Porsche AG is set to break the mold once again.

Having upset purists by adding a cross/utility vehicle to its lineup in 2003, offering its first diesel-engine option earlier this year and unveiling a 4-door sedan set for release this fall, the German auto maker is ready for its next brand-defying trick: a hybrid powertrain.

Despite the early criticism, the Cayenne CUV has become Porsche's volume leader, with more than 250,000 sold worldwide to date, and its optional diesel engine has won converts from critics who initially didn't want to hear Porsche and diesel uttered in the same sentence.

The auto maker now is on its way to converting skeptics of its upcoming Panamera sedan, and it believes the engineering breakthroughs it is touting with its new hybrid powertrain will be enough to convince the faithful that fuel economy and the Porsche badge don't have to be mutually exclusive.

The parallel hybrid powertrain, which will arrive first in the Cayenne sometime next year but also is designed to fit the Panamera, performs flawlessly in a short test loop in suburban Detroit. But while solid engineering is expected, more surprising is Porsche's claim its system will be the least expensive fulltime hybrid powertrain on the market.

It also does something most full hybrids do not: achieve better fuel economy on the highway than in stop-and-go city driving.

The powertrain was developed by Porsche, Volkswagen AG and Audi AG and ultimately is expected to be employed by all three auto makers. Audi already has announced its Q5 CUV will offer a hybrid option in late 2010.

The setup consists of Audi's 3.0L V-6 gasoline engine and single electric motor, both positioned along the same output shaft, not split along two separate paths as in some hybrids.

Unique to the hybrid system — and one of the secrets to its ability to get better fuel economy on than off the highway — is a dry clutch that can disengage the gasoline engine entirely and smoothly reengage it as needed. That allows the gas engine to shut down completely when the vehicle is coasting at speeds up to 86 mph (138 km/h).

Other hybrids on the market shut down their gasoline engines at a full stop, as does the Cayenne, but their engines continue to idle during coasting.

“It won't jar you,” Michael Leiters, project manager-Cayenne, says of the gasoline engine switching off and on. “You can have a cup of coffee in the holder and not a drop will spill.”

The short drive proves him right, as the Cayenne's tachometer settles on 0 rpm while coasting at speed on the freeway, then jumps to life almost imperceptibly with the slightest input from the accelerator.

“We sweated blood (to engineer the clutch),” Leiters says. Engineers at other auto makers have eyed similar designs but have told Porsche “they were not confident they could do it and have it work as quickly and seamlessly.”

Overseeing the system is an electronic control unit that monitors some 20,000 data parameters. That compares with about 6,000 data sets for a conventional engine control module, the auto maker says.

The supercharged V-6 delivers 333 hp and 325 lb.-ft. (440 Nm) of torque, and the electric motor adds another 52 hp and 207 lb.-ft. (280 Nm). Together they deliver a maximum 374 hp and 406 lb.-ft. (550 Nm) of torque. The electric motor can power the wheels alone at speeds up to 32 mph (51 km/h) and for distances of 1.5-2 miles (2.4-3.2 km).

The two are linked to a conventional 8-speed automatic, not a continuously variable transmission as in most other full hybrids on the market.

CVTs produce a “rubber-band effect” Porsche wanted to avoid, says Leiters, noting the lag typical from the time a driver increases throttle pressure and that power actually is translated through the CVT to the driving wheels.

“The revs go up, then (after a slight delay) the thrust comes,” he says. “Our goal was if the customer wants to drive it efficiently, he should know it is a hybrid. But if he wants to drive it sporty, he should know it is a Porsche.”

Porsche says the hybrid Cayenne sprints 0-60 mph (97 km/h) in 6.5 seconds, nearly spot on with the V-8 version. Top speed is 149 mph (240 km/h), just shy of the V-8's 155 mph (250 km/h). That performance comes despite a 353-lb. (160-kg) weight penalty for the hybrid vs. the V-8.

The 38-kw, 288-volt nickel-metal-hydride battery pack mounted behind the rear axle is small enough to preserve much of the Cayenne's cargo space.

Leiters says the current-generation Cayenne was not designed with the hybrid powertrain in mind, so considerable work went into packaging the system, including squeezing in the batteries and feeding the high-power cables from the rear-mounted batteries to the electric motor up front.

But the system is highly compact, with the whole clutch and electric motor module measuring just 5.7 ins. (144 mm).

“In 2005, we decided to do the project, so that's not much time,” Leiters says. “We put the best people from VW, Porsche and Audi together (to get this done).”

The team also had to develop electric motors for the air-conditioning system and transmission oil pump, fit the vehicle with electric power steering and add regenerative braking and stop/start capability. The Cayenne's all-wheel-drive system also had to be modified — the hybrid version no longer has a split second gear to assist in off-roading.

“We don't expect our customers to use a hybrid in that way,” Leiters says.

The OE team had help from key suppliers, including Robert Bosch GmbH (power electronics and electric motor) and Aisin AW Co. Ltd. (transmission). Schaeffler Group's LuK worked with Bosch on the dry clutch.

Porsche doesn't have official fuel-economy numbers but says the hybrid should be about 28% more efficient than a V-6 Cayenne and average about 24 mpg (9.8 L/100 km) city/highway on the Environmental Protection Agency test cycle.

Leiters declines to reveal how much the system costs, but he says the price tag is lower than for other full hybrid powertrains on the market. There is no CVT and only one electric motor, which is smaller, and therefore less costly, than what other hybrids require.

“It is (less) complicated. This is significantly cheaper,” Leiters says.

Later he singles out the 2-Mode system developed by General Motors Co., Daimler AG and BMW AG.

“It is excellent engineering, but expensive,” he says.

Pricing for the Cayenne Hybrid won't be determined until closer to launch next year when it debuts as an '11 model. The vehicle will be marketed worldwide. Porsche says volumes likely will fall in the range of 5% to 10% of Cayenne sales.

The hybrid is just the latest in Porsche's quest to improve fuel economy that has seen the introduction of direct-injection gasoline engines, dual-clutch transmissions and the diesel engine for the Cayenne.

A runaway hit in Europe, despite the initially cool reception from critics, the diesel now is under the hood of about half of the Cayennes sold there. Fuel economy with the diesel, which delivers 240 hp and a whopping 406 lb.-ft. (550 Nm) of torque, is about 28% better in the European test cycle than a comparable gasoline V-6 model.

But so far, the tough business case has kept that engine out of the U.S., though Porsche continues to look for an opening.

“The market demand is not there,” says Leister, adding that Porsche is encouraged by a survey showing 10% of its buyers would like a diesel engine in their next vehicle.

Also suggesting there could be a market in the U.S. are rising penetration rates for the diesel BMW X5 (now 13% of X5 sales), Mercedes GL (20.5%), Mercedes M-Class (17.2%) and Mercedes R-Class (13.5%), he says.

Some development work would have to be performed to homologate the diesel for the North American market, including fitting it with an aftertreatment system. Indications are the earliest it could be offered in the U.S. is about 2012.

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