A "flex-fuel vehicle" in the U.S. and Europe means you can put pure gasoline (sometimes with up to 10 percent ethanol in it) or E85 into the tank. Down in Brazil, Fiat has a hit with the Siena Tetrafuel, a sedan that can burn four types of fuel. The 1.4 liter engine in the Siena Tetrafuel can handle moisturized alcohol, Brazilian gasoline (made up of 25 percent alcohol), pure gasoline and natural gas, according to Italiaspeed. Fiat has sold about 700 units a month during the last quarter. When Fiat introduced the car last October, it expected to sell about 200 a month.
From what I can tell, there are two fuel reservoirs in the car: the standard fuel tank (where any of the liquid fuels go) and then up to 6.5 cubic meters of natural gas in the trunk. The car's computer decides which fuel source to draw from at any given moment and decides what to use based on which will be most economical. Read more at Italiaspeed.
[Source: Claudio Perlini / Italiaspeed, h/t to Demetrio]