Recently, the California Air Resources Board (CARB) has proposed a new set of evaporative emissions and “Useful Life” standards, called LEVII EVAP regulations, which are more stringent than those of the enhanced EVAP emissions regulations. If the new regulations are enforced, it will become increasingly important for the carbon canister to reduce Diurnal Breathing Loss (DBL) and to prevent deterioration of the canister. Therefore, careful studies have been made on the techniques to meet these regulations by clarifying the working capacity deterioration mechanism and the phenomenon of DBL in a carbon canister.
It has been found that the deterioration of working capacity would occur if high boiling hydrocarbons, which are difficult to purge, fill up the micropores of the activated carbon, and Useful Life could be estimated more accurately according to the saturated adsorption mass of the activated carbon and the canister purge volume. As a result, it is presumed that a more adaptable, longer Useful Life can be realized by providing a sufficient purge.
It has been also found that the butane diffusion in a carbon canister during vehicle parking which is loaded to the canister during the DBL test, is the main cause of evaporative emissions from the canister. To prevent such diffusion, it is effective to divide the carbon bed into separated segments and insert some “labyrinth” between such carbon beds. Compared with the conventional canister, the improved canister was able to reduce DBL by half._Furthermore it became clear that DBL is reduced to approximately 1/3 when the gasoline fuel vapor is loaded to the canister instead of butane, which is the main cause of DBL. It was also concluded that the evaluation method should be reconsidered to account for real world conditions.