Photos: Diesel in Detroit

We previously saw this diesel hybrid at the 2007 Frankfurt auto show. It combines an economical 204 horsepower 2.2-liter four-cylinder diesel engine with a small, 20 horsepower electric motor for extra boost. The S300 goes into production in 2010.

The GLK small SUV concept debuted in Detroit with a diesel powertrain, showing Mercedes-Benz's future direction. The GLK is powered by a 2.2-liter four cylinder diesel engine, like that found in the S300 hybrid, producing 170 horsepower and mated to a seven-speed automatic transmission. Although debuted as a concept, the GLK looks ready for production.

The Bluetec diesel engine in the GLK uses a complex system to clean up its emissions. Mercedes-Benz put a clear panel over the AdBlue tank, which holds a solution to break down nitrogen oxides in the exhaust. With this system, the GLK meets U.S. BIN 5 and California LEV II requirements.

Audi didn't have to stretch to create this concept. It took its existing R8 car and replaced the gasoline engine with the diesel V12 TDI engine used in its Le Mans winning R10 race car. TDI stands for turbo direct injection, and the beauty of this engine for endurance racing is that it can keep up with the gas engine cars, but doesn't need to refuel as much.

Audi showed off its diesel system, without a car, in Detroit, paving the way for future imports of diesel vehicles. This demonstration shows all the components of the diesel exhaust cleaning system, including the AdBlue component.

With a twin turbo 3-liter straight six engine, the BMW 335d gets 265 horsepower and 425 ft-lbs of torque, while at the same time getting better than 23 mpg in the city. The 335d will certainly drive a bit different than its gas-engined counterpart, but still gets to 62 mph in only 6.2 seconds. The car uses AdBlue technology, called BluePerformance by BMW, to clean up its emissions. The 335d and a diesel X5 come to the U.S. in 2008.

Japanese automakers have developed clean diesel engines that don't need AdBlue or similar technology. This concept by Mitsubishi demonstrates a sports car powered by a 2.2-liter four cylinder diesel engine, putting out 201 horsepower and 310 ft-lbs of torque, while meeting U.S. emissions requirements. To up the excitement, the concept also uses Mitsubishi's new twin clutch transmission.

Acura forgot to bring a car, but did have this new clean diesel engine to show off in Detroit. The i-DTEC engine doesn't need any sort of AdBlue technology to clean its emissions, and will appear in an as-yet-unnamed Acura model coming to the U.S. in 2009.