DESPITE NEW regulations that could've potentially made Porsche's Cayenne extinct, the large SUV is here to stay. One reason for its survival is it's big enough to accommodate a new hybrid system that Porsche has developed with another VW-owned company, Audi AG.

Audi offered up a drive of its Q7 hybrid a couple years ago at its test track in Neckarsulm. The SUV concept featured a 3.0-liter V-6 coupled to an electric motor and battery and controlled by a clutch that could decouple the engine or motor. The result was that, once up to highway speeds, the driver could "freewheel" at speeds up to nearly 60 mph. Let off the throttle at that speed and, providing several conditions were met, the tach needle would promptly drop to zero, running the vehicle on pure electric power.

Since then, the system has been upgraded, apparently. Cayenne project manager Michael-Hugo Leiters says the Porsche hybrid will freewheel, the engine running at zero rpm, at speeds up to 86 mph. With its nickel-metal hydride Sanyo battery running on a full charge, it can cruise at up to 32 mph on full electric. (Porsche will switch to lithium-ion only after other manufacturers have proven that technology's reliability on the road.) The brain controlling this system is called the Hybrid Manager. It monitors about 20,000 parameters (versus 6000 for a conventional gas engine). Its 3.0-liter Audi direct-gas-injection V-6, combined with a 288-volt battery, makes 374 horsepower (52 of which come from the electric motor at its power peak) and 406 pound-feet. With its Aisin eight-speed automatic, the Cayenne hybrid concept needs 6.5 seconds to accelerate to 60 mph, 0.1 second slower than the Cayenne S V-8. It tops out at 150 mph on the autobahn, 5 mph short of the S.

Porsche expects an EPA fuel mileage rating of 23-mpg city/25-mpg highway. Thanks to its freewheeling feature, it could become the first hybrid with a highway number that's higher than the stop-and-go-dependent city number. And that's in the current model (pictured). The redesigned 2011 Cayenne, expected in about a year, will lose some of its off-road beefiness, just like the 2011 VW Touareg and the next Audi Q7. That new Cayenne will be the first, and so far only, crossover SUV to get this system next year.

Audi has canceled plans for a Q7 hybrid and will build a hybrid version of the smaller Q5 instead, with a four-cylinder engine as part of the powertrain package.

A brief drive of the Cayenne hybrid concept, using the current model's body, revealed the hybrid system to be more innocuous than those in the Honda Insight, Toyota Prius, or even the Lexus RX 450h. You pretty much have to watch the tach to pick up on the transitions between internal combustion and pure electric, whether at a stop or in traffic (our drive didn't get us much above 55 mph, however). Acceleration feels Cayenne S-quick, though still far short of the "omigod, a big SUV can do this?" feel of the Cayenne Turbo.