1、Drive force control of a parallel-series hybrid system Abstract Since each component of a hybrid system has its own limit of performance, the vehicle power depends on the weakest component. So it is necessary to design the balance of the components. The vehicle must be controlled to operate within t
2、he performance range of all the components. We designed the specifications of each component backward from the required drive force. In this paper we describe a control method for the motor torque to avoid damage to the battery, when the battery is at a low state of charge. Society of Automotive Eng
3、ineers of Japan, Inc. and Elsevier Science B.V. All rights reserved. 1. Introduction In recent years, vehicles with internal combustion engines have increasingly played an important role as a means of transportation, and are contributing much to the development of society. However, vehicle emissions
4、 contribute to air pollution and possibly even global warming, which require effective countermeasures. Various developments are being made to reduce these emissions, but no further large improvements can be expected from merely improving the current engines and transmissions. Thus, great expectatio
5、ns are being placed on the development of electric, hybrid and natural gas-driven vehicles. Judging from currently applicable technologies, and the currently installed infrastructure of gasoline stations, inspection and service facilities, the hybrid vehicle, driven by the combination of gasoline en
6、gine and electric motor, is considered to be one of the most realistic solutions. Generally speaking, hybrid systems are classified as series or parallel systems. At Toyota, we have developed the Toyota Hybrid System (hereinafter referred to as the THS) by combining the advantages of both systems. I
7、n this sense the THS could be classified as a parallel-series type of system. Since the THS constantly optimizes engine operation, emissions are cleaner and better fuel economy can be achieved. During braking, Kinetic energy is recovered by the motor, thereby reducing fuel consumption and subsequent
8、 CO2 emissions. Emissions and fuel economy are greatly improved by using the THS for the power train system. However, the THS incorporates engine, motor, battery and other components, each of which has its own particular capability. In other words, the driving force must be generated within the limi
9、ts of each respective component. In particular, since the battery output varies greatly depending on its level of charge, the driving force has to be controlled with this in mind. This report clarifies the performance required of the respective THS components based on the driving force necessary for
10、 a vehicle. The method of controlling the driving force, both when the battery has high and low charge, is also described. 2. Toyota hybrid system (THS) 1,2 As Fig. 1 shows, the THS is made up of a hybrid transmission, engine and battery. 2.1. Hybrid transmission The transmission consists of motor,
11、generator, power split device and reduction gear. The power split device is a planetary gear. Sun gear, ring gear and planetary carrier are directly connected to generator, motor and engine, respectively. The ring gear is also connected to the reduction gear. Thus, engine power is split into the gen
12、erator and the driving wheels. With this type of mechanism, the revolutions of each of the respective axes are related as follows. Here, the gear ratio between the sun gear and the Fig. 1. Schematic of Toyota hybrid system (THS). ring gear is : where Ne is the engine speed, Ng the generator speed an
13、d Nm the motor speed. Torque transferred to the motor and the generator axes from the engine is obtained as follows: where Te is the engine torque. The drive shaft is connected to the ring gear via a reduction gear. Consequently, motor speed and vehicle speed are proportional. If the reduction gear
14、ratio is , the axle torque is obtained as follows: where Tm is the motor torque. As shown above, the axle torque is proportional to the total torque of the engine and the motor on the motor axis. Accordingly, we will refer to motor axis torque instead of axle torque. 2.2. Engine A gasoline engine ha
15、ving a displacement of 1.5 l specially designed for the THS is adopted 3. This engine has high expansion ratio cycle, variable valve timing system and other mechanisms in order to improve engine efficiency and realize cleaner emissions. In particular, a large reduction in friction is achieved by set
16、ting the maximum speed at 4000 rpm (=Ne max). 2.3. Battery As sealed nickel metal hydride battery is adopted. The advantages of this type of battery are high power density and long life. this battery achieves more than three times the power density of those developed for conventional electric vehicl
17、es 4. 3. Required driving force and performance The THS offers excellent fuel economy and emissions reduction. But it must have the ability to output enough driving force for a vehicle. This section discusses the running performance required of the vehicle and the essential items required of the res
18、pective components. Road conditions such as slopes, speed limits and the required speed to pass other vehicles determine the power performance required by the vehicle. Table 1 indicates the power performance needed in Japan. 3.1. Planetary gear ratio The planetary gear ratio ( ) has almost no effect
19、 on fuel economy and/or emissions. This is because the required engine power (i.e. engine condition) depends on vehicle speed, driving force and battery condition, and not on the planetary gear ratio. Conversely, it is largely limited by the degree of installability in the vehicle and manufacturing
20、aspects, leaving little room for design. In the currently developed THS, =0.385. 3.2. Maximum engine power Since the battery cannot be used for cruising due to its limited power storage capacity, most driving is reliant on engine power only. Fig. 2 shows the power required by a vehicle equipped with
21、 the THS, based on its driving resistance. Accordingly, the power that is required for cruising on a level road at 140 km/h or climbing a 5% slope at 105 km/h will be 32 kW. If the transmission loss is taken into account, the engine requires 40 kW (=Pe max) of power. The THS uses an engine with maxi
22、mum power of 43 kW in order to get good vehicle performance while maintaining good fuel economy. 3.3. Maximum generator torque As described in Section 2, the maximum engine speed is 4000 rpm (=Ne max). To attain maximum torque at this speed, maximum engine torque is obtained as follows: From Eq. (3), the maximum torque on the generator axis will be as follows:
