GM

Detroit, Mich—

We've tested the Volt and found that it operates as claimed —with a fully charged battery it behaves like a pure EV and switches to gas propulsion once the battery is depleted. It's in this second, "charge-sustaining" mode where the Volt really displays its technical prowess. GM has finally released all the details about how this powertrain works.

There are two electric motors—a 149 horsepower primary drive motor and a 74-hp motor/generator—and a 1.4-liter internal combustion (ICE) gas engine. All three elements are connected via a planetary gearset.

Planetary gearsets—which are common elements of automatic transmissions—are ingenious devices that have three meshed elements: A sun gear in the middle, a ring gear with internal teeth and smaller planet gears between the two which are joined together by a carrier. Alternating the speed and direction of the various elements changes the ratio between the inputs and outputs.

In the case of the Volt, the main output is the driven front wheels, which are connected to the planet carrier. The large drive motor is affixed directly to the sun gear. The ring gear is where things get interesting.

One clutch connects the ring gear to the transmission case and when engaged, it stops the ring gear from rotating. A second clutch joins the smaller motor/generator to the ring gear. A third clutch combines the engine crankshaft to the motor/generator.

At low speeds and with a fully charged battery, the ring-gear clutch is engaged and the main electric motor routes power to the front wheels. As vehicle speeds increase to about 70 mph, the main electric motor starts to fall out of its most efficient operating range so then two things happen: The ring-gear clutch opens, and the motor/generator clutch connects the second motor to the ring gear. Then both motors power the car.

When the battery is depleted, the operation slightly shifts. Again at low speeds, the ring gear is locked in place via its clutch but now the clutch between the engine and the motor/generator closes, which spins the generator to supply the electricity. This setup is known as a series hybrid because the gas engine does not directly power the wheels.

But the Volt isn't always a series hybrid. Once the speed climbs to about 70 mph, the motor/generator again couples to the ring gear but now—in "charge-sustaining" mode—the smaller electric motor is also affixed to the running gas engine. In effect, the gas engine supplies power directly to the transmission, which is just like a parallel hybrid.

This last mode has caused some consternation because over the Volt's development GM has stated that the gas engine never directly powers the car. While that's not entirely true, there are a few things to keep in mind. The first is that the car can't move solely on the gas engine—if the main electric motor quits, the car will be stranded. Secondly, the engine routes its power through the motor/generator.

In any case, the Volt's technology defines its own new category, a combination of EV and hybrid that defies conventional definition. And as we've experienced, it works.

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