While driving a rental car over Eisenhower
Pass in the Rocky
Mountains a few weeks ago, Pamela Fletcher made a surprising
discovery: By comparison to the soon-to-be-released Chevy Volt, conventional
gasoline-burning vehicles just don't seem as good anymore.
Fletcher should know. As chief
engineer for the Chevy Volt's powertrain, she has driven General Motors' new
extended range hybrid through mountains, across deserts and over frozen tundra.
And she knows what an electric motor and a continuously variable ratio can do
for an otherwise difficult driving experience.
"I've been spoiled," Fletcher says.
"When the Volt transitions from mode to mode, it's seamless. It's smooth. And
that gets emphasized in rolling hills and mountains. You don't have the 'hunting' that goes on when a conventional vehicle tries to find the right
But if Fletcher is surprised by the
Volt's comparative performance versus conventional vehicles, imagine how the
rest of the world will feel when the Volt hits the streets this month. The
Chevy Volt was, after all, supposed to be a "green" vehicle. It was supposed to
be GM's mea culpa for the untimely death of its EV1 electric car a decade ago.
Clearly, though, it's much more
than that. With the Volt, GM has created an exceptionally smart vehicle,
capable of "watching" the driver, battery, generator, clutches, cooling system
and electric motors by monitoring voltage, current and temperature. As such, it
knows when to run purely off battery power. Its supervisory controller knows
how to run the traction motor, when to turn on the generator-motor and when to
give the battery a boost.
"It's basically your co-pilot,"
Fletcher says of the Volt's electronic control system. "It looks at what you're
demanding from the car, does its calculations and then delivers what the driver
is asking for in the most efficient way."
In truth, that's not exactly what the public expected from
the Volt. When GM initially announced the concept in January, 2007, the Volt
was viewed as a rather simple electric car, backed by a series-hybrid
however, is not the best operative term for the Volt. During its
three-and-a-half-year development process, the Volt has evolved into a highly
complex machine that depends heavily on electronic control.
result, its Voltec drive unit now has four basic modes of operation: electric
driving (low- and high-speed); and extended range driving (low- and high-speed).
Here's a quick look at the four
modes: Electric Low-Speed:
and most well-known of the Volt's four modes, this calls for the vehicle to use
the charge from its 435-lb lithium-ion battery pack to power a 111-kW ac traction
motor. When the traction motor turns, it transmits power to a sun gear in a
planetary gear set, which turns the drive axle. Volt owners are expected to
make heavy use of this mode - many will drive the car to work and back, charge
it at night, and never use a drop of gasoline in the process. Electric
The Volt typically hits this mode at about 70 mph. Then, the
supervisory controller splits the power between the large traction motor and a
smaller 54-kW generator-motor (which is still operated by the battery). The
planetary gear set blends the power from the two motors and sends it to the
drive axle. Even as it switches to this mode, however, the Volt still isn't
employing its internal combustion engine - and therefore still isn't burning
any gasoline. Extended Range Low-Speed:
battery's charge drops to an unacceptable level, the 1.4-‚Ąď I-4 engine makes its
debut. At low speeds, the engine spins the generator-motor, which replenishes
the battery. Extended
This is the most complicated and controversial (see
sidebar) of the Volt's four modes. With the battery depleted at higher speeds
(70+ mph), the controller opts for a power split. It calls for power to be
delivered through the big traction motor and
through the smaller generator-motor. Unlike the other modes, however, this one
calls for the generator-motor to transmit power directly into a ring gear in
the vehicle's planetary gear set. GM says it has a good reason for doing this -
electric motor efficiency typically plummets at higher speeds. "You're bringing
down the speed of the main traction motor and supplementing it with the
generator-motor," a GM spokesman says. "As a result, you're improving the
The ability to intelligently blend
power through the planetary gear set also gives the Volt continuously variable
ratios. By commanding the Voltec's clutches to engage various elements (sun
gear, planet gears or ring gear), GM engineers have created a continuously
variable transmission of sorts, capable of accelerating smoothly to 100 mph,
hills and mountain roads notwithstanding.
engineers, the challenge was to develop a control system that could read the
inputs and then select the right mode for the situation, engaging the proper
clutches, and blending power correctly. GM did this by creating an under-hood
inverter module containing four 32-bit microcontrollers (MCUs) from Freescale Semiconductor
. Three of the
microcontrollers serve as the brains for the motors: One operates the traction
motor; another communicates with the generator-motor; and a third drives a
motor for the auxiliary transmission oil pump, which in turn provides hydraulic
pressure to hold the vehicle's clutches.
The inverter module's fourth MCU, however,
is its most important. Known as the supervisory controller, this MCU incorporates
an online optimizer, which serves as the vehicle's "auto-pilot." It watches a
multitude of inputs - including wheel speed, accelerator usage and battery
depletion - and then selects the right mode for the situation.
microcontrollers are taking care of a lot of functions," says Steve Nelson,
vice president of marketing for Freescale. "They run GM's proprietary motor
control algorithms and they communicate with the inverting part of the motor.
They also communicate with the IGBTs (insulated gate bipolar transistors) and
decide when and how to drive the electric motors."
from Freescale and GM agree that one of the greatest challenges was to make the
motors - and hence, the car itself - offer the familiar feeling of a
gasoline-burning engine. "You want to deliver the right driving experience,"
Nelson says. "People are very sensitive to the way cars feel."
At the same time, the Volt's control systems also need to
take care of the costliest and most sensitive part of the vehicle - the lithium-ion
To do that, the GM engineers
employed another dedicated electronic management system. Using a dual-core
Freescale processor, the battery management unit monitors the current and
voltage going in and out of the pack, and watches the temperature of its 288
each about the size of a sheet of loose leaf paper, are heated and cooled by
plates sandwiched between them. Based on commands from the battery management
MCU, coolant is delivered to the cells through passages in the plates.
battery is pampered," Fletcher says. "We don't want it to get too hot or too
cold, so we manage it actively instead of passively. As a result, we get great
performance in all climates."
To be sure,
much of the Volt's electronic innovation was a surprise, even to GM engineers.
"When you start putting together the requirements for any powertrain, you usually
only understand about 80 percent of the problem," says Karla Wallace, senior
manager of electronics engineering for GM Powertrain. "There are always going
to be unexpected developments along the way, with regard to the behavior of the
system and the market's expectations. This vehicle was no different in that
resulting Volt, however, could surprise the general public. "This car feels
better than a conventional vehicle," Fletcher says. "It has a big traction
motor that provides instantaneous torque. So when you step on the accelerator,
you're moving. There's no lag, no lash to take up. It's smooth and fast; you
can drive 100 mph off the traction motor."
that all of that engineering was done in-house, by a hardware and software
staff as massive as any that the automotive giant has ever assembled. GM
engineers say that effort may ultimately be the key to the Volt's success.
"The controls and integration are
the part that most people are missing," Fletcher says. "Competitors can buy the
motors and the planetary gear set, but they can't hook them up and control them
the way we've done here." To see how power is transmitted through the Volt powertrain, read the full story in the November issue of Design News.