does the gas engine in a Volt recharge the battery? or drive the car?
Both.
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.
Chevrolet Volt Hybrid Drive System - General Motors Volt Powertrain - Popular Mechanics
http://www.popularmechanics.com/cars/alternative-fuel/cells/chevy-volt-hybrid-drive-system