Porsche is changing. In 2002 the ultimate sports car company released the ultimate performance SUV, the Cayenne. Later this year Porsche will release its first-ever four-door sedan. And in late 2010 Porsche will offer its first hybrid powertrain.

“If you want to grow, you have to enter new segments,” said Klaus-Gerhard Wolpert, Porsche’s director of Cayenne operations. “We are strongly convinced we can reach a business case with this system.”

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A desire to be socially responsible and a need to offer vehicles that could benefit from European tax incentives led Porsche to investigate developing a hybrid. Company management drove the competition to see if a hybrid system would fit with Porsche’s fun-to-drive character. They decided that hybrid driving does have a unique fun factor.

Coincidentally, Volkswagen saw a need for a hybrid as well. In 2006 the two companies sent about 100 engineers to work together to develop the system. (Note: Porsche has since acquired a controlling interest in Volkswagen.) A working prototype was unveiled in 2007, and now Porsche has upgraded and updated the prototype.

A year or more before a Porsche hybrid will go on sale, Porsche provided two Cayenne S Hybrid test mules equipped with its hybrid system for journalists to drive in and around Stuttgart, Germany. I was part of that group, and this is what I learned.

The Porsche-VW hybrid system is unlike any on the market, but it’s closest to Honda’s Integrated Motor Assist. Porsche’s system is a parallel full-hybrid system. IMA is a parallel system, but it is considered a “power-assist” system, not a full system, because the electric motor can only aid the engine, not propel the vehicle by itself (except in some light cruising conditions). Honda’s is a parallel system because the engine has a mechanical link to the wheels.

The Porsche system has one key feature that Honda doesn’t: a disengagement clutch. Power originates at the engine, the supercharged, direct-injected, 333-horsepower 3.0-liter V-6 from the Audi S4. The power then flows through the disengagement clutch and a 38 kW (52 horsepower) electric motor to an eight-speed automatic transmission. Porsche quotes total output at 374 horsepower and 405 pound-feet of torque.

It is the disengagement clutch that makes Porsche’s system a full hybrid. The clutch can disconnect the engine from the driveline, allowing the electric motor to power the vehicle alone. The electric motor can also aid the engine. The Porsche system works only in parallel because it never sends power on a separate electrical path.

The Porsche disengagement clutch also gives the system the ability to “coast” or “sail” at highway speeds up to 86 mph. The system turns the engine off on the highway when it isn’t needed, such as when traveling downhill. According to Porsche, the start/stop and coasting features allow the Hybrid Manager to turn off the engine 44 percent of the time in the New European Driving Cycle, the equivalent of the Environmental Protection Agency’s fuel economy rating cycle.

The electric motor serves as the alternator and starter, shutting off the engine and restarting it when needed. Like other hybrid systems, regenerative braking recharges the battery.

Two other components are keys to the Porsche hybrid system: a nickel-metal-hydride battery and the Hybrid Manager. Think of the Hybrid Manager as the computer that controls the whole hybrid system. Porsche says getting the Hybrid Manager to control all the hybrid components and provide a smooth driving experience was the toughest part of the project.

The end result? Porsche says the hybrid system increases fuel economy by 25 percent and reduces carbon dioxide emissions by 20 percent versus a vehicle comparable to the Cayenne and with the same amount of power.

Porsche claims its V-6-powered hybrid system provides V-8 thrust (0-60 mph in about 6.8 seconds) with the fuel consumption of a four-cylinder. We agree about the power, but we’ll reserve judgment on the fuel economy until a full test can be completed with a production model.

In my test, the Cayenne S Hybrid was quick from a stop and provided willing passing power. If I hadn’t known otherwise, I could have been convinced it was a V-8. The system works smoothly, too. Even at this early stage, Porsche engineers appear to have made the engine starts and stops transparent to the driver, even on the highway.

With a light foot, the Porsche hybrid system is capable of running on electric power alone up to and past 30 mph. The one negative is weight. Porsche says the hybrid system adds about 400 pounds versus a regular V-6 model and just more than 300 pounds compared to a V-8 model. While we weren’t able to take the Cayenne S Hybrid on many twisty roads, that additional weight is sure to prove a detriment to handling.

The first Porsche hybrid will most likely arrive in the United States next fall. It will debut on the redesigned next-generation Cayenne, which will probably be a 2011 model. Porsche says it will aim to sell 5,000 to 8,000 Cayenne hybrids annually worldwide, with most of those sales occurring in the United States. The system will also be used in the upcoming Panamera four-door sedan some time after it appears in the Cayenne. The hybrid system will not power the 911 or the Cayman/Boxster.

Porsche has yet to determine pricing for the system. Klaus-Gerhard Wolpert confided that the full integration of the hybrid system will cost about $15,000 per vehicle, but the company won’t be able to charge that much. That obviously makes the system a money loser in its first few years, but Wolpert says it will be able to pay for itself in 10 to15 years.

While the payoff may be a long way off for Porsche, customers should reap the system’s benefits immediately. Porsche’s hybrid system works well already, and the design is simple and elegant. While the extra weight isn’t quite in tune with the Porsche brand, it’s good to see that a premium automaker is concerned with fuel economy and social responsibility.