SAE 42 Volt Paper

SAE TECHNICALPAPER SERIES 2001-01-2498 Simulation of Higher Voltage Vehicles to Optimize Performance Erik H. Gaarder and Robert C. Borregard Visteon Corp. Reprinted From: 42 Volt Technology and Advanced Vehicle Electrical Systems (SP–1636) Future Transportation Technology Conference Costa Mesa, California August 20-22, 2001 400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 The appearance of this ISSN code at the bottom of this page indicates SAE’s consent that copies of the paper may be made for personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay a $8.00 per article copy fee through the Copyright Clearance Center, Inc. Operations Center, 222 Rosewood Drive, Danvers, MA 01923 for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. 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Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published in SAE Transactions. For permission to publish this paper in full or in part, contact the SAE Publications Group. Persons wishing to submit papers to be considered for presentation or publication through SAE should send the manuscript or a 300 word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE. Printed in USA 2001-01-2498 Simulation of Higher Voltage Vehicles to Optimize Performance Erik H. Gaarder and Robert C. Borregard Visteon Corp. Copyright © 2001 Society of Automotive Engineers, Inc. ABSTRACT FOCUS OF SIMULATION This paper describes the simulation of a bi-tensional (14 Volt/42 Volt) hybrid vehicle. The focus is on gaining a better understanding of vehicle sub-system interactions through simulation. Sensitivity to assumptions on the simulated performance of the vehicle is examined. Variations in the simulation will focus on energy storage, power generation, motoring, and load management. Optimization of system interactions, relative to the desired vehicle performance and function, are considered in the conclusion. This paper focuses on gaining an understanding of a bitensional Integrated Starter/Generator (ISG) vehicle. This architecture is a likely new higher voltage vehicle system to be introduced in the next decade. Vehicle manufacturers are moving towards higher voltage electrical systems to improve a vehicle's electrical load carrying capacity. Higher voltage electrical systems will assist in handling the steadily increasing vehicle electrical demand in vehicles and enable new technologies. INTRODUCTION SIMULATOR REASONS FOR SIMULATION The modeling tool used for these vehicle simulations is ADVISOR. ADVISOR is an Advanced Vehicle Simulator developed by the National Renewable Energy Laboratory (NREL) [1]. ADVISOR contains models of various vehicle types, such as conventional, series hybrid, and parallel hybrid. ADVISOR has been extensively validated, and numerous studies have been performed using this simulator [2]. Various pre-existing control strategies are available for electric launch assist, engine stop/start, transmission shifting, etc. ADVISOR's Graphical User Interface (GUI) provides easy access to model variables and constants. Changing vehicle parameters for parametric studies, performance assessments and tradeoff studies is expeditious. This simulation package is 1 1 based on Simulink block diagrams and Matlab script data files, allowing one to manipulate model components and easily extend or alter existing models. The advent of higher voltage vehicle systems creates an increased need for vehicle level simulation due to higher system complexity. One simulates a vehicle system to gain an understanding of that system's behavior. If the model adequately describes reality, it will provide insight into the interactions that occur within the system, and assist in evaluating various strategies for the operation of the system. This results in substantial cost and time saving versus actually building alternative vehicle configurations. It is vitally important to validate the simulation results with real world data. This initiates a continuous cycle of simulation, validation of results, additional simulation, etc. 1 Matlab and Simulink are registered trademarks of The MathWorks, Inc. regenerative braking and torque boost strategies, as compared to the base ADVISOR model. Figure 1: ADVISOR's parallel starter/alternator vehicle model LOAD MANAGEMENT Figure 1 shows the top-level Simulink block diagram for a parallel starter/alternator vehicle model. From this diagram one can see how requirements start at the drive cycle and flow through the vehicle system until they reach the fuel converter or energy storage subsystem models. What is achievable by each subsystem model flows back in the opposite direction until it reaches the initial drive cycle. If the vehicle is unable to meet the drive cycle, this is shown by the resulting achievable flow. The ability to dynamically control all the mechanical and electrical loads in the vehicle system was added. Instead of a single number for the mechanical