Fifteen years ago, New York City began rolling out its first hybrid buses in a much ballyhooed program to green up the city’s public transportation program.

But now, three years after the city bought its last diesel-electric buses, it is apparently pulling back on the hybrid program, taking about a fourth of the city’s 1,677 diesel-electric hybrid buses and swapping out their hybrid engines for newer diesel ones.

The news comes as cities across the world have adopted greener technology in public transport vehicles to reduce emissions -- including nitrogen oxide gases that cause ozone air pollution and the micro-particles in vehicle exhaust that cause respiratory problems, especially among children and the elderly. New York City has about 5,700 buses.

The number of hybrids will be reduced gradually from 1,677 to 1,288. The city maintains 14 different bus models, including the Orion VII, which is manufactured by Orion International at facilities in Mississauga, Ontario, and Oriskany, N.Y.

The main reason for the swap to 100 percent diesel is fiscal; as warranties expire on the hybrid engines, the city will have to take on the added cost of maintaining hybrid systems.

“The electric-traction motors are burning out,” a source at the city’s maintenance division told the New York Post in a report published Sunday. “They’re so expensive to replace that it’ll be cheaper to stick a diesel engine in there.”

The Post said Indiana-based engine maker Cummins Inc. has a contract to evaluate the best way to convert the buses to 100 percent diesel, but a company spokesman on Monday was unable to confirm any details about its involvement in the program.

An MTA representative told IBTimes on Monday that hybrid city buses work best with “intense stop-and-go routes where the average speed is 8 miles per hour.” In situations where buses travel longer distances at higher speeds, the hybrid system is less useful because the lithium ion battery harvests power from when the vehicle brakes and when the bus is coasting. The MTA says it will maintain the hybrid engines for buses in Manhattan, which travel much slower and brake more often than the buses in the outer boroughs.

Newer diesel technology -- such as the clean diesel TDI system that Volkswagen AG (FRA:VOW3) uses in many of its sedans -- has become more fuel efficient than many comparable hybrid cars. Like other internal combustion engines, the new diesel technology releases the same pollutants, but not nearly as much as traditional diesels used to put out.