The luxury-car market remains a shambles and the direction of fuel prices as uncertain as ever. Simply because the technology and the US regulatory landscape weren't ready until now for Audi, BMW and Mercedes-Benz to launch their diesel blitzes. And in the meantime, the clearest competitors to the new upscale diesel models haven't lit a fire under American consumers, leaving a broad opening for diesel to reach environmentally conscious buyers who can afford fine rides. The new diesel-powered models are versions of the Audi Q7, the BMW 3 Series small sedan and X5 utility vehicle, and the Mercedes-Benz R-Class crossover, GL-Class SUV and ML-Class SUV. The new diesels' fuel-economy and emissions gains are bigger than what luxury hybrids typically produce compared with conventional vehicles. That's in part because, with pricier hybrids, automakers often simply apply the extra available juice to boosting a vehicle's power rather than just improving fuel economy.