Evaporative Emissions System: Description and Operation

The evaporative emission control system is used to control hydrocarbons emitted to the atmosphere from the fuel system. Hydrocarbon emissions are routed to an activated charcoal trap, where they are absorbed and stored until they can be vaporized and ducted into the intake manifold for combustion. The system consists of the following:

1. Fuel tank with a positive sealing filler cap.

2. Vapor-liquid separator.

3. Check and control valves.

4. Vapor vent line.

5. Carbon canister.

6. Vacuum signal line or hot air line.

7. Canister purge line.

Fuel vapors from the sealed fuel tank and carburetor float bowl are led into the carbon canister. The vapors are absorbed by the activated charcoal in the canister, whenever the engine is stopped or idling. Vacuum pressure from the signal line forces the purge control valve to open, which allows vacuum from the purge line to evacuate the canister. In addition, on some models, warm air from the exhaust manifold is routed through the canister to aid in vaporizing fuel stored in the canister.