Another kind of storage device is a canister of activated charcoal. “Activated” means the charcoal is porous, with a large surface area. It can store large quantities of fuel vapour. It has connections for the fuel tank vent line and the purge line, which carries the vapours to the intake manifold. In some designs on carburetted engines, it is also has a connection from the carburetter bowl.
When the engine is running, the action of the piston during the intake strokes, creates a low-pressure area in the inlet manifold. This can be used to open a purge valve, which draws fresh air into the bottom of the canister. The air collects the vapour and directs it to the inlet manifold where it is drawn into the engine and burned. The purge valve is designed to operate only at speeds well above idle.
If it operated at low speeds, the extra fuel vapours could upset the air-fuel mixture, which could cause poor idling and rough running.
The evaporation of automotive fuel is a major source of hydrocarbon emissions.
The rates of evaporation are higher with gasoline than diesel, because gasoline is more volatile.
Source: CDX Global
