The Chevy Volt is unique in that its engine is really just a generator that produces electricity for the batteries, which then power an electric motor that turns the front wheels for propulsion. While the Volt powertrain is pretty spectacular, it may not be unique for long. British car site AutoCar is reporting that Mazda is not only interested in a similar setup for a future plug-in hybrid, but it already has a working prototype on the road. Mazda engineers told AutoCar that a battery pack charged by a rotary engine is already being used in a Mazda5 mule, and that management is encouraged by the early results but wants to see battery costs go down before proceeding.