Power of 42

FEATURE FOCUS: Advanced Energy Systems

power of 42

Higher automotive voltages may one day empower a car of drive-by-wire applications and amenities. The move could save gas, too.

By Paul Sharke, Associate Editor

An MIT electrical engineering professor, David Perreault, has coaxed more power from an automotive alternator. He and his associates control the alternator so that it always sees the optimum load, irrespective of engine speed.

"What people traditionally did was make it work best at the worst speed—at idle—where you generate the least power," he said.

Perreault's work, sponsored by an MIT industry consortium on advanced auto electrics and electronics, may soon lead to inexpensive ways of switching cars to a 42-volt standard. Although the average automobile purchaser won't care at what voltage the current in his car gets delivered, carmakers clearly do. For them, the benefits are many.

For starters—literally—42 volts supply sufficient oomph to turn over a stopped engine at a traffic light with a touch of the gas pedal (or a release of the brake), and so move a driver briskly away from a standstill. Such stop-start, or idle-stop, systems deliver gas mileage improvements and emissions reduction, especially in city traffic. Toyota has already introduced the first production version of these so-called mild hybrids.

Besides improving efficiency and decreasing emissions, 42-volt systems make implementing electromechanical valvetrains easier, Perreault said. For vehicle control, 42-volts might speed the adoption of brake- and steer-by-wire systems.

As luxury automakers add more of the comforts of home, from video systems to advanced climate controls, the need for electrical power expands. Forty-two-volt systems might be the only practical way of satisfying that demand.

According to Perreault, 42-volt automotive electrics still await the "killer app" through which the systems can saturate the market.

A Ricardo prototype relies on a 42-volt integrated starter-generator to re-start a stopped diesel. On facing page, the system displays its components.

Thomas Keim, who directs the MIT consortium, agreed that 42-volt systems need a single application that requires abundant power, or else a lot of little ones, to really take off. But he predicted that, eventually, 42-volt systems will end up in every new car sold around the world. That forecast might seem a long way off from today, when just a single production vehicle sports 42-volt electrics.

That car—the Toyota Crown Royal Saloon—is sold by the company in Japan as an executive vehicle with a 42-volt system available optionally. The 42-volt system Toyota sells is packaged with a fuel-economizing idle-stop system.

But, it is the same kind of reasoning that Keim said he thought would lead the rest of the world's autos to adopt a new electrical standard: better gas mileage.

"Look at history," Keim said. Fuel needed to move a certain mass down the road has decreased yearly. "In Europe and Japan, you see this as fuel economy. In the U.S., you see it as larger engines on vehicles of given sizes, and a zippier fleet," he explained.

Into the Lap of Luxury

With Toyota's introduction of a 42-volt luxury car, we're seeing the first of several systems that will address the high-end and the sport utility vehicle market, Keim said. In getting to this point, however, there remain some technological needs that must be addressed before the systems begin to appear in high volumes. And when they do, the cost per generated watt may still be higher than it is in today's 14-volt systems.

"There are physical phenomena that need to be dealt with that are not design concerns at the lower voltage," Keim said. Things like arcing and electrochemical corrosion, to suggest but two.

Keim explained that 14-volt arcs are inherently unstable, meaning they'll collapse as soon as they form. Arcs at 42 volts are unstable only if a sufficient minimum gap exists between electrodes. "If the spacing is smaller, then an arc is stable," he said. "It is happy to persist."

"That's an important distinction," Keim continued. "You can find arc energies of 50 or 100 times greater at 42, not three or nine times or the multiples you're used to looking at." But the industry is well on its way to solving the problem, he said.

Electrochemical corrosion doesn't share a similar threshold. "Higher voltage makes things go faster, but the physics is the same," Keim said. "It's just a 'rate question,' " he said. "For arcs, there's a 'phenomenological change.' "

Another concern, according to Keim, is over-voltage clamps needed for 42 volts. Today, all 14-volt parts must be capable of withstanding short exposure to fairly high voltages (75-100 volts) that come about in certain failure modes.

This time, 42-volt-system developers have agreed to limit the voltage that automotive components will see to a value close to nominal. "We can't just change the turns number by three on all our electrical parts and expect to get where we need to go," Keim said. "We're going to have to put some part in the vehicle whose function is to suppress that over-voltage that would otherwise occur during a load-dump event," he said.

Concern remains over 42- to 14-volt faults. If auto electrics transformed straight to 42 volts without stepping into intermediate, dual-voltage systems, there'd be no issue here.
Said Perreault, "Everyone would like a single-volt system." But he didn't think that a one-step leap would happen. While 42 volts is great for stop-start and other energetic functions, it's not the electrical potential of choice for logic devices and lamps.

Perreault's group is working on an alternator that delivers its output at both voltages simultaneously, which he hopes will be cheaper than a system that relies on a dc-dc converter, as Toyota's design does.

With dual-voltage systems come the possibility of being unable to guarantee that fuses will restore a system to a predictable state, Keim added. Everyone in the industry understands this; the challenge is in coming up with a technology that can protect the system during 42- to 14-volt faults.

As a final thought, Keim mentioned idle-stop air conditioning. "Car companies all agree that the market will insist the air conditioner in a car continue to operate if the engine stops. This poses a challenge for batteries because if you're going to run air conditioning through idle-stop, you're going to go through an electric motor, in all probability. Energy will come from a battery.

