Generally, lean burn engines typically operate with an air/fuel (A/F) ratio .gtoreq.18 to obtain improved fuel economy. However, a conventional three-way catalyst (TWC) utilized in the engine exhaust system is ineffective unless operated near a stoichiometric ratio, such as an A/F ratio of 14.5 for gasoline. One solution for overcoming the operational deficiencies of a TWC during lean burn operation is to position a NO.sub.x trap downstream of the TWC. The NO.sub.x trap stores NO.sub.x during lean A/F operation, which can then be subsequently converted to harmless N.sub.2 and O.sub.2 by periodically operating the engine with a rich A/F ratio. This NO.sub.x conversion occurs within an optimum temperature window of approximately 300.degree. C. to 400.degree. C. An NO.sub.x trap is preferably located underbody so that during hard, wide-open throttle (WOT) driving, the trap temperature can be maintained below 800.degree. C.
One problem inherent to an NO.sub.x trap stems from the fact that the use of a fuel which contains sulfur will cause a corresponding accumulation of sulfur in the trap. This accumulation of sulfur decreases the NO.sub.x conversion efficiency. Thus, the sulfur must be periodically "burned" off or desorbed by heating the trap to approximately 675.degree. C. and sustaining that temperature for several minutes. During this desulfation heating phase, it is desirable to control the average A/F ratio reaching the trap to be approximately stoichiometric so as to minimize emission of HC, CO, and NO.sub.x, and to prevent thermal damage to the trap. Stoichiometric operation may be required for periods of 3 to 8 minutes.
One way of controlling the A/F ratio is with an open loop type control system. However, if the A/F ratio is not accurately maintained during the desulfation phase, performance of the trap desulfation process will be significantly compromised by allowing higher levels of HC, CO, and NO.sub.x to be emitted from the exhaust system if the A/F operation is excessively rich, and possible NO.sub.x trap degradation at high temperatures if the A/F operation is excessively lean. This, in turn, degrades the overall efficiency of the engine exhaust treatment system. Since accuracy of an open loop-type control system can be directly impacted by such factors as engine-to-engine variability, and component/engine aging, use of an open loop control arrangement is not completely satisfactory, particularly when higher levels of exhaust treatment efficiency are required for lean burn engines.