When General Motors conceived the extended range electric powertrain for the Chevrolet Volt, the 40-mile electric driving range was specified so that most drivers would rarely, if ever, have to use any liquid fuel. Components like the battery were then sized to match the performance and range specifications. However, the whole point of incorporating the range extending engine was so that the Volt could keep going without the driver having to continuously monitor the battery level, even though GM once told us that the Volt would move without gas in the tank.
In any case, leaving the same gasoline in a tank for months or even years creates a new set of problems. One reason is that there's no such thing as pure gasoline. What's sold at the pump is a blend of numerous hydrocarbon compounds like octane, heptane and other additives that lubricate valves and fuel injectors, among other things. Many of these various compounds will eventually evaporate, reducing the performance of the fuel and possibly leading to engine damage. This is a new problem, as no other car has been asked to operate with gas that might never get used in the tank.
To address this, the Volt has a completely sealed and pressurized fuel tank. Pressurizing the tank helps minimize evaporation from the liquid fuel, forcing it to stay in liquid form. Before the fuel filler can be opened to gas up the Volt, the tank has to be depressurized, which takes a few seconds after pressing the release button. The engine management system also monitors the time between engine running and will periodically prompt the driver to run past the 40-mile electric range before recharging. If the driver doesn't force the Volt to run on gas, the system will eventually start the engine on its own in order to consume some of the aging fuel and circulate the fluids within the engine. Once this maintenance mode is complete, the engine shuts down until it is needed again or enough time has passed again. GM hasn't yet revealed what the time intervals are, but we'll find out soon enough since the start of Volt production is right around the corner.
[Source: Plug-in Cars]