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A vehicle's evaporative emission control system is continuously working, even when the vehicle is not running, due to generation of vapors from the fuel tank during ambient temperature variations. Diurnal temperature cycles cause the fuel tank to breathe the fuel vapor in and out, and thus the activated carbon canister is constantly loading and purging the hydrocarbon vapors. This paper discusses a study undertaken at Ford to evaluate the relationship between carbon canister condition and fuel tank vapor generation during diurnal cycles. The results of an extensive set of experiments are presented, and the data from these experiments are compared to the output of a fuel vapor system model also developed at Ford. Key parameters relating to the migration of hydrocarbons during the experimental conditions studied, including initial canister condition, canister volume, and canister geometry, are discussed.

Automotive fuel vapor emissions that would otherwise evaporate into the atmosphere are being captured in activated carbon vapor storage canisters. Fuel vapor is loaded into the canisters via a direct connection to the fuel tank vapor dome. Hydrocarbons are desorbed from the activated carbon into the engine combustion cylinders using engine intake vacuum. The carbon canister capacity requirements have increased in recent years in order to meet both Enhanced Evaporative Emission regulations and the Clean Air Act emission requirements for On-board Refueling Vapor Recovery (ORVR). The higher capacity requirements have generated the need for larger volume canisters that can meet the emission requirements and still be designed within the space and packaging limits of the vehicle application. This paper describes the simultaneous engineering approach used at Ford Motor Company to design a large volume cylindrical shaped carbon canister.