Noise, Vibration and Harshness, the engineer’s old nemeses, are among the new paradigms that electrification brings to vehicle development. The industry’s success in mitigating NVH in IC-engine cars and trucks – resulting in fewer 11th-hour “band-aid” countermeasures – was achieved in the last decade with new tools and knowledge. Those gains, however, are now the baseline for tackling what NVH experts say are greater challenges inherent in electric vehicles and hybrids.
EV customers are not hiding their displeasure. A review of the “cabin noise” comments in online Tesla owners’ forums from early 2020 reveal significant dissatisfaction in this important metric. Some examples: “Today I drove a Lexus ES 350 on I-15S and noticed it is quieter than Tesla 3,” wrote one Model 3 owner, who recorded decibel readings in both cars. While the Tesla netted a 74 dBA at 70 mph (112.6 km/h) on that interstate road, the Lexus’s cabin was notably more serene, at 67 dBA.
Noted a former Audi owner: “It’s [Tesla Model 3] definitely noisier than my former A7 to a point where I dread driving above 110 km/h [68 mph] on the motorway whereas I routinely drove at 160 km/h [100 mph] with the A7. Worse, listening to classical music via Spotify or Tunein gets near to impossible above 110 km/h. Turning the volume up just makes things worse.”
Admitted a Texas-based Tesla owner: “Yes, the 3 cabin is louder than my previous cars, especially on less-than-perfect roads. Even with the ‘quiet’ tires, it’s undoubtedly louder than my wife’s Acura RDX and my daughter’s Mercedes C300.” Those observations are not unique to the Model 3, or to Tesla. Owners of other EVs who are objective report similar impressions.
“The NVH challenges get tougher with EVs, even though the overall noise levels are lower,” said Kiran Govindswamy, VP of Drivetrain Development, Vehicle Engineering and NVH, with FEV North America. “It puts greater attention on other noise sources because there is no masking of noise as in an IC-engine vehicle. A big part of that is wind and road noise. The focus, in terms of NVH engineering, is balancing the frequencies between powertrain, wind and road noise.”
Favorable NVH behavior is a key indicator of vehicle quality – and a vital attribute in the context of electrified vehicles. Compared with even the silkiest IC-based drivelines, EV propulsion systems inherently exhibit superior refinement during start-up and low speed cruising. At the high rpm where electric-vehicle traction motors typically operate, there is little broadband noise. It’s mostly pure tone, but as rpm rises it becomes a high frequency noise. Power inverters also exhibit high-frequency noise in the range of 10,000 Hz.
“Tonal noise originating from the electric motor is a big concern for engineers designing these vehicles,” said Marius Rosu, product manager on the Physics business unit at Ansys, maker of engineering-simulation solutions. “At varying motor speeds, NVH is unpleasant and may cause fatigue on the vehicle and discomfort for its occupants.”
EV propulsion system noise includes electromagnetic noise coming from the traction motor, or from the generator when in re-gen mode. “It’s also the geartrain orders, the gear whine,” noted Govindswamy. “It’s the mechanical noise from the shafts and bearings. And it’s the dynamics of how the shafts and housings interact to create the sound signature.”
And there are other audibles unique to EV drive units, typically masked in IC vehicles: HVAC noises. Electric heat pumps cycling. Even the sound of coolant circulating in the battery pack when the vehicle is stopped. All are obvious to the customer and must be engineered so that they operate below the noise ‘floor’ that’s already low in an EV.
That’s where experts say simulation, using a combination of finite-element and multi-body-systems tools, enables engineers to optimize component, system and full-vehicle design. The tools help analyze the dynamics of the gear train, shaft deflection, structural elements of the housing and the electromagnetic forces in the drive unit. The ability to predict both the design performance and the level of vibro-acoustic noise through simulation, without the time and cost of constructing prototypes, plays a significant role in mitigating noise and vibration (N&V) at a primary source.
There are tradeoffs: Should you focus on microgeometry changes to gears, for example, versus desensitizing the drive unit structure so that less noise is amplified into the cabin and the outside world? And the related vehicle structure, as it is made lighter to improve fuel economy, can cause the overall vehicle sensitivity to N&V to become worse.
“It forces you to get more creative with your architecture, including precise powertrain and suspension mounting, and more creative with the sound-package development,” FEV’s Govindswamy said. “You like to identify and design your attachment points to minimize the amount of amplification of the force input,” and hence minimize the level of NVH in vehicle.
Delving into root causes
The urgency to reduce NVH in vehicles has spawned a lively domain of specialist suppliers, offering dozens of proven simulation toolsets for the complex task. The most popular of these are interdisciplinary, multi-physics packages that delve deeply into potential root causes.
“The software tools for NVH are boundary-element-based, statistical energy-based, and airflow-based, and each of them have limits in the frequency range,” observed acoustics expert Pranhab Saha, co-founder and Principal Consultant with Kolano & Saha in Waterford, Michigan. Because of this, he said engineers build “bridges” among the various tools that allow them to predict frequency responses within a broader range.
“For example, NVH sound-package materials normally work at mid-to-high frequencies, so the tool their manufacturers use is statistical-energy analysis,” Saha explained. “But what about tire-cavity noise that is at a lower frequency? The statistical-energy-analysis model may not be able to predict this properly. So, there is a need for software tools that can be merged, to enable work in expanded frequency ranges.”
