Fuel vapor leak detection system

Claim 1

In an engine-powered automotive vehicle evaporative emission control system wherein a fuel tank for storing volatile fuel that is combusted in combustion chamber space of the engine is operatively associated with a fuel vapor collection canister that collects volatized fuel from the tank and a canister purge valve that is periodically operated to purge collected vapor from the canister to the engine, and leak detection system is operatively associated with the evaporative emission control system for detecting leakage from that portion of the evaporative emission control system which is upstream of an inlet of the canister purge valve relative to the engine, the improvement in said leak detection system which comprises: pressurizing means having an inlet at which a source of gaseous medium is available to be pressurized by said pressurizing means and an outlet at which positively pressurized medium is made available for delivery to said portion of the evaporative emission control system; a series flow path from said outlet of said pressurizing means back to said source of gaseous medium, said series flow path comprising a first flow sensor for sensing the entire flow from said outlet of said pressurizing means, a second flow sensor for sensing flow through a portion of said series flow path that is downstream of said first low sensor and a calibrated orifice means also disposed in said portion of said series flow path that is downstream of said first flow sensor; a branch flow path that branches to said portion of said evaporative emission control system from said series flow path at a location in said series flow path that is between said first flow sensor and said second flow sensor; and means for sensing differential between flow sensed by said first flow sensor and flow sensed by said second flow sensor.