Advanced Numerical Simulation for Hybrid and Electric Vehicles

Transcription

1 A W H I T E P A P E R F R O M A N S Y S, I N C. Advanced Numerical Simulation for Hybrid and Electric Vehicles Engineering the Future of the Auto Industry with Integrated Multiphysics Simulation Software By Scott Stanton, Technical Director, and Sandeep Sovani, Manager of Global Automotive Strategy, ANSYS, Inc. With concerns over air pollution and petroleum supplies, the use of hybrid electric vehicles (HEVs) and electric vehicles (EVs) have come to the forefront as alternatives to conventional gasoline and diesel engines. Governments worldwide are promoting HEV/EV research. The United States government has announced $2.4 billion in funding for new designs of battery packs, electric motors and other components, setting the goal of 1 million HEVs on the road by the year 2015 [1]. The U.S. Department of Energy predicts that by 2030, alternative vehicles will comprise 28% of the total U.S. light-duty cars and trucks a 20 percent increase from 2005 [2]. To meet these increased demands for HEV/EV applications, competition is intense to develop improved and cost-effective electric powertrains. The potential payoff is enormous in such development efforts, as are the business risks in going to market with flawed, inadequate or suboptimal designs. Clearly, a revolution is underway in automotive technology. The responsibility for leading the charge has been placed squarely in the hands of automotive engineers, who must completely rethink how they approach powertrain design. For engineers at automakers as well as suppliers of major subsystems and components, the challenge is to conduct a massive amount of research and development on an entirely new generation of powertrain in a highly compressed timeframe. In meeting these demands, leading automotive companies with HEV/EV initiatives are focusing on development efforts driven by simulation rather than outdated methods of trial-and-error prototype testing. Indeed, effective implementation of advanced numerical simulation will likely separate winning organizations from less adept competitors in the race for designing the next generation of improved electric powertrains. Numerous software solutions are available for the diverse types of analysis needed in such development work including mechanical, electrical, electromagnetic, electrochemical, WP 41

2 fluid and thermal management applications. In general, these separate packages often are not entirely compatible, hampering engineers in efficiently using the full range of technologies in arriving at optimized electric powertrain designs. This paper discusses the value of implementing a full range of these technologies as an integrated suite of best-in-class software operating in a single unified environment. MAJOR CHALLENGES FACING HEV POWERTRAIN DESIGNERS Today s automotive engineers are challenged with designing new electric powertrain technologies almost entirely from scratch. Key components included in this task are electric battery packs, electric traction motors/generators and power electronics. The design of these HEV components involves complex physical problems and an enormous amount of challenging system integration. Below, the challenges in development of individual components are discussed as well as relevant considerations of electromagnetics EMC/EMI. BATTERY PACKS Electric batteries in HEV/EVs perform the double duties of providing primary drive power for the vehicle as well as energy for numerous electric-powered auxiliary systems. Therefore, they must meet the same reliability, durability and affordability standards and expectations set by petroleum fuel cars beyond that, they must provide orders of magnitude more energy than conventional batteries. HEV battery pack cooling flow paths with As engineers design batteries with large energy capacity and greater power output, they temperature distribution on cells must consider the thermal, structural and electromagnetic influences on the battery pack as well as the cells within. For example, batteries generate heat while charging and discharging. The temperature of all cells within the battery pack must be strictly maintained within a few degrees C of each other. Otherwise, harmful internal current loops can form within the pack that drastically shorten battery life. This necessitates a cooling system whether by air or liquid and sometimes creates a side challenge of minimizing noise close to the passenger cabin. Drivers of HEV/EVs expect an ultra-quiet driving experience, which is not compatible with a loud cooling system. Engineers must also take into account the physical placement of an electric battery pack within an HEV as well as stresses the battery will experience under a range of driving conditions. The battery must be designed to safely withstand multiple variables such as external heating, over-charging, over-discharging, nail penetration, crush or external short. The same safety goals apply to crash scenarios, in which passengers must be protected from toxic acids released from the battery during such an event. MOTOR/GENERATOR For years, automakers invested relatively little time and money in electric machine (that is, the electric traction motor/generator) design because the internal combustion engine was so widely used. These conventional engines accomplished what they needed to: Consumer requirements were met, emissions regulations were not as stringent, and oil prices were not a concern. Today all that has changed, with a huge amount of interest in new motors and a correspondingly huge pressure on companies to develop the most efficient, cost-effective electric design. HEV battery pack cooling flow paths with temperature distribution on cells FEA mesh for shaft motor-generator Courtesy Kato Engineering. Brainpower and investment dollars are flowing into this area, and the electric motor, just as with the electric battery pack, poses its own set of design challenges. WP 4 2

