Cleaning up a transit fleet
The New York Metropolitan Transit Authority (MTA) has been trying out a variety of alternative fuels since the 1990s. Most recently, it has introduced hybrid electric buses to its fleet and has recorded better fuel economy and fewer harmful emissions as a result. Currently running 325 hybrid buses, the transit agency plans to have the largest fleet of the buses in the country — 825 — operating by 2007.
In 1996, New York Gov. George Pataki directed MTA to begin a pilot program to create the cleanest large transit fleet in the nation. Transit planners first considered switching all buses to run on compressed natural gas (CNG) and converted two bus depots to accommodate the fuel. But many of New York’s bus depots are housed in multi-story buildings, so converting all of them to safely use CNG would have been prohibitively expensive. MTA officials turned to hybrid buses, which do not require fuel compressor stations, storage or special fuel handling.
In 1998, New York purchased 10 hybrid electric, low-floor buses from Mississauga, Ontario-based Orion Bus Industries and launched a pilot to test the vehicles. The original 10 buses accumulated more than 700,000 miles in seven years and were rigorously tested for emissions, battery life, noise levels and reliability. The results exceeded expectations, reducing emissions to levels lower than natural gas buses while improving fuel economy by more than 20 percent. Encouraged by those findings and vehicle improvements that extended battery life by 50 percent, MTA ordered 125 Orion VII hybrid models in 2004 and an additional 200 buses in 2005. In late September 2005, MTA ordered 500 more hybrids.
The MTA buses include a hybrid electric propulsion system by Johnson City, N.Y.-based BAE Systems that uses a relatively small diesel engine to drive an electric generator. The engine is not connected mechanically to the wheels, so the peak requirements for power or torque are provided by combining the generator’s output with power from a bank of batteries. When the brakes are applied, the energy from the moving bus is converted to electricity and is returned to the batteries instead of being discarded as heat in the brakes. The electric current from the generator, the batteries or from both is used to power an electric motor that drives the wheels of the bus. During braking the motor becomes a generator that recharges the bus’s batteries. The system does not require a mechanical transmission, which eliminates gear shifting and allows the bus to stop and start smoothly. The hybrid buses experience far less brake wear compared to conventional buses, with roughly double the mileage between brake replacements.
With its current hybrid buses, MTA is experiencing fuel economy improvements of more than 30 percent. Compared to buses that use standard diesel propulsion, MTA’s hybrids emit 90 percent less particulate matter, 40 percent less nitrogen oxides and 30 percent fewer greenhouse gases.
