After lagging in development of hybrid and plug-in cars for years, Mercedes-Benz is now attacking the new technology with a vengeance. The latest proof: reports that it will offer a pricey plug-in hybrid version of its next S-Class large luxury sedan in 2012.

The British magazine Autocar writes that the 2009 Mercedes-Benz Vision S500 Plug-In Hybrid Concept serves as the prototype of a production model that will be offered when the S-Class is completely redesigned for 2012.

The Vision S500 Concept, shown at last fall's Frankfurt Motor Show, is a plug-in hybrid-electric vehicle. The electric energy is stored in a 10-kilowatt-hour lithium-ion battery pack, which gives it a range of up to 18.5 miles on pure electric power.

After that, an unspecified V6 engine combines with a 60-horsepower (44-kilowatt) electric motor to operate as a conventional hybrid, with acceleration of 0 to 62 mph in 5.5 seconds.

Claim: 73.5 mpg

The concept "offers superior driving performance while at the same time consuming less fuel than current compacts," in the words of Mercedes-Benz CEO Dieter Zetsche.

The company claims fuel economy of 73.5 miles per gallon for the concept, though as for all plug-in hybrids, the the fuel efficiency real drivers would get depends entirely on how the car is used.

More important for regulators in Europe, the concept offers CO2 emissions of just 74 grams/kilometer. That's lower than even the highest-mileage subcompact diesel models offered to European buyers under efficiency sub-brands.

But it'll cost ya

The new plug-in hybrid S-Class is likely to be the flagship of Mercedes-Benz's full-size line of luxury sedans. It's the position formerly occupied by long-wheelbase versions with ultra-smooth V-12 engines of 6.0 liters or more. But what's smoother than electric drive?

The plug-in hybrid technology will surely spread, over time, down the range of Mercedes-Benz models.  This is the path that BMW is taking with its mild-hybrid technology, first launched on the 2010 BMW ActiveHybrid 7 and soon to appear, according to reports, in a 2011 BMW ActiveHybrid 5 model as well.

Mercedes-Benz ruled out a purely electric version of the S-Class, though. The car is simply too large and heavy

Two-Mode Hybrid. Not.

Given the Vision S500's technical details, it is clearly NOT Left unclear for the moment is whether the plug-in hybrid S-Class will use an evolution of the Two-Mode Hybrid system jointly developed by General Motors, Chrysler, Mercedes-Benz and BMW.

That collaboration ended this summer, and the Germans are pursuing their own development path. GM developed a Two-Mode Plug-In system, which was to have launched in the now-discontinued Saturn Vue crossover, but it hasn't announced what vehicle it will be used for.

Full size car, compact thirst

The S-Class Plug-In Hybrid's ability to run 15 miles or more on electricity may prove a crucial for buyers in European cities. Some local authorities have proposed levying tolls on high-consumption or non-electric vehicles when they enter congested city centers, where emissions are most harmful.

More conventional versions of the next S-Class will continue to use the V-6 and V-8 gasoline and diesel engines that the S-Class is known for. All are likely to be direct-injected and fitted with stop-start systems as standard.  The company claims they will use 25 percent less fuel while delivering 10 percent more power and torque that's a full 25 percent higher.

[Autocar via Motor Authority]