Optimal Control Strategy Design for Extending All-Electric Driving Capability of Plug-In Hybrid Electric Vehicles (PHEVs) Sheldon S. Williamson P. D. Ziogas Power Electronics Laboratory Department of Electrical and Computer Engineering Concordia University 1455 de Maisonneuve Blvd. W. Montreal, Quebec H3G 1M8, Canada Phone: +1/(514) 848-2424, ext. 8741 Fax: +1/(514) 848-2802 EML: sheldon@ece.concordia.ca URL: http://www.ece.concordia.ca/~sheldon
Outline Introduction ti PHEV topological layout and design issues PHEV energy storage system selection Test setup and control strategy design Simulation results and discussion Conclusion and future work
Introduction In case of a PHEV, the high voltage energy storage system (ESS) is usually of a rechargeable type that serves a dual purpose. It supplements the power delivered d by the internal combustion engine (ICE). At the same time, it provides partial propulsion energy from an off-board source of electricity. Despite the use of an ICE on board the HEV, it is called upon for usage only when the charge on the battery system is below a predetermined set value. Thus, it is clear that the PHEV drive train is predominantly dependant on the energy storage system.
Introduction Since PHEVs use energy from both the electric grid and gasoline or diesel fuels, they provide a distinct transition toward the development of pure electric vehicles (EVs). City driving efficiency concerns of regular HEVs can be eliminated, whereby a PHEV can be designed to be a ZEV for a specific range of miles. If the ZEV range is substantial, it is advantageous to charge the batteries from the electric power grid. This leads to lesser usage of gasoline and a slight usage of electric energy from power plants, which is a comparatively less costly procedure, in any case.
PHEV Topological Layout and Design Issues FUEL TANK INTERNAL COMBUSTION ENGINE CLUTCH (PARALLEL HEV) TORQUE COUPLER TRANSMISSION INVERTER TRACTION MOTOR CLUTCH (PARALLEL ISA HEV) BATTERY OR ULTRA- CAPACITOR BATTERY CHARGER P 1-PHASE AC SUPPLY FROM WALL N Typical layout of a PHEV with a parallel hybrid design
PHEV Topological Layout and Design Issues The PHEV drive train can be modelled either as a regular parallel HEV or as a parallel HEV with an integrated starter-alternator (ISA) option. The main advantage of PHEVs is the slightly larger battery pack (with added charge capabilities) and a slightly larger electric propulsion motor. Therefore, dependence on the low-efficiency internal combustion engine (ICE) is further reduced, compared to a regular HEV design. An important parameter used in the design of a PHEV is the hybridization factor (HF). In order to use the optimal HF efficiently, a suitable control strategy needs to be formulated.
PHEV Based Energy Storage Systems Nickel-Metal Hydride (Ni-MH) and Lithium-Ion (Li-Ion) batteries prove to be the prime options going forward. Hybridization of ESS devices is also promising; for example, high-power density ultra-capacitor (UC) usage in parallel with batteries. Both Ni-MH and Li-Ion batteries have proven cycle life under all- electric driving as well as electric-assist driving patterns. They also depict excellent deep-discharge (EV) as well as shallow-discharge h characteristics ti (HEV).
PHEV Based Energy Storage Systems Ni-MH batteries usually possess a depth of discharge (DoD) in the range of 70-80%. They depict a lifetime of about 1000-1500 charge-discharge cycles at a nominal DoDD of 80%. Li-Ion option is more promising due to its greater DoD, usually in the range of 80-90%. In addition, they also portray a higher lifetime of close to 3000 charge-discharge cycles at a nominal DoD of 80%. Li-Ion batteries also depict higher specific energies, in the range of 100-150 Wh/kg, which is twice that of Ni-MH. The major drawbacks are high cost and poor thermal properties.
Test Setup and Control Strategy Design A conservative design approach is utilized, whereby Ni-MH batteries are used instead of Li-Ion. A parallel HEV arrangement is used for a mid-sized SUV design. The electric traction motor is sized at 36kW and the ICE is sized at 95kW, which h results in a HF of approximately 27%. A total of 28 Ni-MH battery modules are used, each rated at 45Ah capacity, which altogether provide a nominal voltage of 375V. The electric traction motor is a permanent magnet (PM) type of synchronous machine.