3 The motor/generator plays an essential role in the propulsion of the vehicle. It also recharges the battery via regenerative braking. HEV/EV traction motors are different from all other motors because they must work reliably in a very demanding physical environment. Motors must operate consistently under extreme hot and cold temperatures, severe vibrations, hard duty cycles and rough road conditions. In an HEV, an electric motor is also exposed to high temperatures produced under the hood by the engine. All of these variables must be thoroughly addressed in motor design. Reliability is a crucial selling point for auto customers; poor engine performance increases warranty costs and quickly degrades a vehicle brand. Customers also expect high fuel efficiency from HEV/EVs. Fuel efficiency, low emissions, safety and performance aspects of a vehicle drive consumer purchasing decisions and, therefore, directly affect market success. Because the electric motor design determines how much electrical energy provided by the battery is transformed into physical energy used to run the vehicle, designing a highly fuel-efficient motor is one of the most important challenges HEV/EV powertrain engineers face today. POWER ELECTRONICS Power electronics are equivalent to the combined heart and the brain of an electric powertrain system. They must precisely control the power transfer between the battery and the motor/generator and also implement the logic to adjust the powertrain to various driving conditions and driver inputs. To operate at the highest efficiency under a variety of driving conditions, power supplied to the traction motor needs to be carefully controlled at a relatively high switching frequency through devices such as insulated gate bipolar transistor (IGBTs) based on position, speed, temperature, etc. via feedback continuously received from sensors monitoring the motor. Motor controller electronics Thermal management is a major concern with power electronics in HEVs. The entire power delivered by the electric powertrain to the wheels (as well as the power needed to recharge the battery) has to travel through the power electronics. Therefore, even the slightest power loss in the electronics creates a large amount of heat. The heat needs to be carefully managed and dissipated under a wide range of operating conditions (such as driving in a hot desert or in subzero winter conditions) to avoid heat damage to the power electronics and nearby components. Optimally, the electric losses in the electronics need to be accurately calculated, and heat dissipation paths need to be identified and designed to ensure effective cooling. EMI/EMC One of the primary challenges in power electronics development is management of electromagnetic interference and electromagnetic compatibilty (EMI/EMC). Since the power supplied to the motor needs to be controlled at relatively high switching frequencies, the EMI between the various electrical components becomes an important concern. If EMI is unaccounted for, it will destroy signals and prohibit the motor from operating. Therefore, EMI effects need to be carefully studied and accounted for in the control logic. This requires a comprehensive study of the electromagnetic fields in and around the motor, busbars, and nearby components while these components are operating in an interconnected, coupled way as in real-life. EMI/EMC of an IGBT WP 4 3

4 PUTTING SIMULATION TECHNOLOGY TO WORK Multiphysics simulation software allows engineers to understand how a design will perform under various loading conditions before prototyping takes place. Not only can physical, real-life scenarios be modeled with accurate simulation, but the effects of and interactions between fluids, mechanics, thermal physics, electrochemistry and electromagnetic forces can be simulated and the design adjusted based on those models. In this way, designs can be generated faster and systems can be optimized up front in the cycle to avoid surprises and problems that might occur in the later stages of product development. Simulation tools for HEV/EV development span a wide range and include mechanical, fluid dynamics, thermal, electrical and electromagnetic issues. These tools can be used in tackling the challenges of developing individual powertrain components electric battery packs, electric traction motors/generators and power electronics as well as the tremendous complexities when these subsystems are integrated into the complete vehicle powertrain. BATTERY PACK SIMULATION Battery thermal management is a top priority in HEV/EV development to avoid overheating that reduces power-generating efficiency and shortens service life. For cylindrical cells, engineers typically employ an air cooling strategy in which pack housings are shaped for optimal cooling provided by a blower and guiding vanes to direct an adequate airflow. For rectangular cells, cooling generally is accomplished using liquid circulating through heat exchanger elements in contact with cells. A control algorithm is used to vary loads on different cells based on temperatures and charger status. Mesh for air-cooled cylindrical cell module (left) and cooling flow velocity countour(right) In evaluating and optimizing the various thermal management configurations, parameterization and methods such as design of experiments are used in combination with fluid dynamics solvers for analyzing the complex 3-D cooling flows and conjugate (solid-to-fluid) heat transfer. For evaluating pack performance for long driving cycles, the linear time invariant (LTI) method is useful for efficiently performing such real-time simulations. Engineers can apply electronics circuit simulation technology in evaluating the control algorithms for studying overcharging, high-current charging/discharging, external shorts or other electrical problems that could reduce battery life and risk battery explosion. The software is ideal for studying such algorithms due to its ability to tightly integrate 3-D physical models (fluid dynamics and mechanical) into the control circuit simulation. For solving structural problems caused by incidents such as a crash or foreign body penetration of the battery pack, structural mechanics software can be leveraged to evaluate the structural integrity of the assembly to prevent toxic battery contents from escaping or damage to cells that could cause thermal runaway and battery explosion. Such virtual prototyping is also useful in studying vibrations as well as durability and fatigue life of the battery pack. WP 4 4

