Saab, G.M.’s Swedish luxury division, unveiled the 9-4X BioPower, a crossover S.U.V. concept model that’s ethanol compatible. Meanwhile, G.M. Chairman Rick Wagoner used the Detroit show Sunday to announce a breakthrough in producing cellulosic ethanol from waste products, which promises economic and environmental advantages over ethanol made from corn and other food crops. Wagoner announced a partnership and equity stake in Coskata, an Illinois company that claims it can get production costs for cellulosic ethanol to under $1 a gallon—cheap enough to make it competitive with gasoline.

As some automakers prepare to blitz America with the fuel-efficient, clean-diesel cars that dominate the European market, BMW showed the 335d sport sedan and the X5 xDrive 35d sport-utility—diesel-driven versions of familiar, popular BMWs sold in the U.S. Both will reach U.S. dealers this fall, and will utilize urea-injection technology—which uses an ammonialike fluid that neutralizes tailpipe pollution—to meet stringent new federal emissions requirements that are coming in 2009.

The most show-stopping diesel is a concept version of the Audi R8 sports car, the Porsche 911 competitor that has won a slew of international automotive awards. The diesel version, though not yet green-lighted for production, features a 500-horsepower, turbocharged 12-cylinder engine that could easily top 20 m.p.g., versus about 15 m.p.g. for the gasoline version. It draws on the diesel technology Audi used to dominate recent runnings of the 24 Hours of LeMans with its groundbreaking R8 racers.

The drive for economy is being spurred by more than the energy bill provision. California and more than a dozen other states are demanding the power to regulate greenhouse gas emissions linked to global warming, and have sued the Environmental Protection Agency for the right to set their own carbon dioxide standards. Since carbon dioxide emissions are almost entirely a function of fuel economy—meaning a car that gets 40 m.p.g. produces about half the greenhouse emissions of one that gets 20 m.p.g.—any new mandates would effectively force automakers to achieve even more dramatic mileage gains.

Luxury automakers face the same pressures in Europe, where environmental critics are calling for sharp cuts in greenhouse gas emissions. The European Parliament has called for reductions that would effectively require gasoline cars to get 43 m.p.g., and diesels 48 m.p.g., by 2012.

For more than 20 years, some luxury automakers here have chosen to pay fines rather than bring their cars into compliance with U.S. mileage standards. For 2007, DaimlerChrysler’s Mercedes division was fined a record $30 million for its cars that fell short on fuel economy.

Since automakers’ entire fleets must meet fuel economy averages, many avoid fines on luxury and sports cars by offsetting their low mileage with sales of thrifty compacts. Yet the wave of green-minded luxury cars shows automakers preparing for a new reality: one in which even the highest and mightiest machines are no longer immune to environmental demands.