Towards Electrification of Urban Buses Using Model Based Analysis

Paper #:
  • 2018-01-0408

  • 2018-04-03
Vehicle electrification gains increasing interest as a promising solution for both zero emissions in urban environment and minimization of energy consumption. Simultaneously, many European cities focus on cleaner public transport and already started employing zero emission urban buses. Nevertheless, global spread of battery electric buses is currently questionable. During a bus route, vehicle gross weight may significantly vary due to the number of passengers, whereas vehicle energy needs could double when air-conditioning is utilized. Therefore, the electric powertrain design should be analyzed and optimized to achieve a sufficient traveling range. The aim of this work is to give insights into performance aspects of battery electric powertrain systems for urban buses. Instead of exhaustive testing, cost-effective model based vehicle analysis is proposed for quantifying and understanding electric powertrain subsystems performance. The electric vehicle model, developed in this work was utilized to predict electric motor performance, battery pack charge/discharge processes, as well as vehicle performance characteristics. The model was employed to simulate a 12-meter battery electric urban bus under constant speed driving conditions and transient driving cycles. Results show that vehicle energy consumption is restricted to values below 1.5 kWh/km, which corresponds to up to five times higher tank-to-wheel energy efficiency compared to Diesel urban buses. Low energy consumption is mainly assigned to e-motor operation at an efficiency region above 91% and regenerative braking, which enables up to 30% energy recovery. Electric vehicle performance characteristics are improved to those of conventional diesel buses, since the studied bus could achieve 95 km/h top speed and 17.5% gradeability at speed up to 60km/h. On the other hand, energy needs due to auxiliaries and mainly air-conditioning operation seemed to remove significant capacity from the battery pack, reducing vehicle range up to 50% depending on ambient conditions.
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