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978-1-4244-4547-9/09/$26.00 ©2009 IEEE TENCON 2009 A Study on Plug-in Hybrid Electic Vehicles Mojtaba Shams-Zahraei and Abbas Z. Kouzani School of Engineering Deakin University Geelong, Victoria 3217, Australia mshams, kouzani@deakin.edu.au Abstract—Plug-in hybrid electric vehicle (PHEV), which is a hybrid vehicle whose batteries can be recharged by plugging into an electric power source, is creating many interests due to its significant potential to improve fuel efficiency and reduce pollution. PHEVs would be the next generation of vehicles that are expected to replace conventional hybrid electric vehicles. This paper presents a study on PHEV. It gives a review of different drivetrain architectures associated with PHEVs. In addition, different control strategies that could bring about realization of advantages of PHEV capabilities are discussed and compared. Keywords-PHEV; drivetrain architectures; power management I. INTRODUCTION Living in the era of increasing environmental sensibility and rise in fuel price makes it necessary to develop a generation of vehicle that are more fuel efficient and environmental friendly. Hybrid electric vehicles could meet these demands [1]. Plug-in hybrid vehicles have recently created interests among leading automotive industry manufactures because of their potential to replace fuel- generated energy with battery-stored electricity in short daily journeys, and also continuing extended range as a HEV afterwards. This feature makes PHEV very low or zero emission vehicle during their Charge Depletion (CD) or All- Electric Range (AER). A plug-in hybrid electric vehicle (PHEV) is a hybrid vehicle whose batteries can be recharged by plugging into an electric power source. A PHEV combines features of conventional hybrid electric vehicles and battery electric vehicles, possessing both an internal combustion engine and batteries for power. IEEE-USA Energy Policy Committee defines plug-in hybrid electric vehicle as “a hybrid vehicle which contains at least: (1) a battery storage system of 4 kWh or more, used to power the motion of the vehicle; (2) a means of recharging that battery system from an external source of electricity; and (3) an ability to drive at least 16 km (ten miles) in all-electric range, and consume no petrol.” These are distinguished from hybrid cars, which are mass-marketed today, that do not use any electricity from the grid [2]. Benefits of PHEV drivetrain cover both individual and national aspects. Using the energy charged into the energy storage from utility grid to displace part of petroleum is the major feature of plug-in hybrid electric vehicles. This means using a cleaner and between three to four times cheaper energy in comparison to petrol [2, 3]. The widespread use of plug-in hybrid electric vehicles whose battery-generated energy is sufficient to meet average daily travel needs could reduce petroleum consumption between 40 to 50 percent [4-6]. In national point of view, the full penetration of PHEV in society results in its energy dependence shifting from petrol to sources of electricity generation, and from the green house gas (GHG) and other air pollutant emission shifting from high population urban area to electricity plants area. However there is an opportunity to produce the electricity from nuclear energy or other sources of renewable energies [3]. Off-peak charging strategy or more sophisticated vehicle to grid charging technology help load leveling in electricity generation industry which will consequently result in decreasing electricity cost because of reduction in power plant start-up and operation and maintenance costs [5]. However, charging strategy significantly affects the electricity consumption in power generation point of view [7]. The aim of this paper is to present the reader with up-to- date information on PHEVs making it easier for the reader to establish an understanding of the operation principals and applicability of the available architectures, strategies, and technologies. This paper first reviews the different drivetrain architecture of plug-in hybrid vehicles and current manufacturer activities in the field. Then, compatibility of different drivetrains to appeal most advantage of PHEV features is discussed. Different control strategies for newly developed plug-in hybrid vehicles is reviewed and compared. II. PHEV ARCITECTURE All hybrid drivetrain consists of Series, Parallel, Series- Parallel, and Two-Mode Power Split hybrids are compatible to change to a PHEV. However, there are always some potentials and drawbacks in each of them. Series configuration (see Fig. 1-a) is commonly recognized as an electric vehicle which has an onboard engine and generator to recharge the battery so it is easier to upgrade it to a PHEV. This