Evaporative Emission Control

In the uncontrolled vehicles, fuel vapours from the fuel tank and carburettor were vented into the atmosphere that constituted about 20% of all hydrocarbon emissions from a gasoline passenger car.  From 1970, evaporative emission control was required to be employed on production gasoline vehicles in the USA.

• The evaporative emission control system consists of a device to store fuel vapours produced in the fuel system due to evaporation.
• A canister containing activated charcoal is used to store the fuel vapours.
• The vapours produced in the fuel tank normally collect in the fuel tank itself and are vented to the charcoal canister when fuel vapour pressure becomes excessive.  The fuel vapours from the tank and carburettor led to and adsorbed into the charcoal. In the PFI engines only the fuel tank is connected to the canister.
• When engine is running, the vacuum created in the intake manifold is used to   draw fuel vapours from the canister into the engine. Purging air is sucked through the canister which leads the fuel vapours from canister to the engine. An electronically controlled  purge valve is used.
• During engine acceleration additional mixture enrichment can be tolerated and under these operating conditions the stored fuel vapours are usually purged into the intake manifold.
• This system is a fully closed system. A sealed fuel tank filler cap is used and a stable fuel tank pressure is maintained by the purging process of the canister.

A typical schematic layout of evaporative control system is shown in Fig. 5.7. Given below are some of the measures adopted to achieve near zero evaporative emissions as required in California;

• Sealed fuel tank is kept under vacuum to prevent permeation of fuel through walls of a polymer fuel tank and leakage of fuel vapours through filler cap.
• Fuel tubing made of high density polymer or steel to reduce/prevent fuel permeation.
• Better canister technology and more effective activated charcoal.
• Employment of refuelling vapour recovery (ORVR) system as during vehicle refuelling maximum share of fuel evaporative emissions  escape..
• A carbon trap to arrest the escape of fuel vapours from intake manifold. When the vehicle is standing and is under hot soak fuel vapours can escape past the throttle body into atmosphere.