The typical model
If these predictions prove correct, what will a "typical" top-of-the-line automobile in 2005 look like? Although, certainly, variations among manufacturers can be expected, the model car has an alternator, directly driven by the engine, that produces a variable-frequency, variable-voltage output [Fig. 5]. From the alternator, a front-end converter creates two outputs: 25 kHz ac at 48 V rms for the main power bus and 12 V dc for charging the battery. The dc-ac portion of the converter is bidirectional so the battery can supply essential loads when the key is off. The starter is still 12 V and gets its power from the battery; the power-hungry electrically heated catalytic converter takes its power directly from the alternator.
To minimize wiring complexity, the ac main power bus feeds distribution boxes located throughout the vehicle. Containing MOSFET switches and fuses, the boxes are controlled by a separate data communication network that allows each box to convert the ac bus voltage into the voltages needed by the loads it serves. For example, a distribution box near the trunk might provide appropriate voltages to the tail lights, fuel pump, rear window defroster, retractable antenna, and an audio system. Another distribution box in the passenger compartment might control door locks, windows, side-view mirrors, compartment lights, and seat heaters.
Figure 5 can also be used to visualize an exclusively dc high-voltage architecture. In this case, the front-end converter will be an ac-dc-dc converter and dc-dc converters will replace the transformer-rectifier combinations to provide voltages different from the bus voltage, which will probably be 42 V dc.
The details of the models of the future will of course vary, and some manufacturers may choose to eschew ac distribution altogether. But clearly the new models will be more power hungry than today's already complex high-end automobiles, with their 1500 wires, innumerable branch points, as many as three dozen microprocessors, and more than two dozen sensors.
The 1918 edition of Putnam's Automobile Handbook--The Care and Management of the Modern Motor Car (by H. Clifford Brokaw and C. A. Starr: G. P. Putnam's Sons, New York) told readers: "It takes good 'juice' and lots of it to run a modern auto; not the kind Uncle Sam has put a ban upon, [but] the electric 'juice.' " With the possible exception of the nostalgic reference to the Prohibition Era, that observation is pertinent today and will be even more true 90 years after it was written.
About the Authors
John G. Kassakian (F) is professor of electrical engineering and director of the Laboratory for Electromagnetic and Electronic Systems at the Massachusetts Institute of Technology, Cambridge, where he works in power electronics and automotive electrical systems. He is founding president of the IEEE Power Electronics Society and coauthor of the textbook Principles of Power Electronics (Addison-Wesley, Boston, 1991).
Hans-Christoph Wolf is in charge of developing future power train management platforms at Mercedes-Benz AG, Stuttgart, Germany. He was previously responsible for developing advanced electric power distribution systems for the Advanced Engineering Group there.
John M. Miller (SM) is staff technical specialist, Vehicle Electrical Systems Department, at Ford Motor Co., Dearborn, Mich. His principal interests are control of electric machine drives and actuators, and power distribution system architecture. He is active in the PNGV.
Charles J. Hurton is manager, electrical subsystems and planning, at General Motors Corp.'s North American Operations Engineering Center Division, Warren, Mich. In earlier assignments at General Motors, he was manager, electrical component applications, and manager, medium-duty truck vehicle electrical systems.
Spectrum editor: Michael J. Riezenma
For a description of multiplexed digital communication buses in motor vehicles, see "The Thick and Thin of Car Cabling," by Mark Thompson, IEEE Spectrum, February 1996, pp. 42-45.
The authors will present their findings in detail in "The Future of Automotive Electrical Systems," at the IEEE Workshop on Power Electronics in Transportation, to be held in Dearborn, Mich., in October.
Two documents constitute the starting point of the discussions of the working group at the Massachusetts Institute of Technology. One is a seminal report by the Society of Automotive Engineers' (SAE) Dual/High Voltage Study Group, "Dual/High Voltage Vehicle Electrical System," by J. Vincent Hellman and R. J. Sandel, SAE paper 911652. Another is a comprehensive paper on high-voltage automotive systems, "Design Consideration for Higher Voltage Automotive Electrical Systems," by M. Matouka, SAE paper 911654. Both appear in the Proceedings of the SAE Future Transportation Technology Conference and Exposition, Portland, Ore., August 1991.
For information about emerging electrical functions in automobiles, see "The Future of Vehicle Electrical Power Systems and Their Impact on System Design" by G. A. Williams and M. J. Holt, Proceedings of the SAE Future Transportation Technology Conference and Exposition, Portland, Ore., August 1991, and "Control of Engine Load via Electromagnetic Valve Actuators" by Mark A. Theobald, B. Lesquene, and R. Henry, SAE paper 940816, February 1994.
For examples of MAESTrO analyses, see "Alternative Electrical Distribution System Architectures for Automobiles," a paper by K. A. Afridi, R. D. Tabors, and J. G. Kassakian in the Proceedings of the IEEE Workshop on Power Electronics in Transportation, Dearborn, Mich., October 1994.