Hybrid-Electric Transit Bus Performance in North America

Hybrid-electric bus (HEB) technology has been heralded as offering better fuel economy, longer brake life, reduced noise, and lower tailpipe emissions than conventional diesel bus designs. HEB design can employ a variety of architectures, control strategies and energy storage technologies. Energy for electric HEB may be stored in batteries or ultracapacitors, and improvement of energy and power density of storage systems would promote HEB use further. Operation costs of HEB may be attractive through fuel saving and reduced maintenance, but acquisition costs are about 30% to 70% higher for HEB than for conventional diesel buses. Additional HEB operation costs may arise from facility and training investments required by the technology. Previous studies have gathered chassis dynamometer and field data to show both fuel economy and emissions benefits for HEB. To develop a realistic HEB life-cycle cost model, West Virginia University, supported by Transit Resource Center and Battelle Memorial Institute, is conducting the Transit Cooperative Research Program (TCRP) C-15 project, An Assessment of Hybrid-Electric Transit Bus Technology. The project is presently evaluating 40-ft and 60-ft HEB, along with comparable conventional diesel and natural gas buses, at four North America transit agencies. Three major bus hybrid drive technologies are included. The HEB fuels include both diesel and gasoline, and both common propulsion architectures are included. Preliminary data are available on bus fuel economy and maintenance. The diesel HEB show a fuel economy advantage (which is route and site dependent) of 26% to 45% while the gasoline HEB offered a 4% improvement over conventional diesel buses, showing that the hybrid system can offset the lower efficiency of a throttled engine. A life-cycle cost model is being developed which will use these fuel economy data, a wide range of operating data, and technology cost projections. The project, when completed, will provide transit operators with a reliable tool to evaluate impending and future hybrid and conventional bus technology life-cycle costs.