Evaporative Emissions System: Description and Operation

EVAPORATIVE EMISSION CONTROL SYSTEM
An evaporative emission control system is used to prevent emission of fuel vapor.
The vapor generated in the fuel tank while driving or the engine at a stop passes through a tank pressure control valve and enters the EVAP canister where the charcoal absorbs and stores the fuel vapor.
The EVAP canister purge valve is controlled as follows by ECM according to signals from various sensors.
- When the engine is normal operating temperature and the A/F is being feed 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 following factors so as to prevent the A/F from being changed suddenly.
- Engine speed
- Manifold absolute pressure
- Concentration of fuel vapor in purge line (fuel vapor concentration is calculated based on how much the feed back A/F changes when the canister is purged and not purged.)
When the EVAP canister purge valve opens, fuel vapor in the EVAP canister is sucked into intake manifold.
At this time, EVAP canister is purged or cleaned by the air drawn through the air pipe.

Tank pressure control valve operates as follows:
As tank pressure control solenoid vacuum valve closes under the following either condition, vapor passage of tank pressure control valve also closes.
- Engine stops or
- Engine is running, vehicle stops and fuel level is higher than specified.

Therefore, when the pressure of the vapor generated in the fuel tank increases higher than specified pressure, it pushes open the tank pressure control valve and enters EVAP canister.
However, under the conditions that the engine is running and the fuel level is lower than the specified level (or vehicle is running and fuel level is higher than the specified level), tank pressure control solenoid vacuum valve is turned ON by ECM and vacuum is applied to the diaphragm of tank pressure control valve. Thus, the vapor passage of tank pressure control valve opens to allow continuity between the fuel tank and EVAP canister.
The EVAP canister air valve and the fuel tank pressure sensor 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.