LANGLEY — It may be yellow on the outside, but inside it’s going to be a lean, clean, green machine.

The South Whidbey School District’s new bus will employ hybrid technology, running on a diesel engine coupled with two sets of 12-volt lithium-ion batteries that will save big bucks for the district in the years ahead.

Actually, it already has saved money.

The new bus the school board recently approved for purchase will cost $216,000. The state will reimburse $187,000 of that and, thanks to transportation director Veronica Schmidt’s successful $40,000 grant request to the Department of Ecology, the bus is pretty much paid for.

The money helps pay to replace the oldest and most heavily polluting diesel school buses in the state. Using cleaner buses protects children from harmful toxics in diesel exhaust.

South Whidbey will be one of the first school districts to get the new hybrid buses.

“This could be a real statement to the community and our students that we are not just blowing hot air when we talk about our commitment to the environment,” said District Superintendent Fred McCarthy.

At an on-site inspection by the school board at the district’s bus barn, bus manufacturer representative Steve Horn described how the new system will work to board members and school bus drivers.

“The bus runs using a parallel process where either the diesel engine or the batteries can turn the wheels,” he explained. “The diesel is on all the time so if there is a failure, the bus keeps going until it’s brought in for maintenance or repair. In the afternoon or at night, the batteries are plugged in for re-charging for the next day.”

He said the bus can run on batteries alone for roughly 45 miles and the system is most effective on shorter routes with lots of starts and stops.

“The effort it takes to get this massive vehicle up and running is where the fuel costs go up,” Horn noted. “Getting up to speed using batteries will save money.”

Schmidt said she paid an average of $2.56 for a gallon of diesel two years ago. During the fuel spike in pricing last summer, she got her first delivery for the new school year at $4.67 per gallon.

“There’s no reason to believe that fuel costs aren’t going to rise again in the future,” she said.

Horn said the benefits are clear.

The hybrid bus will get anywhere from 30 percent to 65 percent better fuel economy than the current fleet does, the new diesel engine is cleaner resulting in lower emissions, it runs quieter and it will help raise the environmental consciousness of students.

One concern raised is the cost and lifetime effectiveness of the batteries.

“The price of a new set of batteries today is about $20,000 but costs are always coming down as technology improves,” Horn said.

Though the manufacturer claims the batteries will work for at least eight years, no one knows for sure. Board member Fred O’Neal pointed out that Toyota doesn’t know how long its newest Prius hybrid batteries will last, though they’re rated for 160,000 miles.

“Looking at the numbers — battery and electrical costs versus the price of fuel — this hybrid bus is a smart move for the district,” O’Neal said. “Very smart.”