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 by ECM according to signals from various sensors. When none of the following conditions exists, ECM controls ON/OFF (open/close) switching of the canister 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 throttle position.
- When engine coolant temp. is low.
- When engine speed is low.
- When vehicle is stopped.

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 canister is purged or cleaned by air drawn through the filter at the bottom of the canister.

The tank pressure control valve is provided to keep the pressure in the fuel tank constant. When the pressure in the fuel tank becomes positive and reaches its specified value, it opens the valve to let the vapor flow into the EVAP canister. On the other hand, when the pressure in the fuel tank becomes negative and reaches its specified value, it opens the valve to let the air flow into the fuel tank.

Based on the change in the fuel injection volume feed back compensation during the EVAP canister purge flow (while the EVAP canister purge valve is open), ECM checks whether there is an EVAP canister purge flow or not.

At this time, if there is a change in the fuel injection volume feed back compensation, the EVAP canister purge flow is judged to be normal. But if there isn't, ECM performs control as described below while the engine is idling.

ECM stops the idle speed feed back control and then increases opening (ON duty) of the EVAP canister purge valve gradually. At this time, it checks whether there is an EVAP canister purge flow or not based on the change in the engine idle speed.