|The series-hybrid buses offer up to 45% better fuel economy than diesel, and 100% better than CNG.|
Orion VII series-hybrid buses operated by New York City Transit (NYCT) on the city’s most severe duty cycles achieved up to 45% better fuel economy than diesel buses and 100% improvement compared to comparable natural gas buses on an energy-equivalent basis, according to the results of a study released by the National Renewable Energy Laboratory (NREL).
The evaluation is part of a series of evaluations of new propulsion systems in transit technologies performed by the lab. from the U.S. Department of Energy (DOE). NREL recently concluded an evaluation of the GM-Allison parallel hybrid buses in use in Seattle. (Earlier post.)
The Orion VII series-hybrid buses with the BAE HybriDrive combine a 5.9-liter, 260 hp (194 kW) Cummins ULSD (Ultra Low-Sulfur Diesel) engine with a 120 kW traction generator. The electric traction motor delivers 250 hp (186 kW) and 2,700 lb-ft (3,657 Nm) of low-end torque.
The hybrid fleet proved the most reliable in the study, with 7,000 miles between road calls, compared to 5,000 miles for natural gas and 4,000 miles for diesel. The hybrid propulsion system also performed better than the other propulsion systems, with 10,000 miles between calls, compared to 8,000 miles for CNG and 5,000 miles for diesel.
The evaluation compared Orion VII low floor buses at NYCT with CNG propulsion (Detroit Diesel Corporation Series 50G CNG) and hybrid propulsion (BAE Systems HybriDrive propulsion system) against conventional diesel buses.
The CNG buses’ average fuel economy was 25% lower than the diesel baseline buses—a typical difference in fuel economy for low-average-speed operation for the spark-ignited natural gas engines.
The hybrid buses’ average fuel economy was 45% higher than the diesel baseline buses (ranging from 32% to 52% better than the diesel baseline during the evaluation period). The diesel baseline buses for the hybrid bus evaluation have diesel engines without exhaust gas circulation (EGR). The addition of EGR for emissions control would tend to lower the diesel baseline fuel economy.
The reported results represent eight out of a planned 12-month evaluation of these two groups of buses. An additional evaluation of NYCT’s order of 200 Orion and BAE Systems hybrid buses will be reported separately.
The eight-month evaluation period does not include summer months, which could have reduced the hybrid bus fuel economy advantage from air conditioning loading and the ability to collect regenerative braking energy into the batteries. The summer-month fuel economy information will be provided in the final results report on this evaluation. The hybrid buses had an average fuel economy 100% higher than the CNG buses.
In October 2005, New York City transport services ordered 500 more Orion VII series-hybrid-electric buses from DaimlerChrysler Commercial Buses North America. New York City Transit ordered 216 units, and Metropolitan Transportation Authority (MTA Bus) 284. (Earlier post.)
The exploration of alternative fuel technologies for urban transit has been driven, up to now, by imperatives for emissions reductions.
|EPA Emissions Requirements for Transit Buses|
|2004–2006||15.5||2.4 combined or 2.5 with a limit of 0.5 for NMHC||0.05|
Diesel hybrid bus propulsion systems offer improved fuel economy during a time of fuel economy penalties for emissions control.
An issue that requires resolution, however, is the EPA’s current lack of recognition of the emissions reduction from a hybrid bus. Under current regulations, the emissions profile of the bus—or other heavy-duty vehicle—is determined by evaluating the diesel as a stand-alone engine. In other words, from an EPA point of view, the emissions profile of a hybrid bus is the same as the emissions profile of a non-hybrid bus using the same engine.
The California Air Resources Board (CARB) has recognized the emissions savings offered by hybrids, and as granted hybrid bus propulsion systems a 25% blanket reduction in emissions that can be used in the state implementation plan for emissions reductions. Currently, EPA does not recognize this benefit.