Ethanol fuel is an alternative to gasoline. It can be combined with gasoline in any concentration up to pure ethanol (E100). In Brazil, flex-fuel vehicles are capable of running on pure ethanol. In the US, tolerance of ethanol depends on the individual vehicle. Anhydrous ethanol can be blended with gasoline in varying quantities to reduce the consumption of petroleum fuels, as well as to reduce air pollution. In Brazil, by law all fuels are at least 25% ethanol.
   Ethanol is increasingly used as an oxygenate additive for standard gasoline, as a replacement for methyl t-butyl ether (MTBE), the latter chemical being responsible for considerable groundwater and soil contamination. Ethanol can also be used to power fuel cells.
   Ethanol derived from crops (bio-ethanol) is a sustainable energy resource that offers environmental and long-term economic advantages over fossil fuel (gasoline). It is readily obtained from the starch or sugar in a wide variety of crops. Ethanol fuel production depends on availability of land area, soil, water, and sunlight.

Sources

Bioethanol is obtained from the conversion of carbon based feedstock. Agricultural feedstocks are considered renewable because they get energy from the sun using photosynthesis. Although ethanol can be produced from a variety of feedstocks such as sugar cane, bagasse, miscanthus, sugar beet, sorghum, grain sorghum, switchgrass, barley, hemp, kenaf, potatoes, sweet potatoes, cassava, sunflower, fruit, molasses, whey or skim milk, corn, stover, grain, wheat, wood, paper, straw, cotton, other biomass, as well as many types of cellulose waste, current production is primarily from sugarcane, maize (corn) and sugar beets. Technology does not yet exist that makes it economically competitive to produce ethanol from cellulosic feedstock.
   One result of increased use of ethanol is increased demand for the feedstocks. Large-scale production of agricultural alcohol may require substantial amounts of cultivable land with fertile soils and water. This may lead to environmental damage such as deforestation or decline of soil fertility due to reduction of organic matter.
   About 5% (in 2003) of the ethanol produced in the world is actually a petroleum product. It is made by the catalytic hydration of ethylene with sulfuric acid as the catalyst. It can also be obtained via ethylene or acetylene, from calcium carbide, coal, oil gas, and other sources. Two million tons of petroleum-derived ethanol are produced annually. The principal suppliers are plants in the United States, Europe, and South Africa. Petroleum derived ethanol (synthetic ethanol) is chemically identical to bio-ethanol and can be differentiated only by radiocarbon dating.

Production

Ethanol can be produced in different ways, using a variety of feedstocks. Brazil uses sugarcane as primary feedstock. More than 90% of the ethanol produced in the U.S. comes from corn. Crops with higher yields of energy, such as switchgrass and sugar cane, are more effective in producing ethanol than corn. Ethanol can also be produced from sweet sorghum, a dryland crop that uses much less water than sugarcane, does not require a tropical climate and produces food and fodder in addition to fuel.
   Ethanol is produced by yeast fermentation of the sugar extracted from sugarcane or sugar beets. Subsequent processing is the same as for ethanol from corn. Production of ethanol from sugarcane (sugarcane requires a tropical climate to grow productively) returns about 8 units of energy for each unit expended compared to corn which only returns about 1.34 units of fuel energy for each unit of energy expended. Thus sugarcane nets 7/.34 or about 20 times as much energy as corn. (corn produces an additional 0.33 units of energy in the form of high-protein livestock feed).
   For the ethanol to be usable as a fuel, water must be removed. Most of the water is removed by distillation, but the purity is limited to 95-96% due to the formation of a low-boiling water-ethanol azeotrope. The 96% ethanol, 4% water mixture may be used as a fuel, and it's called hydrated ethyl alcohol fuel (álcool etílico hidratado combustível, or AEHC in Portuguese). In 2002, almost 5 billion liters (1,3 billion gallons) of hydrated ethyl alcohol fuel were produced in Brazil, to be used in ethanol powered vehicles.
   For blending with gasoline, purity of 99.5 to 99.9% is required, depending on temperature, to avoid separation. Currently, the most widely used purification method is a physical absorption process using molecular sieves. Another method, azeotropic distillation, is achieved by adding the hydrocarbon benzene which also denatures the ethanol (so no extra methanol/petrol/etc. is needed to render it undrinkable for duty purposes). However, benzene is a powerful carcinogen and so will probably be illegal for this purpose soon.

Ethanol fuel mixtures

Fuel system design must be compatible with the percent of ethanol permitted. All current production spark ignition vehicles are designed to be compatible with up to 10% ethanol. Pure ethanol reacts with or dissolves certain rubber and plastic materials and must not be used in fuel systems that are not designed for it.
   Pure ethanol has a much higher octane rating (116 AKI, 129 RON) than ordinary gasoline (86/87 AKI, 91/92 RON), allowing higher compression ratio and different spark timing for improved performance. To change a pure-gasoline-fueled car into a pure-ethanol-fueled car, larger carburetor jets (about 30-40% larger by area), or fuel injectors are needed. (Methanol requires an even larger increase in area, to roughly 50% larger.)
   In many countries cars are mandated to run on mixtures of ethanol. Brazil requires cars be suitable for a 25% ethanol blend, and has required various mixtures between 22% and 25% ethanol. The United States allows up to 10% blends, and some states require this (or a smaller amount) in all gasoline sold. Other countries have adopted their own requirements. Because of this requirement it is speculated that all cars can run blends up to about 30% (so that manufactures do not have to stock parts incompatible with ethanol next to parts compatible), but it is not known if this is true.

Fuel Economy

For vehicles with current design flexible fuel engines, fuel economy (measured as miles per gallon (MPG), or liters per 100km) is directly proportional to energy content. Ethanol contains approx. 34% less energy per gallon than gasoline, and therefore will get 34% fewer miles per gallon. For E10 (10% ethanol and 90% gasoline), the effect is small (~3%) when compared to conventional gasoline, and even smaller (1-2%) when compared to oxygenated and reformulated blends. However, for E85 (85% ethanol), the effect becomes significant. E85 will produce approximately 27% lower mileage than gasoline, and will require more frequent refueling. Actual performance may vary depending on the vehicle.
   Some researchers are working to increase fuel efficiency by optimizing engines for ethanol-based fuels. Ethanol's higher octane allows an increase of an engine's compression ratio for increased thermal efficiency. In one study, complex engine controls and increased exhaust gas recirculation allowed a compression ratio of 19.5 with fuels ranging from neat ethanol to E50. Thermal efficiency up to approximately that for a diesel was achieved. This would result in the MPG of a dedicated ethanol vehicle to be about the same as one burning gasoline. There are currently no commercially-available vehicles that make significant use of ethanol-optimizing technologies, but this may change in the future.