Evaporative Emissions System: Description and Operation

EVAPORATIVE EMISSION CONTROL SYSTEM (WITH ON-BOARD REFUELING VAPOR RECOVERY SYSTEM)
The component parts of this system are as shown in 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 below. While filling fuel:
The atmospheric pressure is drawn through the fuel filler into the P1 chamber of the refuel vapor control valve. On the other hand, the pressure in the fuel tank P2 rises as the fuel level rises and when it exceeds the total force of the P1 pressure and the return spring force, the diaphragm valve opens. Then, 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.
Except during refueling, the vapor control valve remains closed as the pressure in P1 and P2 is kept almost equal.
When the fuel level is lower than the specified level with the engine running or when the vehicle is running:
As ECM (PCM) opens the tank pressure control solenoid valve, the vapor in the fuel tank flows through the tank pressure control solenoid valve into the EVAP canister and is stored there.
Other than the above:
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.
As the pressure in the fuel tank lowers to the specified level, the vacuum valve in the tank pressure control valve opens and the air is drawn into the fuel tank.

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 (PCM) according to signals from various sensors.
- When the engine is normal operating temperature and the A/F is being fed back, ECM (PCM) 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.)

The EVAP canister air valve and the fuel tank pressure sensor among the component parts are installed so that ECM (PCM) 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 (PCM) when checking such leakage.