Flexible Fuel Emissions

Using ethanol as a vehicle fuel provides local and global benefits—reducing emissions of harmful pollutants and greenhouse gases.

Ethanol has been blended in low levels (10% or less) with gasoline for many years. This use of ethanol as an “oxygenate” promotes more complete combustion of the fuel, which can reduce exhaust emissions of carbon monoxide—a regulated pollutant harmful to human health—by 20% to 30% compared with pure gasoline.

Flexible fuel vehicles (FFVs) fueled with E85 (85% ethanol, 15% gasoline) also emit less carbon monoxide than gasoline-powered vehicles. Emissions from E85-fueled FFVs of other regulated pollutants, such as hydrocarbons and nitrogen oxides (NOx), are similar to those from gasoline-powered vehicles. Visit the U.S. Department of Energy/U.S. Environmental Protection Agency Fuel Economy Guide to compare FFVs and conventional vehicles for environmental impact and petroleum savings.

Tests indicate that vehicles fueled with ethanol blends produce lower emissions of some toxic compounds—such as benzene and 1,3 butadiene—than vehicles fueled with pure gasoline. However, evidence suggests that ethanol might increase toxic aldehyde emissions.

In addition to exhaust emissions, all vehicles emit hydrocarbons due to evaporation of fuel from their tanks and fueling systems, especially in warm weather. The common oxygenate blend E10 (10% ethanol, 90% gasoline) has a higher vapor pressure than pure gasoline and thus produces higher evaporative emissions. E85’s vapor pressure is lower than gasoline’s, so it produces lower evaporative emissions.

Mid-level ethanol blends, such as E15 and E20, can be used in FFVs but currently not in standard gasoline-powered vehicles. See the AFDC’s mid-level ethanol blends page to learn about emissions and other issues associated with these blends.

www.afdc.energy.gov

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