Refueling Vapor Recovery System: Description and Operation

EVAPORATIVE EMISSION (EVAP) CONTROL SYSTEM




WITH ON-BOARD REFUELING VAPOR RECOVERY SYSTEM
The component parts of this system are as shown in the figure. Its role is to prevent the fuel vapor from emitting into the air.

Operation
1. The vapor produced in the fuel tank is stored in the EVAP canister.
The vapor in the tank flows to the EVAP canister and is stored there. When the fuel tank becomes full, the float in the vapor control valve works to close the vapor passage, rejecting further refueling.
When the pressure in the fuel tank rises to the specified level, the pressure valve in the tank pressure control valve opens and the vapor flows into the EVAP canister and is stored there.

2. When the EVAP canister purge valve opens, the fuel vapor stored in the EVAP canister is drawn into the intake manifold together with the air coming in through the suction filter and burned.
The EVAP canister purge valve is controlled as follows by ECM according to signals from various sensors. When the engine is normal operating temperature, throttle valve opens wider than closed throttle position and the A/F is being fed back, ECM controls ON/OFF (open/close) switching of EVAP canister purge valve at a constant rate of 10 time/sec. and the length of this ON time depending on the following factors so as to prevent the A/F from being changed suddenly.
- Engine speed
- Engine load
- Concentration of fuel vapor in purge line (fuel vapor concentration is calculated based on short term fuel trim and vapor purge rate.)

The EVAP canister air valve and the fuel tank pressure sensor among the component parts are installed so that ECM diagnoses leakage of the fuel vapor from the evaporative emission control system. The EVAP canister air valve is usually open but it is closed by ECM when checking such leakage.