Evaporative emission control canister system for reducing breakthrough emissions

A second stage adsorbent filled canister for use in an emissions control system for a vehicle, said system comprising, in combination, a fuel tank for storing a volatile fuel, an engine having an air induction system and adapted to consume said fuel, a first stage canister containing a first volume of fuel vapor adsorbent material for temporarily adsorbing and storing fuel vapor from said tank, a conduit for conducting fuel vapor from said tank to a first stage canister vapor inlet, a fuel vapor purge conduit from a first stage canister purge outlet to said induction system of said engine, and a vent/air inlet for venting said first stage canister and for admission of air to said first stage canister during operation of said engine induction system;

• said second stage canister comprising a second volume of fuel vapor adsorbent material for temporarily adsorbing and storing fuel vapor overflowing said first stage canister, a vapor inlet/purge outlet for receiving said vapor from said first stage canister, a vent/air inlet, a fuel vapor flow path from said vapor inlet/purge outlet through said second volume to said vent/air inlet, a flow path through said second volume for ambient air induced to flow during said engine operation from said vent/air inlet to said vapor inlet/purge outlet and heating means within said second volume adapted to heat said second volume of adsorbent to a temperature at which said air flowing through said second volume removes substantially all adsorbed fuel.

In the method of operating an evaporative fuel emissions control system in combination with an engine adapted to consume said fuel, said method comprising

• directing, during, a period when said engine is not running, the flow of fuel vapor emanating from a fuel tank into a first volume of fuel adsorbent material, said first volume having a vapor inlet and a vent, said vapor flowing in a first direction from said inlet toward said vent, to temporarily adsorb said vapor and, thereafter, during a following period of engine operation,
• promoting a flow of ambient air in an opposing path, through said vent and said first volume of adsorbent material to a air purge outlet, by the action of an engine air induction system, said flow of air removing some, but not all, of the adsorbed fuel from said first volume of adsorbent material and carrying it to said engine in which it is consumed, and in which method said adsorbing and removing of vapor is cyclically repeated, and wherein the capacity of said first volume of adsorbent is sometimes exceeded and fuel vapor escapes the first volume of adsorbent to the atmosphere, the improvement comprising;
• placing a second volume of adsorbent material in a location downstream of said vapor flow vent of said first volume of fuel adsorbent and upstream of said vent, with respect to the direction of said vapor removing air flow, and in the flow path of said airflow, said second volume being no greater than about ten percent of said first volume, and
• prior to each cycle of promoting said air flow, heating said second volume of adsorbent to a temperature enabling the subsequent airflow to remove substantially all adsorbed fuel from said second volume of adsorbent and thereby reduce the escape of fuel vapor from said control system during the subsequent vapor adsorption cycle.