In 2005, a Yale undergrad made so bold as to fill his truck's gas tank with 100 percent bio-diesel fuel, which he'd rendered himself from dining hall grease. The truck didn't explode. The engine didn't sputter. And thus began Yale's romance with bio-diesel.

Giovanni Zinn '05 doesn't try to take credit for the fact that Yale's 19 shuttle buses are now operating on 20 percent soybean-derived bio-diesel fuel. But he thinks his truck served as a catalyst. His senior project, aided by a $25,000 grant from the Yale Green Fund, was building a processor to recycle the waste vegetable oil from Yale's dining halls.

Manufacturers won't warranty an engine that uses more than 20% percent bio-diesel.

Zinn worked with David Johnson, a technical specialist in chemistry. They separated glycerin from the grease to make yellow diesel, which can fuel cars, trucks, or buses. Students from Yale Recycling then began collecting fry grease from dining halls. The oil was strained before processing (to filter out the occasional french fry), and soon, Zinn's 100 percent dining-hall bio-diesel was powering the furnaces at a Yale observatory.

Yale's shuttle buses are run by the New Haven Bus Service, on contract. Company president Dan Miley says he learned of the yellow diesel experiment through Yale's Office of Traffic and Parking, and after consulting Johnson, he decided it was safe to start using some yellow diesel in his fleet. (During commencement 2006, one bus shuttled students and their parents around campus on 100 percent bio-diesel. It ran flawlessly.)

That home-grown bio-fuel is no more: Zinn says his processor was shut down in the fall of 2006 because the administration needed the space for another use. But the use of 20 percent bio-fuel in the shuttles' fuel mix continues.

Will Yale ever go beyond 20 percent? Not yet, says Holly Parker, Yale's new director of sustainable transportation systems, because engine manufacturers won't warranty an engine that uses a higher percentage. But "once the engine manufacturers and regulatory authorities see that 20 percent bio-diesel is working very well," she says, "I think there'll be more acceptance of using a higher proportion in the mix." And Yale may yet see the day of the all-bio fleet."