In its normal operation, the internal combustion engine releases several compounds into the atmosphere. Since most of these compounds are harmful to our health if inhaled or ingested in sufficient quantity for long periods of time, the Federal Government has placed a limit on the quantities of the three main groups of compounds: unburned hydrocarbons (HC); carbon monoxide (CO); and oxides of nitrogen (NOx).
The emissions systems covered in this section are designed to regulate the output of these noxious fumes by your car's engine and fuel system. Three areas of the automobile are covered, each with its own anti-pollution system or systems: the engine crankcase, which emits unburned hydrocarbons in the form of oil and fuel vapors; the fuel storage system (fuel tank and carburetor), which also emits unburned hydrocarbons in the form of evaporated gasoline; and the engine exhaust. Exhaust emissions comprise the greatest quantity of auto emissions, in the forms of unburned hydrocarbons, carbon monoxide, and oxides of nitrogen. Because of this, there are more pollution devices on your car dealing with exhaust emissions than there are dealing with the other two emission types.