An arrangement and a method for controlling a combustion engine, e.g. of the type called HCCI engine. A control unit is operable for controlling the self-ignition of the fuel mixture towards an optimum crankshaft angle (cad.sub.iopt) within a load range (L.sub.tot). The load range (L.sub.tot) can be divided into at least two subranges (L.sub.II, L.sub.III). The control unit is operable to controlling the self-ignition of the fuel mixture towards an optimum crankshaft angle (cad.sub.iopt) within one of the subranges (L.sub.II) by a strategy (II) which entails the effective compression ratio (c) in the cylinder being varied, and within the second subrange (L.sub.III) by another strategy (III) which entails a variable amount of cooled exhaust gases (ceg) being led to the combustion chamber also enabling in the second subrange (L.sub.III) to control the self-ignition of the fuel mixture towards an optimum crankshaft angle (cad.sub.iopt) by variation of the effective compression ratio (c) in the cylinder without it falling below a lowest acceptable value (C.sub.min).