The purpose of a vacuum fuel canister system in an automobile is to prevent the direct release of fuel vapors into the atmosphere, otherwise known as evaporative emissions. Historically, canister systems were located in the engine compartment, but modern systems are normally located near the fuel tank.
As an engine draws fuel from a sealed fuel tank, it creates a slight vacuum in the tank. The tank must be vented to prevent it from collapsing, and a vacuum fuel canister system allows ventilation to take place in a closed system. This means that no hydrocarbons, or compounds containing only hydrogen and carbon, are released into the atmosphere.
Hoses route fuel vapors from the fuel tank to a canister, known as an evaporative canister, which is filled with activated charcoal. The charcoal absorbs vapors until a control valve removes, or purges, them so that they can be burned in the engine.
The evaporative canister also contains a trap that prevents liquid fuel from flooding the canister. A vacuum is created when the level or temperature of fuel in the fuel tank falls and the negative pressure draws fluid from the trap bank into the tank.
