---

The electric-powered, two-seat Audi R8 E-tron could qualify as the world's first zero-emissions supercar.

Scheduled for sale in Europe before the end of 2011, the 2012 R8 E-tron should be available in the United States sometime in early 2012, according to Audi officials.

Whenever it arrives, the R8 E-tron will join the Tesla Roadster in the marketplace for electric-powered sports cars. Yet some car enthusiasts debate the proposition that the Lotus-based Tesla is powerful enough or large enough to merit official supercar status. The Tesla's single electric motor develops the equivalent of about 295 horsepower.

The Audi R8 E-tron will be powered by four separate electric motors to retain Audi's familiar quattro all-wheel-drive system. Combined output will easily surpass 300 hp and 500 pound-feet of torque. Current indications suggest the E-tron will be first to arrive among several green-oriented electric or hybrid-powered supercars from traditional manufacturers, including the Mercedes-Benz SLS E-Cell and Porsche 918 Spyder hybrid.

The R8 E-tron was approved for production in June 2010. Some official statements suggest its model designation will be changed to R4 (for four electric motors) to separate it from the existing, gasoline-powered R8 supercar, which has been widely lauded as one of the world's great sports cars. Other announcements indicate that the R4 designation will be reserved for a subsequent, smaller and less powerful electric sports car from Audi.

Prototypes for the E-tron are based on the conventional R8 chassis. As such, the electric car is nearly identical in dimensions to the gasoline R8, measuring 14.5 feet long, 6.2 feet wide and 4.1 feet tall. The E-tron retains the conventional R8's mid-engine proportions. However, Audi's E-tron show cars have been finished in single-tone paint, with more chrome work than the standard R8, and a single-frame trapezoid grille with thin horizontal slats. As a result, the E-tron looks less overtly aggressive and perhaps a bit classier than the conventional R8.

E-tron's lightweight electric motors are located at each of its four wheels, delivering power to each wheel independently. Each generates 78 horsepower, for a total of 313 hp, with combined torque of 502 foot-pounds in conventional terms, according to Audi. Moreover, the separate motors will allow what's known as torque vectoring, or targeted acceleration of individual wheels and active distribution of traction. That, in turn, allows the motors to actually steer the car, and control understeer and oversteer, by changing power output at any of the wheels for a few milliseconds.

The E-tron will hit 60 miles per hour from a stop in about 4.5 seconds, according to Audi. Theoretically, the car should reach a top speed beyond 150 mph, but Audi says the production car may be limited to 124 mph to preserve the battery charge. In June 2010, an R8 E-tron prototype won a rally for modern electric cars in Austria, beating both a Tesla Roadster and Mercedes SLS E-Cell prototype.

E-tron's lithium-ion battery pack and control electronics will weigh about 880 pounds, according to Audi. The package will be located behind the seats and in front of the rear wheels, where the conventional R8's engine is mounted, to maintain the car's mid-engine balance. The battery will store 53-kilowatt/hours of energy, and can be fully charged on household current in six to 12 hours. Target range for the production E-tron is 150 miles.

Because it has no power-generating engine, the E-tron will technically be a zero-emissions vehicle, just like a child's radio-control car. Of course, that analysis does not account for emissions generated producing the electricity to charge the E-tron. Depending on the power station and fuel source, supplying electricity for the E-tron could generate more emissions than a gasoline engine the produces equal power.

Inside, the E-tron will share the conventional R8's sweeping cockpit-style interior layout, though its instrumentation will be specific and appropriate for an electric vehicle.

Audi will build at least 1000 E-trons on special order. While it has said the car will be available in Europe by the end of 2011, Audi has been less specific about a launch date in the United States.

The E-tron will definitely cost more than the least expensive conventional R8 model, which retails for about $115,400 for 2010. How much more depends on whether Audi considers the E-tron primarily a profit-earning venture or a promotional vehicle and real-world laboratory.