The Mini Cooper isn't the first car you'd associate with the environmental movement, but with its frugal four-cylinder engines and the new Mini E battery-operated version, the Cooper's green credentials are considerable.

Now Mini's buffing the inexpensive, high-miler Cooper it sells in Europe as the Mini One. While other Minis get diesel engines, too, the new Mini One D has a special 90hp (67kW) version of a BMW Group oil-burner that turns in the equivalent of 6mpg fuel economy. The One D also includes regenerative braking that supplies juice for accessory power, and stop-start functionality to save gas in traffic and city driving.

Mini won't actually be selling the diesel One in America, but it's not too early to imagine future Coopers coming with the fuel-efficient drivetrain. For one, BMW's been a huge diesel proponent, from its X5 sport-ute to its newly minted 335d four-door sedan. And in this application, the acceleration's not so bad. The One D's 0-60 mph time of 11.5 seconds isn't that far off a 2010 Honda Insight in acceleration, and fuel economy's comparable, if not better. Top speed in the Mini One D hits 113 mph, too.

More on Mini's new green cars is coming from the 2009 Frankfurt auto show. Stay tuned.