There's no mechanical connection at all between the Volt's 1.4-liter engine and its wheels. But the engine doesn't run just to recharge the batteries either. When battery power drops to a certain point, the engine comes on to generate electricity, as needed, to run the electric motor. If a whole lot of power is needed, the battery can kick in some of its reserve power. The engine will replenish it later.
Driving the Volt under gasoline power is a lot like driving any other gasoline-powered car. When you step on the gas hard, demanding a lot of power, the gasoline engine revs up to supply it. When you let off the gas, it slows back down.
The biggest difference between a gas-power versus an electric-power car is that there's no transmission. Electric motors don't need gears or gear shifts.
NEXT: Easy ride