: electric cars have to be quiet and free from annoying whining noise.

The NVH behavior of an electric vehicle is completely different from one with an internal combustion engine (ICE). For example, in the smart electric drive, not only the electric motor generates noise but also the auxiliary equipment such as battery cooling, high voltage control unit, and all the electrically driven servo devices for HVAC, power-brake or -steering, etc., operating independently from the drivetrain. These devices are extremely noticeable at lower speed or while stopping. And, in addition, new modes of operation have to be considered.

The electric car is dominated by several powertrain orders reaching high frequencies at higher speed while an ICE vehicle noise is characterized by the main engine orders producing a low frequency booming noise and the combustion noise in a wide frequency range. The whining noise from the electric powertrain is especially considerable because of the lower overall sound level, making this more annoying.

An unacceptable noise occurring in the late vehicle development phase can only be reduced with many decoupling and acoustic covering measures. But the better solution is to improve the NVH concept of the noise sources in a very early development phase.

So the task for the NVH engineer is: Improve CAE optimization and define acoustic criteria for whining noise. Both are needed for a successful development to reduce the noise by optimizing the concepts and to set targets for the specific noise characteristic of electric powertrains.

The first NVH analysis of the smart electric drive showed that the whining noise cannot be found looking only at the e-motor; it was necessary to take into account the complete powertrain. So the CAE model was extended up to the driveshafts and the mounts and also improved for a frequency range higher than 5000 Hz. Therefore an accurate modeling of connections and bearings is important. The results from the CAE analysis showed the annoying whining peak at 3000 Hz. Generated from the e-motors main order, this peak is caused by a coupled vibration of the rotating parts and the housings. The solution to reduce this peak to an acceptable level was to optimize the rotor design based on CAE simulation.

To measure and to simulate noise levels and then compare those to target levels is a standard task based on well-proven methods. But to analyze the annoyance of tonal effects in higher frequencies is much more difficult. The peak level or a tone-to-noise ratio gives no sufficient information for the annoyance. So a subjective rating is the usual assessment. To get objective criteria from NVH data more characterizing attributes, such as increasing/decreasing ratios, prominence ratio, and driving conditions have to be taken into account. Using several algorithms for an acoustic assessment, Daimler engineers developed and tested a new ‘whining intensity factor.’ Customer surveys showed a good correlation of this factor to the subjective rating. Together with the standard noise level this ‘whining intensity factor’ is an important tool to guide the NVH development of electric powertrains from concept phase to vehicle testing.

This experience and the defined procedure were used for the development of the Mercedes-Benz B-Class Electric Drive. The noise level is pretty low and a smooth sound characteristic is the result. Compared to current electric cars, the B-Class is the quietest car with the lowest interior noise level. An additional benefit is the lower exterior noise level.

Simultaneously the annoying noise from the electric powertrain is reduced to a minimum. Whining is nearly not noticeable and the sound is characterized by the lowest whining intensity factor compared to competitive electric cars. To achieve premium comfort, the developed tool is an effective way to guide engineering from concept design till vehicle testing. This assessment extracted from acoustic data is unerring and meets the subjective rating precisely.

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