Structure_Design_for_Power-Split_Hybrid_Transmission

Abstract—The HEV plays the key role in the energy saving and carbon emission reduction. Typically an HEV combines two power components, an internal combustion engine and an electrical motor. And the large capacity lithium or the Ni-MH battery is also needed. The power-split power train configuration of an HEV has the individual advantages of the series and parallel types of HEV power train configurations. And the planetary gear set (PGS) plays the key role in it. This paper presents a particular power-split power train configuration. The state of the device can switch in the six modes, according to the driving requirement. This improves the over-all efficiency of the power train. As the mode switching strategy has great effect on the vehicle dynamic performance, the interrelationship between SOC and velocity on the mode switching strategy is researched. I. INTRODUCTION According to the mechanical design, there are three categories of HEVs: Series hybrids, parallel hybrids and power-split hybrids. The serial configuration is shown in Fig.1. A motor is applied to drive the wheels. And the electrical power is supplied by either a battery, or a generator transforming the engine power into electric power, or both. The advantage of this configuration is that the engine could work under its optimal condition most time, as the engine operation is independent from the vehicle speed and road load. But the overall power-train efficiency will be reduced because of the motor drive, since the engine power must be converted to the electrical energy first. Figure 1 The series hybrid configuration Manuscript received March 20, 2010 Aimin Du is with School of Automotive Studies, Tongji University. China. (e-mail: duaimin1971@yahoo.com.cn) Xudong Yu is with School of Automotive Studies, Tongji University. China. (e-mail: yxd20040881@163.com) Junjie Song is with School of Automotive Studies, Tongji University. China. (e-mail: alexsongjj@gmail.com) The parallel configuration as shown in Fig. 2, includes two separate power paths: the mechanical path and the electrical path. Each power path can drive the vehicle individually or collaboratively. The main drawback of the parallel configuration is that a single electric machine is typically used as a generator and as a motor. The electric power assistance must be constrained to avoid draining the battery and frequent role-reversal may be necessary. And the over-all power-train efficiency is lower than the traditional vehicle if driving in the city, where a lot of stop-starts happen. Figure 2 The parallel hybrid configuration The power-split hybrids combine the previous two configurations with a power-split device, as shown in Fig. 3. It is appealing because under proper design and control it can be designed to take advantage of both parallel and series types and avoid their disadvantages. Figure 3 The power-split hybrid configuration The power-split mechanisms were studied as early as the 1970[1]. The Toyota Hybrid system (THS), which is the core of the Prius, the first commercial power-split HEV, is described elaborately [2]. In 2004, Toyota released an improved THS system (THS2). The THS 2 power-split system is adopted and improved for higher-load vehicles as the Toyota Highlander [3][4]. The Planetary Gear Set (PGS) has two degrees for free rotation and it is the ideal mechanics for power-split HEV. There are three components in the PGS: the sun gear set, the carrier, the ring gear set. A simple model of the PGS is shown in Fig.4 . Figure 4 A simple model of Planetary Gear Set In the Power-split HEVs, there are three power components: the internal-combustion engine (ICE), the electrical motor (MOT), and the alternator (Alt). In the THS, the alternator is connected to the sun gear set, the engine is connected to the carrier and the electrical motor is connected to the ring gear set. As described in the literature [5], the components of the power-split hybrid can be configured in six different basic layouts, using a single planetary gear set, depending on what component is linked to what part of the planetary gear set. The six basic configurations are described in the Tab. (1). TABLE 1. DIFFERERNT CONNECTION BETWEEN PGS AND POWER COMPONENTS Configuration Sun gear Carrier Ring gear #1 ICE ALT MOT #2 MOT ALT ICE #3 ICE MOT ALT #4 ALT MOT ICE #5 MOT ICE ALT #6 ALT ICE MOT The configuration 6 is adopted in the THS. The power components include a high-performance engine using the Atkinson cycle, a permanent magnet AC motor and an alternator. When the engine is in high efficiency, the ideal power flow path is the mechanical transmission path, which the engine drives the wheel directly. But in the THS, according to the power balance equation, eq.