Theory of Operation - Fuel Filling and ORVR

The changes made to the fuel system for Onboard Refueling And Vapor Recovery (ORVR) limit the amount of fuel vapors released into the atmosphere during a vehicle refueling event.

Re-design of system components reduces the available path for these vapors into the atmosphere and forces them into an EVAP canister where they are stored for later consumption when the vehicle is running.

All these system changes to meet the legislated requirements for emissions reduction during refueling were met without impacting the fuel fill process. The vehicle can be refueled from any available fuel dispensing equipment, at standard fuel nozzle dispensing rates (4-10 gallons/min.), with any commercially blended grades of fuel, without problems. Changes to the system are transparent to the customer during refueling - no changes from what he or she has been used to.

As fuel is dispensed into the fuel filler pipe and into the fuel tank, vapors are generated. The gasoline pumped up from a cold underground storage tank into a warmer fuel tank expands as it warms up. The fuel tank is designed to have room for expansion of fuel and the collection of this vapor. This vapor is expanding since it is at a higher pressure than the atmosphere.

The primary flow path for the vapor is past an orifice at the top of the Fill Limit Vent Valve (FLVV). From here it flows into the EVAP canister where it is absorbed by the bed of activated carbon. Two other secondary paths for vapor include 1) grade vent valve orifice in the top of the fuel pump module and 2) an orifice in the recirculation valve at the top of the fuel filler pipe.

The vapor passing through the grade vent valve is routed into the same path as the FLVV and into the EVAP canister. The vapor at the top of filler pipe is directed into the stream of fuel dispensed from the fill nozzle and is pulled (venturi action) back into the fuel tank.

The actions described this far continue throughout the refueling process. The vapors generated travel into the EVAP canister (and back into the fuel stream) as they vent through the [1][2]EVAP vent solenoid (normally open). As the fuel fill process approaches the tank capacity, the FLVV and orifice in the grade vent valve began to function to limit the fuel dispensed

The FLVV has a buoyant float which rises on the increasing amount of fuel in the tank and closes off the main vapor path from the tank into the canister. The vapor path is now much smaller, only a small orifice in the FLVV and the grade vent valve admit vapor into the canister. Since the vapor path is now restricted, pressure in the tank rises. This pressure increase raises the level of fuel in the filler pipe back up to the fill nozzle and causes it to shut off. At the same time that the fill nozzle shuts off, the check valve at the end of the fuel filler pipe closes and prevents fuel in the tank from rushing back up the fill pipe and causing fuel "spitback".

The fuel tank is now essentially full. The fill nozzle can be operated several more times (at lower flow rates) and flow a few tenths of a gallon more until no more fuel can be dispensed. At this time, the FLVV is completely closed (to prevent liquid fuel from flowing in to the EVAP canister) and the only vapor path is through the grade vent valve into the canister. Once the fuel cap is reinstalled, the refueling cycle is completed.