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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The carmaker has returned as the world's largest by volume - less than three years after it was rescued from bankruptcy by a US government cash injection - on the back of strong performance in both North and South America, as well as in Asia, including the buoyant Chinese market.

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