One of the more recent challenges that carmakers must address includes developing hybrid components, strategies and calibrations. Hybrids require optimizing the performance of the combined internal combustion engine and electric motor drive to meet the vehicle's fuel economy and emissions goals. Since the electric portion of the hybrid is essential zero emissions, saving fuel and reducing emissions in a hybrid starts with improving the internal combustion engine or diesel control. However, controlling the electrical portion has several new requirements.
Improvements for any aspect of an internal combustion or diesel engine that impacts fuel economy, emissions or driveability such as valve control, fuel injection, spark control, EGR, and others, have been ongoing and frequently quite radical in systems such as homogeneous charge compression ignition (HCCI) gasoline engines. The model-based control (MBC) techniques used to solve these problems are still valid for hybrids but require modifications in various subsystems. Controlling the hybrid portion, merging the two power sources, and optimizing their energy use, just makes the task more formidable. In many cases, carmakers look to the same software tools providers for support but there is room for toolmakers with unique capabilities.
SIMULATING HARDWARE IN THE LOOP
One company that has been involved with testing internal combustion, diesel, and now hybrid powertrains at various stages in the development process is dSPACE (www.dspaceinc.com). The company's simulators have evolved to handle the new technologies in the test and validation side with hardware in the loop (HIL) simulation. The simulators connect to the customer's ECUs or controllers to develop the software.
To execute electric machine models on a simulator, the different characteristics of the electric machine must be considered by the software tools supplier. “The processor hardware, the way you compute these models, the way you simulate these environments, there are drastically different requirements,” said Santhosh Jogi, technical support supervisor for dSPACE. “The tools have had to evolve to support these activities.” Furthermore, the firmware provided five years ago is not capable of performing to customer's expectations requiring improvements for cutting-edge technology.
In addition to evolution in the simulators, electric motors require new interfaces and new types of sensors. For example, resolvers for angular position sensing have been used for many years in the aerospace industry but have not been used in automotive engine control. This sensor is now commonplace in hybrid systems. The interface to resolvers and the simulation of resolvers is an integral part of testing controllers in a simulator. However, this is just one aspect of adding an electric motor to powertrain simulation.
The electric motors and insulated gate bipolar transistor (IGBT) controllers react much faster than an internal combustion or diesel engine and require a very high sampling rate. To simulate this, the motor modules must operate very fast on the order of 50 s step sizes — almost 20 times faster than the engine model. Some portions of the motor model must be simulated much faster to capture the dynamics of items such as the inverter performing the dc to ac conversion. Since the inverter design has a lot of intellectual property associated with it, designers of these systems require a platform that allows them to simulate that portion of the system very fast. “We have special hardware boards that allow us to capture the commands coming from the controller and simulate some of those machine components in hardware, in silicon, because you physically cannot do that fast enough even on a high-end microprocessor,” said Jogi. Figure 1 shows the development tools for testing the engine and the motor and transmission oil pump.
The optimum performance and fuel economy from a combined engine-motor platform allows downsizing the engine. Using dSPACE HIL simulator, FEV evaluated a 1.8-liter turbocharged engine operating with and without electric power boost (EPB) to replace a three-liter, naturally aspirated engine. As shown in Figure 2, the setup included the evaluation of an ultracapacitor (Supercap), new hardware technology for automotive applications. The hybrid control in the dSPACE simulator uses a board for computing real-time simulation and a HIL I/O board for simulating and measuring all engine signals.
Figure 3 shows the torque and vehicle velocity data for full-load acceleration from 30 km/h to 80 km/h in third gear for the 1.8 liter only and the 1.8-liter engine with electric power boost. For this test, the baseline three-liter-equipped vehicle required about seven seconds compared to 8.4 seconds for the 1.8 liter turbocharged engine and 6.4 seconds when the EPB was added. Fuel economy for the turbo-charged EPB version was 24% lower than the three liter on the new European drive cycle.
SIMULATING ELECTRIC DRIVES
Using its experience in electric machine modeling, Opal-RT Technologies (www.opal-rt.com) has developed fully programmable powertrain and vehicle simulation for ECU-in-the-loop testing. The company's RT-LAB EDS is an integrated hardware and software tool, an electric drive simulator (EDS), for electric and hybrid vehicle development. The platform allows simulation of mechanical, electrical, and control system functions including the switching circuits in the power electronics drives.
