On-Board Refueling Vapor Recovery (ORVR): Notes
The ORVR system recovers and stores hydrocarbon fuel vapor that was previously released during refueling. Non ORVR vehicles vent fuel vapors from the tank venting line back to the filler neck and in many states reclaimed by a vacuum receiver on the filling station's fuel pump nozzle.
When refueling an ORVR equipped vehicle, the pressure of the fuel entering the tank forces the hydrocarbon vapor to the top of the fuel tank which is cool (The tank has an "integrated" liquid/ vapor separator). This causes the most of the vapor to condense and stay in the tank. The vapor that is not condensed is sent through the rollover valve and into the charcoal canister. The HC is stored in the charcoal canister, and the system can then "breath" through the LDP and the air filter. The vent line to the filler neck is smaller, but still necessary for checking the filler cap/neck during Evaporative Leak Testing.