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GM's New Electric Vehicle


By Kevin Bullis
Technology Review
January 8, 2007


The automaker tries again, announcing an electric-drive
system that can be paired with gasoline generators or
fuel cells.

Recently, General Motors (GM) has faced criticism for
shutting down production of its EV-1 electric vehicle.
Yesterday at the North American International Auto Show,
in Detroit, it unveiled the EV-1's successor, an
electric-motor-driven concept car called the Chevrolet
Volt. It's the first example of a new "E-Flex" vehicle
platform that the company is moving toward production.

The concept car runs off electricity from the grid,
stored on board in a battery, for the first 40 miles of
a drive. For longer trips, a small gasoline-powered
generator kicks on to recharge the battery pack,
allowing a total range of 640 miles. In between trips,
the battery can be recharged in six and a half hours at
an ordinary wall outlet. For longer trips, the generator
can provide power at the equivalent of 50 miles per gallon.

GM representatives say their reason for including the
gasoline generator is to overcome one of the biggest
limitations of the earlier electric vehicle: the short
range. The original EV-1 had a range of only about 90
miles, and it required an eight-hour recharge at a
dedicated 220-volt electrical outlet. The Volt uses
lithium-ion batteries that take up one-third of the
space of the EV-1's original lead-acid battery pack,
while providing the same total energy storage.

The new vehicle is an example of what's known as a
series hybrid, another in a growing type of
hybrid-vehicle variant. In conventional hybrid vehicles,
such as the Toyota Prius, the car can be propelled by a
gasoline engine, a battery-powered motor, or both. In a
series hybrid, the gasoline engine has no direct contact
with the wheels. It serves only to recharge the
batteries. One of the major advantages of such a system
is that any source of electricity can be used to charge
the battery. This includes electricity from a wall
outlet, but also electricity generated on board by a
gasoline-, ethanol-, or diesel-powered generator.
Because these generators can run at a constant speed,
they are more efficient than an engine that has to ramp
up and down to meet demand.

GM is developing a combination electric motor,
alternator-like generator, and battery pack that can be
used with various power sources. For example, an engine
that runs on diesel might be preferred in Europe. An
engine that runs on ethanol might be favored in Brazil.
In a hydrogen fuel-cell version, the battery pack would
be smaller and primarily used to provide a boost of
power and to recapture energy lost from braking.
Last year at the Los Angeles Auto Show, GM announced
work on a plug-in hybrid. (See "GM's Plug-in Hybrid.")
As with the Volt, the plug-in hybrid can recharge from a
wall socket. But the electric motor will not be the sole
source of propulsion: an internal combustion engine
could provide bursts of acceleration or power for
climbing hills. The plug-in hybrid would not switch
among the different sources of energy as easily as the
series-hybrid system could.

For both the plug-in and series hybrids, GM says the
timeline for commercializing the vehicles will depend on
the development of the battery systems. But such systems
may not be far off. GM representatives say that they
have already seen lithium-ion cells that have the
performance required for both plug-in and series-hybrid
applications. What remains to be done is to combine
these cells into large, complex battery packs and make
sure they work well together in an actual vehicle. Last
week, GM announced that it has a contract with two sets
of companies for building lithium-ion-based battery
packs and control systems for plug-in hybrids.

 

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