or electrical vehicle loads, the enhanced model can control each load separately, and allow it to vary as a function of other variables in the model. This allows one to more accurately study different load management strategies and their effects. To fully utilize this ability, one needs to augment the current standard drive cycles to include information such as steering events, climate conditions, etc. THE MODEL SIMULATION In order to simulate variations to energy storage, power generation, motoring, and load management, the standard ADVISOR models had to be enhanced. ADVISOR was updated in the following way. A small car (~1250kg), 1.6L, 4-cylinder engine, with a manual transmission was chosen as the baseline vehicle. All simulations were performed under the same drive cycle; EPA City Cycle, with accessory loads added to help illustrate sub-system interactions. Accessory loads were considered under worst case operating conditions. Ambient temperature was assumed to be 20 degrees Celsius (293 degrees Kelvin). ENERGY STORAGE ADVISOR was enhanced with the addition of two new energy charge strategies. The enhanced model has the ability to control energy storage by voltage control, with the objective of maintaining the power bus voltage at a certain set point. The enhanced model also has the ability to control energy storage by State of Charge control, with the objective of maintaining the energy storage State of Charge (SOC) at a certain set point. POWER GENERATION & MOTORING The motor model within ADVISOR was enhanced to more accurately simulate an Integrated Starter/Generator (ISG). User control over apportionment of required torque to the engine and motor was added. This allows increased flexibility of Eight simulations were performed. During all eight simulations the primary goal was to meet the EPA City drive cycle. A secondary goal was for the battery to be at a State of Charge (SOC) of 100% at the end of the drive cycle, unless otherwise stated. The resulting performance (fuel economy) achieved for each simulation is tabulated in the Results section of this paper. SIMULATION 1 (BASELINE VEHICLE) A baseline vehicle simulation was performed to establish reference data to compare changes to. This vehicle simulation was performed under the following conditions: Mechanical Accessory Loads - Pulley driven hydraulic steering pump and pulley driven engine water pump. (Note: Air Conditioning was not considered as a load for the base vehicle.) Electrical Accessory Loads - Worst case, rainy day loads including a two speed electric engine cooling fan, all supporting electronic modules, signal lamps, driving lamps, headlamps, fog lamps, instrument panel lamps, instrument cluster, heater blower motor (medium speed), front wiper motor, rear wiper motor, fuel pump, and supporting engine control loads. Battery - 12 Volt, 26 Amp-hour, lead acid, initial State of Charge (SOC) at 99%. Baseline charge strategy pursues 100% state of charge at all times. Alternator - 14 Volt, 65A at 1800 rpm, 110A at 6000 rpm, 64% peak efficiency, pulley ratio of 2.66 to 1. SIMULATION 4 (ISG VEHICLE WITH MECHANICAL A/C) Auxiliary loads can drastically affect the fuel economy of a vehicle [4]. During our initial simulations we did not include the air-conditioning system. Air conditioning is one of the largest accessory loads in the vehicle, but is not run in any of the standard fuel economy drive cycles. In this simulation a mechanically driven air conditioning compressor was added to the torque load of the initial ISG vehicle. All of the air conditioning simulations were performed under worst-case air conditioning compressor load. For these air conditioning simulations it was assumed that the initial interior temperature was 60 degrees Celsius (333 degrees Kelvin) and that the air conditioner, during the drive cycle, worked to bring this temperature down to 22 degrees Celsius (295 degrees Kelvin) and then maintain this temperature. SIMULATION 5 (ISG VEHICLE WITH ELECTRIC A/C) SIMULATION 2 (INITIAL ISG VEHICLE) To illustrate the effect of adding an ISG to the baseline vehicle, and removing the mechanical loads (any load which is driven from the front end accessory drive), an initial ISG simulation was performed. This simulation was performed under the following conditions: Mechanical Accessory Loads - None. Electrical Accessory Loads - Same as baseline vehicle with the exception of added load for electric water pump and electric power assist steering. (Note: As for the baseline vehicle, air conditioning was not considered as a load for the initial ISG vehicle.) Battery - 36 Volt, 26 Amp-hour, lead acid, initial State of Charge (SOC) at 99%. This battery is the equivalent of three baseline vehicle batteries connected in series. The charge strategy used is the same as for the baseline vehicle. Integrated Starter/Generator - 42 Volt, 60A at 700 rpm, 215A at 1200 rpm, 60A at 6000 rpm, 92% peak efficiency, and inline crankshaft