Test Setup and Control Strategy Design In order to design the control strategy (CS), it is assumed that the vehicle is charged once a day, at the end of the day s driving. The main task of the control strategy (CS) is to manage the energy flow to and from the ESS in such a way, so as to minimize overall fuel consumption and emissions. The PHEV based CS has to utilize both BEV and electricassist type HEV operational modes. Essentially, the CS works in both charge-depleting as well as charge-sustaining modes.
Test Setup and Control Strategy Design Urban Dynamometer Driving Schedule (UDDS) or FUDS Driving Schedule Distance = 7.45 miles. Max. Speed = 56.7 mph. Average Speed = 19.58 mph. No. of stops = 17.
Test Setup and Control Strategy Design The PHEV strategy uses the ESS between 95% and 35% state of charge (SOC). Depending on the SOC level, l the vehicle is either driven in the all-electric mode or the parallel HEV electric assist mode. When the battery is at the low SOC level, the all-electric driving speed limit is set at 10 m/s. Alternatively, when the battery is at it s highest SOC level, the all-electric driving speed limit is set at 40 m/s. Tests are conducted for 60 miles and 30 miles of total driving distances.
Test Setup and Control Strategy Design The PHEV performance is compared with a regular parallel HEV design with electric assist. The ESS is allowed to behave liberally ll also in the parallel l HEV case. The electric assist parallel HEV CS uses the ESS between 90% and 50% state of charge (SOC). The electric drive train components are the same, in order to maintain fairness from a comparative point of view. Tests are conducted for 60 miles and 30 miles of total driving distances.
Simulation Results and Discussion 60 miles driving 30 miles driving Avg. Eff. = 24.30% Avg. Eff. = 22.88% ICE operating characteristics for parallel HEV
Simulation Results and Discussion 60 miles driving 30 miles driving Avg. Eff. = 29.30% Avg. Eff. = 22.77% ICE operating characteristics for PHEV
Simulation Results and Discussion 60 miles driving 30 miles driving Avg. Eff. = 78.72% Avg. Eff. = 82.85% Traction motor/inverter operating points for parallel HEV
Simulation Results and Discussion 60 miles driving 30 miles driving Avg. Eff. = 88.28% Avg. Eff. = 89.45% Traction motor/inverter operating points for PHEV
Simulation Results and Discussion Parallel HEV battery performance with 30 miles driving
Simulation Results and Discussion Parallel HEV battery performance with 60 miles driving
Simulation Results and Discussion Plug-in HEV battery performance with 30 miles driving
Simulation Results and Discussion Plug-in HEV battery performance with 60 miles driving
Simulation Results and Discussion Fuel Economies and Drive Train Efficiencies for PHEV and Parallel HEV Drive Train Arrangement Hybrid Type 30 miles driving 60 miles driving PARALLEL HEV 35.3 MPG 30.2 MPG 15.1% Dr. Tr. Eff. 14.8% Dr. Tr. Eff. PHEV 120 MPG 525MPG 52.5 33.4% Dr. Tr. Eff. 23.1% Dr. Tr. Eff.
Conclusions and Future Work PHEV 2007, Winnipeg, Canada A typical PHEV design combines the advantages of a regular parallel l HEV as well as that t of a BEV. Thus, the CS design must incorporate both charge-sustaining as well as charge-depleting characteristics. In addition, the selected ESS must possess excellent deep discharge as well as shallow discharge properties. Future investigative work includes hybridization of the ESS (either Ni-MH or Li-Ion batteries) with high-power density devices, such as UCs. Well-to-wheels analysis and comparison with regular series and parallel HEVs, to evaluate the long-term feasibility of PHEVs.
Conclusions and Future Work PHEV 2007, Winnipeg, Canada PHEVs might prove to be a great asset to a fleet of vehicles rather than serving individual purposes p