Motor vehicles are subjected to wide variations in temperature and air pressure, both while in use and when parked. Elevated temperatures and reduced air pressures, in particular, result in the generation of hydrocarbon vapors within the vapor space of the vehicle's fuel tank. Modern motor vehicles include a gas cap adapted to seal the open end of the tank filler neck, to prevent atmospheric venting of these polluting vapors. Such vehicles also are equipped with a charcoal canister, having an inlet interconnected to the vapor space in the upper portion of the fuel tank, by means of a vapor vent line. The outlet of the canister is interconnected to the engine's air/fuel intake system, such as the intake manifold, or the like.
The function of the charcoal canister is to absorb excessive gasoline vapors generated during high temperature and/or low ambient pressure conditions, while simultaneously avoiding a dangerous vapor pressure buildup in the fuel tank. Thus, in a proper functioning evaporative system, when pressure in the vapor space exceeds atmospheric pressure, fuel vapors migrate through the vapor vent line into the vehicle's canister, where the hydrocarbons are absorbed by the charcoal. If vapor pressure is sufficient, the filtered vapor is safely exhausted to the atmosphere through an air vent in bottom of the canister.
Then, during driving, the vacuum existing in the intake manifold draws fresh air in through the same vent, vaporizing the hydrocarbons. The gas vapor is drawn by vacuum through the purge line, and introduced into the intake manifold for combustion. In this manner, the gasoline is fully utilized, and the charcoal canister is fully purged of vapors, restoring its hydrocarbon storage capacity for the next cycle.
If the charcoal canister is not regularly purged while the vehicle is driven, the charcoal will eventually become saturated and the trapped hydrocarbons will escape directly into the atmosphere through the canister's air vent. When this occurs, the major advantages provided by the evaporative system are defeated. The negative environmental impact of uncontrolled vapor discharge through the canister rivals that of the vehicle's exhaust emissions. Moreover, this vapor emission also decreases the overall fuel economy of the vehicle, by venting hydrocarbons which would otherwise be burned in the engine. Consequently, proper purging of the canister has significance both for the environment and for the conservation of energy.
, and 
