Similar to three way catalysts (TWC), conventional oxidation catalysts (COC) have a “control window” with respect to inlet O2 concentrations. Within this window, which is bordered on one side by poor HC / CO efficiency and low sulfate formation and on the other side by good HC / CO efficiency and high sulfate formation, is a region offering simultaneous control of HC, CO and sulfates.
Starting with an advanced emission control system, which incorporates a TWC with an O2 sensor and closed loop feedback control of the air-fuel ratio (A / F) followed by a COC with secondary air addition for adequate control of HC and CO, an additive control system has been designed which maintains a constant O2 level at the inlet of the oxidation catalyst. With the nearly constant composition of the exhaust gases of an engine with stoichiometric A / F control, the quantity of secondary air addition required to achieve this constant O2 level is a linear function of engine air flow. Using inputs of manifold absolute pressure (MAP) and engine speed, an electric motor servo control system was designed to control the flow of secondary air to the oxidation catalyst on a 2.3 liter, standard transmission vehicle.
Exhaust emission and sulfate results of the 3,000 lb. vehicle are presented, respectively, for the CVS-C/H and the Congested Freeway Driving Schedule (CFDS) test procedures. Details of the electronic servo control system are included.