ST. CLOUD — A switch to compressed natural gas buses appears to be paying off for St. Cloud Metro Bus.

Two years ago, the transit agency started converting its fleet to CNG buses in a bid to save on fuel costs with the cleaner-running buses. Metro Bus also built its own CNG fueling station, which is open to the public.

Sales of the alternative fuel to the public have been slower than hoped, but officials consider the switch a success, according to a press release. The new buses are performing well when it comes to maintenance, and the savings of $1 or more per gallon compared with diesel are better than expected, Metro Bus officials said.

The agency is replacing retired buses with models that run on compressed natural gas. Metro Bus now has 23 fixed-route CNG buses and will soon have 24. In addition, six smaller Dial-a-Ride buses that operate on compressed natural gas will be in use within the next few weeks.

“It is right where we planned on being,” Metro Bus chief finance officer Paula Mastey said.

St. Cloud received a $3.3 million federal grant and a $9 million grant from the Minnesota Department of Transportation to build the facilities necessary for the conversion. A local revenue bond was needed to cover construction cost overruns.

The new buses look similar to their diesel counterparts. The compressed natural gas is loaded into steel tanks mounted on top. The engine functions much the same as a gasoline engine. The fuel-air mixture is compressed and ignited by a spark plug, and the expanding gases produce forces that propel the vehicle.

When Metro Bus made the move to CNG starting in 2014, city officials hoped the buses would save at least $300,000 a year in the first 10 years in fueling costs compared with diesel.

Even though diesel prices have been at historic lows, data provided by Metro Bus show that the agency is close to achieving that goal, with average savings of 24 cents a mile. In the first 22 months since the switch, Metro Bus saved about $545,000 in fuel costs.