Evaporative Emissions System: Description and Operation

Evaporative Emission Canister
The evaporative emission canister consists of a container filled with activated charcoal, the task of which is to "temporarily store" fuel vapors from the tank and then release the vapors via the purge valve to the intake manifold for combustion.

The evaporative emission canister thus has a hose connection both to the fuel tank and to the intake manifold, but is also connected to the surrounding air. Air is drawn via this connection with the surrounding air through the evaporative emission canister and brings fuel vapors into the intake manifold with it.

EVAP canister purge valve The purge valve consists of a solenoid valve mounted on the vacuum hose between the evaporative emission canister and intake manifold after the throttle.

The valve is supplied with current from the main relay and controlled from pin 27 of the control module. The lead is connected to pin 21 of the control module. By measuring the voltage on pin 21, the control module determines whether the function is electrically correct.

An 8 Hz PWM signal is applied to the valve which starts to work as soon as the closed loop is activated, engine coolant temperature is above 60°C (140°F) and intake manifold pressure is below 90 kPa.

The control module starts purging by grounding the circuit to the valve for extremely short periods and then gradually increasing the duration of the periods. The reason for this is that the closed loop must be given time to compensate for the additional fuel. The valve operates for 4 1/2 minutes, stops for 1/2 a minute, operates for 4 1/2 minutes, and so on.