Abstract: The combustion structure of an internal combustion engine comprises a cavity formed in the end wall of the piston and a projection on said piston adjacent its cavity to that the projection in the injection, ignition and combustion phase of the cycle only constricts communication between the two cavities and defines with the cavity in the wall a combustion chamber in which at least three distinct swirls of fluid are induced by the shape of the projection and a resultant three-dimensional circulation sweeps throughout this chamber. The fuel is injected into the circulation in this chamber and ignition takes place therein as well.