(1)(2), a fraction of engine torque has to be transmitted to the sun gear set, which means part of the engine power must be converted to electrical power. The over-all efficiency of the power-train is declined. (1) (2) Here, the are torque of the sun gear, the ring gear and the carrier. The are the speed of the sun gear, the ring gear and the carrier. And the ρ is the characteristic parameter of the PGS, which is the radius ratio of the ring and sun gear. II. POWER-SPLIT POWER TRAIN SCHEME As described above, the series configuration and parallel configuration are complementary. The drawbacks of the parallel configuration are the series configuration’s advantages. The new hybrid transmission scheme presented following combines the excellence of both the series and parallel configuration, as shown in Fig. (5). Figure 5 The New Hybrid Scheme 　Comparing with the THS, there are two main differences. Firstly, the connections between the components and the PGS are different from the THS. The engine is linked to the ring gear. The motor is connected to the sun gear. The carrier is connected to the input of the final drive. And the alternator is Belt Driven Starter Generator (BSG). Secondly, four components are installed. As shown in Fig. (5), the clutch1 is a magnetic clutch, which can shut off the engine power transmission if necessary. The magnetic clutch2 can connect the ring gear and carrier together as required. The brake1 can restrict the sun gear rotation and the brake is used to restrict the ring gear rotation. A geometrical method is used to optimize the characteristic parameter (ρ) of the planetary gear set [6]. And we choose in this article. Before describing the features of the new hybrid structure, it would be necessary to highlight four important key points for HEV design.[7] 1. ICE is downsized, but is still the main power source. This means it cannot alone provide maximum power during a driving cycle. So the power assistance function is always present. 2. The important role of the alternator is to start the engine quickly in order to reduce the engine emission and fuel consumption. 3. Energy recovery: when the power request is negative and SOC is below a lower limit, the electrical motor is used as a generator to recharge the batteries. 4. Battery recharge: when SOC is too low, a fraction of ICE’s torque is used to generate electricity to recharge the batteries. In the new hybrid structure, different operation modes of the power-split device are obtained by changing the state of the four components: clutch1, clutch2, brake1 and brake2. It is summarized in the Table 2. TABEL 2 OPERATION MODE OF THE HYBRID SCHEME Mode Mode Clutch1 Clutch2 Clutch3 Clutch4 Drive1 0 0 0 1 Drive2 1 0 1 0 Drive3 1 1 0 0 Drive4 1 0 0 0 Brake1 1 0 0 0 Brake2 0 0 0 1 　In this table, state ‘0’ means the clutch is separated and the brake is idle. And the state ‘1’ indicates the clutch is combined and the brake works. The mode switching of power-split device could be obtained by changing the state of four components above. Each mode can meet a specific driving demand. When the vehicle is started or the velocity is very low, the power-split device enters into the mode Drive1. The clutch1 is separated the engine and the ring gear and the Brake2 work to restrict the ring gear rotation. The motor’s torque is transferred to the final drive, through the sun gear and the carrier. The transmission ratio is 4, as shown in the Eq. (3). While SOC is low, the battery needs to be charged, the engine is started and drives the alternator to generate. Drive1 is a typical series hybrid configuration. (3) Both the Drive2 and Drive3 are parallel hybrid configurations. In Drive2, the Brake1 works and the sun gear is locked. The engine’s power is passed to the final drive through the ring gear and the carrier of planetary gear, during which the engine speed is reduced and the transmission ratio is 1.33, as shown in the Eq. (4). The torque of the alternator couples with the engine’s at the front-end of the crankshaft. (4) In Drive3, the ring gear and the carrier combine together, and then the engine power is delivered to the final drive directly. Meanwhile, the motor can work as an electrical motor or a generator, depending on the driving condition. When the velocity is high, the power-split device will enter into Drive4. At this point, the engine works at the maximum speed, and the motor’s torque and speed should be adjusted to balance the power demand of the vehicle. When the vehicle decelerates, the