The Chevy Volt: A big step in the right direction

Nov 1, 2010 12:00 PM,

Steve Mraz, Contribution Editor

Whether you call it a series hybrid, an electric vehicle, a plug-in, or just a plain old sedan, the Volt’s powertrain embodies creative engineering that lowers mpg.

Chevrolet’s Volt’s evolutionary powertrain combines a lithium ion battery pack with a 1.4-liter gas engine, letting drivers get up to 45 miles of all-electric driving and giving them the option to travel 310 more miles on gasoline.
Select figure to enlarge.

The Chevy Volt is in the headlines, and with good reason. The most intriguing feature of the 2011 Chevrolet Volt five-door sedan is its powertrain. It includes two electric motors, three hydraulic clutches, and a planetary gear set, according to Chevy. But some might say it also includes a 1.4-liter engine, which has emerged a point of controversy. More on that later.

From a driver’s perspective, the Volt decides which motor(s) to use, when to rely on the battery pack, and when to switch the engine on or off. The Volt always tries to use the battery first until it is depleted, because this is the least expensive way to power the vehicle. And if you drive the car only 40 miles per day, you may never need to take it to a gas station. But if you want to travel farther — up to 310 miles farther — the Volt comfortably handles that with its gas engine.

A 350-mile drive

Assuming the car starts with a full charge and a full tank (9.3 gallons) of premium gas, it completes the first 40 miles or so on battery power. An inverter turns battery dc into ac, which powers a three-phase traction motor. The motor cranks out almost 150 hp (111 kW) and 273 lb-ft of low-speed torque. That’s enough to get the Volt going from 0 to 60 mph in nine seconds and through a quarter mile in less than 17 seconds. The Volt’s top speed, which is governed, is 100 mph.

The T-shaped battery pack fits under the seats of the Volt. It acts as a structural member to increase the car’s stiffness and improve the ride. The 435-lb pack also lowers the Volt’s center of gravity, which also improves handling. The orange wires denote high-voltage lines.

Total range on just battery power varies between 25 and 50 miles, depending on temperature, terrain, number of passengers, and driving style. And using the car’s heat or air conditioning, as well as the stereo and other electronic devices, also reduces the range.

When the battery drains down to its minimum charge level, the 84-hp gas engine starts up and spins a 54-kW generator/motor. Electricity from the generator/motor goes to the inverter for conversion to ac, which powers the traction motor. This electricity never recharges the battery -- the car is designed to be recharged strictly from household current, which is always less expensive than recharging via the gas engine. The engine extends the range of the Volt by 310 miles, which varies with driving style and conditions.

There’s more mileage available if the regenerative brakes send enough electricity to the battery pack during Regen Power Recovery mode. Regen kicks in whenever the driver lets off the accelerator to brake or coast. During those times, the traction motor becomes a generator, slowing the wheels slightly and recharging the battery pack. Braking recoups up to 0.2 g of stopping power to generate electricity.

The Volt powertrain includes a 1.4-liter combustion engine.

If the driver accelerates while driving at highway speeds, some of the torque from the engine is sent to the wheels, bypassing the generator, a fact which has sparked some controversy. In several earlier press releases and statements, Chevy indicated the Volt’s wheels would always be powered by an electric motor. But during development, engineers discovered that at high speeds, sending more torque into the generator from the gas engine was less efficient than sending that torque directly to the wheels.

If the driver insists on driving after travelling more than 350 miles and eventually runs out of gas, Chevy engineers designed a little mercy into the Volt; it gives you five more miles on battery only. However, doing so runs the battery past its lower charge limit may shorten the life of the battery pack.

The battery pack

The 435-lb lithium-ion battery pack plays a major role in the Volt’s powertrain. GM, along with partner LG Chem, perfected the 16-kW pack at GM’s Battery Lab in Warren, Mich.. It’s now being built by a GM subsidiary in Brownstone Township, Mich. The T-shaped pack measures 5.5-ft long and runs between the driver and passenger seats and under the rear passenger seats. In fact, the size and shape of the battery dictated that the rear seats be two buckets rather than a bench.

The pack consists of 288 prismatic cells, each measuring about 5 × 7 × 0.25 in. and weighing about a pound. The pack also includes electronics for battery management, diagnostics, and thermal controls.

Continue on next page