Whether the distribution is ac, dc, or a mixture of both will depend on manufacturers' individual choices, and their assumptions about cost, performance, manufacturability, controllability, repairability, adaptability, and reliability. Whatever the electrical distribution architecture, it should be compatible with existing 12-V dc loads. This design will allow the present 12-V dc infrastructure to be used while new, more efficient, or functionally improved loads at other voltages are introduced.
As happens now, lamps and motors will account for most of the electrical load. Incandescent lamps will continue to be powered at 12 V, regardless of whether the power supply is ac or dc, because tungsten filaments for low voltages are shorter and less fragile than those that operate at higher voltages. In fact, short-filament 6-V incandescent lamps are so rugged and so much easier to focus than 12-V units that some manufacturers may return to them, especially for cars with ac distribution, where transformers make downconverting from the high-voltage bus especially easy.
For headlights, high-intensity gas or metal-vapor discharge technology will probably replace tungsten. They are already available as an option in the new Mercedes-Benz E-class car, providing twice the light at half the power.
Motors and actuators will definitely benefit from a higher, regulated voltage; at 12 V, motors lose 15 percent of their energy in the brushes alone. The dc motor, brushed or brushless, will be the choice for most applications such as fans and pumps because of its efficiency at high voltage, as well as its easy control, low cost, and high quality and reliability.
Where high torque at low speed is needed--in window lifts and windshield wipers, for instance--the ultrasonic motor with its low-profile pancake-like shape is promising. The Lexus already uses ultrasonic motors to adjust its headrests.
The upper limit on motor voltage will be set by safety concerns and semiconductor device voltage limitations. Many auto makers favor 42 V dc, which can readily be handled by power MOSFETs made by the standard 60-V process. It corresponds to the average full-wave rectified output of a single-phase 48-V rms ac distribution source. Also, it is not so high as to present a serious shock hazard to humans who accidentally touch it.