Evaporative Emissions System: Description and Operation

Fig. 2 Evaporative emission control system.:

The evaporative emission control system collects vapors from both the fuel system and the engine crankcase. The vapors are stored in a charcoal canister until they are drawn into the engine intake system to be burned in the engine, Fig. 2. An engine breather filter prevents crankcase by products from being drawn into the vapor control system. A vapor separator prevents liquid fuel from entering the vapor system.
The fuel tanks have a 10% expansion area built in, thereby allowing for fuel and vapor expansion. This is done by extending the filler pipe into the fuel tank, thereby making a liquid/vapor barrier at the filler neck line.
A 1 mm orifice at the top of the filler neck allows the expanding fuel to displace the air from the tank into the vapor storage system.
A pressure relief valve controls the transfer of vapor from the vapor separators to the charcoal canister by preventing transfer until a preset pressure is achieved. To allow a flow from the canister to the tank, a vacuum relief valve is used.