IEEE Spectrum July, 2007 - 10

20 IEEE Spectrum | July 2007 | NA

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.
A DiffERENt spiN: the
motor's rotor [green] turns
about the stator [purple],
driving the wheel's rim
[gold] and the tire.

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
up-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 selfpowered 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."
-WILLIE D. JONES

www.spectrum.ieee.org

E-tRACtIoN WoRLdWIdE S.C.A.

NEWS

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 dieselpowered 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

http://www.spectrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum July, 2007