More Power

The auto industry prepares for a major overhaul of its electrical system

by Richard Mandel

In the mid-1950s, roughly halfway through the First Century of the Automobile Era, American automakers ended production of vehicles with 6 volt electrical systems and re-started their lines with 12 volt-based designs. Now, the industry is targeting a new standard, and the change will redefine several aspects of automotive design.

Within the next half-decade, the first vehicles with 42 volt electrical systems will take to the nation's highways. The upgrade from the present 12 volt systems (actually 14 volts, the output of a fully charged 12 volt battery) comes as a response to demand for more electrical systems in an automobile platform.

The load increases

Vehicles today operate at power levels of about 1 kW. It's not merely gadgets like video entertainment systems, GPS receivers and cell phone power jacks that are placing new demands on vehicle power systems -- besides, most of those items run on a thimble-sized diet of watts, anyway. Devices like antilock brakes, electronically-controlled suspensions and electronic engine controls, while reducing mechanical elements and improving fuel efficiency and handling, require of themselves as much power as was used overall in the typical family car of 1960.

For years, manufacturers have had on their wish lists items like electronic clutches, heated catalytic converters to reduce emissions, non-hydraulic steering and brakes, and electronic engine valving, all adding to the present parade of watt-hungry motors and solenoids that operate car seats, windows, door locks and radio aerials. Refinements in motors are helping to fuel these concepts to reality, as sizes go down and torque ratings go up. There is an additional convergence with the present state of sensor technology and control chips, which have the fast response times and the physical heartiness necessary for components used in automotive applications.

Another area in vehicles under consideration for electric motors lies under the hood. The water and power steering pumps could be operated on demand, rather than slaved to the constantly varying speed of the engine. The air conditioning compressor can be reduced in size because it won't require an electromagnetic clutch to engage power from a perpetually rotating front pulley. And, as any auto racer can tell you, fewer accessory components powered by the engine translates to more power transferred to the action of propulsion.

It's anticipated that the trend to completely electrify electromechanical systems may result in power demands within the next five years of 12 kW and more. These levels are beyond what the present 14V system can bear. Switching to 42V systems will boost the power generating and handling ability of alternators, batteries and cabling. It will also mean that smaller wire sizes can be used, resulting in proportionally smaller wiring harnesses and a cable weight reduction that some engineers have estimated will be as much as 40 to 50%.

To make decisions as to architectures and standards for the new protocol, a consortium was formed in 1996, comprised of automakers and their suppliers, and the Laboratory for Electromagnetic and Electronic Systems at MIT. The MIT lab provides resources for research and development, as well as a ready supply of engineering students adding to discussions. The list membership list of the MIT/Industry Consortium reads like an industry Who's Who, with representatives from most of all the world's major automakers and diverse companies like Dow Automotive, International Rectifier, Lear and Yuasa.

Important decisions

One of the initial discussions for the new standard was how to phase in 42V technology. Industry could conceivably just start producing vehicles with 42V systems, but it would require an immense effort to have ready stock of new components -- there could be no carryover from the previous model year, incurring a huge investment cost for retooling throughout the entire supply chain. Instead, the Consortium agreed that the first models will be dual voltage. This protocol has additional benefits for present incandescent lighting technology, which works better at 14V. Producing the same lamp for a 42V system would necessitate using a thinner filament, but then the filament won't stand up to the bounces and jolts of the auto environment. One proposal has been to supply pulsed 42V energy to 14V lamps in a 1/9 duty cycle square wave, which appears to be effective as long as average voltage is kept down.

The choice of a dual voltage system raises other problems, however. Should the 14/42 vehicle have one or two batteries? A dc-dc converter would be required in either case because the vehicle will be generating at one voltage but supplying two for operation. Additionally, in a dual battery configuration, there will be added weight and cost to the second vehicle for the extra battery. On the plus side (no pun intended), a dual battery system would use a smaller dc-dc converter. There's also an issue in a single battery system whether to generate 14V with a single, centralized converter, or use multiple point-of-use converters configured in a distributed power architecture.

The industry appears to be leaning toward a single-battery package. A battery configuration workgroup within the Consortium is recommending a new connection for 42V batteries that can be used in single or dual battery vehicles. The polarized connector will ensure that plus and minus won't be accidentally reversed.

Tradeoff

Changes to an automobile's non-appearance systems often entail compromise. While the mass of a new wiring harness will be lighter, the weight reduction may be countered by the addition of motors and other new electronic components. It's also been pointed out that every 350 watts added to a vehicle's load results in the loss of one mile-per-gallon in fuel economy.

Norman Traub, technology information manager for Delphi Automotive's Packard Electric Systems division in Troy, MI, also represents Delphi for the Consortium. He observes that "converting systems like power steering (or replacing the hydraulic pump with an electronic steering system) and the engine's cooling pump to an on-demand basis would give back much of the efficiency," enough to counter the new electronics. He also points out that new components would use the smaller wiring of the 42V system, which would also tend to even out any weight penalty.

Iftikar Khan, manager, Vehicle Electrical Systems and Generator Development at Delphi in Kokomo, IN, adds that items like an "engine-off-at-idle" function and regenerative braking can lead to a net gain of fuel mileage for vehicles, primarily those used in urban areas. A 42V system lends itself well to engine-off-at-idle (which actually shuts off the engine at traffic lights and restarts it when the pedal is depressed), because a 42V starter could spin the engine at idle speed rather than the 80-200 rpm of the present 14V starter. Simulations presented at conferences indicate 10-20% improvements in economy, depending on the scenario.

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