Gasoline, diesel, LP and natural gas are all hydrocarbon compounds.
Hydrocarbon emissions react with other compounds in the atmosphere to produce photo-chemical smog. Hydrocarbons are a major source of motor vehicle emissions.
Gasoline needs to evaporate easily to burn properly in an internal combustion engine. But this property also means it evaporates easily into the atmosphere at ordinary temperatures and pressures.
When a vehicle is being refuelled, hydrocarbon vapors can escape from the filler neck into the atmosphere. When the vehicle is left in the sun, its temperature increases, and fuel evaporates from the tank and, if there is a carburetter, from it as well.
An evaporation control system is fitted to modern vehicles to collect, and store the hydrocarbon vapors from the tank and the carburetter. Then, when conditions are suitable, these vapors are then drawn into the intake manifold, and burned, as part of the combustion process.
In a gasoline-engined vehicle, during an engine cycle, some of the gases escape past the piston rings into the crankcase. This is called blow-by. Some of these blow-by gases mix with heated, vaporized oil and can usually escape out into the atmosphere.
Modern vehicles direct these gases and vapors back through the inlet system to be burned during normal combustion.
