Method and control device for a combustion engine

The method involves detecting an ion stream for a cylinder, while an internal combustion engine is operated with an air-fuel mixture. Another ion flow is detected for the cylinder, while the internal combustion engine is operated with another air-fuel mixture. The former air-fuel mixture and the latter air-fuel mixture are different to each other. The internal combustion engine comprises multiple cylinders. An independent claim is included for a control apparatus for an internal combustion engine.