The HIL digital simulation for motor drives consists of a permanent magnet synchronous motor (PMSM) controlled by a three-phase IGBT inverter, a dc link capacitor, and a three-phase diode bridge. With features such as 10 s simulation cycle time for detailed switching models of a power-conversion module and interpolation techniques that allow 10 ns precision for modeling pulse-width modulated (PWM) carrier frequencies of up to 10 kHz, the simulator's dynamic models are especially effective in HIL testing.
The complete setup of the HIL application is shown in Figure 4. The controller portion has a control module and a PWM generator board. The vector control operates at about 50 s and the PWM carrier is varied from 2 kHz to 9 kHz. Two target processors simulate the drive circuitry for implementation in Simulink. One processor handles the ac-dc portion and the other handles the dc-ac control including the motor. A third processor is used for data acquisition.
RT-LAB integrates with The MathWorks' Simulink and simPowerSystems blockset for virtual prototyping. Other simulators from RT-LAB that model vehicle subsystems such as fuel cells, IC engine, and transmission can be merged with the EDS to study system interactions and trade offs.
Plug-in hybrid electric vehicles (PHEVs) add even more complexity to the system. At the recent SAE Hybrid Vehicle Symposium in San Diego, Michael Duoba, lead engineer for Argonne National Laboratory's (ANL) Advanced Powertrain Research Facility explained that battery modeling is more complex for a PHEV than a conventional HEV due to the discharge requirements for extended operation. Figure 5 shows the approach the lab used to establish a model for PHEV batteries as part of its efforts in battery development and testing as well as vehicle modeling and simulation and control strategy development. The battery data came from large-capacity SAFT cells and was applied to a VL41 Li-Ion cell.
To determine the all-electric range of a specific configuration, ANL performed simulation tests with a 41 Ah SAFT VL41M (10 kWh) test battery in an HIL mode. As shown in Figure 6, the 1633 kg virtual vehicle had a simulated Ford Duratec engine, five-speed transmission, and a UQM 75 kW permanent magnet brushless dc motor. The control strategy depleted the charge until the battery was 30% and then sustained this level. The lab's data is being used to modify SAE J1711, Recommended Practice for Measuring Fuel Economy and Emissions of Hybrid-Electric and Conventional Heavy-Duty Vehicles, to include testing for PHEVs.
MANAGING MOUNTAINS OF DATA
With all of the added calibration items and new data tables required for hybrid development, data management becomes an even more critical aspect of the simulation and testing process. To address this, Vector CANtech created a product lifecycle management (PLM) tool specifically targeting software called eASEE. The preconfigured module of the entire suite is called eASEE.cdm (www.vector-cantech.com/va_easee_cdm_2_us,,4165.html), where cdm stands for calibration data management. The tool recognizes all of the standard file formats for software with a built-in hierarchy.
“When powertrain calibration engineers want to create versions and variants and have traceability and relate artifacts to other artifacts or artifacts to platforms, they are able to do it with a couple of mouse clicks or key strokes,” said John Cain, vice president sales and marketing of Vector North America. With eASEE.cdm, engineers can assign parameters for a multitude of engine calibrations without requiring spreadsheets and spending hours manipulating the data.
The enterprise PLM solutions in place today have been developed around hardware instead of software, and address part number formats that accompany CAD drawings and mechanical parts. Vector's approach simplifies the design in powertrain and other automotive systems by targeting software file formats. As Cain observed, “All the automotive manufacturers want to be able to manage the life cycle of all of these software files, the objects and artifacts, in a database environment that creates hierarchical relationships between all of these artifacts.” Clearly, software provides the means for tackling one more aspect of complex system design.
ABOUT THE AUTHOR
Randy Frank is president of Randy Frank & Associates Ltd., a technical marketing consulting firm based in Scottsdale, AZ. He is an SAE and IEEE Fellow and has been involved in automotive electronics for more than 25 years. He can be reached at [email protected].