A hybrid powertrain combines two different propulsion systems to move the vehicle, hence the term hybrid.
While the Volt can be run on just electricity, it cannot be run on just gasoline. There is no way for the car to move from a stoplight without electricity. There is no torque converter. Then you run into the problem of providing full rated 111kW (149hp) power at 0 mph. The problem is the ICE generator isn't nearly 111kW, it's only like 63kW but only at peak RPM. So when the battery is drained to zero miles, how does it turn 84 hp into 149 hp? CARB and the EPA banned nitrous oxide for emissions reasons on production cars.
The beautiful legacy and downfall of Saleen
There is not a massive flywheel or compressed air tank. Nor does it need it. All 149HP at 0 mph is electric motor. Even with the traction battery reading empty. Well, duh, the gas engine can't move the car at 0 mph.
It's magic. There are elves with enormous amounts of flatulence hidden under the bumper that provide jet propulsion.