Older models of generators would come with a vented fuel cap to allow the fuel tank vent to adjust to changes in temperature. The vented fuel cap would let air and evaporated fuel (hydrocabons) escape into the air. In an effort to reduce emissions the EPA required fuel tanks to no longer allow evaporated fuel to escape into the air. This article gives an explanation of the different parts of the fuel system that make this possible.
What happens in the tank?
To trap the hydrocarbons inside the tank we converted from plastic to steel fuel tanks. The plastic was permeable allowing hydrocarbons to escape though the tank wall. In addition, the vented cap was replaced with a non-vented sealed cap.
Plastic fuel tanks with vented fuel caps allow pressure and hydrocarbon to escape
Steel fuel tanks with non-vented fuel caps do not release pressure or hydrocarbon
What did we do to release the pressure?
If the tank is not vented, in hot weather it will bulge out and in cold weather collapse in. Eventually this would rupture the tank defeating the purpose of trying to prevent fuel from evaporating. The solution was to install a separate vent in the tank that leads to a carbon canister. This allows the tank to vent and the carbon canister catches the hydrocarbons and lets the air escape. If liquids get inside the carbon canister it will be ruined. To protect fuel from splashing out of the vent a slosh valve is provided that automatically closes if liquid splashes against it.
Where do the hydrocarbons go?
The carbon canister will store the hydrocarbon and only the filtered air will be released. Once the engine is started, it creates a vacuum and will pull the hydrocarbon through the engine, burning them through the normal combustion process.
