控制发动机离合器的转矩以改善混合动力电动车的特性.pdf

International Journal of Automotive Technology, Vol. 12, No. 5, pp. 763768 (2011) DOI 10.1007/s1223901100887 Copyright 2011 KSAE 12299138/2011/06016 763 TORQUE CONTROL OF ENGINE CLUTCH TO IMPROVE THE DRIVING QUALITY OF HYBRID ELECTRIC VEHICLES H. S. HWANG1), D. H. YANG2), H. K. CHOI2), H. S. KIM2) and S. H. HWANG2)* 1)Korea Automotive Technology Institute, 74 Yongjeong-ri, Pungse-myeon, Cheonan-si, Chungnam 330-912, Korea 2)School of Mechanical Engineering, Sungkyunkwan University, Gyeonggi 440-746, Korea (Received 28 June 2010; Revised 24 March 2011) ABSTRACTAs a powertrain for hybrid electric vehicles (HEVs), the automatic transmission (AT) is not only convenient for the driver but also reduces hybridization costs because the existing production line is used to produce the AT. However, it has low fuel economy due to the torque converter. To overcome this disadvantage, this paper studies HEVs equipped an AT without a torque converter. In this case, additional torque control is needed to prevent the driving quality from deteriorating. This paper suggests three different torque control methods and develops a simulator for an HEV that can simulate the dynamic behaviors of the HEV when the engine clutch is engaged. The HEV drive train is modeled with AMESim, and a controller model is developed with MATLAB/Simulink. A co-simulation environment is established. By using the developed HEV simulator, simulations are conducted to analyze the dynamic behaviors of the HEV according to the control methods. KEY WORDS : Hybrid electric vehicle (HEV), Engine clutch, Driving quality, Drive shaft torque, Torque control, Engine control, Motor control 1. INTRODUCTION The need for eco-friendly vehicles such as hybrid electric vehicles (HEVs) has increased rapidly due to resource exhaustion and increased awareness of environmental issues. The Toyota Prius first went on sale in Japan in 1997, making it the first mass-produced hybrid vehicle. It is sold in more than 40 countries and regions, with its largest markets in Japan and North America. In 2008, global cumulative Prius sales reached the 1 million sales milestone, and by early 2010, the worldwide cumulative sales reached 1.6 million units (Toyata, 2009; International Business Times, 2009). The Toyota Prius has the Toyota Hybrid System (THS), which has a single power-split device (incorporated as a single 3 shaft planetary gearset); thus, it can be classified as an input-split system because the power of the engine is split at the input to the transmission. This power-split system makes the setup very simple in mechanical terms but has some drawbacks. For example, the maximum speed is mainly limited by the speed of the smaller electric motor. Additionally, the transmission efficiency depends heavily on the amount of power being transmitted over the electrical path because multiple conversions cause the path to have a lower efficiency than the purely mechanical path. In particular, in higher speed regions, the efficiency, therefore, drops below that of a generic automatic transmission with a hydrodynamic coupler. To overcome these drawbacks, it is necessary to develop a new hybrid electric system that can reduce additional costs due to hybridization and can be produced using the existing production line effectively (Muta et al., 2004). As a powertrain for the HEV, the automatic transmission (AT) has some advantages, including a smooth start, convenience for the driver and safety. Furthermore, because the existing production line can be used to produce the AT, the hybridization costs can be reduced. AT has excellent accelerating and shifting performance because of the torque converter. However, because the torque converter transmits power with low efficiency, it serves as an obstructing element in the improvement of fuel economy when it is applied to the HEV. To overcome this defect, some studies have focused on an AT without the torque converter. Without the torque converter, additional torque control is required to prevent the deterioration of driving quality upon engagement of the engine clutch. Oil pressure control methods upon engagement of the engine clutch have been investigated (Kim et al., 2008a). Improvement of the shift quality (SQ) with motor torque control when shifting gears has also been investigated (Kim et al., 2008b). In this study, an HEV simulator is developed that can simulate the dynamic behaviors of the HEV equipped with an AT without a torque converter upon engagement of the engine clutch. To reduce the torque fluctuation that causes *Corresponding author. e-mail: hshme.skku.ac.kr 764H. S. HWANG et al. low driving quality upon engagement of the engine clutch, three different torque control methods are suggested: sychronized control of the engine and motor speeds before engagement of the engine clutch; motor torque control after engagement of the engine clutch; and combined control of the engine and motor torques. Simulations were performed to analyze the dynamic behaviors of the HEV according to the control methods. 2. HEV SIMULATION MODEL Figure 1 illustrates the structure of the test HEV. It is a 2- shaft parallel hard HEV with an AT. It can be operated in electric vehicle (EV) mode, in which only the motor is driven, and can be changed into the HEV mode by controlling the engine clutch installed between the engine and the motor. 