As a way of reducing weight, gas tanks in recent years have been made of lightweight plastic. They also have been equipped with charcoal canisters to trap evaporating gasoline and then feed it back into the engine. In the Volt, the engine may not fire up for extended periods of time, potentially building up hydrocarbons and a large amount of pressure. This extra pressure meant the plastic tanks of modern vehicles had to be replaced with a lightweight steel tank in the Volt.
"Gasoline readily evaporates at normal ambient temperatures and it also degrades over time from oxygenation and condensation," said fuel system integration engineer for the Volt, Jon Stec. Engineers pressure sealed the 9.3 gallon steel fuel tank to contain the gasoline vapor. This ensures that the gasoline in the tank doesn't hurt the Volt's performance or emissions when it is needed. "Using a sealed tank limits this evaporation when the engine is off."
"Volt engineers and supplier Spectra Premium Inc. developed the tank from 1.4 millimeter thick hot-dip tin-zinc coated steel to resist corrosion from both inside and outside. Despite the strength of the tank, it has a mechanical pressure relief valve that begins opening at 3.5 psi and a vacuum relief that opens at -2.3 psi, levels that are rarely exceeded.
Even with a tank that resists fuel vapors escaping or humidity getting in, the gas inside still needs to be used up and replenished periodically. That's where the Volt's "maintenance mode" comes in. If the engine hasn't started after six weeks, the powertrain controller sends a message to the driver telling him the engine needs to run for maintenance.
Volt drivers can defer the engine maintenance mode for up to 24 hours, after which the engine will run for a while on its own to use up some of the gas and keep the internals lubricated and ready for use. If a driver manages to go a full year between fill-ups, the fuel maintenance mode will run the engine until the old gas is used up or the driver adds fresh fuel."
A driver who starts the year with a full tank of gas (9.3 gallons) and drives 15,000 miles on electricity, maintenance mode will use just enough gas to average over 1,600 miles per gallon.
This site is not affiliated with GM-Volt.com but the site as always and writer Jeff Cobb are doing a great job over there keeping all of us up to date on the Chevy Volt and other EV news.