mounted. SIMULATION 3 (ISG VEHICLE WITH STOP/START) A main benefit touted for ISG vehicles is the ability to provide stop/start operation. Stop/Start is a control strategy that turns off the engine when the vehicle speed is zero and the engine coolant has reached normal operating temperature. When the vehicle is about to move the engine is quickly restarted. Stop/Start operation provides both fuel economy and emissions benefits, under certain drive cycle and vehicle conditions. In this simulation the initial ISG vehicle was run with stop/start operation enabled. In this simulation an electrically driven air conditioning compressor was added to the electrical load of the initial ISG vehicle. SIMULATION 6 (ISG VEHICLE WITH ELECTRIC A/C AND STOP/START) A simulation of the initial ISG vehicle with an electrically driven air conditioning compressor added to the electrical load along with allowing stop/start operation was performed. During stop conditions, it was assumed that the air conditioning continued to operate, and maintain the interior vehicle cabin temperature. SIMULATION 7 (ISG VEHICLE WITH TORQUE BOOST) A simulation of the initial ISG vehicle was performed with a modified vehicle control strategy so that 10% of required driveline torque was provided by the ISG during engine accelerations between 600 rpm and 1500 rpm. This strategy reduces the demand upon the engine to meet the required drive cycle by utilizing the ISG to provide torque boost. SIMULATION 8 (ISG VEHICLE WITH REGENERATIVE BRAKING) A simulation of the initial ISG vehicle was performed with a vehicle control strategy to provide regenerative braking during all vehicle decelerations. A battery state of charge control strategy was used, with SOC targeted at 0.98, and bus voltage limited to 48V. The targeted SOC was set at 0.98 to allow room for accepting energy from regenerative braking and not degrade the life of the battery. RESULTS Fuel economy results for the simulations are tabulated below. Simulation (1) Baseline vehicle Fuel Economy % Change from Baseline vehicle 30.0 mpg (7.8 L/100 km) (2) Initial ISG vehicle 31.4 mpg 4.7% (7.5 L/100 km) (3) ISG vehicle with Stop/Start 32.7 mpg 9.0% (7.2 L/100 km) (4) ISG vehicle with mechanical A/C (5) ISG vehicle with electric A/C (6) ISG vehicle with electric A/C and stop/start (7) ISG vehicle with torque boost (8) ISG vehicle with regenerative braking 26.0 mpg -13.3% (9.1 L/100 km) 22.0 mpg -26.6% (10.7 L/100 km) 23.6 mpg 2 -21.3% (10.0 L/100 km) 31.1 mpg 3.7% (7.6 L/100 km) 32.6 mpg 8.7% (7.2 L/100 km) loads have on this hybrid vehicle. Numerous studies have been done on the effects of auxiliary loads on hybrid vehicles [3]. This paper shows the sensitivity associated with load management. In Simulation #2, the main contributor to the fuel economy gain was the conversion of two mechanical parasitic loads to electrical loads. Smaller average accessory torque loads due to changing the engine coolant water pump and steering assist from mechanically driven to electrically driven have a positive 1.2-mpg (0.30 L/100 km) effect on fuel economy. The other two main factors that affected the performance of the vehicle in simulation #2 were increases in mass and generator efficiency. The increase in mass (bigger battery, ISG, and inverter) had a negative 0.4-mpg (0.09 L/100 km) effect on fuel economy performance. The increased generator efficiency of the ISG compared to the alternator had a positive effect on fuel economy. Due to potential sub-system interactions, this value was not calculated. Simulations 4, 5, and 6 show air conditioning's negative impact on vehicle fuel economy. A change from mechanically driven to electrically driven accessories, with no change to duty cycle or component efficiency, has a negative effect on fuel economy. This is due to an increased loss associated with additional power conversions in the ISG and electric motor. An ISG running at a higher voltage can support more electrical loads than the traditional alternator. This ability allows one to take mechanical loads, such as the steering pump, and convert them to electrical loads, which only load the vehicle when needed. The resulting fuel economy gain more than offsets the loss in power transmission efficiency due to changing from a 98% efficient belt to an electric drive. As seen in Simulation #5, one should not arbitrarily switch a mechanical load to an electrical load. In this simulation the switch from mechanical A/C to electrical A/C was detrimental. STOP/START OPERATION DISCUSSION By examining in detail the results achieved from the previous simulations, one can highlight some of the vehicle's fuel economy sensitivities. In this paper the focus is on the direction and cause of the change to the results, as opposed to the specific fuel economy number itself. LOAD MANAGEMENT Simulations 2, 4, 5, and 6 show the large effect that 2 In this simulation the vehicle was not able to maintain 100% SOC. Battery