Spy Photos:Mercedes Benz SLK caught at the Nurburgring

The SLK roadster has served Mercedes Benz well over its life since introduction in 2004, and like all cars its once-cutting-edge design has become somewhat tired, despite a mid-cycle facelift. An all-new SLK is in the works, however, and it's been spotted testing in the still-wintry German weather.

Aside from the car's general proportions not much is clear thanks to generous slabs of camouflage cladding. Nevertheless, the car's LED daytime running lights, common to all of Mercedes' most recent models, are obvious. The door-mounted side mirrors stand out further on thinner stalks than the outgoing model, while the general proportions of the car with the top up seem to reflect a longer hood and more rearward seating position.

The sharper angle between the rear of the hard top and the rear deck area also mimics the 2010 SLC to a degree, though the real design cues shared with that car are expected to be located around the front fascia. At any rate, it's clear that Mercedes has dumped the controversial front end of the current models for something new.

Powertrains expected for the next-generation SLK include the 2.2L four-cylinder diesel found in the current C250 CDI. Rated at 204hp (152kW) and 369lb-ft (500Nm) in its C-Class application, the engine would be sporty, but whether Americans would find it palatable is an open question. Mercedes has discussed bringing diesels to the U.S., but in the GLK and C-Class, not a sporty model like the SLK.

Petrol engines featuring evolutionary developments on the current range are expected, though there is also talk of a possible twin-turbo 3.5L V6 generating over 470hp (350kW) for an AMG version. More details and specifics will emerge as the car moves forward in its development cycle.

The next-generation SLK is expected to make its debut in late 2010. Pricing for the current model starts just under $46,000 in the U.S., and the next model is expected to stay near that mark.