The last of SEPTA’s old-school diesel-belching buses will be retired within two years in favor of new hybrids, a milestone on the path to the agency’s goal of a zero-emission bus fleet.

It will pay $178 million to buy 222 hybrid diesel-electric buses from New Flyer of America Inc., a purchase the transit authority’s board approved Thursday.

SEPTA bought its first hybrid buses in 2006, and only 133 diesels are left in a fleet of 1,447. Hybrids use 30% less fossil fuel and also reduce greenhouse gas emissions.

Executives want a zero-emission bus fleet as soon as possible to meet sustainability goals, and battery-powered electric buses have been the preferred option. But SEPTA’s first 25 such coaches, bought in 2019, had to be yanked off the road in February 2020 after cracks were discovered in their frames.

They’re still not back in service as Proterra, the manufacturer, works on fixes.

The buses ran on Routes 29 and 79 between South Philly and Center City — relatively short routes on level ground. Even so, fluctuating temperatures shortened the range of the batteries.

To go all-in on battery-powered buses, “we would need to have significant charging infrastructure in place,” said Jody Holton, SEPTA’s assistant general manager for planning. She estimated that the cost could add as much as $300 million.

The agency would need to invest in new transformers, substations, and links to the electric grid at its eight bus depots for electric buses to be a viable replacement for its current fleet, Holton said. And some routes might need charging stations along the way, in addition to those at the depots.

The new federal infrastructure law contains billions to help transit agencies switch to zero-emission buses, and SEPTA officials hope that would help. Using hybrids will help the agency figure out what would work best.

“If battery density and performance improve over time and range improves over time, that obviously makes it easier,” said Sheth Jones, a project manager in the authority’s planning and strategic initiatives office. “What SEPTA does will partly depend on how the technology advances over the next decade or more.”

SEPTA is also studying hydrogen power as an alternative, said Tyler Ladd, an engineer who is the agency’s project manager for power.

“The energy storage medium differs between the batteries and hydrogen cells, but they’re both zero-emission, electric buses,” Ladd said, adding the technology for hydrogen fuel is also advancing. “It also would alleviate some of the need for some of the infrastructure improvements.”

Some transit advocates say SEPTA already has the means to get to zero emissions much faster: its roughly three dozen trolley buses, which use overhead electrical wires but don’t run on tracks.

“With a minimal amount of investment toward stringing up wires the agency could swiftly operate buses with zero emissions and immediately lower its climate footprint,” said Dan Trubman, who works on transit issues as a member of the 5th Square urbanist group.

He said many European transit systems have expanded their trolleybus networks.

The hybrid deal includes a $95 million option for SEPTA to buy an additional 120 hybrid buses, if it needs to do so. Executives said they hope the next bus procurement will be for zero-emission models.

“What is going to get us there in a way that is cost-effective for the agency and is also going to afford climate-action results?” said Ayanna K. Matlock, business transformation manager for SEPTA.