A method in a combustion engine 100 for determining a desired fuel setting λ+. The method comprises configuring a fuel setting λ, determining a combustion engine parameter β in dependence of the configured fuel setting λ, wherein the combustion engine parameter is related to the fuel setting by a convex function relationship, and updating the fuel setting λ towards the desired fuel setting λ+, based on the determined combustion engine parameter β and on the convex function relationship between the fuel setting λ and the combustion engine parameter β.