drivetrain already has a sized electric motor to coupe maximum power demand of drive cycle. The increase in power capability of the battery provides the maximum power demand of drive cycle which means all electric range and zero emission could be met even in vigorous driving situation. The advantage of engine operation independent of wheel speed offers engine operation on its most efficient point. However, the known drawback of this drivetrain which is twice conversion of engine mechanical power to electrical and again to mechanical in electric motor reduces overall efficiency of drive train after depletion of batteries [1, 1 8]. General Motors is planning Chevrolet Volt PHEV with series drivetrain for 2011. In parallel drive train, both engine and electric motor can propel the wheel directly (see Fig. 1-b). A sized electric motor as well as batteries are necessary to upgrade a parallel drivetrain to a PHEV. In pre-transmission parallel architecture similar to Honda Insight, Civic, and Accord hybrids a small electric motor is located between engine and transmission replacing flywheel. It is also possible for a parallel hybrid to use its engine to drive one of the vehicle's axles, while its electric motor drives the other axle. DaimlerChrysler PHEV Sprinter has this powertrain configuration [8]. Series-parallel or power split drivetrain, the most commonly used drivetrain, is shown in Fig. 1-c. Toyota Prius, the most sold hybrid vehicle, Toyota Highlander, Lexus RX 400h, and Ford Escape and Mariner benefit from this architecture. The series-parallel hybrid powertrain combines the series hybrid system with the parallel hybrid system to achieve the maximum advantages of both systems. In this powertrain, mechanical energy passes through the power split in two series and parallel paths. In the series path, engine power output is converted to electrical energy via a generator which runs the electric motor to drive the car. In the parallel path, on the other hand, there is no energy conversion and the mechanical energy of the engine is directly transferred to the final drive block, through the power split which is a planetary gear system [10]. From the PHEV compatibility point of view, same as Parallel drivetrain, this architecture does not have a sized electric motor for the maximum demand [11]. Pure electric mode is defined for series-parallel drivetrain which means the engine can turn off completely during AER or CD. However, during vehicle high speed, there is still a generator maximum speed constraint for continuing in AER without turning the engine on since the generator speed increases sharply proportional to motor speed with ring to sun gear teeth number ratio in planetary gear when the engine does not rotate. This is due to the speed equation between series-parallel components which is as follows: ( ) MotEngGen RR ωωω −+= 1 (1) where ω is angular velocity and R is ring to sun gear teeth number ratio. One of the generator rolls is engine starting when necessary so generator should have the torque capability to propel the engine in generator high speed which coincides with vehicle high speed. This leads us to the point that the definition proposed by IEEE-USA Energy Policy Committee for PHEV about the ability to drive at least 16 km (ten miles) in all- electric range and consume no petrol, is not consistence because this series-parallel physical constraint needs engine operation in vehicle high speed. Currently, EnergyCS, EDrive, and Hymotion companies offer PHEV upgrade kits for Toyota Prius and Ford Escape and Mariner [5]. Saturn Vue Green Line SUV with Two Mode Hybrid drivetrain used in GM hybrid vehicles will be the first commercialized PHEV in 2010. (a) (b) (c) Figure 1. (a) Series, (b) parallel, and (c) series-parallel drivetrain PHEVs [9] III. DRIVETRAIN COMPATIBILITY FOR PHEV Different simulation tools with backward and forward approaches or most of the time combination of them are applied for modeling of HEV and PHEV to evaluate their characteristics and compatibilities. Advance Vehicle Simulator (ADVISOR) developed in National Renewable Energy Laboratory [9] and Powertrain System Analysis Toolkit (PSAT) developed in Argonne National Laboratory [12] are two dominant simulation tools for advanced vehicles. However 2 Gao et al. [6] used a simulation software developed in The Advanced Vehicle Systems and Research Program at Texas A&M University. Other researcher have used their own simulation modeling developed in Matlab/Simulink [13]. Li et al. [14] have compared series and parallel drivetrains of an assumed mid-sized SUV with completely same sized components in ADVISOR. They have utilized a charge depletion control strategy which sets a large SOC envelop between the maximum and minimum SOC levels. Two simulations with different battery capacity, which