Burke, Andrew and E. Blank (1995) Electric/Hybrid Transit Buses Using Ultracapacitors. Institute of Transportation Studies, University of California, Davis, Presentation Series UCD-ITS-RP-95-32
The use of electrochemical capacitors (ultracapacitors) in the electric driveline of full-size electric and hybrid transit buses was studied for buses with performance comparable to standard diesel-powered buses (0-20 mph acceleration times of 6-8 seconds). The capacitors were used in conjunction with or instead of batteries. For the hybrid buses, a turbo-diesel generator or fuel cell was used for on-board generation of electricity. It was found that the diesel generator or fuel cell could be load leveled on the CBD 15 driving cycle with an ultracapacitor storing 2.5-3.0 kWh of energy. Using the energy density (6-10 Wh/kg ) of state-of-the-art, small (20 cm2) ultracapacitors, the weight of the ultracapacitor unit would be 300-400 kg, which is much-less than a battery, pack to provide the same power (200 kW). Simulations of the 14500 kg hybrid buses on the CBD 15 cycle using ultracapacitors indicated a fuel economy of 4.4 miles/gallon diesel fuel with the turbo-diesel generator and 7.5 miles/gallon diesel fuel equivalent with a fuel cell. The projected emissions of the bus using the diesel generator were .7 gm/mi HC, 3.4 gm/mi CO, and 14.1 Gm/mi NOx. The low resistance and high round-trip efficiency of the ultracapacitors resulted in significantly higher (10-30%) fuel economy using the ultracapacitors instead of batteries in the hybrid/electric driveline. Quantitative conclusions concerning the improvements to be expected in the fuel economy and emissions from utilizing an electric/hybrid driveline in transit buses must await the availability of chassis dynamometer data for hybrid and standard diesel-powered buses on the same urban driving cycles.