Evaporative Emission Control System (EVAP)

Fig. 1 EVAP Circuit:

Evaporative Emission Control, (EVAP), limits the escape of fuel vapor from the fuel tank into the atmosphere. The fuel tank is capable of being filled to approximately 90 % of its volume, the additional 10 % is a volume of air which allows for expansion for the fuel without it escaping from the tank. The air above the fuel in the tank is charged with fuel vapor, which if the tank was allowed to vent to atmosphere, would escape as a pollutant.

In order to collect the vapor, the tank is vented through a canister (1) packed with activated carbon. The fuel vapor is desorbed onto the carbon and held there until the ECM commands the EVAP control valve (2) to open and allow manifold depression to draw air through the carbon via the controlled entry port (3). The flow of air through the carbon picks up vapor from the fuel which has been desorbed by the carbon, so carrying the vapor to the inlet manifold (4) to supplement the fuel injected, and be consumed.

When the engine is at rest the fuel tank is maintained at a positive pressure of 0.069-0.092 Bar (1.0-1.33 psi) by the pressure control valve (5) pressure above this setting is released, by the valve, to the carbon canister When the engine is running, the line (6) from the inlet manifold to the pressure control valve is subject to manifold depression which activates the valve, so opening the line to the canister, allowing air drawn through the canister to replace the fuel withdrawn, to maintain atmospheric pressure in the fuel tank. If the pressure control valve is inoperative, over pressurization of the fuel tank is prevented by the fuel filler cap having a pressure release setting of 0.14-0.17 Bar (2.0-2.5 psi).