The engine only helps to power the drive train in mountain mode. Otherwise it charges the battery, which drives the electric motor. The Volt is a pretty awesome car. I would still be driving it if I didn't need something more rugged, or could afford two separate vehicles. But I loved owning it. I even took it on a long distance trip over the mountains and it did great! I barely had to put gas in the thing unless it was freezing outside and I had a lot of driving to do that day, or for long-distance trips. The first year I owned it I only put gas in it twice, and the gas tank is only 9 gallons.