The effects of urban driving conditions on the operating characteristics of conventional and hybrid electric city buses Seref Soylu ⇑ Sakarya University, Faculty of Engineering, 54187 Sakarya, Turkey highlights  Operating characteristics of conventional and hybrid electric buses were examined.  Recovery of braking energy offers an excellent opportunity to improve fuel economy.  Speed and altitude profiles of routes have dramatic impacts on the energy recovery.  Capacity of the auxiliary power source has a dramatic impact on the energy recovery.  Round-trip efficiency of the regenerative braking system was calculated to be 27%. article info Article history: Received 29 May 2014 Received in revised form 25 August 2014 Accepted 27 August 2014 Keywords: Hybrid bus Regenerative braking Real-world driving Energy efficiency Traction energy Braking energy abstract The basic operating characteristics of a conventional bus (CB) and a hybrid electric bus (HEB) were exam- ined under urban driving conditions. To perform this examination, real-time operating data from the buses were collected on the Campus-Return route of the Sakarya Municipality. The main characteristics examined were the traction, braking, engine, engine generator unit (EGU), motor/generator (M/G), and ultracapacitor (Ucap) energies and efficiencies of the buses. The route elevation profile and the frequency of stop-and-go operations of the buses were found to have dramatic impacts on the braking and traction energies of the buses. The declining profile of the Campus-Return route provided an excellent opportunity for energy recovery by the regenerative braking system of the HEB. However, owing to the limits on the capacities and efficiencies of the hybrid drive train components and the Ucap, the bus braking energies were not recovered completely. Braking energies as high as 2.2 kW h per micro-trip were observed, but less than 1 kW h of braking energy per micro-trip was converted to electricity by the M/G; the rest of the braking energy was wasted in frictional braking. The maximum energy recovered and stored in the Ucap per micro-trip was 0.5 kW h, but the amount of energy recovered and stored per micro-trip was typically less than 0.2 kW h for the entire route. The cumulative braking energy recovered and stored in the Ucap for the Campus-Return route was 52% of the available brake energy, which was 13.02 kW h. Consequently, the round-trip efficiency of the regenerative braking system, between the wheels and Ucap, was determined to be 27%. Finally, although the brake engine energy (BEE) of the CB was 1.18 times higher than its positive traction energy (PTE), the BEE of the HEB was only 1.07 times higher than its PTE. In fact, it is normal to expect the BEE to be higher than the PTE owing to power train losses, but the energy recovered by the regenerative braking system was found to cover most of the power train losses and even improve the energy efficiency of the HEB. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Both the European Commission (EC) and the United States Department of Energy (USDOE) are taking serious actions to dras- tically reduce CO 2 emissions, with the goal of keeping climate change below 2 °C. The EC has suggested that to reach that goal, http://dx.doi.org/10.1016/j.apenergy.2014.08.102 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved. Abbreviations: CB, conventional bus; HEB, hybrid electric bus; EGU, engine- generator unit; M/G, motor/generator; Ucap, ultracapacitor; PE, power electronics; Gen, generator; FE, fuel energy; BEE, brake engine energy; GE, generator energy; PTE, positive traction energy; NTE, negative traction energy; EC, European Commission; USDOE, US Department of Energy; CO 2 , carbon dioxide; GHG, greenhouse gases; VTP, vehicle technology program; PEMS, portable emission measurement system; GPS, global positioning system. ⇑ Tel.: +90 5359788982. E-mail addresses: ssoylu@sakarya.edu.tr, serefsoylu@gmail.com Applied Energy 135 (2014) 472–482 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy