Mobile Sources - Cleaner Cars

Control of pollutants from mobile sources

In response to tighter standards, manufacturers equipped new cars with even more sophisticated emission control systems. These systems generally include a "three-way" catalyst (which converts carbon monoxide and hydrocarbons to carbon dioxide and water, and also helps reduce nitrogen oxides to elemental nitrogen and oxygen), plus an on-board computer and oxygen sensor. This equipment helps optimize the efficiency of the catalytic converter. The catalytic converter is an anti-pollution device located between a vehicle's engine and tailpipe. Catalytic converters work by facilitating chemical reactions that convert exhaust pollutants such as carbon monoxide and nitrogen oxides to normal atmospheric gases such as nitrogen, carbon dioxide, and water.

Figure: Typical Catalyst system for Exhaust Emissions

In diesel exhaust, the addition of a particulate filter as an anti-pollution device traps particles in the exhaust before they can escape into the atmosphere. A vapor recovery system, also an anti-pollution system, captures gasoline vapors that would otherwise escape into the atmosphere from hot vehicle engines and fuel tanks.

Finally, auto makers must build some cars that use clean fuels, including alcohol, and that release less pollution from the tailpipe through advanced engine design. Electric cars, which are low-pollution vehicles, are currently being used as clean cars. Electric cars produce essentially no pollution from the tailpipe or through fuel evaporation. Car manufacturers are also beginning to sell "hybrid" vehicles that combine an electric motor with a separate gasoline or diesel engine. Hybrid vehicles can more than double the gas mileage of conventional gasoline or diesel-powered cars and can cut emissions significantly.