Electronics Boost Chevy Volt's Efficiency

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 gear."

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."

Power Split

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 powertrain.

Simplicity, 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.

As a 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: The simplest 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 High-Speed: 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: When the battery's charge drops to an unacceptable level, the 1.4-â„“ I-4 engine makes its debut. At low speeds, the engine spins the generator-motor, which replenishes the battery.

Extended Range High-Speed: 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 efficiency."

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.

For GM 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.

"The 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."

Engineers 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."

Massive Effort

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 battery packs.

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 cells.

The cells, 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.

"The Volt 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 respect."

The 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."

GM stresses 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.