Evaporative Emissions

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Control of evaporative fuel vapors (hydrocarbons) from fuel tank is important for overall reduction in vehicle emissions. Evaporative system has been combined with ventilation of fuel tank, which allows tank to breathe (equalization). The overall operation provides:

An inlet vent, to an otherwise "sealed" fuel tank, for the entry of air to replace the fuel consumed during engine operation.
An outlet vent with a storage canister to "trap and hold" fuel vapors that are produced by the expansion/evaporation of fuel in the tank, when the vehicle is stationary.

Canister is then "purged" using engine vacuum to draw the fuel vapors into the combustion chamber. This "cleans" the canister allowing for additional storage. Like any other form of combustible fuel, the introduction of these vapors on a running engine must be controlled. The ECM(s) control the evaporative emission valves which regulate purging of evaporative vapors. See Fig 1.

Fig 1: Evaporative Emission System (V12)

Courtesy of BMW OF NORTH AMERICA, INC.