5 MOTOR/GENERATOR SIMULATION In developing the motor/generator, the design team must focus considerable attention on the electromagnetics of this electric machine. From initial CAD drawings and related engineering specifications of the assembly, electronics design optimization software can be used to define the main elements of the motor/generator including magnet materials, coil configurations, number of turns, air gaps and more. Parasitic extraction tools can be used to compute the machine s electrical properties. These outputs can be entered into electromagnetic field simulation software, which computes the torque profile of the machine that is, how the torque ramps up over time for driving the vehicle in motor mode as well as electrical resistance in stopping the vehicle in brake mode. Vehicle weight is brought into the analysis to determine acceleration as well as stopping time for various scenarios. Based on this output, the team can then modify the design by changing any of the basic design para-meters (magnet size, for example) to balance machine performance against its size, weight and cost. The computed torque output may be used further in structural mechanics software for computing stresses, loads, deformations and vibrations of the physical parts of the powertrain including the driveshaft and gearing. Vibration analysis is important because tractions can be a promiment source of noise in EVs, which are expected to be be quiet by nature. Fluid dynamics analysis may be used for studying thermal management issues, mapping energy losses, and determining heat distributions in the motor/generator assembly. Throughout the electromagnetics and mechanical development processes, integrated multiphysics software coordinates the actions and exchanges of data between the various tools in the many computations performed for different load scenarios and in comparing various design alternatives. This multiphysics cosimulation process is facilitated by the software all running on a single unified environment with a smooth flow of data between programs. POWER ELECTRONICS SIMULATION For thermal management of the HEV power electronics, engineers enter representations of IGBT characteristics (switching voltages, current waveforms, etc.), control algorithms (for turning the IGBT on and off) and the motor/generator into a power electronics circuit simulation software for virtual analysis. From this data, the software determines how the levels of electrical current flowing through the entire system vary at given times for vehicle acceleration, cruising and braking. Using electronic thermal current tools, engineers then specify the geometry of the major heat sources in the powertrain system (IGBTs and current-carrying parts of the motor/generator). Through parametric analysis, each heat source is applied individually IGBT temperature distribution at major points of interest in the system, with air circulation and conducted thermal energy taken into consideration. The software then processes this data and generates a thermal model, which engineers use to determine overall temperature profiles of each IGBT together with temperature-dependent performance variables, such as energy drained from the batteries to ensure that heat levels do not exceed specified limits to adversely affect IGBT performance. From this temperature profile, engineers can utilize the thermal structural analysis capabilities of FEA software to determine the resulting thermal stresses. Electronic design analysis tools can be applied to calculate electromagnetic forces acting on motor/generator components to determine deformations and mechanical stress distributions on the structure. Engineers can then modify the structure to eliminate stress concentrations and excessive deformation, or conversely, to lighten regions that may have been overdesigned with excess material. WP 4 5

