What is "Oxygenated Gasoline"?
"Oxygenated gasoline" is a mixture of conventional gasoline and one or more combustible liquids which contain oxygen ("oxygenates"). At present, the most common oxygenates are ethanol and MTBE (methyl tertiary-butyl ether). ETBE (ethyl tertiary-butyl ether) and TAME (tertiary-amyl methyl ether) have seen some use and their use may increase.
Why Oxygenated Gasoline?
The National Ambient Air Quality Standard (NAAQS) limits the maximum concentration of carbon monoxide in ambient air to nine parts per million (8 hour average). Thirty to forty metropolitan areas exceed this limit on some days, primarily during the winter months. The government classifies these areas as "carbon monoxide nonattainment areas."
Most of the excess carbon monoxide comes from gasoline vehicles. While exhaust from gasoline engines consists mostly of non-toxic gases (nitrogen, carbon dioxide and water), it contains some carbon monoxide. Older vehicles without catalytic convertors are the worst carbon monoxide emitters. Newer vehicles with a catalytic convertor in the exhaust system are much better because the convertor changes most of the carbon monoxide to carbon dioxide. But, these vehicles emit more carbon monoxide right after they are started than after they have warmed up; it takes a little time for the exhaust to heat the catalyst to the temperature at which it is most effective.
All owners can help reduce carbon monoxide emissions by properly maintain their cars. This is particularly true for older cars which lack a catalytic convertor to moderate the higher amounts of carbon monoxide produced by an out-of-adjustment engine. Proper maintenance includes keeping the carburetor or fuel injection system clean and properly adjusted and keeping the engine tuned. For newer cars, it also includes replacing the catalytic convertor when the catalyst is no longer sufficiently active.
Oxygenated gasoline also reduces carbon monoxide emissions. This is how it works: Engines emit more carbon monoxide when they are fed "rich" air/fuel mixtures -- mixtures containing more fuel than can be completely "burned" by the oxygen (from the air) in the mixture. Rich air/fuel mixtures are used during engine startup and warmup and at full throttle (for rapid acceleration). Oxygenated gasoline requires less oxygen (from the air) for complete burning than the same volume of conventional gasoline. Adding oxygenate is like adding more air. So, for the same carburetor or fuel injection setting, changing an engine's fuel from conventional gasoline to oxygenated gasoline produces a "less rich" air/fuel mixture and, therefore, one which generates less carbon monoxide when it burns. The carbon monoxide reduction obtained from oxygenated gasoline is much larger for older cars than for newer, well-maintained cars.
To help reduce the concentration of carbon monoxide in the air in the problem areas, the Clean Air Act Amendments of 1990 established the following standard for the states:
All gasoline sold in the specified carbon monoxide nonattainment areas during the winter months when the carbon monoxide level is the highest must be oxygenated gasoline with a minimum oxygen content of 2.7 weight percent.
The Clean Air Act is administered by the Environmental Protection Agency (EPA). The EPA gave each state with nonattainment areas the responsibility of writing regulations to implement the program, which began November, 1992.
Since different oxygenates contain different amounts of oxygen, the volume needed to achieve the target of 2.7 weight percent oxygen varies:
| Oxygenate |
Volume % Oxygenate In Gasoline Needed to Obtain 2.7 Weight % Oxygen |
| Ethanol |
7.8* |
| MTBE |
14.8 |
*10 vol % ethanol (3.5 wt % oxygen) is often added to gasoline because tax laws encourage it and an EPA waiver allows it.
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