Evaporative Emission Control For S.I Engine

Petrol Vapors from fuel tank escape into atmosphere by evaporation ( Or breathing ). This Vapor comes through filter cap vent or the tank vent tube. When the engine is not running , petrol will evaporate in the carburetor float chamber. It is estimated that the loses from the fuel tank are about 10% of the total Hydrocarbon Emission.

In this , fuel tank and carburetor float chamber are vented to a carbon canister. The canister absorbs the vapor and stores it. "Absorb" means the petrol vapors are trapped by stricking to the outside of the charcoal particles.

A vapor-liquid separator is provided on the top of fuel tank. Vapors go to the tap of the separator is filled with filter material that allows vapor, but not liquid to pure. A float type separator in which the float goes up and closes the orifice to the canister hose.

Carburetor float bowl has two vents. One is connected to the air cleaner and helps to compensate a clogged air cleaner. The other vents connect to charcoal canister. Various arrangements may control this vent. A special filter cap is provided on the fuel tank which maintains some pressure in the tank, that forces fuel vapors from the tank to the canister. Flow of Vapors from the fuel tank to the canister. Flow of vapors from the fuel tank may be controlled by a mechanically operated vent valve or an electrically operated solenoid valve. The throttle linkage operates the mechanical valve. By that, it allows the vapors from float chamber to canister during idling.

When the throttle is opened ,vent valve is closed. Likewise the electrical vent valve is opened with the ignition switch off. When the ignition switch is on the vent valve is closed by the energization of solenoid.

When the engine is operated, the fresh vapor enters through the canister and picks up the petrol vapor from the carbon in the canister. This air then flows into the intake manifold and becomes part of air-fuel mixture entering the engine cylinders to be burnt there. This action of clearing the trapped petrol vapor from the cannister is called purging.

Some Evaporative control systems have the purge valve in the purge line. It connects to the throttle body and discharges vapors just above the throttle valve into the intake air.

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