While upgrading the BS-III to BS-IV Stage Government have made mandatory of EVAP system in all two wheeler, before going into deep first understand what it means.
In BS-III Type two wheeler there was a small pin hole located in the centre of the fuel tank cap. The purpose of this hole is simple just to release the fuel vapors from the fuel tank. If we park the vehicle under the sun light or outside gasoline get heat and fuel vapors are created and escaped through this vent to atmosphere.
Why it harmful ?
Basically Gasoline is a Petroleum product, it contains variety of different carbon contents and also many chemical which are harmful to the environment and health. If the vapors of Petrol enters in the atmosphere it leads to adverse effect on environment and human health. It causes skin problems, asthma and many more disease. These lighter vapors reacts with air and creates smogs.
How to control ?
By adopting simple system called EVAP (Evaporaive Emission Control system) we can control this. If you check the below image you will get rough idea of the system.
First Check the fuel tank cap it does not contains the air vent hole. Now think what happens to the vapor created inside the tank ? Instead of air vent a small tube provided inside the tank, it carrier all vapors created inside the tank to outside and other end of this tube is connected to the small sealed container called as Charcoal Canister.
Working of Charcoal Canister - is simple, it contains charcoal, the fuel vapors escaped from the fuel tank enter into the canister and stored inside.
In above figure you can check another tube coming from the carburattor bowl also looped in this circuit, this carries vapors from the carburettor to the canister (only for carburattor type and not applicable in FI System).
Out vent of the Canister is connected to the Purge Valve. This purge valves controls when these vapors has to be re feed to the engine through the carburettor. Let us know how the Purge valves operate.
Purge Valve - is a simple construction. It contains two chambers and a simper valve compressed against its seat by a spring. In normal condition i.e engine off or running in idle or slow speed the valve is in closed position. In this position fuel vapors which travels from the canister stopped in the purge valve chamber, the valve does not allows to enter it into the carbuettor.
At the bottom side of the purge valve a small tube is connected to the Intake manifold, when engine RPM increases it leads to increase vacuum in the lower area of purge valve. This action cause to pull the Purge valve against its spring tension.
As soon as valve comes down fuel escapes through the passage and supplied to the carburettor. Now the fuel vapors stored in the Charcoal canisters escapes through the purge valve and enters into the combustion chamber through carburettor.
Now the cycles completes, In this system fuel tank is fully sealed and there is no vent for vapor escape to the atmosphere, what ever created inside it directly recycle through the charcoal canister and feed back to the engine.
