Evaporative Emissions System: Description and Operation

The Evaporative Emission system is designed to limit the amount of fuel vapors released into atmosphere, to meet Federal and State requirements for fuel evaporation.
All gasoline engines are required to have Evaporative Emission systems. The system consists of a sealed fuel tank, carbon canister, hoses and associated valves, fittings, to connect system together. Vapors are stored in the carbon canister until engine has been started, then at a pre-determined time the vapors are purged into the intake manifold to be burnt.


Purge System - Fuel Tank Venting
Fuel vapors trapped in the sealed fuel tank are vented through the orificed vapor valve assembly in the top of the tank. The vapors leave the valve assembly through a single vapor line and continue to the carbon canister (located in the engine compartment or along the frame rail), for storage, until they are purged to the engine for burning.

Typical EEC-IV Evaporative Emission System:

Canister Purging - EEC-IV
Purging the carbon canister removes the fuel vapors stored in the carbon canister. With an EEC-IV controlled purge system, the flow of vapors from the canister to the engine is controlled by a purge solenoid or a vacuum controlled purge valve. Purging occurs when the engine is at operating temperature and off idle.

Typical Combined EEC-IV And Purge Ported System:

Canister Purging - Combined PORT and EEC-IV
Purging the carbon canister removes the fuel vapors stored in the carbon canister. With a purge port system, the vapors flow from the carbon canister to the throttle body. With an EEC-IV controlled purge system, the flow of vapors from the canister to the engine is controlled by a purge solenoid or a vacuum controlled purge valve. Purging occurs when the engine is at operating temperature and off idle.

Carbon Canisters, Typical:

CARBON CANISTER
The fuel vapors from the fuel tank and carburetor and fuel bowl are stored in the carbon canister until the vehicle is operated, at which time, the vapor will purge from the canister into the engine for consumption. There are two canisters sizes, 925 ml and 1400 ml carbon. Canisters are sometimes used in pairs when the vehicle has a large fuel tank or dual carburetor bowls.