Volkswagen and Audi have been most committed to diesel in the U.S. market, with Audi planning this year to stretch its TurboDirectInjection 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.
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.