performed on four urban dynamometer driving schedule (UDDS) and one highway fuel economy test (HWFET), resulted in different outcomes in term of overall powertrain efficiency. The first simulation with 290 kg and 60 Ah Nickel-Metal Hydride battery pack resulted in 11.2% better overall drivetrain efficiency for parallel architecture. This caused by better efficiency of electric motor operation in propelling and regenerative braking modes in parallel drivetrain. Another simulation with upgraded battery to a 418 kg and 80 Ah power showed that series powertrain passed all the drive cycle in AER and it was not necessity to turn the engine on. While the overall efficiency of Parallel configuration did not improve with upgraded battery pack, the series powertrain showed 30.5% better overall efficiency in comparison with parallel drivetrain. The series powertrain had less pollutant operation while had sluggish acceleration performance due to electric motor and battery power limitations. The study has concluded with limited onboard electric energy, the parallel PHEV overall efficiency and acceleration performance are more than series drivetrain. However, by increasing the battery capacity the series drivetrain is completely preferable [14]. Jenkins et al. [11] have investigated the correlation of the motor and battery size with fuel economy of Prius series- parallel HEV in ADVISOR. The aim of the investigation was to check the compatibility of series-parallel drivetrain to change to a PHEV. As mentioned in Introduction section, neither parallel nor series-parallel drivetrains have sized motor and battery to run in AER in high power demand of drive cycles, therefore when changing these drivetrain to PHEV, the effect of upgraded motor and battery size on efficiency should be considered. Jenkins et al. simulations showed that there is a slight fuel economy improvement if the motor upgraded to up to 75 kW and its mass goes up to 60 kg while the battery is remained unchanged and depleted from 70% to 50% SOC during the test. The other simulation showed fuel efficiency improved up to 80% by upgrading the batteries to up to 25 kW and its mass to up to 400 kg. After these optimum points, the upgrading of batteries resulted in lower efficiencies. Hymotion Prius and EnergyCS Prius were tested in the Advanced Powertrain Research Facilities (APRF) at Argonne National Laboratory (ANL) in UDDS and HWFET [15]. Hymotion Prius utilizes a Lithium polymer battery parallel to Prius NiMH battery and EnergyCS replaces Prius battery with a higher capacity Li-ion battery. The tests showed that the engine during charge depletion mode ignited in higher vehicle speeds and remained on less frequently than charge sustained mode. The operation of PHEV Prius is similar to OEM Prius during charge sustained mode. The test showed about two third and half of fuel consumption replaced by electricity in UDDS and HWFET respectively in charge depletion mode. However, the engine efficiencies of PHEV were 20% and 24.5% for cold and hot start respectively while 30.8% and 34.1% during charge sustained mode. As mentioned in Introduction section, series-parallel drivetrain needs engine operation in higher vehicle speeds because of generator maximum speed constraint so the electric energy consumption is reduced if the vehicle is mostly used in highway drive cycles. The Hymotion battery has enough energy capacity to run in four UDDS or HWFET cycles in charge depleting modes. The temperature of the engine has significant effect on its combustion efficiency and emission and is important to maintain the catalyst operative temperature. This factor should be considered in power management of PHEVs. Freyermuth et al. [8] have compared all three PHEV configurations in PSAT. The components of a midsize sedan in each drivetrain sized to meet following performances: • 0-60 mph < 9 s • Gradeability 6% at 65 mph • Maximum speed > 100 mph Two different 16 km (10 mile) and 64 km (40 mile) all- electric range in UDDS were assumed for sizing of battery. The component sizes were different because of the mentioned sizing procedure which is unlike similar component sizing in [14]. In urban driving condition, series-parallel showed best fuel economy in comparison with series and parallel configuration. Parallel drivetrain had completely better efficiency in 16 km AER in comparison with series one whereas in 64 km AER parallel and series performances were almost similar. In highway driving condition, series-parallel and parallel architectures showed similar and better efficiency in comparison to series architecture. The engine efficiency of series PHEV was the best since the engine performs independent of wheel speed. Series-Parallel as well had better engine efficiency in comparison with parallel but, because of power recirculation especially