Detailed Analysis of Regenerative Energy when the Electric Bus Driving on Expressways
Authors:   Waseda Univ., JAPAN, Fang Yiyuan , Tianqi Xu , Yang Wei-hsiang , Ihara Yuto , Kamiya Yushi 
Published: April 2022

Electric vehicles have many advantages over traditional ICE (internal combustion engine) vehicles, such as zero emissions, noise and vibration reduction, and excellent acceleration performance. In addition, when decelerating or downhill, the ICE vehicles need to use friction braking or engine braking to control speed, but it will waste kinetic or potential energy during the braking. In contrast, electric vehicles driven by the motor can use regenerative braking to control speed and recover that energy to battery for running use. The general view about the regenerative energy is that the energy when running on ordinary roads is much more than expressways, due to the frequent acceleration and deceleration. However, expressways are routes where artificial/ topographical elevation differences occur easily. Especially in Tokyo, Metropolitan Expressways and its surrounding areas, ramps and underground/ undersea tunnels exist, and this situation causes a frequent uphill/ downhill run. It means the regenerable energy running on expressways cannot be able to ignore. Through a large-sized electric bus verification test, the unique situation in Japan is confirmed. In this paper, by analyzing the data obtained from the test that the test route named Misono is 11 km, including the ordinary road and the expressway, a peculiar phenomenon related to the regenerative energy when driving on the ordinary road and the expressway is summarized. Also, to validate the generality of the phenomenon observed, the states of regenerative energy of the other three driving routes are analyzed by the established vehicle simulator. First of all, two energy sources of regenerative energy are investigated: (a) recoverable energy through deceleration, (b) recoverable energy through downhill. And two energy losses: (c) air resistance loss, (d) rolling resistance loss. From the above four kinds of energy, the (e) theoretically regenerative energy (=(a)+(b)-(c)-(d)), which is the upper limit of the regenerative energy, can be calculated. And because of the mechanical loss such as the transmission and the motor-generator loss, the (f) actual regenerative energy is smaller than that. We define the ratio of two above as (g) energy regeneration efficiency (=(f)/(e)), which is used to evaluate the efficiency of the vehicle's energy recovery. Using the above method to analyze the data obtained from the test of Misono route, by comparing the expressway and the ordinary road, we found that there are more (a) recoverable energy through deceleration on the ordinary road. However, there are more (b) recoverable energy through downhill on the expressway, and the (g) energy regeneration efficiency is also significantly higher. Last of all, by the established vehicle simulator, through the simulation results of the three other routes, including the heavy-vehicle test mode of Japan, the same phenomenon about the regenerative energy on expressways and ordinary roads were verified. Through the analyses, the ordinary roads have the more regenerative energy as the general view, however, on the consideration of the unique circumstance of the expressways in Japan, the regenerative energy on expressways is also essential.