Putting Electricity Where The Rubber Meets the Road

In-wheel motors improve fuel economy and safety

4 min read

Illustration: E-Traction Worldwide S.C.A.
A Different Spin: The motor’s rotor (green) turns about the ­stator (purple), driving the wheel’s rim (gold) and the tire.
Illustration: E-Traction Worldwide S.C.A.

In-wheel motors improve fuel economy and safety The electric motors in hybrid vehicles do a lot to save fuel and cut emissions, providing the push that gets a hybrid going from a dead stop and the extra getâ''up-and-go for rapid acceleration or hill climbing. They also recapture some of the energy that goes to waste every time a driver applies the brakes. But engineers would like to make hybrids even more efficient by further exploiting the electric motor's virtues. One way is to increase onboard electricity storage so none but the longest trips require turning on the gasoline engine at all. Another approach is to eliminate the losses that occur as the torque generated by the motor is transferred to the wheels.

Several companies are proposing to curb these losses by squeezing electric drive motors into the wheels [see diagram, "A Different Spin"]. Generating propulsion power right where the rubber meets the road makes a surprisingly big difference.

A motor housed inside a wheel hub can shunt up to 96 percent of the torque it generates directly to the patch of tire that touches the road, says Peter le Comte, CEO of e-Traction, an Apeldoorn, Netherlands–based maker of wheel-hub-motor direct-drive systems. With a conventional drivetrain, roughly 20 percent of the power generated by the motor is lost to friction.

To prove this point, the Dutch company has built two hybrid diesel-electric city buses, each with twin 40-kilowatt electric motors in its rear wheels and lithium-ion battery packs that store enough power to run the buses for close to an hour without any input from their diesel engines [see photo, "Where the Action Is"]. The buses cost US $500 000 each. Though this is more than twice what standard diesel-powered buses sell for, le Comte says that a strong economic case can still be made for trucks and buses featuring two or more of e-Traction's handmade 45 000 wheels.

Le Comte notes, for example, that the average bus traversing New York City's congested streets, which sucks up fuel at roughly 67 liters per 100 kilometers (3.5 miles per gallon) and gives off more than 150 metric tons of carbon dioxide each year, is a textbook example of inefficiency. ”But when [buses are] fitted with our wheels, fuel economy is improved to 16 L/100 km, resulting in a nearly 80 percent decrease in fuel consumption.” The reduction in emissions is directly proportional to the fuel savings.

A good deal of the efficiency boost comes from regenerative braking. When the driver steps on the brake, the motors, acting as generators, convert up to 70 percent of the kinetic energy pushing the bus down the road back to electrical energy. The popular hybrids from Toyota and Honda purposely shunt less than 20 percent of kinetic energy back to their battery packs, because their motors, batteries, and wiring—all meant to be compact and lightweight—are not designed to handle any more.

With e-Traction's superior regenerative braking system, says le Comte, a single bus could reduce annual fuel consumption by 55 000 to 75 000 liters (15 000 to 20 000 gallons). And when other savings on large vehicles, including those provided by less frequent maintenance and parts replacement, are accounted for, these systems could pay for themselves in two to three years, says le Comte.

Another advantage of wheel-hub motors is the ability to independently control the power generated at each wheel. For example, if when braking, one of a vehicle's outside wheels cannot prevent a spinout, the control software can tell one or more wheels to spin in reverse without any action by the driver. This, say the companies developing these systems, will make today's antilock braking systems and electronic vehicle stability-control systems unnecessary. Antilock braking systems allow the driver to maintain steering control during sudden hard stops or at least to avoid spinning out. In electronic vehicle stability-control systems, ABS is combined with traction control, which individually brakes wheels that slip on, say, wet pavement, and yaw control, which corrects for forces that cause spinouts. Drivers may also see other benefits, such as the ability to turn all the wheels 180 degrees in either direction, which will make parallel parking much less of a headache.

The obvious question, then, is: When will wheel-hub motors appear on hybrid-electric versions of the average family car? Companies working on these systems admit that they'll be a tougher sell as a passenger-car option. ”These systems are very expensive,” admits le Comte. ”That is why our initial focus has been on city buses and garbage trucks, where, comparatively speaking, the front-end cost is not exorbitant.”

With the average person driving 24 000 kilometers a year, ”getting Toyota Prius–type fuel economy in a Range Rover would save you about US $2000 a year,” says le Comte. Still, at that rate, it would take 15 years or more to recover the system's upâ''front cost.

Another unsolved challenge relates to road handling. If too much of a car's weight is moved below the suspension system—which is what happens when motors, control hardware, and inverters are moved into the wheel hubs—the car's ride is a lot rougher. Maintaining passenger comfort therefore requires tremendously responsive electronic suspension systems, adding even more to a car's sticker price.

Still, e-Traction is among the host of companies working on miniaturized versions for the light-duty market comprising passenger cars, SUVs, and pickup trucks. Fuel economy will still be a major thrust, but in the smaller vehicles, performance and safety will be even bigger selling points. For example, GM is still refining a version of the wheel-hub motor system it installed in a Chevy S-10 pickup truck, primarily a front-wheel drive vehicle. Twin 25-kW motors added to the rear wheels provide a 60 percent increase in the vehicle's propulsion power from a dead stop.

WaveCrest Laboratories, headquartered just outside Detroit, was founded in 2000 as a maker of electric bicycles but has since turned its attention and its 30 patents to the production of cars outfitted with self-powered wheels. Gary Gloceri, WaveCrest's vice president of product development, notes that there has been ”a lot of interest” in the system it installed in a Saturn Vue SUV. As in GM's S-10 pickup, it is a secondary drive system whose rear-wheel-mounted motors supplement the power the gasoline engine provides to the front wheels. The Saturn and a one-off DaimlerChrysler Smart roadster have provided a wealth of data on how the electric motors, energy storage systems, and the vehicle control systems should be optimized to work together.

Though cars with wheel-hub motors won't roll off assembly lines any time soon, companies like GM and Siemens VDO, the automotive arm of the German manufacturing conglomerate, say electric wheels are the future of automotive propulsion. ”The move of populations into megacities will require very clean, very flexible vehicles that can get into tight spaces,” says Brad Warner, a spokesman for Siemens. ”The driving force behind projects like [our electric wheel-hub system] is our belief that zero-emission propulsion systems are where the future lies.”

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