Early vehicles vented the fuel tank through the filler cap into the atmosphere. Some of the fuel in the tank would vaporize. Some vapors escaped from the filler cap, some from the carburetter.
Non-vented filler caps are designed to stop the exit of vapors. A vacuum relief valve can relieve low pressure in the tank when the temperature drops. This will also stop the tank from collapsing if its internal pressure falls below atmospheric pressure.
The fuel cap may also incorporate a pressure relief valve. If the tank’s internal pressure exceeds the set value of the relief valve, it can stop the tank from rupturing. Some modern caps have no valves at all, and are completely sealed to stop the entry of air and water, as well as the emission of fuel vapor.
Modern tanks also contain an expansion volume either directly in the shape of the tank, or in a separate chamber connected to the fuel tank by tubing.
A liquid vapor-separator may be connected to the tank by a number of tubes. This separator allows liquid fuel to separate from the vapors, and return to the tank.
A vapor line is connected to the vapor space in the tank, or the liquid vapor separator. It carries fuel vapors from the tank to a storage volume. This vapor line can incorporate check valves. If the vehicle is tilted too far from the horizontal, they stop liquid fuel entering the storage volume.
A storage device is used to store the fuel vapors. The fuel tank breathes through this storage device. Some vehicles use the engine crankcase.
When the temperature of the fuel in the tank increases, fuel vapors are forced along the vent line, past a liquid check valve, and into the crankcase.
When the engine starts, the Positive Crankcase Ventilation system flushes vapors out of the crankcase and into the intake manifold where it joins with the inlet air-fuel mixture. Once in the inlet manifold, the vapors are drawn into the engine where combustion can convert them into carbon dioxide and water vapor.