Evaporative Emissions System: Description and Operation

Fig. 7 EVAPORATIVE CONTROL SYSTEM:

PURPOSE
The evaporative control system prevents gasoline vapors from escaping into the atmosphere. Vapors from the gas tank are routed to the evaporative control canister. Carbon in the canister stores the gasoline vapors until the engine is started. The intake manifold vacuum of the running engine draws the vapors into the combustion chamber. Fig. 7.

Fig. 65 EVAPORATIVE CONTROL SYSTEM (TYPICAL):

CONSTRUCTION
The evaporative control system consists of a charcoal canister (which contains activated charcoal), vacuum valve, roll-over valve and connecting hoses. Fig. 65.

OPERATION
The evaporative control system controls pressure variations within the gas tank resulting from changes in fuel volume and temperature. The vacuum valve controls the fuel vapors going to the intake manifold. The vacuum valve is controlled by the throttle valve upstream vacuum.

When the engine is idling, the vacuum valve is closed. Fuel vapor is absorbed by the activated charcoal in the canister. As engine speed increases, upstream vacuum also increases, opening the vacuum valve. This allows gasoline vapors to be drawn into the combustion chamber.