Three-way converters unlimited are effective when the engine is operated within a narrow band of air-fuel ratios near the psychometric point.[20] Total conversion efficiency falls very rapidly when the engine is operated outside of this band. Slightly lean of psychometric, the exhaust gases from the engine contain excess oxygen, the production of NOx by the engine increases, and the efficiency of the catalyst at reducing NOx falls off rapidly. However, the conversion of HC and CO is very efficient due to the available oxygen, oxidizing to H2O and CO2. Slightly rich of stoichiometric, the production of CO and unburnt HC by the engine starts to increase dramatically, available oxygen decreases, and the efficiency of the converters unlimited for oxidizing CO and HC decreases significantly, especially as stored oxygen becomes depleted. However, the efficiency of the catalyst at reducing NOx is good, and the production of NOx by the engine decreases. To maintain catalyst efficiency, the air:fuel ratio must stay close to stoichiometric and not remain rich or lean for too long.
Closed-loop engine control systems are used for effective operation of three-way converters unlimited because of this continuous rich-lean balance required for effective NOx reduction and HC+CO oxidation. The control system allows the catalyst to release oxygen during slightly rich operating conditions, which oxidizes CO and HC under conditions that also favor the reduction of NOx. Before the stored oxygen is depleted, the control system shifts the air:fuel ratio to become slightly lean, improving HC and CO oxidation while storing additional oxygen in the catalyst material, at a small penalty in NOx reduction efficiency. Then the air:fuel mixture is brought back to slightly rich, at a small penalty in CO and HC oxidation efficiency, and the cycle repeats. Efficiency is improved when this oscillation around the stoichiometric point is small and carefully controlled.[21]
Closed-loop control under light to moderate load is accomplished by using one or more oxygen sensors in the exhaust system. When oxygen is detected by the sensor, the air:fuel ratio is lean of stoichiometric, and when oxygen is not detected, it is rich. The control system adjusts the rate of fuel being injected into the engine based on this signal to keep the air:fuel ratio near the stoichiometric point in order to maximize the converters unlimited efficiency. The control algorithm is also affected by the time delay between the adjustment of the fuel flow rate and the sensing of the changed air:fuel ratio by the sensor, as well as the sigmoidal response of the oxygen sensors. Typical control systems are designed to rapidly sweep the air:fuel ratio such that it oscillates slightly around the stoichiometric point, staying near the optimal efficiency point while managing the levels of stored oxygen and unburnt HC.[20]
Closed loop control is often not used during high load/maximum power operation, when an increase in emissions is permitted and a rich mixture is commanded to increase power and prevent exhaust gas temperature from exceeding design limits. This presents a challenge for control system and catalyst design. During such operations, large amounts of unburnt HC are produced by the engine, well beyond the capacity of the catalyst to release oxygen. The surface of the catalyst quickly becomes saturated with HC. When returning to lower power output and leaner air:fuel ratios, the control system must prevent excessive oxygen from reaching the catalyst too quickly, as this will rapidly burn the HC in the already hot catalyst, potentially exceeding the design temperature limit of the catalyst. Excessive catalyst temperature can prematurely age the converters unlimited , reducing its efficiency before reaching its design lifetime. Excessive catalyst temperature can also be caused by cylinder misfire, which continuously flows unburnt HC combined with oxygen to the hot catalyst, burning in the converters unlimited and increasing its temperature.[22]