Tucson, Ariz.-based Sun Tran recently placed 47 new alternatively-fueled buses — 10 using hybrid-electric technology and 37 utilizing B5 biodiesel — over a six-month period. The new buses replace almost 19% of the fleet and will be used throughout the system on regular fixed routes.

“In the late 1980s, there were growing concerns about Tucson’s air quality, which led to Sun Tran and the City of Tucson to begin experimenting with alternative fuels,” Sun Tran GM Kate Riley explains. “In 1987, Sun Tran converted a 35-foot GMC bus to use both compressed natural gas (CNG) and diesel fuel. This was one of the first such buses in the country, and the start of the transition to utilize alternative fuels.”

By 1997, nearly half of Sun Tran’s fleet used CNG technology, and by the spring of 2006, 100% of Sun Tran’s fleet ran on cleaner-burning fuels — specifically, CNG and biodiesel at that time.

Currently three-quarters of Sun Tran’s fleet exclusively utilizes biodiesel fuel, which is a domestically produced, renewable fuel that can be manufactured from vegetable oils, animal fats or recycled restaurant greases. Biodiesel-fueled buses also emit significantly fewer particulates than traditional diesel-fueled vehicles.

The 57 buses purchased since 2010 are equipped with selective catalytic reduction technology that utilizes a urea solution injected into the exhaust gases to meet tighter U.S. Environmental Protection Agency emissions standards for nitrogen oxides.

The buses, built by Gillig, have replaced 15-year-old vehicles in Sun Tran’s fleet, decreasing the average age from 6.8 to 4.6 years.

In addition to the 197 B5 biodiesel buses, SunTran’s fleet includes 45 CNG and 11 hybrid-electric/biodiesel vehicles. Riley explains that SunTran plans on keeping flexibility in their future alternative-fueled vehicle choices.

“There is no plan to completely run any one fuel type. Sun Tran prides itself on the use of alternative fuels, but wants to ensure we continue to be flexible as additional vehicles are purchased,” she says. “As fuel technologies improve, we will continue to procure the best fuel options available at that time.”