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.
Salem, VA 24153
Phone 540 375 3154
Fax 540 387 2787