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07-11-2006, 01:29 AM


NEW YORK - A new transmission being developed by General Motors, DaimlerChrysler AG and BMW AG could move big SUVs and large luxury sedans out of the gas-guzzler category.

The "Two-Mode" heavy-duty, rear-wheel-drive hybrid transmission is a scaled-down and modified version of the gearbox used in GM's diesel-electric hybrid city buses. It is expected to boost the fuel economy of big SUVs such as the Chevrolet Tahoe and Dodge Durango by at least 25 percent.

That means the 5,265-pound Tahoe could deliver roughly 26 mpg in combined city/highway driving. That compares with 21 mpg for a two-wheel-drive 2007 Tahoe. The fuel-economy gains could be even greater if the gearbox is used in a lighter luxury sedan or combined with a fuel-saving diesel engine.

GM plans to launch the Two-Mode next fall in the 2008 Chevy Tahoe, GMC Yukon and Cadillac Escalade. Dodge plans to install the gearbox in the Durango in 2008. BMW and Mercedes-Benz have not said which vehicles will get the Two-Mode or when they will be in production.

At an engineering conference in Vienna, Austria, on Friday, April 28, the three automakers were scheduled to make public the technical details of the innovative gearbox.

Tim Grewe, GM's chief engineer for the Two-Mode, said the transmission combines the best traits of an EVT, or electronic variable transmission - such as the gearboxes in a Toyota Prius or a Ford Escape Hybrid - with the best traits of a regular four-speed automatic.

In stop-and-go city driving and at low speeds, the Two-Mode drives the vehicle on electricity using two electric motors encased in what looks like a regular rear-wheel-drive transmission. Power can be sent to the wheels at all speeds by both the electric motor and gasoline engine at the same time.

But cruising at highway speeds, power from the gasoline engine alone is transmitted through a series of planetary gears and clutches.

Unlike other hybrid transmissions, the Two-Mode has been designed to pull heavy loads, such as boats and horse trailers.

A powerful computer constantly monitors the vehicle's speed and load and the driving conditions, such as the grade of the road, and keeps the transmission in the most fuel-efficient mode, Grewe said.

"The EVT has a fundamental limitation in that you can't hook it up to big engines. It can't handle the torque of a V-8," he said. "What we did was use the right technology to handle the torque of a V-8, and that's a clutch. We overlay a four-speed transmission on top of an EVT."

All three automakers have set up shop in a three-building operation in Troy, Mich., a Detroit suburb. Each company is working to tailor the gearbox to its individual needs.

Larry Nitz, GM's executive director of hybrid powertrain engineering, said the Two-Mode can be used in rear-wheel-drive or four-wheel-drive SUVs, as well as in large luxury cars. He also said the transmission is strong enough to be teamed with a diesel engine, favored by European drivers in large SUVs and full-sized luxury sedans.

Although GM designed the Two-Mode's basic architecture and plans to build and sell the transmission to DaimlerChrysler and BMW, engineers from both German companies have made improvements.

"It's a joint development," said Andreas Truckenbrodt, executive director of hybrid powertrain programs for DaimlerChrysler. "The basic concept was developed by GM, but we are adding so much expertise and resources that it's really a joint hybrid system.

"The engineers from all three companies are sitting next to each other, back to back. We've bundled the electric-motor experts, the controls experts and the battery experts so that they can work together."

But not everything is shared, said Wolfgang Epple, BMW's vice president of hybrid powertrain programs. Each automaker has a private area in the Troy center where it works to integrate the Two-Mode transmission into specific vehicles.

07-11-2006, 01:30 AM
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