6 EMI/EMC SIMULATION In HEV/EV development, switching speeds of IGBTs ranging from tens to hundreds of khz with turn-on rise times and turnoff fall times in the order of 50 nanoseconds to 100 nanoseconds can cause two major electromagnetic problems. Conducted emissions (through current-carrying structures) can cause power integrity issues or set up reflected waves of energy that can potentially damage the inverter and the motor. Radiated electromagnetic fields (through air) can affect the rest of the vehicle s many electronic systems. Both types of interference problems must be considered. Engineers must design for electromagnetic compatibility/electromagnetic interference (EMI/EMC) in vehicles. To accurately characterize the behavior of a switching device such as an IGBT, engineers typically begin by using a parameterization wizard that takes into account performance curves and tabular data from vendor-supplied specification sheets. This process automatically extracts the required parameters to aid in creating a semiconductor circuit model of the IGBT. Next, the physical layout of the power inverter is imported from CAD geometry into parasitic extraction software, which then computes the frequency-dependent resistance, partial inductance and capacitance (RLC) along the conduction paths. The tool is used to create an equivalent circuit model for system simulation. Results of these simulations can be used to examine radiated emissions, enabling engineers to calculate the field intensity at any given point in space to determine if the inverter package is in compliance with federal and international standards. If allowable limits are exceeded, EMI/EMC problems in the inverter system can be traced to the source in the physical layout of the device. The design can then be parametrically varied and a series of simulations performed until conducted and radiated electromagnetic emission levels are acceptable. SYSTEM INTEGRATION System integration is perhaps the largest challenge in electric powertrain development. Each component of the electric powertrain has unique characteristics, attributes, strengths and other complexities that must be taken into account. The objective is to ensure that the entire electric powertrain performs at the highest overall efficiency under a wide variety of loads and operating conditions experienced in real-life driving scenarios. Since subsystems and components work together in a coherent, tightly coupled way, they cannot be developed entirely in isolation from one another. Rather, the performance of each subsystem must be carefully matched with those of all others. System simulation spanning electromagnetics, thermal, fluid, and structural physics To successfully simulate such complexities in the HEV/EV powertrain, simulation solutions must be in the form of an integrated platform capable of multidimensional, multiphysical and multiscale simulation, providing the technology needed in addressing the many mechanical, fluid, electrical, electrochemical and electromagnetics issues of these complex powertrain systems. Multidimensional indicates a system comprising subsystems and components governed by a mixture of physical phenomena that could be 0-D (circuit logic and block diagrams, for example), 1-D (such as modeling flow through long channels), 2-D (stresses on shells, etc.), 3-D (problems such as flow through complex 3-D passages), or 4-D (having time variation of 3-D flow, stress, thermal magnetic, etc. fields). WP 4 6

7 Multiphysical indicates that a system or component is governed by more than one physics. A battery pack, for example, is governed by fluid flow, heat transfer, electrochemistry, structural stress/strain distributions, electric and magnetic fields. Multiscale means a system has important physical phenomena occurring at different physical scales. In a battery pack, for example, electrochemical reactions occur at a nanoscale, whereas heat transfer and cooling flow are at a macro scale; the battery controller works at the pack level; and the pack has to operate coherently with the rest of the powertrain at the vehicle level. Simulation in Action: Toyota Prius Powertrain To demonstrate the speed and accuracy of their electromagnetics simulation approach, engineers at ANSYS, Inc. performed a study based on design and performance data published by Oak Ridge National Laboratory in a report on the Toyota Prius THSII traction motor [3]. Engineers first created an FEA model of the powertrain system including battery, IGBT inverter, traction motor and control system. Next, they parameterized the model, which they applied to Maxwell software from ANSYS for a range of different boundary conditions such as applied current, voltage and rotor position. From this information, Maxwell then calculated multiple values of motor output including torque, inductance and mechanical losses in the system. Together, the data comprises a physics-based solution domain map essentially a series of points defining motor performance output for given electrical inputs. Motor transient FEA Using the solution domain map as a look-up table to characterize the motor, the team then created an equivalent circuit model for a parametric system-level simulation in Simplorer software from ANSYS to calculate motor torque and the other output parameters. Simulation predictions from this system-level approach correlated well with the published motor output data, demonstrating the accuracy of the method [4]. The advantage with this system-level approach is that motor output predictions can be performed almost instantaneously and with the same accuracy as the FEA models, which each take hours to mesh and solve. The approach thus has the potential to be used by automotive development teams to quickly evaluate and optimize electric traction-motor designs in lieu of numerous physical prototype test cycles. Predictions are viewed as especially beneficial in the early stages of motor development to quickly explore various combinations of control strategies, shaft sizes, magnet types, etc. in optimizing motor WP 4 7

