Biologically produced alcohols, most commonly ethanol, and less commonly propanoland butanol, are produced by the action of microorganisms and enzymes through the fermentation of sugars or starches(easiest), or cellulose(which is more difficult) Alcohol fuels are produced by fermentation of sugars derived from wheat, com, sugar beets, sugar cane, molasses and any sugar or starch. The ethanol production methods used are enzyme digestion(to release sugars from stored starches), fermentation of the sugars, distillation and drying Ethanol can be used in petrol engines as a replacement for gasoline by mixing with gasoline to any percentage. Most existing car petrol engines can run on blends of up to 15% bioethanol with petroleum gasoline. Ethanol has a smaller energy density than that of this means it fuel(volume to produce the same amount of work. An advantage of ethanol(CH-CH10R) is that it has a higher octane rating than ethanol- free gasoline available at roadside gas stations, which allows an increase of an engine's compression ratio for increased themal efficiency. In high altitude(thin air locations. some states mandate a mix of gasoline and ethanol as a winter oxidizer to reduce atmospheric pollution emissions Ethanol has roughly one-third lower energy content per unit of volume compared to gasoline. This is partly counteracted by the better efficiency when using ethanol(in a long-term test of more than 2,1 million km, the BEST project found FFV vehicles to be 1-26 %more energy efficient than petrol cars The BEST project), but the volumetric consumption increases by approximately 30% so more fuel stops are required