Evaporative Emissions System: BMW: Operation
When the vehicle is stopped and engine is off, or while standing on a non-level position, the gas vapors are collected in the liquid-vapor separator where part of them condense and flow back to the fuel tank. The vapor continues to the charcoal canister where it is absorbed and retained until the engine is started again.
After starting, a suction effect causes a flow into the intake manifold and the gas vapor is burned by the engine. The liquid-vapor separator is also capable of compensating for fuel expansion of a completely filled gas tank when ambient temperatures are about 80°F (27°C).