Method for purifying exhaust gas of an internal combustion engine

Method for purifying the exhaust gas from an internal combustion engine (1) which is operated under lambda-based closed loop control and which has an exhaust gas tract (4) in which is located a catalytic converter (5), whereby

• a lambda value for the exhaust gas is continuously sensed upstream of the catalytic converter (5), from which a pre-converter lambda signal is generated,
• this pre-converter 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 by a binary lambda probe, from which a binary post-converter lambda signal (9) is generated, this being a monotonically decreasing function of the lambda value for the exhaust gas downstream from the catalytic converter (5) and having a two-point type graph around lambda=1, and
• a trimming controller (8) is used to apply a correction to the lambda control loop, characterized in that
• a broadband lambda probe is used to generate a linear post-converter lambda signal (9) which is a strictly monotonic increasing function of the lambda value of the exhaust gas downstream from the catalytic converter (5),
• this linear post-converter lambda signal (9) is used for trimming control, and
• if the binary post-converter lambda signal has a defined level the actual signal level for the linear post-converter lambda signal is simultaneously determined, the lambda value which has been assigned to the defined binary post-converter signal level is used to determine a corresponding set level for the linear post-converter signal and the trimming control takes into account any difference between the actual signal level and the set signal level as a correction factor.