Vehicle Emissions Reduction

Blending small percentages of oxygenated compounds such as ethanol, methanol, tertiary butyl alcohol (TBA) and methyl tertiary-butyl ether (MTBE) with gasoline has the effect of reducing volumetric energy content of the fuel, while improving the antiknock performance and thus making possible a potential reduction in lead and/or harmful aromatic compounds. Assuming no change in the settings of the fuel metering system, lowering the volumetric energy content will result in a leaner air-fuel mixture, thus helping to reduce exhaust CO and HC emissions.

     i. MTBE

It appears that MTBE or methyl tertiary butyl ether can be added to gasoline up to 2.7% without any increase in NOx. There are two opposing effects taking place with the addition of oxygenates: enleanment, which tends to raise NOx, and lower flame temperatures, which tend to reduce NOx. With MTBE levels above 2.7%, the lower flame temperature effect seems to prevail.

While the use of MTBE has been found to be very attractive from an air pollution point of view, recent evidence in the US has shown that leaks and spills are a serious threat to drinking water. This has led to a movement toward a ban on its use in gasoline in the future.

      ii. Ethanol

Available data indicates that ethanol can be added to gasoline at levels as high as 2.1% oxygen without significantly increasing NOx levels but above that point levels could increase significantly. For example, EPA test data on over 100 cars indicates that oxygen levels of 2.7% or more could increase NOx emissions by 3-4%. The auto/oil study concluded that there was a statistically significant increase in NOx of about 5% with the addition of 10% ethanol (3.5% O2).

Since ethanol has a higher volatility than gasoline, the base fuel volatility must be adjusted so as to prevent increased evaporative emissions. As a general rule, without adjustment, volatility will increase by about 1 psi when ethanol is added to gasoline.