Evaporative emission control system with reduced running losses

Abstract

An evaporative emission control system that operates in a running loss mode and an active weathering mode during purge to substantially eliminate running losses during operation of an internal combustion engine. The evaporative emission control system includes a charcoal canister, a canister vent valve, and a purge valve that permits fuel vapors from the canister and engine fuel tank to be purged into the engine's air intake manifold. The running loss mode operates to close the canister vent valve when the gas pressure within the fuel tank increases above a threshold. The vent valve is maintained closed until the fuel tank pressure drops below a lower limit. This prevents running losses by closing the vent when higher pressures are detected that cannot be reduced by purging under the current engine operating conditions. The active weathering mode cycles the canister vent valve open and closed when the volatility of the fuel is determined to be too high for the current ambient temperature. This cycling forces air changes within the fuel tank to accelerate the weathering of the volatile components in the fuel. Fuel volatility is estimated based on tank temperature and fuel vapor concentration. The maximum desired volatility is determined for the ambient temperature and the active weathering mode is begun when the estimated volatility exceeds the maximum desired volatility.