in high vehicle speed, had similar overall efficiency to parallel configuration. The difference in results in comparison with [14] is due to different sizing approach and control strategies. As PHEV idea solved the problem of low efficiency of series powertrain at least during AER, we can say that similar to what Li et al. [14] asserted, if the high capacity battery is available, the series drivetrain is appealing. Volt GM is selected this drivetrain option with a 64 km AER battery capability. As a PHEV customer, if your daily commute is less than all electric range of series PHEV, you rarely pay for petrol refills. Series-Parallel has a more complicated configuration that we can strongly say has the most efficient charge sustained mode when acts as a traditional HEV and completely competitive charge depletion mode. In spite of the fact that we cannot define AER for this drivetrain in high speed where engine ignition is inevitable, the charge depletion mode is completely efficient. When talking about the overall performance of PHEV, performance will vary dramatically, depending on driving style and driving conditions than conventional hybrids. This is due to the added weight of a large 3 battery that once depleted in all electric range or charge depletion mode is just an extra load. IV. POWER FLOW CONTROL STRATEGY After The full advantage of the PHEV powertrain is gained through an appropriate power flow control strategy. The controller determines operating points for each component and transfers the adequate commands to the local controller of each subsystem. In conventional HEV, the aim of power strategy is to maintain the battery SOC in adequate range with consideration for the battery health. However, Grid charged battery of PHEV offers the option of using electricity and fuel energy simultaneously in which using of stored electric energy is preferable. The first option for power management of PHEV is to run the vehicle on pure electric mode until all energy stored in battery depleted, which is the definition of AER. Afterward, the vehicle acts as a conventional HEV in charge sustained mode to steady the SOC. If the distance of journey between recharging is less than the defined PHEV AER, then the most efficient mode of operation is just electric mode which does not use a drop of petrol. However, in real condition, many commutes are longer and sometimes the power demand is higher than battery capability which means inevitable engine operation. The surveys in [6, 16] have presented charge depletion (CD) strategy, using both battery and engine simultaneously, would be more efficient in comparison with simple AER followed by CS control strategies if the journey exceeds AER. Gao et al. has suggested two different Electric Vehicle/Charge Sustained (EV/CS) and blended control strategies for parallel configuration [6]. They have suggested a manual shifting option between EV and CS for driver. In EV, the vehicle uses the stored energy in battery aggressively and in CS, SOC of battery sustained around specific value. In blended control strategy or charge depletion mode, both engine and motor operate simultaneously. In this strategy, engine is constrained to operate in its efficient region as illustrated in Fig. 2. The engine is controlled as no surplus energy remains to charge the battery to prevent charging and discharging waste. When required torque is higher than top torque boundary, and the engine is controlled to operate on this boundary so that the remaining demand power is supplied by electric motor. The engine solely propels the wheel if demanded torque is between the boundaries. Engine bellow the bottom torque boundary turns off and the vehicle runs in pure electric mode. In [16], four different control strategies were simulated and compared for a series-parallel PHEV with 16 km AER battery pack in PSAT for a vehicle with similar performances with Freyermuth et al. model in [8]: 1. Electric Vehicle/ Charge Sustaining (EV/CS) 2. Differential Engine Power 3. Full Engine Power 4. Optimal Engine Power In EV/CS mode, engine only turns on when the power demand is higher than available power of battery. Differential Engine Power is similar to EV/CS but the engine-turn-on threshold is lower than maximum power of electrical system. In Full Engine Power, if the engine turns on it will supply all the power demand of the drive cycle and no power will drain from battery. The aim of this strategy is to force the engine to operate in higher power demand and consequently in higher efficiency. Optimal Engine Power Strategy similar to previous strategy seeks to propel the engine more efficiently in higher power by restricting the engine operation close to peak efficiency. Engine-start threshold can be derived from simulation for different predetermined journey distances.