Volkswagen and Audi have been most committed to diesel in the U.S. market, with Audi planning this year to stretch its Turbo Direct Injection clean-diesel option across most of its highest-volume models, and VW pushing to get diesel penetration of its Passat mid-sized sedan to around 25 percent.

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Automakers have been achieving incremental improvements in the efficiency of their conventional engines with new technologies like direct-injection, six-speed transmissions, turbo-chargers and start-stop systems.

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