42 Volts. Right Answer Wrong Question - Delphi - Brief Article

Automotive Manufacturing & Production, August, 2001 by Christopher A. Sawyer

Forty-two volts. It's the future for cars and trucks given the drain on-board electronics will be putting on vehicle electrical systems in the coming years...Or so we've been told.

"The drive for 42 volts came out of the electrical power needed to drive the valvetrain in camless engines," says Jean Botti, Delphi Europe's director of Engineering. The combination of the camless valvetrain and all of the other electronics expected in the next five years would have produced a power draw of up to 12 kW. So 42 volts looked like a good answer to the problem." (An American home draws 1.3kW, on average.)

"But," he observes nonchalantly, "camless engines aren't here yet, and mechanical activation of the valvetrain doesn't draw any power, so we have a solution meant for one eventuality being adapted to a different set of circumstances. The 42-volt system still has a major advantage in wiring harness mass reduction when compared to the current 12-volt design. As more electronics migrate onto the vehicle," he continues, "this advantage will grow in importance."

Maybe so, but that doesn't explain the continuing work Delphi and others are doing on 12-volt electrical systems. "Well," Botti smiles, "until the industry goes totally to 42-volt systems, you have to continue to improve what you have. And this has had an ironic side effect. The power capacity of 12-volt systems continues to grow, which makes it less imperative for manufacturers to switch to a 42-volt architecture." And? "And the fact is that the filaments must be so thin on lights running at the higher voltage, he sighs. "that their life expectancy is much less that of the current designs. So you have an incentive to have a dual-voltage system--whether by separate architectures on the same vehicle, or a 42-volt system running the 12-volt accessories through a DC-to-DC converter--because it gets very expensive to change all of the electrical and electronic components over to the higher output."

What may tip the balance in favor of 42-volt electrical systems is drive-by-wire technology. "The power draw from these systems, when acting together, can be quite high," says Botti, "and a 42-volt would be more than robust enough to handle the demands placed on the electrical system by this technology." Currently, the big drawback is the need for an uninterruptible power supply for those rare situations when power is needed to drive these systems and drive from the engine is interrupted. "We think our solid oxide fuel cell is perfect for this, and it's fully compatible with this architecture," says Botti. It also doesn't hurt that stop-start systems that shut the engine off when the vehicle is stopped, and restart it when either the clutch or accelerator is depressed, need 42-volt systems to handle engines above 2.0-liters.

One last item. Why 42 volts and not some higher output? According to Delphi engineers, 42 volts is the upper limit before the electrical system becomes safety critical. As one engineer wryly noted, "Electric welders work at 48 volts. At that power level, the arc created doesn't self-extinguish. Grounding becomes a real issue above 42 volts."