Driven: 2009 GMC Yukon Denali Hybrid

#GM, #Yukon

Green Machinery

Denali buyers obviously care about exterior image, but we're more impressed with what lurks under the gussied-up skin.

For the most part, these changes are invisible. Don't believe us? Pop the hood -- the most you'll see poking from beneath the sea of plastic beauty covers are traces of GM's 332-hp, 6.0-liter V-8, along with a few bright-orange high-voltage cables. Third-row passengers will also catch a glimpse, as the big plastic bulge encountered while entering and exiting their seats is the 300-volt nickel-metal-hydride battery pack, tucked nicely beneath the second-row bench seats.

But the pièce de résistance -- the 2-Mode transmission itself -- is hidden from view. A shame, for as we've noted time and again, the complicated transmission is a technological wonder, combining a CVT, fixed gearing, electric motors, and generators all into one succinct package.

We'll try to describe it in an equally succinct manner: At low speeds -- say, below 27 mph -- the gearbox functions as an electrically variable transmission, much like the unit in the Toyota Prius. The Denali Hybrid can be powered by either the V-8, one of the two electric motors, or a variable combination of the two power sources.

In the second mode, power primarily comes from the brawny V-8, with one electric motor occasionally working to provide boosts of power during hard acceleration or when towing heavy loads (the second motor acts as a generator to recharge the battery packs). Here, power is transferred to the wheels via four fixed gearsets, which shift and act like a normal four-speed automatic transmission.

Those who tow -- and the Denali Hybrid can pull up to 5700 pounds -- will note the absence of a tow/haul switch on the transmission. Instead, drivers can switch into "manual mode," which gives control over the range of "gears" -- 1 through 4 -- used.

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