Dual-Speed and Variable-Winding Systems Boost EV Efficiency
The pursuit of enhanced performance and energy efficiency in battery electric vehicles (BEVs) continues to drive innovation in powertrain design. A recent study by researchers from Chang’an University and Zhejiang Institute of Communications has provided a comprehensive comparison of three distinct BEV drive systems, revealing significant performance differences that could influence future vehicle development strategies. The research, published in the Chinese Journal of Automotive Engineering, evaluates single-speed, dual-speed automated manual transmission (AMT), and variable-winding permanent magnet synchronous motor (PMSM) systems under identical vehicle parameters, offering a clear benchmark for the advantages and limitations of each configuration.
The study addresses a critical challenge in the BEV industry: balancing the often-competing demands of dynamic performance and economic efficiency. While single-speed gearboxes have become the de facto standard due to their simplicity, low cost, and high mechanical efficiency, they force a compromise. A single gear ratio cannot simultaneously optimize the vehicle for rapid acceleration from a standstill and efficient high-speed cruising. This limitation arises because the electric motor must operate across a vast range of speeds and torques, and a fixed gear ratio means the motor frequently operates outside its peak efficiency zone, particularly during urban driving cycles with frequent stops and starts. As a result, despite the inherent efficiency of electric motors, a significant portion of the battery’s energy can be lost to suboptimal motor operation, directly impacting the vehicle’s range. This fundamental trade-off has prompted automakers to explore more complex drivetrain solutions to extract every possible percentage point of efficiency and performance.
The research team, led by Liu Yongtao from Chang’an University, selected a 4.5-tonne pure electric light-duty truck as the baseline vehicle for their analysis. This choice provides a relevant case study, as commercial vehicles have stringent requirements for both payload capacity and operational range, making efficiency paramount. The study focused on three single-motor drive systems to ensure a fair comparison, isolating the impact of the transmission and motor technology. The first system is the conventional single-speed AMT, which uses a single fixed gear reduction to connect the motor to the wheels. The second is a dual-speed AMT, which incorporates a two-gear transmission that can shift between a lower ratio for high torque at low speeds and a higher ratio for efficient cruising at high speeds. The third system is a novel variable-winding PMSM, which retains a single-speed gearbox but features a motor with two distinct stator winding configurations that can be electronically switched.
The variable-winding motor represents a sophisticated approach to the same problem. Instead of changing the gear ratio, it changes the fundamental electrical characteristics of the motor itself. In its “full-winding” mode, the motor is optimized for high torque output at low and medium speeds, ideal for launch and hill climbing. In its “half-winding” mode, the motor is reconfigured for high-speed operation, allowing it to achieve a higher maximum speed and operate within a more efficient constant-power region. This technology effectively gives the motor two distinct performance profiles within a single physical unit, expanding its high-efficiency operating range without the mechanical complexity of a multi-speed gearbox. The switching between windings is designed to occur in milliseconds, minimizing any disruption to power delivery.
To conduct a rigorous and realistic comparison, the researchers employed advanced optimization techniques. They used the elite-preserving genetic algorithm and dynamic programming theory to determine the optimal gear ratios for the single-speed and dual-speed AMT systems. This process involved simulating the vehicle’s performance over the China Heavy-Duty Commercial Vehicle Test Cycle (CHTC-LT), a standardized driving cycle that includes urban, suburban, and highway segments. The primary optimization goal was to minimize the vehicle’s energy consumption per 100 kilometers. For the dual-speed system, this optimization was particularly complex, as it required not only finding the best gear ratios but also determining the optimal shift schedule—the precise points at which the transmission should change gears to maintain the highest possible efficiency. Similarly, for the variable-winding motor, the team had to design the optimal “switching schedule” for when to transition between the full-winding and half-winding modes.
The simulation results delivered clear and actionable insights. In terms of dynamic performance, measured by the 0-50 km/h acceleration time, the dual-speed AMT system emerged as the clear leader. It outperformed the single-speed system by 2.63%, a significant improvement for a commercial vehicle. This advantage stems from its ability to use a very short first gear ratio, which maximizes the torque multiplication at the wheels for rapid acceleration. The single-speed and variable-winding systems, constrained by a single gear ratio that must also accommodate the vehicle’s top speed, cannot achieve the same level of launch performance. The variable-winding system, while still using a single-speed gearbox, showed a modest 1.38% improvement in acceleration over the baseline single-speed system. This gain is attributed to the motor’s ability to deliver higher torque in its full-winding mode, providing a slight edge in low-speed responsiveness.
