EMISSION CONTROL SYSTEM

Atmospheric pollution, or smog, is what results when any type of fossil fuel, gasoline, for example, burns and enters the atmosphere. One way these pollutants get up there is through incomplete com-bustion in your car engine.

When your car engine is running, the gasoline that serves as its fuel is not destroyed completely. The unburned portion, containing hydrocarbon compounds, reaches the atmosphere by way of the engine exhaust system. Hydrocarbons are also released as unburned fuel escapes into the crankcase during the compression stroke of the piston. A third source of hydrocarbon pollution is the carburetor. With the engine switched off, the fuel in the carburetor evaporates and enters the atmosphere through the air vent. A poorly-adjusted carburetor will also expel hydrocarbons; if the fuel-and-air mixture is set too lean, the fuel may be so diluted with air that the mixture will not ignite. The result is a misfire and further HC emission. And finally, hydrocarbons find their way into the atmosphere in the form of vapor escaping from the fuel tank.

Nitrogen oxide (NOx) emissions form as the engine operates. Air is mixed with fuel, then drawn into the engine, and ignited in the com-bustion chamber, where the temperature can exceed 4,500°. Above 2,500°F, nitrogen combines with oxygen and forms NOx, a colorless gas which combines further with oxygen when it enters the atmos-phere, creating the compound nitrogen dioxide (NO2). NO2 chemically activates hydrocarbons in the presence of sunlight, adding to photochemical smog. As the sunlight interacts with the smog, a new pollutant is created, ozone (03). 03 is an odorous gas which irritates the lung tissues and the eyes, affects the growth of some crops and plants, and causes deterioration of rubber products.

Carbon monoxide (CO), a by-product of combustion, is a poisonous gas having no color or odor. When inhaled, it takes the place of the oxygen in the bloodstream, and prolonged exposure to concentrated CO will cause death. Carbon monoxide chemically speeds the production of photochemical smog.

On most cars built since 1973, HC, NOx, and CO emissions are re-duced by the installation of the following emission control devices: a charcoal canister controls the emission of HC from the fuel tank and the carburetor. A positive crankcase ventilating system (PCV) controls HC emissions from the engine's crankcase. An exhaust gas recir-culation system (EGR) controls NOx from the engine's exhaust. Air injection reaction (AIR), thermostatic air cleaner, and catalytic con-verter systems control HC & CO in the engine exhaust.