Reduction of accessory overdrive and parasitic loading on a parallel electric hybrid city bus

This paper reports on the first phase of a project that explores the trends and dependencies of the input power to the major mechanically-driven accessories including hydraulic pumps, air compressor, air conditioning (AC) compressor, and alternator on a modern parallel hybrid city bus. In this first phase, the impact of accessory electrification is 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. Data collection and processing methods of this study are described, the shortcomings of mechanically-driven accessories are discussed, and an estimation of savings for accessory electrification is performed. Over 11 days and 145.6 hours of data collection on routes through Minneapolis, the average input power to the accessories was 11.0 kW (AC off) and 19.3 kW (AC on). By approximating the impact of accessory overdrive and parasitic loading, it was calculated that replacing mechanically-driven accessories with their electrically-driven counterparts would reduce the accessory power demand by about 34% (AC off) and 31% (AC on). Under the conservative assumption that, with the air conditioning on, 50% of the bus's fuel is consumed by its accessories, it is estimated that accessory electrification would result in a 13% (AC off) and 15% (AC on) improvement in overall fuel economy.