Tooxygenate means to impregnate, combine, or supply something with oxygen,[1] while oxygenates are hydrocarbons containing at least one oxygen atom that are used as fuel additives[2] to promote complete combustion in fuel mixtures.[1] Oxygenates can also be employed to reduce air pollution by reducing carbon monoxide production and soot particle size and compounds such as aromatic hydrocarbons, polyaromatic hydrocarbons, and nitrated polyaromatic hydrocarbons that are side-products caused by incomplete combustion.[3]
The most common oxygenates are either alcoholsorethers, but ketones and aldehydes are also included in this distinction.[4] Carboxylic acids and esters can be grouped with oxygenates in the simple definition that they contain at least one oxygen atom.[4] However, they are usually unwanted in oils, and therefore likely fuels, due to their environmental toxicity and tendency to cause catalyst poisoning and corrosion during oil production and refining.[5]
In the United States, the Environmental Protection Agency (EPA) had authority to mandate that minimum proportions of oxygenates be added to automotive gasoline on regional and seasonal basis from 1992 until 2006 in an attempt to reduce air pollution, in particular ground-level ozone and smog. As of 2023, the EPA continues to require the use of oxygenated gasoline in certain areas during winter to regulate carbon monoxide emissions; however, the programs to fulfill its conditions are implemented by the states. In addition to this North American automakers from 2006 onwards promoted a blend of 85% ethanol and 15% gasoline, marketed as E85, and their flex-fuel vehicles, e.g. GM's Live Green, Go Yellow campaign. US Corporate Average Fuel Economy (CAFE) standards give an artificial 54% fuel efficiency bonus to vehicles capable of running on 85% alcohol blends over vehicles not adapted to run on 85% alcohol blends.[6] There is also alcohols' intrinsically cleaner combustion, however due to its lower energy density it is not capable of producing as much energy per gallon as gasoline. Much gasoline[citation needed] sold in the United States is blended with up to 10% of an oxygenating agent. This is known as oxygenated fuel and often (but not entirely correctly, as there are reformulated gasolines without oxygenate) as reformulated gasoline. Methyl tert-butyl ether (MTBE) was the most common fuel additive in the United States, prior to government mandated use of ethanol. Typically, gasoline with added MTBE is called reformulated gasoline, while gasoline with ethanol is called oxygenated gasoline.[7]