2.1. Engine, Motor and ISG Simulation Model An engine has complex characteristics depending on the fuel injection time, ignition period and combustion characteristics, for example. In this paper, a simple model of the engine is used that approximates a torque-generating device with steady state characteristics. The vibration characteristic of the crank shaft and that of the simple model are considered to simulate the explosive torque vibration of the engine. The driving motor and integrated starter generator (ISG) are modeled as a 1st order transfer function by approximate fitting curves with a steady state torque characteristic with respect to speed considering the time delay. Figure 2 shows the characteristic curves of the motor and ISG used in this paper. 2.2. Torsional Damper and Engine Clutch Model The engine clutch of the HEV in this paper uses the AMESim model, which is a wet multi-disk clutch. The transmitted torque and friction coefficient are calculated using the following equations (Deur et al., 2005; Yang et al., 1998). (1) (2) where is the friction coefficient, s and k are the static and dynamic friction coefficients, N is the number of clutch plates, P is the acting pressure on the clutch, Ac is the acting area of the clutch, Ro and Ri are the external and internal radii of the clutch, is the relative rotational velocity due to slip, and s is the rotational velocity in the steady state. The operation of the engine clutch has the following 3 different ranges: the disengaging range from the power shaft of the engine; the slip range as power is transmitted; and completely engaged range between the engine and motor axes. A torsional damper is modeled as a spring and a damper with various stiffnesses w.r.t. the relative twisting angle between the torsional damper ends. Figure 3 shows the hysteresis characteristic of the torsional damper. 2.3. Automatic Transmission and Vehicle Dynamics Model Vehicle dynamics modeling is performed using AMESim, which is used widely with vehicle and hydraulic models. In the vehicle model, the inertia, stiffness and damping effect are considered to describe the dynamic behaviors of the TtransNAP2 3 - - Ro 3 Ri 3 () Ro 2 Ri 2 () -= sk() s - kexp= Figure 1. Structure of a parallel HEV with AT. Figure 2. Characteristic curves of the motor and ISG. Figure 3. Characteristic curve of torsional damper. Figure 4. Schematic diagram of an automatic transmission. TORQUE CONTROL OF ENGINE CLUTCH TO IMPROVE THE DRIVING QUALITY 765 HEV in detail. The HEV has adopted the 6 speed automatic transmission, as shown in Figure 4. Figure 5 shows the HEV powertrain model that is used in AMESim. The transmission was constructed with three PGs (Planetary Gears), OWC (One Way Clutch), LRB (Low and Reverse Brake), UD/B (Under Drive Brake), and OD/C (Over Drive Clutch). Figure 6 shows the results of the comparison of the drive shaft torque between experiment and simulation, which are used to validate the vehicle model. The simulation results were obtained with the same input conditions for the engine torque, motor torque, and oil pressure profile. In the test condition, the accelerator position is 100%, which is the wide open throttle (WOT) condition, and the driving mode and gear shifting conditions are included. 2.4. Controller Model The controller was modeled using MATLAB/ Simulink, and the co-simulation environment was established. The hybrid control unit (HCU) determines the torques demanded for the engine management system (EMS) and motor control unit (MCU); these torques are calculated according to the drivers command and driving strategy (Ahn et al., 2009). This study applied a simple driving strategy because we were only concerned with the effect of variations of drive shaft torque on the driving quality during engagement of the engine clutch. The driving strategy used is shown in Table 1, which shows the operating conditions and actions of the four driving modes. The EMS transmits the throttle open signal, which is calculated according to the demand torque from the HCU to the engine. The TCU transmits an oil pressure profile signal according to the gear shifting schedule, which depends on the driving mode (e.g., the EV or HEV mode), to the engine. The MCU transmits the demand torques, which are transferred from the HCU to the motor and ISG. Figure 7 shows the HEV controller model built with MATLAB/ Simulink. Figure 5. HEV Powertrain model used in AMESim. Figure 6. Comparison between the experiment and the simulation. Table 1. Driving strategy. Driving mode Condition and action EV mode Condition 0 0 Action Regenerative braking according to conditions APS: Accelerator Position Sensor (%) BPS: Brake Position Sensor (%) Figure 7. HEV controller model built with MATLAB/Simulink. 766H. S. HWANG et al. 3. SIMULATION RESULTS Various simulations were performed under the following conditions with the applied torque control algorithms to reduce the torque vibrations of the drive shaft. Table 2 shows the various simulation conditions according to the combination of engine and motor controls. The input condition of the accelerator pedal is a step function of 100% APS. 3.1. Without Control Figure 8 shows the simulation results without the control algorithm to decrease the vibrations upon engagement of the engine clutch. During the mode change, the engine torque is constant according to the throttle position, and the motor torque is determined according to the APS. Figure 8 (a) shows the speeds of the engine and the motor as well as the drive shaft torque, which is a reference value for the driving quality in this paper. In this figure, the point at which the engine speed coincides with the motor speed is the point of complete engagement of the engine. The variation of the drive shaft torque can be observed from the slip range just prior to the complete engagement point. Figure 8 (b) shows the mean output torques of the motor and the engine. The negative values of initial engine torque are the torque losses due to the friction loss of the engine, and the ISG covers the loss torque. 