Evaporative Emissions System: Description and Operation

PURPOSE AND OPERATION
The system is designed to prevent harmful fuel vapor from escaping to the atmosphere. The system consists of a new vapor separator tank connected to the fuel tank, and located between the body inner and outer panels on the right hand side of the vehicle near the rear wheel arch. An adsorption canister, containing activated charcoal, is positioned in the engine compartment attached to the front right hand fender valance. The two components are connected by a pipe running the length of the chassis.





A Pressure relief to atmosphere.
B From fuel tank to separator.
C To adsorption canister.
D Pressure relief valve.
E Pressure relief valve.
F Shut-off valve.
G "Speed Fit" connectors.


A pressure relief valve is fitted in the hose open to atmosphere, which would act as a safety valve should a build-up of pressure occur in the system, for example if a hose became blocked or kinked. The volume of vapor emitted, in such an instance, would be acceptable.


A pressure relief valve is also fitted in the hose to the adsorption canister and releases vapor to the canister when the pressure in the separator reaches between 0.75 and 1.0 psi.


In the top of the separator a shut-off valve is incorporated in the vapor exit port to prevent the possible presence of any liquid fuel being transmitted to the adsorption canister should the vehicle roll over.





The adsorption canister, which is connected by hose to the plenum chamber, absorbs and stores the fuel vapor from the fuel tank while the engine is not running. The vapor is purged from the canister by air drawn through an orifice in the base of the canister and by the influence of vacuum at the top. The vapor drawn into the plenum chamber through a solenoid operated purge valve is finally burnt in the combustion chambers.

The purge valve, which is attached to the adsorption canister support bracket, is controlled by the fuel injection electronic control unit (ECU) which determines the most emission acceptable time at which purging should take place. This will normally be at engine speeds above idle and when the vehicle is in motion. A signal from the ECU to the purge valve operates the solenoid and opens the valve to purge the canister of fuel vapor.