Drive force control of a parallel-series hybrid systemAbstractSince 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 the 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 Engineers of Japan, Inc. and Elsevier Science B.V. All rights reserved.1. IntroductionIn 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 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 expectations 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 engine 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. In 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 CO 2 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 limits 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 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 transmissionThe transmission consists of motor, 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 generator and the driving wheels. With this type of mechanism, therevoluti ons of each of the respective axes are related as follows. Here, the gear ratio betwee n the sun gear and the—MEJthanical PowerElectrical PowsrFig. 1. Schematic of Toyota hybrid system (THS).ring gear is p :2-上'where Ne is the engine speed, Ng the gen erator speed and Nm the motor speed.Torque tra nsferred to the motor and the gen erator axes from the engine is obta ined as follows:motor axis torque：二 ------- Te.r 1 +pwhere 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 proporti on al. If the reducti on gear ratio is n , the axle torque is obta ined as follows:aide torque : (■-_-—Te + Tm jff. (4(where Tm is the motor torque.As show n above, the axle torque is proporti onal to the total torque of the engine and the motor on the motor axis. Accordi ngly, we will refer to motor axis torque in stead of axle torque.2.2. EngineA gasoline engine having 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 setting the maximum speed at 4000 rpm (= Ne max).2.3. BatteryAs sealed nickel metal hydride battery is adopted. The advantages of this type of battery are highpower density and long life. this battery achieves more than three times the power density of those developed for conven ti onal electric vehicles [4].3. Required driving force and performanceThe THS offers excellent fuel economy and emissions reduction. But it must have the ability to：2iHybrift transmissionoutput en ough driv ing force for a vehicle. This secti on discusses the running performa nee required of the vehicle and the essential items required of the respective 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 n eeded in Japa n. 3.1. Planetary gear ratio pThe planetary gear ratio (p ) has almost no effect on fuel economy and/or emissions. This is becausethe required engine power (i.e. engine condition) depends on vehicle speed, driving force and batterycondition, and not on the planetary gear ratio. Conversely, it is largely limited by the degree of in stallability in the vehicle and man ufacturi ng aspects, leav ing little room for desig n. In the curre ntly developed THS, p =0.385.3.2. Maximum engine powerSince the battery cannot be used for cruising due to its limited power storage capacity, most driving is relia nt on engine power only. Fig. 2 shows the power required by a vehicle equipped with the THS, based on its driv ing resista nce. Accord in gly, the power that is required for cruis ing 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 maximum power of 43 kW in order to get good vehicle performance while maintaining good fuel economy.TjhJe 1ui red lieitkms Ebr Yfhi 讨己IkmRcquiriMi abdiiy CniLsin^ iiminiLjin AbiJal^lew uphill rc^ad AbUily faruphillroad 3l higJi speed 30% {11 ) 5% (3 > it 105 km h5% (3 ) it 130 krn-li nnttintancnuft33 Maximum gen erator torqueAs described in Section 2, the maximum engine speed is 4000 rpm (= Ne max). To attain maximumtorque at this speed, maximum engine torque is obtained as follows:1000 x maxl2TT '60f x Ar ma\ From Eq. (3), the maximum torque on the gen erator axis will be as follows:- --- Te msx = 26-5 (Nm). i + p2(kJ-琢.3 Required dnvc torque st the moio-r 日ELE ”Te mat = 95.5 (Nm).Vehicle Speed (kni/h} Fig. Z Rirquired drive power.5% slof>eI 思啊I Hoad 1 議 0 kni/h105 km/hBo 140 km/1A chicle Sjta^ed (koi/h} 一日也.EH<SSNS nnE-h-.This is the torque at which the generator can operate without being driven to over speed. Actually, highertorque is required because of acceleration/deceleration of generator speed and dispersion of engine an d/orgen erator torque. By add ing 40% torque margin to the gen erator, the n ecessary torque is calculated asfollows:---- Te ma\x. L4 = 7 JTIJK- 37,1 (Nm|. + p3.4. Maximum motor torqueFrom Fig. 3, it can be see n that the motor axis n eeds to have a torque of 304 Nm to acquire the 30% slope climb ing performa nee. This torque merely bala nces the vehicle on the slope. To obta in en ough starting and accelerating performanee, it is necessary to have additional torque of about 70 Nm, or about370 Nm in total.From Eq. (2), the transmitted torque from the engine is obtained as follows:y!