Should EVs use a sibling's architecture?

Automotive News analyzes upcoming electric cars and whether they should be dedicated models or repurposed.

Tesla Model S
Tesla says positioning the large, flat battery pack on the floorpan of the Model S--separated from the frame here for display purposes at the Detroit auto show--improves the EV's ride, handling, and interior space. Josh Miller/CNET

DETROIT--Eric Kuehn makes a strong case for Ford Motor Co.'s decision to develop the Focus Electric on an existing vehicle architecture--even one designed for an internal combustion drivetrain.

Kuehn, Ford's chief engineer for electric vehicles, said using the existing architecture has many benefits: Ford reaps economies of scale; the automaker can still market the vehicle under the well-known Focus name instead of under a new nameplate; and a single architecture allows Ford to build the electric vehicle alongside other Focus models at the Michigan Assembly Plant. That means Ford can shift its mix quickly after production begins late this year.

"As customers move more to electric, we'll be able to flex our production and move more to electric," Kuehn said during an interview at the recent Detroit auto show. "If gas prices go down, then we can flex our overall production to gas-powered power trains."

At other automakers, that sort of talk is anathema.

Peter Rawlinson, vice president and chief engineer for Tesla Motors, said, "There are profound advantages to having a ground-up approach to building electric vehicles."

That's why EV-maker Tesla is creating a new platform for its Model S midsize car, due for production in mid-2012.

Hot debate
The two approaches highlight a hot issue among EV builders: whether it is better to build an EV from the ground up or to retrofit. Underlying the choices are sharply differing views of the sales potential of EVs and the investment they merit: hedge your bet or go all in?

Strategies vary from automaker to automaker. Nissan's Leaf, for instance, is built on its own architecture, and BMW is developing a dedicated EV platform for the MegaCity small EV due in 2013.

Meanwhile, General Motors Co. put its Chevrolet Volt plug-in on a front-wheel-drive platform shared with the Chevrolet Cruze. Even Tesla buys a chassis from British sports car maker Lotus for its initial model, the Roadster.

But Tesla CEO and EV evangelist Elon Musk has said EVs won't reach their potential without dedicated platforms. That's because retrofitters tend to stuff the large, heavy battery packs in the trunk "like a sack of potatoes," in Musk's words.

You can see the difference in Tesla's vehicles. In the Roadster, the 990-pound battery pack is positioned behind the passenger compartment. By contrast, the Model S will have a low, flat pack covering the bottom of the car.

Rawlinson said that design distributes weight evenly and improves ride and handling.

"There is a significant ride and handling advantage because it means the car has a low center of gravity," he said. "When the car corners, it doesn't roll so much."

Rawlinson said starting from scratch allowed Tesla to create a light aluminum structure with minimal use of high-strength steels. Tesla discovered some of the advantages of a dedicated architecture during the design process, he added.

"When we embarked on this program, we embarked upon a voyage of discovery in a sense," Rawlinson said. "The advantages of electric vehicle architecture were not self-evident; they had to be teased out and explored by the vehicle engineering team."

Rawlinson cited other benefits the team found:

  • Having the flat battery pack on the car underside improves underbody aerodynamics, something other automakers achieve with shielding to prevent air pockets.

  • Body stiffness is enhanced by cross members in the battery pack, which improves handling.

  • The pack enhances side- and front-impact safety.

  • Getting the pack out of the trunk allows Tesla to put two rear-facing children's seats there, making the Model S a seven-seater.

Retrofit advocates counter that their choice provides significant cost savings. Ford's Kuehn said using a high-volume platform is a "critical part of our strategy."

Likewise, GM's Micky Bly, executive director of global electrical systems, hybrids, EVs and batteries, said EVs--and plug-ins like the Volt--require a large investment.

"You've got to have the volume to pay it off," Bly said. "That's why everybody's been hedging their bets with more of what I call a retrofittable design instead of a pure redo."

Paul Haelterman, managing director of IHS Automotive Consulting, said it costs about $1 billion to develop a new architecture. That's a big outlay for EVs, Haelterman said, considering that EVs are likely to make up only 1.5 to 2 percent of the market this decade.

EVs could become a better investment by 2020 if battery costs drop and gasoline prices rise, he said. But for now, retrofitting is sensible.

"I think the short-term model is that that is going to be a profitable track for people to follow," Haelterman said.

Others say automakers eventually will design new architectures to accommodate both internal combustion and electric drivetrains.

Herbert Kohler, vice president of e-drive and future mobility for Daimler, said the company's small-car architecture is a good model. The platform, used for the Mercedes A and B class and the Smart ForTwo, was designed in the 1990s to accommodate battery-powered EVs.

Putting an EV on a platform designed for internal combustion isn't ideal, Kohler added: "Different technology requires different car concepts."

(Source: Automotive News)

 

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