The term “emission” normally refers to the pollution produced by a light vehicle during normal use. Emission control systems are designed to limit the pollution caused by the harmful products of storing and burning fuel.
Emissions from a gasoline driven motor vehicle usually come from 4 sources:
The fuel tank and carburetor allow fuel to evaporate and escape to the atmosphere. These are called evaporative emissions.
The crankcase and exhaust system emit pollutants directly from the engine into the atmosphere. They are caused when hydrocarbons, lead compounds, and oxygen and nitrogen from the air, are burned in the combustion chamber.
In a compression-ignition engine, emissions originate from the engine, and escape to the atmosphere from the exhaust, and the crankcase breather.