Evaporative Emission Control System (EVAP)

To reduce the emission of fuel vapor, the fuel tank is vented to atmosphere through activated charcoal adsorption canister(s) which collects the fuel droplets. The charcoal is periodically purged of fuel when the EVAP Canister Purge Valve opens the vapor line between the canister(s) and the air intake induction elbow. This action allows manifold depression to draw air through the canister atmospheric vent, taking up the deposited fuel from the charcoal absorber and burning the resulting fuel vapor in the engine.

The EVAP Canister Purge Valve is controlled by the engine management system ECM. Purging is carried out in accordance with the engine management fueling strategy (see below)

The fuel tank vapor outlet is via a removeable flange assembly on the top surface of the tank. The vapor storage canister or canisters are fitted on the underside of the vehicle below the rear seats.

There are three variants of the evaporative system. All systems use the charcoal absorber storage canisters and purge valve and operate as described above. The specific features of each system are described below. The evaporative systems are designated as:

single canister system
running loss system
running loss with On-board Re-fueling Vapor Recovery (ORVR) system

EVAP Canister Purge Valve:

The EVAP canister purge valve controls the flow rate of fuel vapor drawn into the engine during the canister purge operation. The valve is opened by a vacuum feed from the induction elbow: the vacuum feed is controlled by the integral valve solenoid and is applied when the solenoid is energized. The solenoid is pulsed on (energized) and off by a fixed frequency (100 Hz) variable pulse width control signal (pulse width modulation). By varying the pulse on to off time, the ECM controls the duty cycle of the valve (time that the valve is open to time closed) and thus the vapor flow rate to the engine.

With no ECM signal applied to the valve solenoid, the valve remains closed.

Canister Purge Operation
The following pre-conditions are necessary for purging to commence:

After battery disconnection/reconnection, engine management adaptations must be re-instated.
Engine has run for at least 8 seconds.
Engine coolant temperature is not less than 70 °C.
Engine not running in the fuel cut off condition (eg overrun).
The adaptive fuel correction function has not registered a rich or lean failure
The evaporative emission leak test has not failed
No faults have been detected in the relevant sensor and valve circuits - Air Flow Meter (AFM), Engine Coolant Temperature sensor, Evaporative Canister Purge valve and Canister Close Valve (CCV).

If these conditions have been satisfied, purging is started. If any failures are registered, purging is inhibited.

The canister(s) is purged during each drive cycle at various rates in accordance with the prevailing engine conditions. The engine management software stores a map of engine speed (RPM) against engine load (grams of air inducted/rev). For any given engine speed and load, a vapor purge rate is assigned (purge rate increases with engine speed and load).

The preset purge rates are based on the assumption of a vapor concentration of 100%. The actual amount of vapor is measured by the closed loop fueling system : the input of evaporative fuel into the engine causes the outputs from the upstream oxygen sensors to change, the amount of change providing a measure of the vapor concentration. This feedback causes the original purge rate to be adjusted and also reduces the amount of fuel input via the injectors to maintain the correct air to fuel ratio.

Engine speed/load mapping and the corresponding purge rates are different for single canister, running loss and ORVR evaporative systems.