Internal combustion engine

An internal combustion engine comprising:

• a cylinder in which a piston is reciprocably fitted, the cylinder having a cylinder axis;
• a cylinder head formed with an intake port having a first port and a second port including, respectively, a first inlet slot and a second inlet slot opening into a combustion chamber;
• a valve mechanism for opening and closing a first intake valve and a second intake valve for opening and closing the first port and the second port, respectively, the first valve and the second valve each having a longitudinal axis;
• and
• intake air control means for generating a swirl in the combustion chamber by intake air flowing from the first port;
• and
• a camshaft provided with a valve motion cam for opening and closing the first intake valve, the camshaft having a rotational centerline perpendicular the cylinder axis,
• wherein a perpendicular line orthogonal to a first plane, which is a plane including the first inlet slot, the perpendicular line being either parallel to or coincident with the longitudinal axis of the first intake valve,
• that a plane including the cylinder axis and extending in parallel with the rotational centerline of the camshaft is a first reference plane,
• that a plane including the cylinder axis and extending orthogonally to the rotational centerline is a second reference plane, and
• that a plane extending in parallel with the first reference plan is a first orthogonal plane,
• that a line of intersection n1 between the first plane which includes the first inlet slot and the first orthogonal plane inclines upwardly as it approaches from a position near an outer periphery of the combustion chamber toward the second reference plane,
• wherein the first port includes a first port section having a passage shape which extends substantially along the perpendicular line from the first inlet slot in an upstream direction by a predetermined length of the first port in a plan view, and
• wherein the first port section having the passage shape has a cross section which gradually and smoothly increases in size as it extends from the first inlet slot in the upstream direction.