Calculating the magnetic forces in an e-motor, for example, is a critical input for mechanical and acoustic analyses. It is the first step towards predicting NVH. For that process Ansys, for example, offers simulation of e-motor noise caused by the magnetic forces acting inside the motor’s air gap.
These excitation forces can be directly transferred from a transient electromagnetic simulation in one platform used to create and test digital prototypes (Ansys Maxwell) to a harmonic vibration analysis in another (Ansys Mechanical), according to Rosu. “While it had been possible in previous releases to generate an acoustic spectrum for a single operating point, we are now able to walk through a range of rotational speed points in a so-called multi-rpm simulation,” he explained.
Full-motor design is automated using Maxwell’s template-based capabilities. The engineer need only specify the values for typical motor parameters – number of phases and poles, slots, materials, motor dimensions, coil pitch, etc. Once the parameters are specified, Maxwell automatically generates the motor design. (Housing and motor models also can be imported directly into Maxwell.) “Simulation of the motor in Maxwell over wide-ranging operating conditions involving varying speeds, current, power, torque, etc., provides useful insights into the motor’s electromagnetic performance as well as the magnetic forces that are generated during operation of the machine,” Rosu said.
Results are utilized in another tool, Ansys VRXPERIENCE Sound, to synthesize and evaluate motor noise and its impact on human perception. The multiphysics approach combining Maxwell, Mechanical and VRXPERIENCE Sound outputs an audio file of the motor’s acoustic configuration. Engineers can hear its sound at varying rpm. “With the complete acoustic profile of the motor,” Rosu said, “electrical and mechanical engineers can make changes to the design to reduce NVH while satisfying electrical performance requirements.”
Suite SIMULIA up front
OEM engineers developing EVs say their two biggest NVH challenges are wind/tire noise and vibration. “Road noise was always there in IC engine vehicles, but the spectrum of road noise and some of the IC engine noise are all in the same frequencies,” said Saha. “With the IC engine gone [in EVs] the tire-road noise becomes more critical.”
Tire companies are developing quieter tires, adding more elasticity and damping into the tire structure, to reduce radiated noise. When testing shows the tire falling short of noise targets, vehicle OEMs must add stiffeners or sound-package materials inside the vehicle and in the wheelhouse area, which add weight and complexity. “There is also tire cavity noise, which has existed since the advent of tubeless tires,” Saha noted. “It has become more critical with EVs. There is a lot of design and engineering focus inside the tire, to address the cavity noise issue which is around 250-300 Hz. We want to make that less audible.”
He and other experts assert that effective NVH mitigation is best handled at the systems level. “We could optimize at the component level, but once it’s all integrated into a vehicle the NVH behavior often does not meet the customer’s targets,” said Siva Senthooran, Technical Director, Acoustics, for Dassault Systemes.
Dassault’s SIMULIA brand offers a comprehensive simulation toolset for NVH work. It was created to help engineers focus their NVH investigations as far up front in the design stage as possible—before prototypes exist. It addresses the two key bogies: sources and paths. WithinSIMULIA, the PowerFLOW suite is used to “predict how the wind behaves as it flows around the vehicle, and the types of sources it will generate going into the cabin,” Senthooran said.
For electromagnetic noise sources there is CST. Dassault’s Simpack predicts forces generated by multi-body systems. Abaqus is for vibration and acoustic structural coupling at low to mid frequencies. Wave6 addresses paths of noise transmission and radiation over a broad frequency range.
To analyze wind noise, NVH engineers need an exterior shape. But clay models are not suitable for this type of study, so the team has to wait for an acoustic quality prototype. “PowerACOUSTICS is a design tool that’s part of the PowerFLOW suite, used at an early stage when the design is fluid and you don’t have all of the vehicle details, to find out how much noise transmission you’re going to get from the structure,” Senthooran explained. “With PowerACOUSTICS you can actually predict what somebody is going to hear inside the cabin.”
When a design is ready for prototyping is finally ready, engineers have had more time for optimization. “By then, they’ve discovered most of the issues in the design,” he said. “And when the design is close to the surface-freeze stage, engineers have the visual evidence showing the NVH-related issues and worked together with the studio to address the changes.”
Database supports development
When asked to name a supplier asset that he deems to be “fundamental” for his N&V work, a Detroit-3 body engineer involved with a new EV program replied, “That would be FEV’s NVH database.” As a full-vehicle engineering services provider, FEV routinely benchmarks new vehicles from across the industry for their NVH behavior.
The benchmark data is fed into a proprietary database the currently covers more than 600 vehicles and engines, with more than 100 metrics used by customers for component, system and vehicle target-setting on development programs. From the data, FEV engineers generate “scatterband” plots that present the state of the industry for a given metric. “A scatterband field might be radiated noise from an engine, or from a drive unit,” Govindswamy explained. “It might be isolation across mounts, or the vehicle acoustics sensitivity function.
“The NVH database has allowed us to support our customers by telling them where their products stand in both CAE and in the development state for testing,” he said. “Are you best in class? Mid-pack? Or is there room for improvement? Extensive development work supported by benchmarking is at the core of what we do.”
NOTE: SAE’s 2020 Noise and Vibration Forum remains scheduled (as of May 8) for Sept. 22-23 in Shanghai, China.Continue reading »