Learning from MAESTrO
In examining the cost/benefit attributes of active suspensions, electromechanical valves, and other electric loads, the MIT working group used a computer program called MAESTrO (for Multiattribute Automotive Electrical System Tradeoff), which MIT's Laboratory for Electromagnetic and Electronic Systems originally developed for Mercedes-Benz. The program takes as its input the network topology and load types, voltages, powers, and duty ratios (load factors). It then determines the parameters of the loads (such as cost, weight, and efficiency), wires (such as gauge and loss), converters (weight and loss), transformers, and other components from its built-in models, and produces as output the system attribute values for specific designs showing how they compare [Fig. 4].
For example, design A may be lighter than design B but may cost substantially more. Design B may cost half as much as C, but C may be considerably more energy-efficient. With these MAESTrO-generated plots, it is easy to identify a "Pareto-optimal frontier"--that is, a line along which all designs are equally "good" but have different degrees of acceptability.
With this program, unacceptable designs can be eliminated, narrowing the field of acceptable possibilities and making final selection a tractable and structured task. With the help of this program, the MIT working group was able to probe the key issues and reach a consensus with deliberate speed.
And what is the consensus? Not surprisingly, the group quickly decided that the present 12-V dc system cannot be upgraded to handle future electrical loads; it would cost too much, weigh too much, and be too inefficient. Instead, the 12-V alternator will be replaced by a more efficient, higher-voltage design. Instead of being internally regulated at a constant voltage; the its voltage (and power) will increase with speed, so that its full capacity is exploited. An external power electronic interface will take that unregulated output and produce well-regulated ac or dc--or both.
The battery will still be a 12-V unit, though, because it will be the most reliable and economical electricity storage medium available. New battery technologies may eventually be practical for automobiles, but not by 2005. Doubling the battery voltage to 24 V would have a disproportionate effect not only on cost per ampere-hour, but also on reliability because of the greater number of cells and thinner plates.
An essential part of the new electrical distribution system will be load management. It will ensure that advanced loads, such as the active suspension, get the high power they need for the brief time they need it, coordinating power demands so that the alternator and power converters will not have to be sized unrealistically. It will also ensure that safety-critical loads such as power brakes and steering take precedence, and that "key-off" loads--the clock and security system--can draw power when the engine is off, but not discharge the battery to the point where the car cannot be started.
Loads will be switched on and off by semiconductor switches controlled through a data bus of optical fiber or copper. The power MOSFET is the switching device of choice because it is efficient and rugged and its manufacturing technology is well understood and widely practiced. Moreover, MOSFET power switches can be driven directly by a car's electronic control units.