Evaporation of gasoline from the fuel system is potentially a much greater source of airborne hydrocarbons than tailpipe emissions, so modern gasoline vehicles are equipped with fuel systems which are fully closed and designed to maintain a stable fuel tank pressure without allowing vapour to escape to the atmosphere. A canister of activated charcoal absorbs excess fuel vapour which builds up in the system, especially in hot conditions, and holds it until engine operating conditions are suitable to accept a fuel-rich mixture. Then the vapours are purged into the engine intake manifold where they mix with incoming air and are burned in the engine. The canister is then cleared of fuel and ready for further use.