The Structure and Optimal Gear Tooth Profile Design of Two-Speed Transmission for Electric Vehicles

Abstract

As environmental regulations have been strengthened worldwide since the Paris Climate Agreement, the automobile industry is shifting its production paradigm to focus on eco-friendly vehicles such as electric vehicles and hydrogen-battery vehicles. Governments are banning fossil fuel vehicles by law and expanding the introduction of green vehicles. The energy efficiency of electric vehicles that use a limited power source called batteries depends on the driving environment. Applying a two-speed transmission to an electric vehicle can optimize average speed and performance efficiency at low speeds, and achieve maximum speed with minimal torque at high speeds. In this study, a two-speed transmission for an electric vehicle has been developed, to be used in a compact electric vehicle. This utilizes a planetary gear of a total of three pairs, made of a single module which was intended to enable two-speed. The ring gear was removed, and the carrier was used in common. When shifting, the energy used for the speed change is small, due to the use of the simple method of fixing the sun gear of each stage. Each gear was designed by calculating bending strength and surface durability, using JGMA standards, to secure stability. The safety factor of the gears used in the transmission is as follows: all gears have been verified for safety with a bending strength of 1.2 or higher and a surface pressure strength of 1.1 or higher. The design validity of the transmission was verified by calculating the gear meshing ratio and the reference efficiency of the gear. The transmission to be developed through the research results of this paper has a simple and compact structure optimized for electric vehicles, and has reduced shift shock. In addition, energy can be used more efficiently, which will help improve fuel economy and increase drive range.

Review of the Methods to Optimize Power Flow in Electric Vehicle Powertrains for Efficiency and Driving Performance

“…Within this class, the two-speed transmission is the most popular, which can be described in the form of either AMT or DCT. Most of the works related to the application of two-speed transmission reviewed here involved optimization of the gear ratios and shifting strategy to achieve optimum power consumption and driving performance, in terms of acceleration rate and top speed [33][34][35][36][37][38][39][40]. As a result, the EV powertrain becomes more efficient and more capable, and this opens up the possibility of optimizing the size and capacity of the motor and the battery.…”

“…In short, it can be concluded from the work, that highly flexible gear shifting strategy is crucial in optimizing EV driving mileage, and such flexibility is possible with the optimized multispeed transmission in the EV powertrain. Other works involving the optimization of multispeed discrete transmission in EV can be found in [34][35][36][37][38][39]. However, unlike [33,34] they presented the optimization of twospeed transmission in an electric truck subjected to specific gradient route without heavy traffics, and the motor's efficiency mapping also included regenerative braking efficiency.…”

“…Based on the aforementioned shifting strategy and driving cycle, the two ratios of the transmission were optimized for efficiency and acceleration using particle swarm optimization (PSO). The results showed that, when compared against one-speed trans- Other works involving the optimization of multispeed discrete transmission in EV can be found in [34][35][36][37][38][39]. However, unlike [33,34] they presented the optimization of two-speed transmission in an electric truck subjected to specific gradient route without heavy traffics, and the motor's efficiency mapping also included regenerative braking efficiency.…”

“…Based on the model, the optimum gear ratios were selected for the EV powertrain, and its performance was compared against a single-speed EV powertrain for Worldwide Harmonized Light Vehicles Test Procedure (WLTP) cycle, which showed an efficiency improvement around 10.7 to 12.1%. Regarding the transmission efficiency, research in [37] explained a possible method to improve it by modifying the tooth profile of the gears which can potentially reduce not only the loss in the gear meshing, but also the required effort for the gearshifting. In [38], on the other hand, a two-speed EV powertrain model were optimized using genetic algorithm (GA) with the objectives to achieve quick 0-100 km/h acceleration (driving performance) and minimum power consumption (efficiency) under NEDC.…”

“…A summary of the works presented in [33][34][35][36][37][38][39][40] is provided in Table 2 and they highlighted the significance of optimizing the gear ratios and the gearshifting strategy to achieve powertrain efficiency and driving performance. Some of these works have started to discuss on the gearshifting mechanism, but analysis on jerking and actuation power usage during shifting was still limited.…”