The results for economic efficiency, however, paint a different picture. When evaluating energy consumption over the CHTC-LT cycle, the variable-winding PMSM system proved to be the most efficient, reducing energy consumption by 2.0% compared to the single-speed system. The dual-speed AMT system also showed an improvement, with a 0.6% reduction in consumption. These findings highlight a crucial distinction: the dual-speed system excels at maximizing performance, while the variable-winding system is superior at maximizing efficiency. The variable-winding motor’s ability to keep the motor operating in its high-efficiency zones across a wider range of speeds gives it a decisive advantage in minimizing energy losses. The dual-speed system improves efficiency by allowing the motor to operate at lower speeds for a given vehicle velocity in second gear, but this benefit is less pronounced than the fundamental efficiency expansion achieved by the variable-winding technology.
The study also provides a direct comparison between the two advanced systems. While the dual-speed AMT offers the best acceleration, it does so at a higher energy cost. Conversely, the variable-winding system achieves the lowest energy consumption but does not match the dual-speed system’s dynamic prowess. The single-speed system, serving as the baseline, performed the worst in both categories, confirming the limitations of a fixed-ratio drivetrain. This clear performance hierarchy provides valuable guidance for vehicle manufacturers. For applications where rapid acceleration and high performance are paramount—such as high-end passenger cars or performance-oriented commercial vehicles—the dual-speed AMT is the optimal choice. Its mechanical complexity and cost are justified by the significant performance gain. For applications where maximizing range and minimizing operating costs are the top priorities—such as long-haul delivery trucks or fleet vehicles—the variable-winding motor system offers the greatest benefit.
The implications of this research extend beyond the immediate comparison of these three systems. It validates the effectiveness of using advanced optimization algorithms like dynamic programming to design control strategies for complex powertrains. The study demonstrates that the performance of a system is not just a function of its hardware but also of the intelligence of its control software. The optimal shift and switch schedules are critical to unlocking the full potential of both the dual-speed gearbox and the variable-winding motor. This underscores a growing trend in the automotive industry, where software and control systems are becoming as important as mechanical engineering in defining a vehicle’s capabilities.
Furthermore, the research contributes to the ongoing debate about the future of BEV transmissions. While some industry experts have argued that the wide speed range of electric motors makes multi-speed transmissions unnecessary, this study provides empirical evidence that they can offer tangible benefits. The 2.63% improvement in acceleration is a compelling argument for their use in performance vehicles. At the same time, the success of the variable-winding motor suggests an alternative path to improved efficiency that avoids the added weight, cost, and potential reliability concerns of a mechanical gearbox. This technology could be particularly attractive for manufacturers seeking to improve efficiency without increasing the mechanical complexity of their drivetrains.
The findings also have important implications for cost-benefit analysis in vehicle development. A dual-speed AMT system adds cost and complexity through additional gears, synchronizers, and a more sophisticated control unit. A variable-winding motor may have a higher initial manufacturing cost due to its more complex stator design and switching circuitry. The study’s results allow manufacturers to quantify the performance and efficiency gains of each system, enabling them to make informed decisions based on their target market and vehicle application. For instance, the 2.0% energy savings of the variable-winding system could translate into a meaningful increase in range or a reduction in battery size, potentially offsetting its higher component cost.
In conclusion, the research by Liu Yongtao and his colleagues provides a robust and detailed analysis of next-generation BEV drive systems. By systematically comparing a single-speed, dual-speed, and variable-winding configuration under identical conditions, they have established a clear performance benchmark. Their work demonstrates that there is no one-size-fits-all solution for BEV powertrains. The choice between a dual-speed AMT and a variable-winding motor depends on the primary design objective: maximizing dynamic performance or maximizing energy efficiency. This nuanced understanding is essential for engineers and designers as they navigate the complex trade-offs involved in creating the next generation of electric vehicles, which must be both powerful and efficient to meet the demands of a rapidly evolving market.
Liu Yongtao, Liu Yongjie, Gao Longxin, Zhou Zijia, Wang Zheng, Chen Yisong, Wang Taiqi, School of Automobile, Chang’an University, Zhejiang Institute of Communications, Chinese Journal of Automotive Engineering, DOI: 10.3969/j.issn.2095-1469.2024.02.10