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# Box **S.** Use of Oxygenates in Motor Gasolines

Oxygenated supplements, covering a range of lower alcohols and ethers can substitute for lead additives in gasoline. While these supplements have high octane numbers, they are not as effective as lead in raising base fuel octane numbers nor do they offer valve seat protection. The behavior of oxygenates in terms of blending and vehicle performance is different from HC-only gasolines. The commonly used oxygenates in gasoline blends are methanol, ethanol, isopropanol, tertiary butyl alcohol (TBA), methly tertiary butyl ether (MTBE), and iso-butyl alcohol. Tertiary amyl methyl ether (TAME) has also been used in small amounts. The volume of oxygenates in gasoline blends varies from 3-10%. The oxygen content of the blend is generally about 1-2% by weight. Oxygenates serve three basic objectives: extending the gasoline stock or serving as a fuel substitute (e.g., ethanol in Brazil), boosting octane value, and providing an effective means of reducing harmful emissions. Some of the important vehicle performance characteristics of oxygenated fuels are summarized below: _Anti-knock Performance._ Oxygenated fuels perform better than hydrocarbon-only fuels at low olefin levels. They give better anti-knock performance in unleaded gasolines; this is particularly the case with increasing MTBE content. MTBE has proven to be an effective substitute for lead in gasoline as it has a high octane rating (RON 119, MON 101) and is less water-sensitive than alcohol. It is, however, not clear if oxygenates give better anti-knock performance under lean-bum conditions.

_Driveability._ Vehicles fitted with fuel-injection systems have better tolerance in cold weather conditions for low-volatility fuels containing oxygenates. At high altitudes and in hot weather conditions, oxygenated fuels (except for certain methanol blends), give similar or better handling perfornance compared to wholly hydrocarbon fuels.

_Exhaust Emissions._ CO levels are progressively reduced as oxygen content is increased whilst NOx and HC emissions are marginally affected. Ethanol blends increase aldehyde emissions with increasing concentration; with other oxygenates, there is only a marginal increase, which may be corrected by exhaust oxidation or three-way catalysts. Oxygenates are particularly useful in lowering emissions from older vehicles.

_Fuel Economy._ In case of commercial oxygenated gasoline (which must comply with existing gasoline specifications), fuel economy is essentially unchanged with increasing amounts of oxygen. There could be marginal benefits in terms of reduced energy consumption with increased fuel oxygent content.

Source: C.J. Lang and F. H. Palmer. uTbe Use of Oxygenates in Motor Gasolines, in Gasoline and Diesel Fuel Additives, (Editor: K. Owen), John Wiley and Sons (1989).

technique currently available to reduce sulfur emissions. The aromatic content of diesel fuel is also important as an increase in aromatics increases particulate, HC, and NOx emissions. Another important characteristic is the ignition quality which is characterized by the cetane number. Maintaining current cetane number levels may prove difficult in the future because of the expected reduction in the quality of crude oils and the increasing demand for diesel fuel.