Modelling and control of three-way catalytic converters

Three-Way Catalytic Converters (TWC) have been in use for the exhaust gas aftertreatment of spark-ignited engines for more than two d eca es now. During this period, emission limits enforced by legislation in Eur ope and elsewhere have been roughly cut in half every five years. In addition to t hese limits, regulations were introduced, which make the persistent mon itori g and the indication of malfunctions of the exhaust gas aftertreatment system including the sensors compulsory. In order to meet the stringent regulations, various concept s have been developed which not only aim at the stoichiometric control of the r aw exhaust gas entering the TWC, but also at internal states of this device, u sually the oxygen storage level. Often, the stored oxygen is estimated from th e information provided byλ sensors which are located upstream and downstream of the TWC. Problems arise frequently because the sensor signals are co nsiderably distorted by hydrogen. The hydrogen concentration of the raw exhaust g as from the engine can be predicted quite accurately using static functio s or maps. However, its production and/or oxidation in the TWC is a highly dynamic process, which changes both with the engine operation point and over the TWC’ s lifetime. In this thesis, an observer-based control and diagnosis con cept has been developed, which not only accounts for the oxygen storage dyna mics of the TWC, but also predicts the hydrogen concentration in the exhaust gas and its distortion of theλ sensor signals. In order to identify the mechanisms driving the composition of the most important exhaust gas species in terms of legislation and sens or signal distortion, a detailed 1-D model of a TWC has been derived, which reflects th e dynamical behaviour during low-frequency λ excitations very well. It has been found that during rich engine operation, i. e., with excess fuel, the TWC is deactivated by the adsorption of hydrocarbons. This leads to a significan t decrease of the water-gas shift and steam-reforming activities, which in t urn changes the ratio between hydrogen, carbon monoxide, and hydrocarbons, inde pe ntly of the air-to-fuel ratio. This effect is more pronounced with an in creased ageing level of the TWC. The influence of the exhaust gas composition on the response o f a switchtypeλ sensor, which is usually located downstream, has been inves t gated by