As the Volt drives using the electricity stored in the battery, it gradually drains that battery just like any electronic device. But if you're driving the Volt with the batteries nearly depleted and find yourself facing a long, steep grade, the gasoline generator alone may not be able to produce enough electricity to maintain normal performance. Mountain Mode is the remedy for this possible problem.
Mountain Mode is a driver selectable feature that runs the gas engine in order to store up energy that the drive system can later pull from when traveling up a long steep hill. The catch is you have to know ahead of time that you'll be driving up hill because if you wait until you're at the bottom of that long, steep grade to select Mountain Mode, it's too late. The Volt will still get up the hill but the car's performance will be diminished.
The Volt's 1.4 liter gas engine is good for about 55 kilowatts; however the drive system has a capacity of 110 kilowatts. To make the most of the gas generator and the battery's combined power, Mountain Mode runs the gas engine (even if the battery isn't fully drained) in order to bank the extra capacity the car will soon need. Once that big hill comes up, the electric drive system can pull more power from the batteries to make it to the top while maintaining the expected performance. If the Volt is already in extended range mode, meaning you've used up all the plug-in power and the gas engine is now making the juice, selecting Mountain Mode will cause the engine to run at a higher rpm in order to make more electricity.
According to Larry Nitz, General Motors electric and hybrid powertrain engineering executive director, the Volt's battery always has a little bit of extra power banked anyway just in case the driver needs to pass or just wants to be a little more aggressive with the accelerator. Mountain Mode essentially increases that buffer.
When asked if the Volt could just make its own electricity by constantly putting it in Mountain Mode and never plugging the car in, Chevrolet reps said "Yes, but that would be pointless because then you'd be using the car in the most expensive and inefficient way possible."
But, it's possible. Chevy's Volt will use its internal combustion engine to charge its battery.
Check out episode 5.4 below and jump to the 4:00 minute mark for a demonstration of how Mountain Mode works: