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Study evaluates hybrid bus performance in New York City

28 February 2006

The US DOE’s National Renewable Energy Laboratory (NREL) has published a report titled “New York City Transit Hybrid and CNG Transit Buses: Interim Evaluation Results”. In the study—which focused on fuel economy and costs, maintenance and reliability—Orion VII buses with the BAE Systems HybriDrive series-hybrid powertrains received the highest score when compared with conventional diesel and natural gas (CNG) buses.

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The NREL study evaluated average in-use fuel economy for buses in regular revenue service in New York City. The fuel economy of hybrids was 45% better than that of conventional non-EGR diesel buses . In another bus depot, CNG and conventional diesels were compared. On an energy-equivalent basis, the CNG buses showed 25% lower fuel economy than diesel . The CNG buses had a fuel cost per mile 53% higher than the hybrid buses.

The report also indicated bus drivers liked the increased power output of the hybrid buses, whose electric drive motor offered superior torque to help with acceleration and hill-climbing.

In a series hybrid, the vehicle utilizes a smaller diesel engine coupled with an electric generator. The vehicle is propelled by an electric motor in an all-electric drive (i.e., there is no mechanical connection between the diesel engine and the wheels). A regenerative braking system recovers waste energy, making the powertrain very efficient in a stop-and-go service, such as that encountered in urban bus operation. The regenerative braking, as well as the absence of a transmission, also contributes to the reduced maintenance.

The Orion VII HybriDrive buses utilize a 5.9 liter, 260 hp (194 kW) Cummins engine and a 120 kW traction generator. The electric traction motor delivers 250 hp (186 kW) and 2,700 lb-ft (3,657 Nm) of torque.

New York City operates a fleet of more than 4,000 transit buses including diesel, CNG, and hybrids. Currently, 325 Orion VII hybrid buses operate in revenue service, and 500 more are on order.

The New York results are contained in an interim report based on data measured between September 2004 and May 2005. A second, final report planned for release later in 2006 will incorporate data from a wider range of depots and more months of operation.