Online Estimation of the Oxygen Storage Level of a Three-Way Catalyst

Very stringent limits for exhaust gas emissions as well as high claims for onboard diagnosis (OBD) of the three-way catalytic converter (TWC) demand a sophisticated control and observer strategy which can both further reduce the exhaust gas emissions and also estimate the relevant parameters allowing to monitor the decreasing performance of the TWC over its lifetime. The most crucial parameter and state, respectively, are widely believed to be the oxygen storage capacity (OSC) and the relative oxygen level (ROL) of the TWC. The TWC's performance decreases with a diminishing OSC. Therefore, an accurate estimation of the OSC can be used for OBD. Keeping the ROL at an optimal level by means of control enhances the TWC's performance significantly, even during transients of the air/fuel ratio imposed by the driver.

In order to monitor both the ROL and the OSC, an observer has been derived from a complex TWC model. The observer consists of an extended Kalman filter (EKF), which estimates the ROL of the TWC from the exhaust gas mass flow, the temperature and the lambda-sensor signal upstream of the TWC. Additionally, the gas composition and the signal of a sensor downstream are estimated in order to close the loop for the EKF. The EKF also estimates the OSC by means of online estimation.