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Gas-Guzzling Hybrids

Big engines get a green sheen.

In December, General Motors and DaimlerChrysler showed off the technology at the heart of their recently ­announced hybrid-car partnership. The companies said that the contraption – a transmission packaged with two electric motors – would be in vehicles for sale in 2007, boosting their fuel economy by 25 percent. GM’s announcement claimed it would “advance the state of hybrid technology in the industry.” But the system will, in the end, produce an SUV that averages about 20 miles per gallon instead of 16; the Toyota Prius hybrid averages 55.

Still, experts say the new technology is a real advance. It’s just not one aimed at changing the gas guzzler culture. GM and Daimler have built a hybrid system that’s geared to squeezing a few extra miles per gallon from the big engines inside beefy pickup trucks, SUVs, and luxury sedans.

The Prius has a small gas engine and, overall, leans significantly toward the electric side of the hybrid equation. An electric motor propels the car at low speeds and works with the gas engine at higher speeds. The catch: much of the motor’s electricity is generated by a second motor tethered to the gas engine, a process that’s only 70 to 85 percent efficient. That’s a good bit worse than the 98 percent efficiency with which a gasoline engine can transfer its torque to the road when only the gears of its transmission mediate. This loss is often not a big problem, since at low speeds, the gas engine is less efficient at producing torque. But when, say, the car is pulling a heavy load or traveling at highway speeds, it would be better for the gas engine to do the work directly. And trimming that loss of efficiency becomes more important when the engine is larger.

Enter the GM-Daimler transmission. The eight-cylinder engines it will be paired with feature “displacement on demand” technology, which shuts down individual cylinders at steady or low speeds. Its control systems put more emphasis on direct use of the gas engine, but in ways that keep it operating at peak efficiency, depending on how many cylinders are in use. And with an additional set of gears, the control systems can transfer nearly all of the engine’s torque to the asphalt when the efficiency numbers call for it. “This is the next contribution to a radical change in the way drive power is provided,” says Thomas Keim, an engineer at MIT’s Laboratory for Electromagnetic and Electronic Systems. If only its impact at the pump were more radical.