Two weeks before the Detroit motor show opens it doors, Audi’s star of the North American International Auto Show has leaked onto the internet – and according to early reports, it’s a 500bhp diesel-powered sports car.The German car brand will show a design concept called the R8 V12 TDi at Detroit and, just as its name suggests, it’ll be a twelve-cylinder turbodiesel version of the critically acclaimed mid-engined R8. Motor industry watchers have been anticipating a hotter version of the 414bhp R8 for some time now. Most of the speculation has been concerning a V10 petrol-engined option; Quattro GmbH boss Werner Frowein even confirmed that a ten-cylinder R8 will go into production. This is the first indication that a third R8 derivative is on the cards, though, which could also be fastest and most expensive.Although there has been no official confirmation from Audi yet, the R8 V12 TDi should use the same 6.0-litre, twin-turbocharged V12 as the Q7. That motor produces a heady 500bhp and 738lb ft of torque between 1750- and 3000rpm. Bluetec technology means it’ll also be clean enough to meet Euro V emissions standards.And performance should be beyond question. In the two-tonne-plus Q7, the 500bhp V12 TDi engine is strong enough for 0-62mph in 5.5sec; in a 1600kg R8, think 0-62mph in around 4.0sec dead, twinned with incredible in-gear acceleration and overtaking potential.If the response to the hot diesel R8 is positive and Audi puts it into production, the R8 TDi will be a line in the sand in the development of the diesel motor car – the first true diesel-powered supercar, Audi could argue. Expect more on the car to emerge at its unveiling next weekend.