IEEE Electrification Magazine - December 2013 - 9

new behavior patterns consistent with the characteristics
of the new technologies.
traditional autos meet the needs of both short commutes and longer-distance travel with one vehicle and have
thus set that expectation for most of the 1 billion auto
owners around the world. While an electrically powered
vehicle is far more efficient than an ice-powered vehicle,
storing enough electricity for long-distance travel demands
several new strategies for future transportation vehicles.
an hev, such as the popular toyota prius, uses an ice in
combination with two more-efficient electric motors to
propel the vehicle. this strategy demonstrates an effective
way to increase fuel efficiency without compromising longdistance travel. this blending of efficient electric motors
with a smaller ice that can operate at a more efficient
engine speed uses the advantages of both technologies and
overcomes the obstacle of the very large battery packs
required of electric-only vehicles for long-range travel.
When the prius was released in north america, its average
fuel economy was about 44 mi/gal,
roughly double that of the fleet average (22 mi/gal) at the time. its rise in
popularity placed a renewed emphasis
on fuel economy in north america
and, along with emboldened policy
makers and consumer demands,
spurred other automakers to improve
their fuel economy in response.

All-Purpose Vehicles Versus
Specialty Vehicles

current needs, often priced as a monthly service fee. this
concept offers driving flexibility without the cost of ownership of multiple vehicles. one example is the smart2go program, a car-sharing program in europe from daimler ag.

Vehicle Weight and energy consumption
energy consumption is determined by a combination of
the efficiency of the powertrain and the weight of the
vehicle. While powertrain design and optimization are
topics beyond the scope of this article, a typical electric
powertrain might be on the order of 75-90% efficient. the
vehicle mass or weight also has a significant impact on
the overall energy consumption. a reasonable (perhaps
overly simplified) rule of thumb is that every 10 lb (4.5 kg)
of vehicle weight adds 1 Wh/mi of energy consumption;
so a 3,000-lb vehicle is likely to have an energy consumption on the order of 300 Wh/mi, while a 1,000-lb vehicle is
likely to consume around 100 Wh/mi. the actual consumption will depend on the driving conditions, speed,
aerodynamics, rolling resistance, etc.,
but assuming constant values for
those variables, the vehicle weight is
clearly the most important variable in
determining the energy consumption
of the vehicle at lower speeds. at
highway speeds, aerodynamic drag
also becomes important. a vehicle
that consumes less energy can
achieve a given range with a smaller
battery, reducing the battery cost and
the overall cost of the vehicle. therefore, reducing the vehicle weight
reduces energy consumption and,
thus, the battery capacity requirement for a given range and, in turn, the cost of the vehicle.
if lower costs and fuel savings are not enough of an
incentive, this "lighter is better" relationship also extends
to performance. the lighter the vehicle, the better the
acceleration with a given power. the conventional
approaches toward lightweighting, as a means of improving fuel economy, target a reduction in the vehicle weight
of 100-300 lb. in the example above, we considered a
weight reduction of 2,000 lb, with spectacular results.
therefore, to eliminate any confusion between minor
reductions in the weight (a few hundred pounds) of a
3,000-4,000-lb vehicle and vehicles designed to perform at
a weight of 1,000-1,200 lb, perhaps we should refer to our
1,000-lb vehicle concept as a superlight vehicle. based on
acceleration models, with all other variables being equal, a
3,000-lb vehicle with the power to accelerate from 0 to 60
mi /h in 9 s would accomplish the same 0-60-mi/h acceleration in roughly 3 s if the vehicle weighed 1,000 lb. conversely, if such high performance is not necessary, one
could achieve the same 9-s acceleration as the 3,000-lb
vehicle with one-third the power applied to the 1,000-lb
vehicle. to summarize, superlightweight designs improve

Energy consumption
is determined by
a combination of
the efficiency of
the powertrain
and the weight
of the vehicle.

the current ice vehicles are designed
to accommodate local commutes and
distant travel with one vehicle. if consumers require one vehicle to meet all of their driving
needs, then a vehicle designed to travel locally on electricity and use petroleum fuel in a range-extended ev powertrain for longer trips is a logical solution. a current
effective example of this type of vehicle is the chevy volt.
another strategy for meeting different driving needs
might be to commute locally with an all-ev but fly or rent a
hybrid electric car or range-extended ev suitable for longer
trips when long-distance travel is required. this notion arises
because evs are more suited to local use, where they can
easily return to a place for charging, which is likely to require
some time to complete. that said, the question is whether to
choose different vehicles for different missions or a single
vehicle designed to meet most drive cycles or missions. both
vehicle designers and consumers will likely face this question for another decade or so.
similarly, a strategy for a family with varying needs
might be to own a vehicle for the daily local commute, such
as an all-ev, as well as a second car suitable for longer trips.
some forward-thinking companies are offering transportation services that combine the availability of several types
of vehicles for use by customers, depending on their

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Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2013