Audi is working on a diesel engine for the latest Audi R8. The engine is the same as that just launched in the Q7 and introduced in the A8, a 6.0-liter V-12 with 500 horsepower and 738 pound-feet of torque.

Although the R8 TDi hasn't been approved for the production, if it will be unique among sports cars.

Audi insiders admit the diesel R8 has a couple hurdles to overcome. Though compact, the V-12 diesel is bigger and heavier than the V-10 and V-8 gas engines. Another issue is whether the diesel particulate filter trap would fit between the R8's mid-engine and its rear bumper. But if the R8 TDI is okayed, look for it in Audi showrooms no earlier than model-year 2009.