Diesel passenger car purchases continue to increase, which is great news for the biodiesel industry. Diesel car sales in the U.S. increased by more than six times the rate of overall car sales during the first six months of 2014 -- a 25 percent increase from last year -- according to Edmunds data and the Diesel Technology Forum.

Diesel sales in 2014 show six consecutive months of increases: +6.8 percent in January; +4.5 percent in February; +39.5 percent in March; +60.4 percent in April; +26.8 percent in May and +8.8 percent in June.

"Sustained and mostly double-digit increases in sales each month over a four-year period prove that U.S. consumers are embracing the benefits of clean diesel technology and its proven, high fuel efficiency, great driving performance, and long-term value," said Allen Schaeffer, executive director of the Diesel Technology Forum.

Clean-diesel vehicles are about 30 percent more efficient than those with gasoline engines, says the DTF, and the group expects automakers to introduce even more diesels to help meet federal fuel-efficiency standards that mandate a 54.5 mpg corporate average by 2025.

Current clean-diesel offerings in the U.S. include 27 models of cars and SUVs, nine vans and 10 pickup trucks. Click here for a list of the clean diesel cars and pickups available in the U.S.