In theliquid fuel industry,oxygenates arehydrocarbon-derivedfuel additives containing at least oneoxygen atom[1] to promote completecombustion.[2] Absent oxygenates, fuel combustion is usuallyincomplete, and the exhaust stream pollutes the air withcarbon monoxide,soot particles,aromatic andpolyaromatic hydrocarbons, andnitrated polyaromatic hydrocarbons.[3]
The most common oxygenates are eitheralcohols orethers, butketones andaldehydes are also included in this distinction.[4]Carboxylic acids andesters 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 causecatalyst poisoning andcorrosion during oil production and refining.[5]
In theUnited States, theEnvironmental 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 particularground-level ozone andsmog. As of 2023, the EPAcontinues 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 asE85, and their flex-fuel vehicles,e.g.GM's Live Green, Go Yellow campaign. USCorporate 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 lowerenergy 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. Methyltert-butyl ether (MTBE) was the most common fuel additive in the United States, prior to government mandated use ofethanol. Typically, gasoline with added MTBE is called reformulated gasoline, while gasoline with ethanol is called oxygenated gasoline.[7]