Evaporative Emissions System: Description and Operation

EVAPORATIVE EMISSION (EVAP) CONTROL SYSTEM




WITHOUT ON-BOARD REFUELING VAPOR RECOVERY SYSTEM
The evaporative emission control system used in this vehicle consists of those parts as shown in figure. When the pressure of the vapor generated in the fuel tank increases higher than specified value, it pushes open the tank pressure control valve and enters EVAP canister.

However, under the conditions that the ignition switch is ON, the fuel level is lower than the specified level, possibility of the fuel liquid flowing into the vapor line is little, tank pressure control solenoid valve is turned ON by ECM (PCM) and vacuum is as applied to the diaphragm of tank pressure control valve while the engine is running.

Thus, the vapor passage of tank pressure control valve opens to allow continuity between the fuel tank and EVAP canister. As a result, under above conditions, the vapor in the fuel tank flows into EVAP canister without being stopped by tank pressure control valve.

The EVAP canister purge valve is controlled by ECM (PCM) according to signals from various sensors.When none of the following conditions exists, ECM (PCM) controls ON/OFF (open/close) switching of the purge valve at a constant rate (10 times per second) and its opening (amount of purge air) by increasing and decreasing its ON time.
- When throttle valve is at closed position.
- When engine coolant temp. is low.
- When engine speed is low and engine load is low.
- When vehicle is stopped.
- For specified time after engine start.

The higher the engine speed rises and the more fuel mixture is drawn in, the larger amount of air is purged. When the purge valve opens, fuel vapor in the canister is sucked into intake manifold.

In this state, the EVAP canister is purged or cleaned by air drawn through the EVAP canister air valve. Also, ECM (PCM) seals the EVAP system by turning ON (closing) the EVAP canister air valve and checks the EVAP gas leakage from the EVAP system at this time by checking variation in the output voltage of the tank pressure sensor.




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.