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Silent EVs Cause Industry to Ponder NVH Issues

Not only will the relative silence of an EV allow potentially annoying noises to emerge from behind the mask of the engine sound, but the vehicle itself may create new high frequency noises.

PARIS – Noise, vibration and harshness will be a critical customer issue for electric vehicles.

Not only will the relative silence of an EV allow potentially annoying noises to emerge from behind the mask of the engine sound, but the vehicle itself may create new high frequency noises.

And, while engineers from major auto makers and suppliers spoke at a recent conference held by the French automotive engineering society SIA about how to tackle many of the NVH issues in EVs, there is no consensus on what to do about adding sounds at low speeds to warn pedestrians and cyclists that a car is silently approaching.

“NVH will probably be a major player in the success of the electric vehicle,” says Benedicte Le Nindre, who works in Renault SA’s research department.

Customers surveyed last year “expect silence, peace of mind and comfortable riding, a windy sound quality, a fluid driving experience like a skipper enjoying a sailboat,” she says.

Emerging sounds include the noise of a ventilator fan, which at the lowest speed is masked in most cars by low frequency engine sounds. New sounds include the power electronics, whine of the electric motor, ventilation for the battery and gear noise during regenerative braking.

The issues apply equally to full hybrids that can be driven on electricity alone.

Jan Horak of Reiter Automotive Systems in Switzerland notes Toyota Motor Corp. uses more underhood sound insulation in its hybrids than in diesel versions of comparable vehicles, as well as plastic sound deflectors, to improve silence in the cabin.

While such traditional approaches to insulation work – the Toyota hybrids required no extra insulation in the cabin – the pressure to reduce mass in EVs has engineers looking for ways to suppress noises at the source.

Coils are the biggest source of noise in power electronics, says Xavier Benoit of French electronic supplier Adetel Group. So to minimize their noise, they must be correctly designed as small as possible and should be placed away from mechanical structures and mechanically decoupled with, for example, a rubber mounting.

Such solutions mean “acoustics must be considered at the start,” Benoit says. “Everything eliminated at the source is won for the future.”

Ventilation is a particular problem for batteries. The nickel-metal-hydride batteries to be used by PSA Peugeot Citroen in its diesel hybrids in 2011 will lose 40% of their durability if they are just 5° F (15° C) too hot.

PSA chose to place the exhaust fan behind the battery in the trunk, pulling air from the cabin past the battery to cool it, on the assumption that on hot days, drivers of the Peugeot 3008 and Citroen DS5 hybrids will be running the air conditioning.

Engineers at the auto maker found different locations of the air inlet in the cabin had remarkably different effects on the noise that would be heard by rear-seat passengers. Using a porous vent tube also can reduce noise, says PSA engineer Laetitia Morel, but the system must be designed carefully because the tube can require a faster fan to maintain air flow.

Ventilation noise from the HVAC system can be reduced by using larger, slower fans, says Said Naji of French supplier Valeo SA, but that means “the customer must give us more packaging space.”

More space is also needed to lengthen the air inlet tube that would eliminate the problem of an annoying change in ventilation sound when a customer switches from outside air to recycled air.

Historically, customers dislike vehicles that have a pollution sensor that automatically switches to recycled air because the change in sound is not related to the driver’s decision. The same problem occurs in hybrid vehicles, where the engine goes on and off without a direct relationship to the driver’s activity.

“The best way to use additional energy (to reduce carbon-dioxide emissions) is to let the system decide” when to use battery power, rather than the driver, says PSA’s Ladimir Prince.

But drivers don’t always understand all the compromises that engineers must make when they devise the computer’s algorithms.

Most engineering today is done with computer models. David Quinn, manager of NVH at the Nissan Technical Center Europe in England, says noise problems are very complex.

Francois Gerard, an engineer at LMS International, suggests adapting a fuel-economy model that simulates an entire driving cycle to study comfort questions.

“Torque and acceleration of the car body can be predicted,” he says. “And that can be used to find transient phenomena and the importance of different components.”

PSA developed a digital tool to specify the level of acceptable whining noise at the beginning of a project.

But both Jan Horak of Reiter Automotive and Georg Eisele of FEV Motorentechnik GmbH in Germany agree the independent action of the engine annoys drivers.

FEV developed an EV based on a Fiat 500 and added a Wankel engine as a range extender because the rotating piston makes far less noise at the source than a reciprocating engine.

Even with a double muffler, however, the wind and rolling noise didn’t completely cover the engine sound.

“No customer wants to hear this start and stop of the engine because he is not the one who decides,” says Eisele. “It is the system, so he doesn’t want any feedback.”

Meanwhile, sounds from regenerative braking are new to the automotive industry.

“Below 50 km/h (31 mph) you don’t have wind noise, and road noise goes down, so you have this disturbing whining noise,” Eisele says. “Some are magnetic noises, but a lot of other orders of noise come from the gear train.”

The unexpected quiet in an EV has engineers thinking about adding “designer” sounds. Renault’s Le Nindre says it will be important for all the human-machine interfaces to have the right sounds.

“We will probably have to work on the ‘dingle’ for starting,” she says, because on or off, a parked EV makes no noise.

And designers need to compose sounds for a low-battery alert, which in the Renault Fluence ZE and Kangoo ZE will sound when there is 12.5% of the battery left. “It will not be very appreciated to get a bad beep telling you that you will stop soon.”

Renault is also considering designing acceleration sounds for electric sports cars, “not a Ferrari sound, but something to help drivers adjust their driving,” Le Nindre says.

Sound design is nothing new. The Audi TT, for instance, has used an electromagnetic actuator to enhance engine sounds for several years.

French supplier Hutchinson SA has developed such a system, and engineer Pascal Audrain says it could be used on the structure of an EV to create sound that would mask unwanted noises or provide acoustic feedback to a driver.

Electronics also could be used to add sound to the exterior of the vehicle at low speeds, to make up for the fact blind people, for example, rely on their ears to check for approaching traffic in city crosswalks.

“At less than 30 km/h (18 mph), there is a risk of deadly silence,” says Le Nindre. “There is no consensus on how to counterbalance the risk.”

Suggestions include treating road surface in cities to create more tire noise, using pedestrian sensor technology to emit a noise only when there is a pedestrian in view, or simply adding an electronic or mechanical noise to the car to be directed forward at low speeds.

Renault even found that some people in its customer clinics last year look forward to personalizing their EV’s sound, using a noise that appeals to them.

At the same time, the idea of urban cacophony coming from silent EVs is disturbing.

“Everyone who has a chance to sail does not miss the sound of a combustion engine,” says FEV’s Eisele. “Everybody who has made a transatlantic flight with the annoying noise of the turbines would like to do it with a glider if it were possible.”

Committees in the U.S. and Japan are working on the question.

“If exterior noise is needed, we have many questions to answer,” says Le Nindre, trying to achieve a solution that is audible to pedestrians but not to drivers and residents.

“We need to understand how an electric vehicle sounds, what is required for safety, and work with manufacturers, suppliers, architects and territorial administrations. And we need to organize lobby activities.”

Meanwhile, the Renault Fluence goes on sale next year, sans any added exterior sound.

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