"garry parker" mumbled:
Emmisions legislation forbids the venting of fuel vapours to the air. Fuel releases vapour all the time. When the engine's running it is vented to the intake and burnt. When the engine's off it's vented through a charcoal filled cannister. When the engine is next started air is sucked through the cannister to purge the vapour. This polluted air is then passed to the inlet.