"Increasing energy requirements of batteries is a major concern in idle-stop vehicles. The trade-offs are just unpleasant," Keim said.

Idle-stop remains atop the list of 42-volt benefits. Delphi Automotive Systems of Troy, Mich., offers automakers a couple of ideas for improving fuel economy with these systems, said A.J. Lasley, chief engineer at the company's Energenix center.

One system can fast-crank an engine after it shuts off at a stop. The system integrates an inverter into a conventional alternator, using the belt for restarting the engine. It is a low-cost, 14-volt solution for the near term, Lasley explained.
Named Energen 5, the system is the first of other versions anticipated by Delphi. Energen 10, intended for the sport utility vehicle fleet, will boost the voltage of Energen 5 to 42 because of the higher displacement in the engines for which it is destined. It will use an integrated starter generator—either an induction or dc brushless machine—that acts directly on the engine crank, Lasley said.

"On the crank, it's now a tool that can be used on the powertrain," he said. In addition to idle-stop, the system promises to economize on fuel usage through a palette of tactics, including engine-off during coast down and idle, early torque converter lockup, deceleration fuel cutoff, regenerative braking, and electrical launch assist.

Toyota's mild hybrid system uses a belt-operated motor-generator to restart a stopped engine, move the vehicle while starting, operate auxiliary equipment when the engine is stopped, generate power while motoring, and recapture energy from braking. According to a paper presented during the 2002 SAE Congress in Detroit, the system improves fuel economy by 40 percent in the Japanese urban standard drive cycle compared with similar vehicles not fitted with the system. The system uses a smaller motor-generator than that found in the hybrid gas-and-electric Prius, making the system more adaptable to installation in conventional cars, Toyota said.

A power control unit switches between drive and generate modes. Key to the system is a solenoid clutch between the engine crankshaft and pulley, which the controller engages or disengages to accommodate any of four scenarios: stopped, starting, driving, and stopping.

When stopped, the clutch disengages, and the generator turns auxiliary equipment through the 42-volt system. When moving away from a stop, the clutch engages to reestablish combustion (a starter tied to the 14-volt system provides an initial cold-engine start).

During normal driving, the engine propels the vehicle and drives the generator to feed power to the 42-volt system (which feeds the 14-volt system through a dc-dc converter). When stopping, the wheels drive the generator by way of the transmission and engine to capture some of the energy ordinarily converted to heat during braking.

The paper's authors reported undesirable vibrations as a vehicle slows under motor-generator control. These vibrations emanate from torque fluctuations in the engine during the slow-speed compressions and expansions taking place in the cylinders. To alleviate them, Toyota employs two operating modes on the generator as the engine's speed drops. First, it runs continuously with the throttle valve closed to reduce engine pressure. Below 300 rpm, the generator operates only during specific intervals of the ignition cycle.

In developing the system, Toyota engineers needed to address the difficulties in restarting the engine and transferring drive smoothly from the motor-generator to the engine. They also had to solve the problem of a car coasting downhill backward in the time between a brake's release and an engine's gaining sufficient speed to move the car forward.

Over the past two years, United Kingdom-based Ricardo plc built a prototype C-class vehicle that halves the current Euro IV emission levels and uses fewer than 4 liters of fuel for every 100 km traveled (the equivalent of 56 mpg in the United States). According to Peter Miller, Ricardo's director of electrical and electronic design, C-class vehicles are a kind of everyman car, targeted for Europe's mass market.

Toyota's mild hybrid restarts its engine through a belt-driven motor-generator. The engine image depicts the belt arrangement and the inverter.

The goal of the research program was to produce a vehicle that achieved significant fuel economy and emissions reductions without compromising comfort, space, performance, ride, or price. Using 42 volts was not an objective of the program, although it turned out to be the crucial "enabling technology" in meeting program goals, Miller said.

Using system modeling to optimize design choices and control strategy was important to the program's success, Miller said.
Ricardo engineers modeled the integrated powertrain — simulating various electrical, engine, and transmission systems — to determine how energy was used over an entire drive cycle, for example. They used a smaller diesel engine than was furnished in the stock Opel Astra 2.0-liter DTi to ultimately consume 20 percent less fuel and save 30 percent in engine weight. To the engine, they coupled a 42-volt flywheel-mounted starter-alternator from Paris-based Valeo, for stop-start operations, regenerative braking, and torque boosting at low engine speeds. A Ricardo-designed control program oversees the works.

Vehicle system simulation enabled Ricardo researchers to predict what the fuel consumption and emission
levels would be at every conceivable operating condition. The control system evaluates the potential "cost" (not dollars, per se, but a term in optimization that defines a relationship, in this case between fuel consumption and NO_x emissions) of charging versus not charging during any given moment. Miller said, "The cost function tells you what the impact of generating electricity or using it is."

The resulting vehicle, which the company calls the i-MoGen, competes quite favorably with the docket of existing Astra variants. The company claims that the i-MoGen prices out cheaper than an Astra 1.6i dual fuel model, beats all the other versions in combined mpg rating, betters the CO₂ output of the 1.7 DTi eco4, and ranks comparably with the other models on acceleration and top speed.

The step up to 42 volts is not the first time the automotive industry has made the grab for more on-board power. It moved from 6- to 12-volt batteries in the 1950s. Perhaps a few farsighted engineers back then saw the day in the distance when even their new systems would be unable to meet demand. That day is coming.