Improving fuel economy through vehicle electrification is a critical technology for meeting increasingly strict fuel efficiency regulations. However, only a limited number of small vehicles, such as Class B models, incorporate electric systems due to the relatively modest improvements in fuel economy compared to the increased costs and the need for additional installation space. This paper explores the best solution for a strong hybrid system tailored for small vehicles.
The study focused on maximizing energy efficiency by comparing engine and transmission efficiency across different driving modes, ultimately selecting a suitable automatic transmission for small vehicles. The hybrid system's design also involved determining the motor-generator connection configuration and its output power, balancing both fuel economy and drivability. To ensure smooth torque during gear shifts and reduce shaft length, the arrangement of the motor-generator and transmission gears was carefully designed.
A prototype of a mechanical automatic variable speed hybrid system (HV-AMT) was developed along with a test vehicle. The shift sequence that enables uninterrupted torque and flexible driving performance was analyzed, and the results were validated through testing.
In recent years, stricter fuel economy regulations have emerged due to global warming and rising energy consumption concerns. Electric power units have been introduced to improve fuel efficiency, with several hybrid systems developed based on this approach. Despite these efforts, only a small percentage of small vehicles utilize advanced hybrid technologies like strong hybrid systems. This is largely because the cost-benefit ratio is not favorable, and the integration of electric components requires additional space.
This study presents the verification results of a hybrid system specifically designed for small vehicles, developed by Aisin Seiki Co., Ltd. The focus was on selecting an appropriate hybrid system structure, emphasizing energy efficiency by combining engine and transmission efficiency. The research considered future fuel economy standards and explored the potential of strong hybrid power systems.
To enhance engine efficiency, the engine is operated within its optimal range, which can be achieved using a continuously variable transmission (CVT). Simulation results for a 1.0L engine vehicle under LA4 mode operation are shown in Figure 2, highlighting the efficiency of the engine under typical driving conditions.
Figure 1 illustrates the classification of electric power units based on their level of electrification and functionality. The study focused on strong hybrid systems, which offer greater potential for improving fuel economy. Transmission efficiency plays a crucial role in overall system performance, and various types of transmissions were evaluated.
Table 1 outlines the efficiency characteristics of different transmissions. Mechanical automatic transmissions (AMTs) and dual-clutch transmissions (DCTs) show higher efficiency due to minimal hydraulic pressure usage during shifting. In contrast, automatic transmissions (ATs) rely on torque converters, resulting in lower efficiency. Continuously variable transmissions (CVTs), while offering flexibility, suffer from lower efficiency due to friction-based power transfer.
AMT was chosen as the base transmission for the hybrid system due to its high energy efficiency and cost-effectiveness. However, AMT can cause torque interruption during gear shifts, which could affect driving comfort. To mitigate this, a motor was integrated to provide auxiliary power during shifting, ensuring smooth operation.
The design concept of the hybrid system included selecting an optimal motor-generator (M/G) connection point. Various configurations were analyzed, including input shaft, output shaft, and combined input-output shaft connections. The latter offered the best balance between hybrid functionality and system performance.
Figure 4 compares different M/G connection points, showing how each configuration affects the system's capabilities. The input and output shaft connection was selected for its superior mixing function and ability to support hybrid operations effectively.
Additionally, the motor-generator's drive performance was optimized to ensure it could contribute significantly to both propulsion and regenerative braking. This helped maintain high efficiency and improved the overall driving experience.
By integrating these elements, the study demonstrated that a strong hybrid system with an AMT can achieve significant improvements in fuel economy while maintaining good drivability and cost efficiency. The findings highlight the potential of such systems for small vehicles in the context of evolving environmental and regulatory requirements.
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