8 MOVING AHEAD OF THE COMPETITION The pace of HEV/EV development is incredibly fast, with OEMs and suppliers alike having to meet challenging demands in a market with huge potential gains for cutting-edge companies and monumental risks for stragglers. In this intense environment, automotive engineers are challenged with designing extraordinarily complex next-generation electric powertrains within demanding timeframes that cannot be met using the slow and inefficient trial-and-error prototype testing methods. Instead, multiphysics-based simulation-driven development must be used to balance the intricate, interdependent and often conflicting mechanical, electrical, electromagnetic, fluidic and thermal management requirements. Such an approach allows design teams to efficiently evaluate hundreds of alternatives within multiple domains, conduct numerous what-if studies, predict vehicle behavior in real-life driving scenarios, and optimize final designs faster than is otherwise possible. The process enables engineers to best develop the batteries, motor/generators, power electronics, and other various components and to tightly integrate these diverse parts so they all work together in a coherent, coupled manner. The key to such an optimal powertrain design is a multiphysics-based simulation solution in which the different software programs all work together in an equally integrated and coupled manner. Piecing together fragmented and otherwise separate and often incompatible programs simply won t make the grade in such complex applications. As the demand for HEV/EV continues to grow, a simulation solution with the breadth and depth of multiphysics technologies is not only a competitive advantage but an absolute necessity for companies that want to stay in the game. References [1] [2] Annual Energy Outlook 2007 with Projections to 2030, Report DOE/EIA 0383 (2009). [3] J.S. Hsu, C.W Ayers, C.L. Coomer, R.H. Wiles, S.L. Campbell, K.T. Michelhaugh, Report on Toyota/Prius Motor Torque Capability, Torque Property, No-Load Back EMF, and Mechanical Losses, Oak Ridge National Laboratory, Oak Ridge Institute for Science and Education, ORNL/TM-2004/185. [4] S. Stanton, D. Lin, Z. Tang, Interior Permanent Magnet Machine Analysis Using Finite Element Based Equivalent Circuit Model, IEEE Vehicle Power and Propulsion Conference 2009 proceedings pages , ISBN WP 4 8

9 ANSYS is the only simulation software provider with industry-standard mechanical, fluid dynamics, magnetics and electrical tools for complete multiphysics simulation. Tools integrated on the ANSYS Workbench platform that are used extensively in HEV powertrain development include: Simplorer multidomain system simulation software for design, modeling, analysis and optimization of high-performance systems that include electrical, thermal, mechanical, electromechanical, electromagnetic and hydraulic designs Q3D Extractor computational field solver for the calculation of frequency-dependent resistance, inductance, capacitance and conductance parameters of electrical current-carrying structures for engineers designing printed circuit boards, electronic packaging and power electronic equipment HFSS, a full wave solver for 3-D full-wave electromagnetic field simulation, providing electric and magnetic fields, currents, scattering parameters and near- and far-radiated field results. From specified geometry, material properties, and output type, the tool automatically generates an appropriate, efficient and accurate mesh for solving the problem using the finite-element method. Maxwell, a low-frequency electromagnetic field simulation tool that uses the finite element method to calculate static, frequencydomain and time-varying electromagnetic and electric fields for designing and analyzing electromechanical and electromagnetic devices such as motors, actuators, transformers, sensors and coils RMxprt, which speeds the design and optimization of rotating electric machines such as motors and generators. Templates for specific machines enable users to easily create models, assign materials, calculate machine performance, make initial sizing decisions and perform hundreds of what-if analyses in a matter of seconds. ANSYS Icepak, computational fluid dynamics software for thermal management of electronics systems that predicts heat flow and thermal transfer at the component, board or system level. Simulations include fluid flow and all modes of heat transfer (conduction, convection and radiation) for both steady-state and transient thermal flow. SIwave, which analyzes entire PCBs and IC packages for performing complete signal- and power-integrity analysis from DC to beyond 10 Gb/s. The tool extracts frequency-dependent circuit models of signal and power-distribution networks directly from electrical CAD layouts. ANSYS Mechanical, a comprehensive solution for structural linear, nonlinear and dynamics analysis including stress, deflection and vibration. A complete set of element behaviors, material models and equation solvers are provided for a wide range of engineering problems as well as thermal analysis and coupled-physics capabilities involving acoustics, piezoelectric, thermal structural and thermal electric analysis. ANSYS CFD, a product suite that offers a wide range of general-purpose and application-specific modeling and fluid flow analysis capabilities. Modeling capabilities are included to represent fluid flow, turbulence, heat transfer, laminar-to-turbulent modeling, incompressible-to-fully-compressible, and isothermal analysis for stationary and rotating devices. ANSYS Multiphysics, a wide-ranging set of engineering analysis tools for simulation of complex coupled-physics behavior. Solver technology for wide range of physics disciplines including structural mechanics, heat transfer, fluid flow and electromagnetics within the open and adaptive ANSYS Workbench framework. ANSYS, ANSYS Workbench, Ansoft, AUTODYN, CFX, FLUENT and any and all ANSYS, Inc. brand, product, service and feature names, logos and slogans are registered trademarks or trademarks of ANSYS, Inc. or its subsidiaries in the United States or other countries. ICEM CFD is a trademark used by ANSYS, Inc. under license. All other brand, product, service and feature names or trademarks are the property of their respective owners. ANSYS, Inc. Southpointe 275 Technology Drive Canonsburg, PA U.S.A ansysinfo@ansys.com Toll Free U.S.A./Canada: Toll Free Mexico: Europe: eu.sales@ansys.com 2010 ANSYS, Inc. WP 4 9