Electric Vehicle Drivetrain Development in China

Abstract

Despite great progress recently made on applications of in-wheel motors in electric vehicles, almost all production or near-production electric vehicles still utilize mechanical speed reduction systems for transferring torque from the traction motor to wheels for the purposes of torque augmentation and speed reduction. These systems in general fall into three categories, i.e. fixed ratio, stepped variable ratio, or continuously variable ratio. In China, most electric cars retrofitted from internal combustion engine propelled vehicle models use gear reduction systems of a fixed speed ratio, in order to minimize the time to market. Typically a conversion is made to the original 5-speed manual transmission by taking out a few unused gear sets. With the rapid growth in electric vehicle industry, some gearboxes of fixed speed have been engineered and they typically have a layshaft configuration. Most of them still do not come with a “park” gear due to a lack of understanding on customer’s needs. As an exception, a transmission of fixed speed ratio dedicated for electric vehicle applications has been developed at the Electric Vehicle R&D Center, Chinese Academy of Sciences (UCAS). Among electric vehicles announced by domestic vehicle manufacturers in China, some employ 5-speed manual transmissions (MTs) or automatic transmission (ATs) that typically found in traditional vehicles. From the driving convenience, transmission efficiency, or cost standpoints, these transmissions are, in general, not appropriate for applications in electric vehicles. The “misusage” of these transmissions has often something to do with their availability rather than suitability. A great deal of effort has been put into the research and development of automated mechanical transmissions (AMTs) in China to date. Significant progress has been made to the reduction of shift time, improvement of shift quality, and optimization of the mechanical components. Continuously variable transmission (CVT) is considered to be an important trend in drivetrain technology. However, the pulley-belt types of CVT commonly seen in traditional vehicles are not proper for electric vehicle applications. An EVT dedicated for electric vehicles is under development at UCAS, in which the power from an electric motor of dual-rotors is coupled by means of a planetary gear set, allowing continuous variable of the output speed. In summary, the electric vehicle drivetrain technology in China is undergoing rapid advances, which will impact the development of electric vehicle industry at home and abroad.