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Abstract

A method for purifying the exhaust gas from an internal combustion engine, which is operated under lambda-based closed loop control and which has an exhaust gas tract in which is located a catalytic converter, is provided. According to this method, a pre-converter lambda value for the exhaust gas is continuously sensed upstream of the catalytic converter, from which a pre-converter lambda signal is generated, and this pre-converter lambda signal is used as the reference variable for the lambda control loop. A post-converter lambda value for the exhaust gas is continuously sensed downstream from the converter, from which a post-converter lambda signal is generated, this being a monotonically decreasing function of the lambda value for the exhaust gas downstream from the catalytic converter. The post-converter lambda signal is used in a trimming control loop to apply a correction to the lambda control loop, whereby a measurement signal is generated which, at least below a certain value of lambda close to lambda=1, is a strictly monotonically increasing or monotonically decreasing function of the lambda value for the exhaust gas downstream from the catalytic converter. For levels of the post-converter lambda signal which are above a threshold value, this supplementary measurement signal is used for trimming control. For levels of the post-converter lambda signal which are below the threshold value, the post-converter lambda signal itself is used for trimming control.

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