Corey Brasselmon hops the No. 5 Sun Tran bus on East Pima Street on his way to work. The hybrid "green" bus, which has a battery-pack hump toward the back of the roof, runs on a mix of electricity and biodiesel. A.E. Araiza/Arizona Daily Star

There's a beast on the streets of the Old Pueblo. It's blue and green with a humped back, and it lumbers by with a distinctly Earth-friendly ethos.

It's Sun Tran's new hybrid bus, and it wants you to ride green.

The hybrid bus, which has been cruising the streets since the beginning of March, runs on a combination of biodiesel fuel and self-generated electricity. The hump that makes it look like a whale that took a wrong turn somewhere around Cabo San Lucas, Mexico, is actually a 600-volt battery pack that stores up electricity generated when the bus slows down.

Michele Joseph, director of marketing for Sun Tran, says Sun Tran has been using some form of alternative fuels in its fleet since 1987. The hybrid bus, she said, is "just another way to demonstrate our commitment to clean-burning fuels."

In the hybrid bus, heat energy produced in the braking system when the bus slows down is transferred to the battery pack, or Energy Storage System (ESS), and stored for use when the bus accelerates.

Joseph says an onboard computer system runs calculations to determine the most efficient means for making the bus move in different situations. When accelerating from zero to 17 mph, the bus is powered by 100 percent electric power, she said, and from 17 to 40 mph a combination of electric power and biodiesel fuel pushes things along. After 40 mph, everything runs on biodiesel.

Electric power - which doesn't need a plug, because the braking system generates all the juice it needs - and biodiesel fuel, which burns cleaner than traditional diesel, combine to make the bus run 60 percent more efficiently than buses with a standard diesel engine, Joseph said. Hydrocarbon, carbon monoxide and particulate emissions are also greatly reduced.

The hybrid bus came with a price tag of $596,478. A grant from the Federal Transit Administration paid $495,077 of that, and the city put up the other $101,401, said Joseph.

There has been no analysis on operating and maintenance costs for the bus yet, she said, but the improved fuel efficiency is expected to lower the overall cost compared with a traditional bus.

The bus faces some limitations, however.

Underpasses pose a problem for the hybrid because of the size of its battery pack, so you probably won't see it downtown or on any Sun Tran route that has to go under the Stone or Sixth avenue underpasses.

The bus runs on route No. 5, between the intersection of Sabino Canyon and Tanque Verde roads and the Pima Community College West Campus. Keep an eye out for it on Speedway and East Pima Street.

Contact reporter Clayton R. Norman at or 573-4142.