—Te max = 70.0 (Nm).Con seque ntly, a motor torque of 300 Nm (=T m max) is n ecessary.3.5. Maximum battery powerAs Fig. 2 shows, driv ing power of 49 kW is n eeded for climb ing on a 5%slope at 130 km/h. Thus, thenecessary battery power is obtained by subtracting the engine-generated power from this. As alreadydiscussed, if an engine hav ing the mini mum required power is in stalled, it can only provide 32 kW ofpower, so the required battery power will be 17 kW. If the possible loss that occurs whe n the batterysupplies power to the motor is take n into accou nt, battery power of 20 kW will be n eeded. Thus, it isnecessary to determine the battery capacity by targeting this output on an actual slope. Table 2 lists therequired battery specificati ons.Table 3 summarizes the specifications actually adopted by the THS and the requirements determ inedby the above discussi on. The required items represe nt an example whe n mini mum engine power isselected. In other words, if the engine is changed, each of the items have to be changed accord in gly.Tub-le 2Required batter) KpecificatiansItem.Value2SS VPower20 IcWEnergy 1.8 kWhTable 3Required and. actual E-pcciOralions of the TI1W compDaienEsSpcciiication Required vdlue THSt inline EH髯imum pox弋丁44)k.WCicncnit口:r m^ximuinni lorquc17.1 Nm Ss N]nhi 口tor maiimium torque Nm305 Mmliallery mdiiinnium21 LWuodl erwrgy(inKtantantnusI LH kWh4. Driving force con trol The THS requires controls not necessary for conventional or electric vehicles in order to control the engine, motor and generator cooperatively. Fig. 4 outlines the control system. Fig. 4. Control diagram of the THS.In puts of con trol system are accelerator positi on, vehicle speed (motor speed), gen erator speed and available battery power. Outputs are the engin e-required power, gen erator torque and motor torque.First, drive torque demanded by the driver (converted to the motor axis) is calculated from theaccelerator position and the vehicle speed. The necessary drive power is calculated from this torque and the motor speed. Required power for the system is the total of the required drive power, the required power to charge the battery and the power loss in the system. If this total required power exceeds the prescribedvalue, it becomes required engine power. If it is below the prescribed value, the vehicle runs on the battery without using the engine power. Next, the most efficient engine speed for gen erati ng engine power iscalculated; this is the engine target speed. The target speed for the gen erator is calculated using Eq.⑴ with engine target speed and motor speed. The generator torque is determ ined by PID con trol. Engine torque can be calculated in reverse by using Eq. (3) and the torque transferred from the engine to the motor axis can be calculated from (2). The motor torque is obtained by subtracting this torque from the initiallycalculated drive torque. Since it is not possible to produce a torque whereby the motor con sumpti on power exceeds the total of the gen erator-ge nerated power and the power supplied by the battery, it is n ecessary to con trol the motor power (torque) with in this total power. Fig. 5 shows the control method. The sum of the power form the generator and the available battery power become the power that can be used by the motor. The available motor torque can be obtained by dividing this combined power by the motor speed. When the motor speed is low, if the calculated motor torque exceeds the motor specificati on of torque the motortorque is determ ined by the specification. By controlling the motor torque requirement with this limited torque, the motor con sumpti on power can be con trolled to within the available power. If the available battery power is large eno ugh, the available motor torque hardly limits the motor torque. Con versely, whe n the charge is low, the motor torque is frequently limited.Fig. 6 shows the respective maximum drive torque of the battery, the engine, and the engine plus thebattery while running based on the controls above, when the THS has the components as specifiedAccelomlor PoddPxitiQ 口 Trq.1 h Acc. SpeedM-olor 、 Motor Tortjue Motor f by Power ------ kA Avnilflible Ballery Power , L __ Power Genera SpwdRequired Drive Torque At Mow Axk Vehicle (Motor) 9peed Driwt Tgrgue fr^m Engine Genera lor . ContToller **Battery SJharg'e PowerRequired Power Engine Powf r Motor SpeedPID (kmtrolTarget hjlBltK! Tar^eLGanfiratOf Spood. Geinerftlflr * T&rq«5__in Secti on 3.5. Con clusi onsThis paper discussed the control of drive power in the Toyota Hybrid System. The followingcon clusi ons were obta in ed:The performa nee required for each comp onent can be determ ined by reversely calculati ng powerperforma nee required for a vehicle.The available battery power varies according to its state of charge. However, by limiting the motortorque, the battery power can be con trolled to within the battery's available power.混合动力系统驱动力的串并联控制摘要由于混合动力系统的每个部分都有自己的极限性能，所以汽车动力取决于最脆弱的哪一个组成部分。