Abstract: An internal combustion engine of the four- stroke combustion engine type is disclosed, comprising an engine block provided with at least one cylinder (2) having a piston movable reciprocally therein for the purpose of bounding a combustion chamber, feed means for air (3) debouching in the combustion chamber, feed means for fuel (4) debouching in the combustion chamber, discharge means for combustion gases (5) connected to the combustion chamber and an ignition mechanism, wherein at least two inlet valves (6,7) and at least one outlet valve (8) are arranged per cylinder, wherein the feed means for air (3) are connected to the one inlet valve (7), the air inlet valve, the feed means for a first fuel (4) are connected to the other inlet valve (6), the fuel inlet valve, and that it is arranged in order to allow mixing of first fuel and air only inside the combustion chamber during operation. Also an adaptation method is disclosed for an engine.