FUEL TANK AND CARBURBTTOR VENTILATION

The gasoline vapours from the fuel tank escape into the atmosphere by evaporation or ‘breathing’. With the increase of temperature, the air inside the fuel tank which carries gasoline vapours, expands and is thus forced out through either the filter cap vent or the tank vent tube. When the temperature decreases the air from outside enters the tank. Thus with the increase and decrease of temperature, the fuel tank 'breathes', thereby discharging gasoline vapours into the atmosphere. Gasoline is also lost from the carburettor float bowl by evaporation when the engine is not running. It is estimated that the losses from the fuel tank and carburettor are about 10% of the total hydrocarbon emission of the engine into the atmosphere. A simple system to control these losses. The fuel tank and the carburettor float chamber are not vented into the atmosphere. They are, instead, vented to a carbon canister, which adsorbs the vapour and stores it. The vapour- liquid separator is a chamber on the fuel tank. Vapours go to the top of the separator while liquid gasoline is returned to the tank. Flow of vapours from the fuel tank is controlled by a mechanically operated vent valve or an electrically operated solenoid valve. The throttle linkage operates the mechanical valve so that it is open during idling which makes the vapour flow from the float chamber to the canister. When the throttle is opened, the vent valve is closed. Likewise, with the ignition switch off, the electrical vent-valve is open. When the ignition switch is on, the vent-valve is closed by the energization of the solenoid. A special filler cap is omployed on the fuel tank which maintains some pressure in the tank, that forces fuel vapours from the tank to the carbon canister. When the engine is operated, fresh air removes the stored vapours from the carbon in the carbon canister and carries them through the inlet manifold into the engine combustion chamber to be burnt there. The control of flow of fuel vapours to the carbon canister is done in some cases by a special purge control valve which is operated usually by EGR port (discussed later) vacuum. The purge control valve is closed when the engine is not running. when the engine is idling, the valve remains closed. However, when the throttle is opened, vacuum applied to the purge control valve opens it, allowing the vapours to be purged from the canister into the intake manifold.