If each new generation of computer processor is able to double the speed of the previous one, is it any surprise that GM's next-generation gas-electric hybrid powertrain for its big pickups and SUVs will be four-mode instead of the current two-mode?

We first heard a four-mode hybrid powertrain was in the works in 2007, during a conversation with then-vice-chairman and product czar Bob Lutz. Now, we've got a bit more information after our latest visit to GM earlier this week for a product and technology preview.

"It's all about any hybrid system," said John Turzewski, GM's lead development engineer for full-size truck hybrids. "As you evolve and go through your next generation, it's cost reduction, smaller, lighter and better packaging. There's lots of improvement all along those lines."

The two-mode hybrid powertrain uses an electrically variable transmission that houses two 60kW electric motors that can power a truck on their own up to about 25 mph, depending on driving conditions. It provides an almost infinite range of gears, but it also has four fixed gears, like a conventional automatic transmission, that can be manually selected with the column shifter. The electric motors seamlessly support a 6.0-liter V-8, individually or in tandem, at low and high speeds, helping the engine enter fuel-saving four-cylinder mode sooner and stay in it longer to achieve maximum miles per gallon.

Will the four-mode hybrid have twice as many electric motors as the two-mode? Turzewski isn't saying, but we're willing to bet there will be more than four fixed gears to help run the truck more efficiently in a wider array of driving conditions.

GM has already promised that its next-gen hybrids will be able to tow 50% more than today's 6,100-pound maximum. Today's trucks also rate the best fuel economy in the segment at 21/22 mpg city/highway with two-wheel drive.

The four-mode hybrid system is expected to arrive by 2013.