system will turn into braking mode. If the velocity is high, the power-split device will enter into the Brake1 mode. The engine is idle. The engine’s brake torque and the alternator’s regenerative braking torque are combined through the planet carrier and delivered to the final drive. This will prevent the motor to be over speed. The residual brake torque is provided by the mechanical braking system. When the velocity is low, the device enters into the Brake2 mode during which the engine stops and Brake2 works to restrict the ring gear’s rotation. The regenerative braking torque is supplied by the motor alone. From the discussion above, it is definite that such a power-split device is very suitable to the HEV. When the velocity is low, the series mode is adopted to improve the engine’s efficiency under normal driving condition, the parallel mode runs. Meanwhile, according to the velocity and the load, Drive2 or Drive3 could be chosen. It enters into combined driving mode during the high velocity. Towards the braking condition, there are two braking modes selected according to the velocity of the vehicle. III. Mode Switch Strategy The mode switching strategy has a significant impact on the over-all efficiency of the power train. And both the SOC and the driver’s load have a significant influence on the mode switch. But in this paper, we just show the research work on the interaction between the SOC and the optimal mode switching velocity. Drive1 is suited for the low velocity condition. And if the SOC is low, the engine should be started to drive the alternator to recharge the batteries. As the velocity rises up to 30km/h, the power-split device changes to the Drive2. Both the Drive2 and Drive3 are parallel hybrid configuration. In Drive2, the transmission ratio from the engine to the final drive is 1.33 and the engine torque and the alternator torque couple at the front-end of the crankshaft. But in Drive3, the transmission ratio between the engine and final drive is 1 and the torque couple at the input of the final drive. The switch strategy ensures the dynamic of the vehicle when the device changes from Drive2 to Drive3. In other words, the acceleration of the vehicle must remain stable when switching. The acceleration is determined as follows, eq.(5): (5) Here, theis the maximum torque supplied by all the power component: the engine, the motor and the alternator. But when the SOC is low, the battery cannot provide adequate power to meet the motor’s requirement. And it means, the SOC has an effect on the max torque of the motor supplied (). In Drive2, only the engine and the alternator work and the max torque is calculated as in Eq.(6). (6) And in Drive3, the engine and the motor work, the max torque is calculated in Eq.(7), (7) Here, theis the maximum torque of the engine. According to the Eq.(5), we can get the acceleration under the driving mode and SOC. For example, when SOC=0.8, we can draw the acceleration curves under the Drive2 and Drive3, which are shown in the Fig.(6). And the intersection point is desired velocity for the device switching from Drive2 to Drive3. Figure 6 The Acceleration curve under SOC=0.8 As the velocity increases, the engine runs out of the economic range gradually. And it is necessary for the power-split device switching from the mode3 to mode4. Here we choose 120km/h as the up limit for Drive3. IV. CONCLUSION A new structure for the power-split hybrid transmission is developed in this paper. Although this power train uses a single planetary gear like the THS, it functions differently as there are four extra components. Six driving modes of the power-split can be obtained by changing the state of the clucth1, cluthc2, brake1, brake2. If the velocity is below 30km/h, the power-split device enters into the Drive1 and it works as a series hybrid. As the velocity rises, the device enters into the Drive2. The SOC has great influence on the mode switch from the mode2 to mode3. The mathematical function is found to analyze the relation between SOC and desired mode switching velocity. And we choose the 120km/h as the up limit for mode3. When the velocity is above, the power-split device would enter into the mode4. REFERENCES [1] G. H. Gelb, N. A. Richardson, T. C.Wang, and B. Berman, “An electromechanical transmission for hybrid vehicle power trains—design and dynamometer testing,” SAE, Warrendale, PA, Tech. Rep. 710235, Jan. 11–15, 1971. [2] S. Sasaki etc. Toyota’s Newly Developed Hybrid Power train. Proceedings of 1998 International Symposium on Power Wemiconductor Devices & Ics,Kyoto [3] K. Muta, M. 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