Improving the fuel economy of vehicles through electrification of power units is a key technology to meet stringent fuel economy regulations. However, only a small number of small vehicles such as Class B use electric devices because the increase in fuel economy is very limited with respect to cost, and an additional installation space for the electric device is required. The best solution for a strong hybrid system for small vehicles was studied. First, in terms of maximizing energy efficiency, engine efficiency and transmission efficiency distribution in different drive modes were compared, and a suitable automatic transmission was selected for small vehicles. When comparing the functions of the hybrid system, the connection mode of the motor generator and the output power of the motor for both fuel economy and drivability are determined. In addition, in order to achieve uninterrupted torque during shifting and a shorter shaft length than conventional manual transmissions, motor generator and transmission gear arrangements were designed. A prototype of a mechanical automatic variable speed hybrid system and a test vehicle were developed. Finally, the shift sequence that can achieve uninterrupted torque and flexible driving performance and its evaluation results on the vehicle are introduced.
In recent years, fuel economy regulations have become more stringent due to global warming and energy consumption issues. Electric power units have been used to improve fuel economy, and several hybrid units have been developed based on this. However, only a small number of small vehicles employ electric power units such as strong hybrid systems. The reason is that the increase in fuel economy is very limited relative to the increase in cost, and an additional installation space for the electric device is required.
In this study, Aisin Seiki Co., Ltd. introduced the verification results of a hybrid system suitable for small vehicles and its mechanical automatic transmission (HV-AMT).
1 hybrid system selection
Electric power units are typically classified based on electrification and function (Figure 1). Considering the strict fuel economy regulations in the future, this paper focuses on the study of strong hybrid power. To select the type/structure of the system, energy efficiency combines engine efficiency and transmission efficiency as a means of improving fuel economy.
In order to improve engine efficiency, the engine is concentrated in an efficient area (optimized operating conditions). A continuously variable transmission (CVT) can do this. Figure 2 shows the simulation results of the 1.0 L engine vehicle during LA 4 mode operation.
Figure 1 Electrification and function
Figure 2 Engine operating point during LA 4 mode operation
As can be seen from Figure 2, small vehicles typically employ an engine efficient zone, even during the certified drive mode, such as LA 4 . For small vehicles, the CVT has a slight advantage over a step-variable transmission.
Table 1 shows the transmission efficiency characteristics of a typical transmission. The mechanical automatic transmission (AMT) and the dual clutch automatic transmission (DCT), which use hydraulic pressure only in the case of shifting, have the highest transmission efficiency, while the automatic transmission (AT) transmission efficiency of the transmission ratio is maintained by the torque converter. The CVT that transmits torque through the pulley and the steel belt friction has the lowest efficiency.
Table 1 Typical transmission characteristics
Note: [0] reference; [+] advantage; [-] disadvantage; [--] great disadvantage
AMT is most suitable in terms of cost and form factor. However, ATM will have a shift shock due to the interruption of torque during shifting, and other transmissions will not.
Based on the above, FIG. 3 shows simulation results of energy efficiency (engine efficiency & TImes; transmission efficiency) of a 1.0 L engine vehicle.
Figure 3 Energy efficiency simulation results
As shown in Figure 3, AMT, DCT, and CVT have the highest energy efficiency. AT's engine efficiency is better than AMT and DCT. Because the transmission ratio of the torque converter of the AT is set larger.
Based on the above results, AMT has the advantages of high energy efficiency and low cost compared with other transmissions, so it is selected as the base type transmission of the hybrid system to achieve maximum fuel economy of small vehicles. In order to eliminate the shifting shock caused by AMT, the motor is used as an auxiliary power during the shifting process.
2 design concept
2.1 Motor/generator connection point is high
When determining the structure of the drive unit, the functions that can be realized by various motor/generator (M/G) connection points are studied. The comparison of various connection points is shown in Fig. 4.
The input shaft connection means that the M/G is between the engine and the transmission, and the engine can be disconnected by the clutch. The output shaft connection means that the M/G is located on the transmission output shaft. The input and output shaft connections mean that the M/G is placed on the third axis and the two clutches can be disconnected from the input or output shaft. The rear wheel connection means that the M/G is mounted on the rear drive shaft.
As mentioned above, auxiliary power is required during HV-AMT shifting. In addition, the "Input and Output Shaft Connections" with the best mixing function are selected.
Figure 4 M/G connection point comparison
2.2 M/G drive performance is high
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