When cat­a­lysts were first put on Amer­i­can vehi­cles in the fall of 1974 (1975 mod­el year), their func­tion was to cat­alyze the oxi­da­tion of CO and unburned and par­tial­ly burned hydro­car­bons to CO₂ and H₂O. All that was need­ed was to be sure that there was enough O₂ present when the A/F ratios were too low to pro­vide it ‘nat­u­ral­ly’. For some vehi­cles, there was enough O₂ present in the exhaust at most oper­at­ing con­di­tions to meet the emis­sion stan­dards (espe­cial­ly the eas­i­er 49-state Fed­er­al stan­dards) and there was no extra hard­ware (or soft­ware) need­ed. On oth­er vehi­cles, or in Cal­i­for­nia in gen­er­al, more O₂ was need­ed when the A/F ratios were low than was in the exhaust, and pro­vi­sions were made to add O₂ by a ram air ven­turi horn or by an engine dri­ven air pump, deliv­er­ing air to the inlet to the cat­alyt­ic con­vert­er. Doing this dur­ing start­up of the engine would hin­der the warmup of the cat­a­lyst, and there would have been some sort of con­trol to pre­vent deliv­ery of the extra air to the con­vert­er until some time had elapsed or some tem­per­a­ture had been achieved. In the sim­plest sys­tem, the air deliv­ery might have been acti­vat­ed by the same con­trol that closed the choke.

Meet­ing the NOx stan­dards in the late 70s led to the three-com­po­nent con­trol cat­a­lysts, which were capa­ble of using the CO, H₂ and hydro­car­bons in the exhaust enter­ing the con­vert­er to reduce the NOx to N₂ while they were being removed by reac­tion with the NOx and O₂. How­ev­er, removal of all three pol­lu­tants depend­ed on pro­vid­ing an exhaust mix­ture with just exact­ly enough reduc­tants as oxi­dants; this required the A/F ratio of the air/fuel mix­ture being exact­ly that required to the­o­ret­i­cal­ly burn all of the fuel to CO₂ and H₂O, or sto­i­chio­met­ric. This was achieved with a closed-loop sys­tem which includ­ed an oxy­gen sen­sor exposed to the gas leav­ing the cat­a­lyst that pro­vid­ed a volt­age sig­nal pro­por­tion­al to the devi­a­tion of the exhaust mix­ture from sto­i­chiom­e­try, and a feed­back sys­tem (com­put­er) to effect a change in the rate of feed­ing fuel to the engine to dri­ve the exhaust com­po­si­tion (and sen­sor out­put) back to sto­i­chiom­e­try. The oxy­gen sen­sor was devel­oped as the best way to do this, although work was done to try to devel­op CO sen­sors, etc., as alternatives.

Their are a num­ber of good reviews which describe these changes, includ­ing those by Lester, Tay­lor, Hege­dus, Brig­gs, etc. I would par­tic­u­lar­ly sug­gest, for your pur­pos­es, L. L. Hege­dus and J. J. Gum­ble­ton, CHEMTECH, 10 (10) 630 (1980).
 
Con­tributed by George Lester
Adjunct Pro­fes­sor, North­west­ern Uni­ver­si­ty and Pres­i­dent, George Lester, Inc.
1200 Pickwick,
Salem, VA 24153
Phone 540 375 3154
Fax 540 387 2787
Skygeorge@aol.com