BusTalk

[Mr. Linsky]

Here's an interesting piece that appeared in the NY Times in April of last year.


Two-Mode Technology: Borrowed From a Bus


By LINDSAY BROOKE
Published: April 27, 2008 New York Times

FOR its large S.U.V.’s — and later this year, for full-size pickups — General Motors needed a hybrid system that would save fuel without sacrificing the hauling, towing and highway performance that endears these trucks to customers.

Those critical requirements called for a new type of hybrid system, and for design inspiration G.M. had only to look as far as its hybrid-drive system used first in transit buses. Those vehicles featured an approach known as the two-mode system, which G.M. engineers downsized from bus duty to fit in the smaller trucks.

The two-mode differs from the single-mode systems popularized by Toyota’s Prius in its design and function. In the Prius, a single planetary gearset within the variable-ratio transmission splits the gasoline engine’s power, routing it to drive the wheels or to charge the batteries for both city and highway driving.

Single-mode systems work best at low speeds because they can propel the car without running the gas engine. But when higher speeds or greater loads call for the engine to kick in, the single-mode’s electric motors provide less benefit. In fact, highway driving and heavier payloads are not the single-mode’s baliwick unless fortified with vastly larger, heavier, costlier batteries and electric motors.

Two-mode hybrids also have a variable transmission with two planetary gearsets added. These multiply the torque and provide two operating modes for the motors. The first mode accelerates the vehicle from a standstill to second gear. The other takes it from second gear to overdrive.

And the system’s two clutches can engage different gear combinations, providing an overlay of four fixed (not variable) gear ratios. This provides the hauling and towing flexibility that truck owners crave. To the driver, the fixed ratios feel and function like a conventional automatic.

On the highway, the two-mode design lets the electric motors deliver extra boost while using less battery power. That helps to keep the gas engine operating at peak economy.

The two-mode’s mechanical path (gears) for multiplying torque is roughly 20 percent more efficient than the single-mode’s electrical path, G.M. engineers say. Its electric motors can thus be made smaller, to fit inside more compact transmission cases in both rear- and front-drive vehicles. Such benefits attracted BMW and the former DaimlerChrysler (now the separate Mercedes and Chrysler L.L.C.) to join with G.M. in an alliance to share the two-mode hybrid system among their fleets.

Mr. Linsky - Green Bus Lines, Inc., Jamaica, NY

[ripta42]

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