3.2. Engine Speed Control Prior to Engagement The engine speed control method controls the tracking of the motor speed prior to engagement of the engine clutch after the engine is started. This control algorithm calculates the engine output torque using the feedback on the motor speed and then generates the command signal for the engine throttle position based on the calculated torque (Figure 9). Figure 10 shows the simulation results from engine speed control. These results illustrate that the drive shaft torque with control has less fluctuation than one without control after engagement of the engine clutch. 3.3. Motor Torque Control during Engagement Motor torque control is applied during engagement; its block diagram is shown in Figure 11. Figure 12 shows the simulation results with motor Table 2. Simulation conditions. NoSimulation conditions 1Without control 2Engine speed control prior to engage 3Motor torque control during engaging 4Combination of engine and motor torque controls Figure 8. Simulation results without control. Figure 9. Control block diagram of the engine torque control. Figure 10. Simulation results with engine speed control. Figure 11. Block diagram of the motor torque control to reduce torque fluctuation. TORQUE CONTROL OF ENGINE CLUTCH TO IMPROVE THE DRIVING QUALITY 767 torque control from the feedback on the drive shaft torque during engagement. Before engagement of the engine clutch, the results on the engine speed, motor speed and drive shaft torque are similar to those without control, but after engagement of the engine clutch, they show an increase in the reduction in the torque fluctuation because of the motor torque control. 3.4. Combination of Engine Speed and Motor Torque Controls Figure 13 shows the simulation results obtained from the application of the combined engine speed and motor torque controls. The results are similar to those obtained from the application of engine speed control before the engagement, and they are similar to those from the motor torque control after the engagement. The remaining vibration of the drive shaft torque disappears after the engagement of the engine clutch. 4. ANALYSIS OF SIMULATION RESULTS The simulation results of the various control methods above are compared in terms of vehicle speed and acceleration of the HEV. Figure 14 shows the oil pressure profile of the engine clutch from the start of the engine to the complete engagement of the engine clutch. All the same pressure profiles are provided as simulation inputs with the various control methods. Figure 15 and Figure 16 show the vehicle speed and acceleration according to the various control methods. The trend in the variation of the acceleration results is similar to that of the drive shaft torque. From the vehicle speed graph, Figure 12. Simulation results with motor torque control. Figure 13. Simulation results with a combination of engine speed and motor torque controls. Figure 14. Oil pressure profile acting on the engine clutch. Figure 15. Comparison of vehicle speeds. Figure 16. Comparison of vehicle acceleration. 768H. S. HWANG et al. the acceleration performances of the vehicles with control are worse than those without control. This performance decrease is caused by the reduction of the engine torque according to the control algorithm after the start of the engine. In the case of torque control, the vehicle speed increases slowly due to driving torque reduction after complete engagement of the engine clutch. Table 3 shows the fluctuation in vehicle acceleration, which indicates differences in peak to peak values while the engine clutch is being engaged and the speed when the engagement of engine clutch is completed at a relative time of 1. The values are listed alongside the control methods, and they are expressed as a percentage of the results without control. As a result, although the engine and motor control may decrease the accelerating performance of a vehicle a little, they can improve driving quality by reducing the torque fluctuation of the drive shaft. Therefore, automotive engineers can design the controller using an appropriate control method based on their judgment of the tradeoff between acceleration performance and driving quality. 5. CONCLUSION In this paper, an HEV equipped with AT without a torque converter is modeled, and the dynamic characteristics of the HEV are investigated based on the control method of the engine and motor. An HEV simulator was developed in which AMESim was used to model the HEV drive train and MATLAB/Simulink was used to develop the controller model; a co-simulation environment was established. To reduce the torque fluctuation, which is responsible for low driving quality at the engagement of the engine clutch, three different torque control methods were suggested. The drive shaft torque and vehicle speed were compared and analyzed according to the control method. From the simulation results, although the engine and motor controls may reduce the acceleration performance of a vehicle a little, they contribute to the improvement of driving quality by reducing the torque fluctuation of the drive shaft. In the future, automotive engineers will be able to design the controller considering design factors such as acceleration performance and driving quality by using the HEV simulator. ACKNOWLEDGEMENTThis research was financially supported by the Ministry of Knowledge Economy(M