Impact and Control of Canister Bleed Emissions

Current EPA and CARB regulations allow a maximum of 2.0 g/test for Hot Soak + Diurnal evaporative emissions. The State of California has adopted LEV II regulations that will decrease the evaporative emissions standard to 0.5 g/test starting in the 2004 model year. These regulations also include a Zero Emission Vehicle or ZEV program. The ZEV program allows car manufacturers to substitute vehicles that meet the SULEV tail pipe emission standards and have zero fuel evaporative emissions for electric vehicles. The increased stringency of these regulations has necessitated significant decreases in hydrocarbon emissions from evaporative emission canisters. For example, canister vent emissions may be at levels of 100-300 mg/test for a vehicle that meets the current regulations. However, canister emission targets should be 50 mg/test and less for LEV II and 10 mg/test and less for zero evaporative emission vehicles. Emissions at this level are not due to a lack of adsorptive capacity in the canister, but rather are due to diffusion of hydrocarbon species. These emissions are often referred to as bleed emissions.

A technique was developed to study the level of bleed emissions specifically from canister vent ports. Canister design and purge volume were shown to have a significant impact on bleed emissions. Further, the incorporation of a small auxiliary chamber in series with the primary canister was shown to decrease bleed emissions significantly.