Evaporative Emissions System: Description and Operation

On Board Refueling Vapor Recovery (ORVR) AWD




1. Canister purge (CP) valve
2. EVAP canister
3. Shut-off valve
4. Air cleaner (ACL)
5. Fuel tank pressure sensor
6. On board refueling vapor recovery (ORVR) valve
7. Roll-over valve
8. ECM
9. Expansion tank

System description
When refueling starts the check valve in the fuel tank opens. The fuel flow creates negative pressure at the top of the fuel filler pipe. This results in an air flow in the fuel filler pipe. It is important to have low resistance between the fuel tank and atmospheric pressure (via the EVAP canister shut-off valve filter) to stop fuel vapor escaping from the fuel filler pipe.
The principle of the system is called a dynamic seal.
When the fuel tank is full the ORVR valve closes and the fuel tank pressure increases rapidly. This closes the check valve in the fuel tank, preventing fuel coming out of the filler pipe when the fuel pump pistol is shut off.
Fuel vapor in the fuel tank (together with air) reach the EVAP canister via the ORVR valve. Fuel vapor is absorbed by the carbon in the EVAP canister. If the ORVR valve is closed the fuel tank is vented via the roll-over valve.
When the canister purge (CP) valve is closed the fuel vapor remains in the EVAP canister. When the engine control module (ECM) transmits an opening signal to the valve, a vacuum from the intake manifold or from the fresh air intake creates a flow through the EVAP canister. The EVAP canister is connected to the outside atmosphere via the canister shut-off filter which allows the flow to evacuate the EVAP canister. The air flow through the EVAP canister removes stored fuel vapor which flows to the engine and enters the combustion process.

On board refueling vapor recovery (ORVR) valve
There is a combined float roll-over valve (called the ORVR valve) by the connector for the large EVAP line to the fuel tank As its name implies it has two functions:
- a roll-over function which closes the large EVAP line connector so that fuel cannot leak out if the car is on its side or roof.
- a float valve function which closes the large EVAP line connector when the fuel level reaches the valve - when the fuel tank is full.

Roll-over valve
A small EVAP line runs from the ORVR valve to a separate roll-over valve located as high as possible on the fuel tank. The roll-over valve ensures that the fuel tank can breath if the ORVR valve is closed when the car has been refueled.

Expansion tank
The expansion tank is in two sections. One section takes care of shut-off at full tank (when refueling), the other is the expansion tank. Cold fuel expands as it heats up and therefore there must be 7% air left in the fuel tank system.