We've known for a while that BMW would launch its first hybrid this year, and now we know when: The 2010 BMW ActiveHybrid X6 will reach dealers this December.

We'll be driving the hybrid X6 in a couple of weeks, but meanwhile, here's our advance preview of BMW's hybrid-electric sport-activity vehicle, unveiled last month at the Frankfurt Motor Show.

2011 BMW X6 ActiveHybrid

2011 BMW X6 ActiveHybrid

BMW X6 ActiveHybrid

BMW X6 ActiveHybrid

2010 BMW X6 ActiveHybrid

2010 BMW X6 ActiveHybrid

2010 BMW X6 ActiveHybrid

2010 BMW X6 ActiveHybrid

Sport utility + fastback = sport activity coupe

BMW calls the X6 a "sport activity coupe," seeking to distinguish it from the more traditional and upright X5 SUV. And with a 4.4-liter twin-turbo V8 engine, this is one fast and heavy all-wheel-drive sports luxury activity coupe utility vehicle.

The hybrid version is distinguished by a hump in the hood, known as a "powerdome". Earlier spy shots of the X6 hybrid being tested in Death Valley showed a huge box sitting on the hood, so the sleek bulge comes as something of a relief. The hybrid also offers a new and exclusive shade of metallic blue paint.

World's most powerful hybrid

But BMWs are known for their driving qualities, and the ActiveHybrid X6 will be the world's most powerful hybrid-electric vehicle. Its twin-turbocharged 4.4-liter V8 puts out 400 horsepower, and two electric motors contribute 91 and 86 horsepower respectively.

Add that all together and maximum power output among the three separate power sources is 478 horsepower, with a maximum torque of 575 foot-pounds. That gives the European version a 0-to-62-mph time of just 5.6 seconds.

The penalty comes in weight. The standard 2010 BMW X6 xDrive50i (yes, that's its real name) tips the scales at an already-porky 5,000 pounds, and the hybrid gear adds another 400 pounds.

20 percent better on fuel

As for fuel efficiency, BMW says "reduces fuel consumption by approximately 20 percent versus a comparable vehicle powered by a combustion engine alone."

The EPA rates the six-cylinder X6 at 16 mpg city / 20 mpg highway, and the European ActiveHybrid X6 achieves 22.8 miles per gallon on the European test cycle.

BMW claims the ActiveHybrid X6, like many "full hybrids", can run in full electric mode at speeds up to 37 miles per hour, for distances up to 1.6 miles. It most likely can't do both at the same time, however.

The Two-Mode's many parents

Built in BMW's plant in Spartanburg, South Carolina, alongside its sibling the X5, the ActiveHybrid X6 will use a nickel-metal hydride battery pack. The hybrid X6 uses a version of the Two-Mode Hybrid system jointly developed by General Motors, Daimler, Chrysler, and BMW.

So far, that system is sold in the US only on GM vehicles, including the full-size 2009 Chevrolet Silverado Hybrid and GMC Sierra Hybrid pickup trucks, and higher-end full-size sport utilities, including the ultra-luxe 2009 Cadillac Escalade Platinum Hybrid.

Twin-turbos and hybrids and diesels, oh my!

But the Two-Mode system will soon appear in the 2010 Mercedes-Benz ML450 Hybrid sport utility as well, making the 2010 BMW X6 and the 2010 Mercedes-Benz ML two of the very few vehicles to offer gasoline, clean diesel, and hybrid variants in the US market.

The production version of the 2010 BMW ActiveHybrid X6 was unveiled at September's Frankfurt Motor Show along with a mild hybrid version of the 7 Series sedan.

We'll bring you our driving impressions of the hybrid X6 as soon as we have them, and a full review soon after that.