Detection of EVAP purge hydrocarbon concentration

A method of operating a vehicle having

• an internal combustion engine with an air induction system,
• a fuel tank connected to the engine to supply fuel to the engine,
• an electronic engine control module comprising a programmed microprocessor controlling fuel delivery to the engine, and
• a canister to adsorb vapor from the fuel tank comprising a vapor inlet coupled to the fuel tank, a purge outlet coupled to the air induction system, and an air inlet having a mass flow meter and a purge valve operable by a signal from the electronic engine control module, comprising steps of:
• adsorbing fuel vapor from the fuel tank into the canister through the vapor inlet;
• desorbing fuel vapor from the canister through the purge outlet by opening the purge valve through a signal from the electronic engine control module and drawing air through the canister into the air induction system;
• measuring the air flow rate at the air inlet with a mass flow sensor that provides a value of the air flow rate to the electronic engine control module;
• calculating in the electronic engine control module an approximate hydrocarbon flow rate from the canister by subtracting the value of the air flow rate at the air inlet from a known value for maximum flow rate from the canister;
• using the electronic engine control module to adjust fuel delivery from the fuel tank to the engine in response to the calculated approximate hydrocarbon flow rate.