Principles of Operation
WARNING: This page is about a different car, the 2018 Ford EcoSport. However, it is still accessible from the selected car via links, so may be relevant.
The system prevents fuel vapours from the fuel tank escaping into the atmosphere. Fuel vapours are stored in the evaporative emission canister until they can be delivered via the canister purge valve to the intake manifold and burnt as part of the normal air/fuel mixture.