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Fuels
The emissions that come out of a vehicle depend greatly on the fuel that goes into it. Consequently, programs to control air pollution have centered around changing fuel composition as well as around improving vehicle technology or performance. One of the first, and most successful programs was the removal of lead from gasoline. More recently, EPA has proposed to limit the sulfur in gasoline to coincide with tighter emissions standards for automobiles and trucks. More information is available on the Tier 2 Vehicle & Gasoline Sulfur Program.
Other recent fuel system changes include:
Limits on gasoline volatility: Volatility is a measure of how easily a liquid evaporates. Limits on gasoline volatility have been imposed over the last several years to control evaporative emissions of both hydrocarbon and toxic compounds.
Reformulated gasoline (RFG): The 1990 Clean Air Act Amendments requires reformulated gasoline to be introduced in the nation's most polluted cities beginning in 1995. From 1995-1999, these gasolines must provide a minimum 15% reduction in air toxics emissions over typical 1990 gasolines. This increases to a 20% minimum reduction beginning in the year 2000. The air toxics reductions will be achieved mainly by reducing gasoline volatility and by reducing the benzene content of the gasoline.
Limits on diesel sulfur: Regulations limiting the amount of sulfur in diesel fuel took effect in 1993. Today's lower-sulfur diesel fuels are important in reducing emissions of particulate matter and other air toxics from diesel-fueled buses and trucks.
Oxygenated gasoline: Carbon Monoxide emissions occur from incomplete combustion of fuel and is emitted directly from vehicle tailpipes. Incomplete combustion is most likely to occur at low air-to-fuel ratios in the engine. Carbon Monoxide emissions increase dramatically in cold weather. This is because cars need more fuel to start a cold temperatures, and because some emission control devices (such as oxygen sensors and catalytic converters) operate less efficiently when they are cold. Oxygenated fuels has the effect of "leaning out" the air to fuel ratio, thereby promoting complete fuel combustion. The most common oxygen additives are alcohols or their derivatives. Currently, there are no areas that use oxygenated fuels because Carbon Monoxide levels have decreased to safer levels. But should Carbon Monoxide level increase to unsafe levels again, oxygenated fuels could possibly be used.
More Information on Fuels - EPA's Office of Transportation and Air Quality