Conventional lean burn engine control systems include an air/fuel controller that delivers fuel to the engine intake proportional to measured air mass to maintain a desired air/fuel, lean of stoichiometric. The typical three-way catalytic converter provided in the engine exhaust passage does not convert the NO.sub.x produced while running lean and in order to reduce NO.sub.x emission to the atmosphere, it has been proposed to locate a NO.sub.x trap downstream of the three-way catalyst.
A typical NO.sub.x trap utilizes alkali metal or alkaline earth materials in combination with platinum in order to store or occlude NO.sub.x under lean operating conditions. The mechanisms for NO.sub.x storage involves the oxidation of NO to NO.sub.2 over the platinum followed by the subsequent formation of a nitrate complex with the alkaline metal or alkaline earth. Under stoichiometric operation or operation rich of stoichiometric, the nitrate complexes are thermodynamically unstable, and the stored NO.sub.x is released and catalytically reduced by the excess of CO, H.sub.2, and hydrocarbons (HCs) in the exhaust.
Accordingly, in the prior art, the amount of NO.sub.x introduced to the trap since the last purge is estimated and when the trap is estimated to be full the engine is switched to a relatively rich air/fuel to purge the NO.sub.x trap. After a predetermined purge time interval, the engine is returned to lean operation. The prior art relies on engine speed to determine NO.sub.x accumulation, see for example U.S. Pat. No. 5,473,887. However, engine speed alone does not provide an accurate estimation of NO.sub.x accumulation since several other variables that affect NO.sub.x accumulation are different at the same engine speed depending on other engine operating condition, thereby causing wide swings in the rate of NO.sub.x accumulation. It is important to obtain as accurate an estimate of NO.sub.x accumulation as possible since underestimation will permit lean operation to continue after the trap is full and tailpipe NO.sub.x emission will result. On the other hand overestimation of the accumulated NO.sub.x will cause purging at a rich A/F at a higher frequency than required, reducing fuel economy.