Evaporative Emissions System: Description and Operation

Evaporative Emission Control System Description (with On-Board Refueling Vapor Recovery System)

The component parts of this system are as shown in the following 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 as described in the following. While filling fuel:

The vapor in the tank flows to the EVAP canister through the vapor control valve 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 fuel tank is filled with fuel:

The vapor produced in the fuel tank passes through the tank pressure control valve and flows into the EVAP canister through the refuel vapor control valve and is stored there.

When the fuel tank is not filled with fuel:

The vapor produced in the fuel tank flows into the EVAP canister through the refuel vapor control valve 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 times/sec. and the length of its 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 vent valve and the fuel tank pressure and temp. 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 vent valve is usually open but it is closed by ECM when checking such leakage.