Demystifying the Chevy Volt’s Mostly-Electric Engine

A look at the inner workings of GM's plug-in car

In principle, the Chevrolet Volt is simple: It’s an electric car with a gas-powered generator for backup. Yet the machinery under the hood is complex. The overarching goal is to wring maximum miles out of the Volt’s lithium-ion battery and, when gasoline is necessary, to use it frugally. This requires the Volt’s drivetrain—composed of two electric motors, a gasoline engine, three clutches and a planetary gearset—to blend power from different sources to find the perfect balance in every driving situation.

Here’s how it happens.

One-Motor EV Mode

Driving around town at up to approximately 40 mph, power flows from the 400-pound, 16-kilowatt-hour lithium-ion battery to a large permanent-magnet motor, which moves the car.

Two-Motor EV Mode

The harder you push the car, the more stressed the primary motor becomes—until a second, smaller motor joins in, allowing the larger one to spin more slowly. The two motors connect
to the planetary gearset, which blends power from the pair into torque that drives the car.

One-Motor Extended-Range Mode

Between 25 and 50 miles into your drive, the battery will dip to 35 percent charge, and the gas engine will start. Here, the gas engine drives the small electric motor, using it to generate electricity (an electric motor run backward is a generator) to sustain the battery’s charge.

Two-Motor Extended-Range Mode

With the battery low, at highway speeds, the primary electric motor continues to drive the car. But now the gasoline engine mechanically connects to and powers the secondary electric motor, which in turn helps the primary motor drive the wheels.

Verdict

Critics have argued that, because under some circumstances the gas engine helps power the wheels, the Volt is nothing new—a glorified Prius. But that’s not true. Efficient as it is, the Prius is a primarily gas-powered car. The Volt, in contrast, always drives like an electric car, and for up to 50 miles a day you’ll never burn a drop of gas.