David Kittelson, Professor, Mechanical Engineering
The electrical load for transit buses continues to grow, and increased electrical demand challenges even the most robust conventional charging systems. It has been shown that electrically operated accessories and support systems offer greater efficiency than typical belt-driven or hydraulically operated systems. The "Superbus concept? would use an onboard auxiliary power unit (APU) to supply all electrical power for bus operations (excepting vehicle propulsion), allowing the diesel engine to shut down at traffic stops and at bus layovers while maintaining all passenger comforts and other support systems. This research undertook the first phase of a project exploring the trends and dependencies of the input power to the major mechanically driven accessories, including hydraulic pumps, air compressor, air conditioning compressor, and alternator, on a modern parallel hybrid city bus. In this first phase, the impact of accessory electrification was estimated by considering the near-elimination of accessory overdrive and parasitic loading. In addition to reducing accessory fuel consumption, accessory electrification can also serve as a bridge to the eventual use of a diesel or fuel cell auxiliary power unit to generate electricity for accessories on transit buses. In this study, researchers described data-collection and processing methods, discussed the shortcomings of mechanically driven accessories, and performed an estimation of savings for accessory electrification.