An internal combustion engine includes a piston reciprocally moving in at least one cylinder. An intake valve controls the supply of fresh air into a power chamber defined within the cylinder by the moving piston. An ancillary chamber is fluidly connected with the power chamber via a flow-through valve. An auxiliary piston is reciprocally movable in the ancillary chamber and determines the volume of the ancillary chamber. A fuel supply valve introduces fuel into the ancillary chamber. The flow-through valve, the fuel supply valve and the auxiliary piston are moved such that no fresh air from the power chamber goes into the ancillary chamber and such that fuel is expelled from the ancillary chamber into the power chamber via the flow-through valve for combustion with fresh air compressed in the power chamber.