PURPOSE: To raise the thermal efficiency of an internal combustion engine, by forming a baffle plate having air holes and disposed radially in an elongated plate shorter than the radius of a cylinder in a combustion chamber on the lower surface of a cylinder head, and thereby imparting turbulences or the like to the swirl flow of intake air produced at the time of low-load operation of the engine.

CONSTITUTION: Intake air introduced from a subsidiary intake passage 2 into a cylinder 4 at the time of suction stroke produces strong swirl and is charged into the cylinder 4 along with downward movement of a piston 8. Since, in this state, direct collision of the main intake-air flow against a baffle plate 24 is not caused, the swirling energy of intake air is kept high and the strong swirl does not produce turbulences during the suction stroke. Subsequently, when the piston enters into compression stroke, strong swirl produced at the time of suction stroke is weakened gradually, and the intake air is compressed along with upward movement of the piston 8, so that flat swirl of intake air is left in a combustion chamber. When the piston comes near to the top dead point of the compression stroke, the swirl collides against a turbulence generating baffle plate 24 and produces turbulences, so that many fine turbulences, i.e., vortices are separated. Thus, since fuel is burnt rapidly, it is enabled to raise the thermal efficiency of an engine.