Power Control Strategy to Improve the Fuel Economy and Driveability of Automatic Transmission Systems

Abstract

The issues of rising in oil price and improving driveability in a vehicle have increased the need for research to improve the fuel economy and the driveability in automatic transmission vehicles. For these issues, multi-speed transmission systems resulted from the engine downsizing and their operational strategies can be the realistic alternative. Although many research have been conducted about a control model for multi-speed transmissions, there are not enough to present a control strategy considering both the fuel economy and the driveability. This dissertation proposes the power control strategy for automatic transmission systems to improve the fuel economy and the driveability based on the drivers requirements. For the proposed control strategy, this dissertation is studied in consideration of fuel economy, driveability and drivers intention, respectively. In the fuel economy aspect, Dynamic Programming (DP), which guarantees the global optimal solution, is utilized to obtain the optimal gear shift pattern based on the required power. DP algorithm is developed to obtain the operating points of the optimal gear that minimize the fuel economy within the required power band as a forward recursive way, which aims to find the optimal path from the initial state to the present state. In order to evaluate the fuel economy and the dynamic performance, the vehicle simulation model for a vehicle equipped with 8-speed automatic transmission is implemented on co-simulation by using AVL cruise and Matlab simulink. The results of the co-simulation on FTP 72 cycle are validated with those of the experiment and it is confirmed that the developed simulator is applicable to the other strategies. Using this co-simulation model, the optimal gear shift pattern developed from DP algorithm is evaluated for the fuel economy on FTP 72 cycle, and verified the effectiveness compared with the production shift pattern, which has been applied to the target vehicle. However, this process has limitations in a construction of down-shift line and is required for calculation of pre-process. For improvement of the driveability in vehicles, we analyze an objective evaluation method of the driveability and then introduce the driveability index according to the predefined driving condition. In addition, this dissertation analyzes the reason of an abnormal vibration occurred by a damper clutch operation for the improvement of the fuel economy at higher gear and low speed. Based on the analysis about the engine operating regions, the vehicle simulation is conducted to realize an abnormal vibration by a damper clutch operating. Proper engine operational regions are confirmed to avoid the abnormal vibration, which affects to deteriorate the driveability, by the damper clutch engagement. For analysis of the drivers intention, the operational conditions are classified and analyzed according to variation of an accelerator pedal. Moreover, the engine power generated from the drivers intention is classified into the vehicle driving power and the powertrain loss power. From this analysis, the concept of the power ratio, which consists of the power for acceleration and the power for resistance, is introduced. The power ratio for the target vehicle is generated in a look-up table within the feasible speed range, and then the power ratio pattern is developed by using the production shift pattern to be applied in the power ratio map. From the simulation results of the power ratio pattern, it is verified that this pattern is applicable to the vehicle. It is also resulting in the concept of driver acceleration demand. Based on the previous research, the power control strategy for automatic transmission systems is constructed that it starts to generate the required acceleration and calculates the candidates for the gear speed and throttle control while satisfying the requested acceleration. Among these candidates, the power control strategy determines the optimal gear speed and throttle value while minimizing the fuel consumption considering the effects of the torque converter. As a result, the performance of the proposed power control strategy for multi-speed transmissions is compared to those of the production shift pattern in vehicle simulations. The proposed control strategy can result in the fuel economy improvements of up to 12.69% on UDC cycle versus the production shift pattern. That is, comparing to the production shift pattern, the proposed strategy can estimate the drivers intention, thus it is possible to improve the fuel economy. In addition, it is verified that the power control strategy has better acceleration performance and shifting shock than the production shift pattern at fixed APS conditions. This control strategy can result in driveability improvement by reducing the number of APS operation, which is related to the drivers fatigue.

Compared to the current development process, the proposed gear shifting strategy effectively reduces the development time and effort required for developing a gear shift pattern for multi-speed transmissions. This strategy is applicable to the various powertrain systems because the gear shift and throttle control are obtained through real-time calculation to achieve the requested acceleration in the most fuel efficient way. Following the required acceleration based on the drivers intention, this strategy has an advantage of a fast response. Consequently, the proposed procedure can be available and useful in multi-speed transmission systems to improve the fuel economy and the driveability.