High flying hybrids

Hybrid electric transit buses are raising the industry standard for fuel economy, emissions, and reliability, according to a recently published independent study.

Hybrid buses tested the US National Renewable Energy Laboratory (NREL) on New York’s most demanding routes achieved up to 45 percent better fuel economy than diesel buses and 100 percent improvement compared to natural gas on an energy-equivalent basis. The report also said bus drivers liked the increased power output of the hybrid buses, whose electric drive motor offers superior torque to help with acceleration and hill climbing.

New York’s hybrid fleet also proved 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. BAE Systems’ HybriDrive system, which is used on Orion VII buses, is said to have 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.

In a series hybrid, the vehicle utilises a smaller diesel engine, reducing weight and cost. All-electric drive maximises efficiency and reduces emissions, and a regenerative braking system recovers waste energy and extends brake life. The design also eliminates the transmission, a significant maintenance item on transit buses.

The NREL study examined the operation, maintenance, performance, emissions, cost, and safety of urban transit buses operating in normal revenue service in New York, where usage and duty cycle are extremely demanding. The city operates a fleet of more than 4,000 transit buses including diesel, natural gas, and hybrid electric varieties. 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 and final report planned for release later in 2006 will incorporate data from a wider range of depots and more months of operation.