Investigation of regenerative braking efficiency in different drive cycles

Abstract

As being an important fact, climate change threatens the world population and its impact has become crucial recently. Thus, the world is being prepared for a very fresh era: green energy and electrification era. The European Union proposes the EU7 emission regulation, which has lower limits compared to prior regulations; and net zero emission policies are already adopted by several governments throughout the world. These developments increase the importance of electric vehicles (EVs), which have already start to replace the conventional vehicles due to their zero tailpipe emissions. Furthermore, the high efficiency, quiet operation and improved performance characteristics of EVs make them more preferable, and the recent advancements in battery technology bring this preference to the fore. Though, EVs still have significant disadvantages such as limited driving range and long charging time. The concept of regenerative braking becomes crucial in increasing the battery state of charge, which improves the driving range and reduces the required charging duration. Thus, the optimization of the regenerative braking system operation becomes critical. The chief objective of this study is to investigate the utilization rate of regenerative braking in different drive cycles, which depends on braking rate and road conditions. Consequently, seven well-known different drive cycles are selected, which vary with distance, urban / highway scenario, traction per kilometres, etc. An E class sport utility battery electric vehicle is selected for modelling purposes and it is subjected to all drive cycles. The driving resistances are calculated and the instantaneous electric motor torque demand is obtained for all drive cycles. Furthermore, the total braking and traction force ratios are evaluated for all drive cycles. It is observed that the comparison of these drive cycles based on total braking and traction force values does not provide reasonable conclusions due to the variation of drive cycle range. Thus, results are compared by normalizing the traction and braking forces by the range of each drive cycle. Finally, a braking to traction ratio is determined for all drive cycles, and the tendency of regenerative braking utilization rate change is investigated. Based on the results, it is observed that the braking to traction ratio of the vehicle significantly reduces when the vehicle moves from urban to highway drive cycle. Furthermore, the utilization rate of the regenerative braking also drops down to %13 from %53, when the drive cycle transition from urban to highway occur. The low braking to traction ratio shows that the recovered energy begins to be insufficient in highway driving conditions, and enough energy cannot be provided for the charging of the battery pack. In addition, the test vehicle's weight has increased to its gross weight. Subsequently, results were reanalyzed in the view of weight change. Finally, it is observed that weight change has not a significant effect on utilization rate in city conditions, whereas it increases the efficiency around %6 in highway conditions.