Potential and pitfalls in the use of dual exhaust gas oxygen sensors for three-way catalyst monitoring and control

Although it is known that exhaust gas oxygen (EGO) sensors are sensitive togas composition, even at a constant air-fuel ratio (AFR), its significance in dual EGO sensor based catalyst control and on-board diagnostic systems has not been fully recognized. The (time varying) difference in gas composition across the catalyst gives rise to a dynamically changing bias component at the sensor output, which is not readily distinguishable from the oxygen storage and release effects the sensor is intended to monitor. Unless treated explicitly, this is likely to degrade the performance of dual EGO sensor based systems. However, the distortion itself also reflects catalyst activity and is strongly correlated with a reversible catalyst deactivation effect which dominates hydrocarbon and NO conversion efficiency under rich conditions. A method for exploiting the biased signal to obtain both improved estimates of the true AFR and an insight into the reversible deactivation effect is therefore outlined.