was discharged to 85% SOC. The ISG also facilitates stop/start operation. Simulation # 3 shows that there is a fuel economy gain from stop/start operation. The main factor, which contributes to this change, is the difference between engine frictional loss and battery charge/discharge loss [4]. While the car is idling (no stop/start) there is an engine frictional loss and during stop/start operation the battery undergoes more charging and discharging cycles. It is important to note that the magnitude of the change in fuel economy, due to stop/start operation, is drastically affected by the drive cycle chosen due to its dependence on the number and length of vehicle stops. As mentioned above, the benefit from stop/start operation can be negated by other system interactions, such as loads, which are on during stop conditions. MOTORING An ISG can provide torque boost. In simulation # 7, motoring by the ISG had a negative impact on fuel economy, as compared to the initial ISG vehicle. This was due to discharging the battery during torque boost operations, recharging the battery between boosts, and increasing ISG electrical generation. These all have a negative effect on fuel economy as each incurs energy losses. This negative effect outweighs the lower fuel consumption during boosting operations. It is more efficient to propel the vehicle directly with the engine than indirectly via the ISG and battery. losses exceed the battery losses incurred during vehicle stops and subsequent recharging. If one is considering regenerative braking then the limitations imposed by the battery need to be taken into account. Torque boost, or motoring with the ISG has a negative effect on fuel economy. Opportunities for further study would be to downsize the engine and utilize torque boost to meet the drive cycle requirements. A vehicle using a downsized engine and employing torque boost to meet drive cycle requirements would theoretically have a fuel economy benefit. ENERGY STORAGE AND POWER GENERATION ACKNOWLEDGMENTS The limitations imposed by the vehicle's energy storage and power generation devices play a role in fuel economy benefits. Simulation #6 shows how the ISG was unable to support the additional loads. With an electric A/C the vehicle was unable to recover from stop situations and the battery was depleted to 85% SOC. With the power generator and energy storage device chosen in this simulation one cannot combine electric A/C and stop/start operation over this drive cycle and chosen strategies and expect satisfactory vehicle performance. Simulation #8 shows the fuel economy gain due to regenerative braking. This positive change is attributed to the energy recovered from vehicle deceleration, which would otherwise be lost during a braking operation. The gain, due to regenerative braking, was limited to the ability of the battery to receive charge. A higher charge acceptance battery would be able to recoup more of the vehicle's deceleration energy and result in better fuel economy. CONCLUSION A systems viewpoint of the vehicle is necessary when optimizing for vehicle performance. Not only should the conflicting objectives of each sub-system be analyzed, but also the synergies to be gained should be exploited. The authors would like to thank Arthur J. Gajewski, Roy D. Schultz, and Shawn H. Swales for their assistance with this paper. REFERENCES 1. Wipke K.B., Cuddy M.R., Burch S.D., “ADVISOR 2.1: A User-Friendly Advanced Powertrain Simulation Using A Combined Backward/Forward Approach”, IEEE Transactions on Vehicular Technology: Special Issue on Hybrid Electric Vehicles, Nov. 1999. 2. Ogburn M.J., Nelson D.J., Wipke K.B., Markel T., “Modeling and Validation of a Fuel Cell Hybrid Vehicle”, FutureCar Congress 2000, Washington D.C., SAE Paper # 2000-01-1566. 3. www.ott.doe.gov/coolcar/ 4. Schmidt M., Isermann R., Lenzen B., Hohenberg G., “Potential of Regenerative Braking Using an Integrated Starter Alternator”, SAE 2000 World Congress, Detroit, MI, SAE Paper # 2000-01-1020. DEFINITIONS, ACRONYMS, ABBREVIATIONS This paper has shown that for an ISG vehicle one should pay particular attention to load management, motoring, energy storage, and stop/start strategies and their interactions. Accessory loads should be minimized by reducing parasitic accessory loads whenever possible. Electrically driven air conditioning compressors should be avoided or the engine should be left running with a mechanically driven compressor during vehicle stops. Energy storage capabilities and strategies play a crucial role in enabling new higher voltage technologies. Stop/Start operation should be used if engine frictional A/C – Air Conditioning ADVISOR – Advanced Vehicle Simulator, developed by the National Renewable Energy Laboratory (NREL) Bi-tensional – Dual voltages (14 Volt/ 42 Volt) ISG – Integrated Starter Generator, combines the function of a generator (alternator) with that of a starter motor SOC – State of Charge