Exhaust-flow-rate control valve, and two-stage supercharging system provided with same

An exhaust-flow-rate control valve for controlling a flow rate of exhaust gas flowing through an exhaust flow passage, the exhaust-flow-rate control valve comprising:

• a valve rod;
• a valve body having a flat plate shape and configured to rotate about the valve rod;
• and
• a valve housing having a cylindrical shape and defining a part of the exhaust flow passage inside the valve housing, the valve housing including:
• a valve seat surface being formed on an inner peripheral surface of the valve housing, extending along a direction intersecting with an axial direction of the exhaust flow passage, and being configured to make contact with an outer peripheral portion of the valve body in a fully-closed state in which an opening degree of the valve body is zero degree;
• and
• a flow passage surface being formed on the inner peripheral surface of the valve housing, extending along the axial direction of the exhaust flow passage, and defining a passage region through which the exhaust gas passes between the flow passage surface and the outer peripheral portion of the valve body in a valve-open state in which the opening degree of the valve body is greater than zero degree,
• wherein a recessed portion is formed on the outer peripheral portion of the valve body, closer to a seat surface which is to be in contact with the valve seat surface than a center line in a thickness direction of the valve body, the recessed portion being disposed on a position recessed from a virtual shape defined by a virtual line Ia passing through the seat surface of the valve body in the fully-closed state and a virtual line Ib passing through a tip of the valve body disposed away by a shortest distance from the flow passage surface in the fully-closed state and extending in a direction perpendicular to the virtual line Ia, in a cross-sectional view taken in a direction perpendicular to a center axis of the valve rod, and
• wherein, provided that H1 is a shortest distance between the valve body and the flow passage surface when the valve body is in the fully-closed state,
• the shortest distance H1 is a distance such that a corner portion of the outer peripheral portion of the valve body makes contact with the flow passage surface when the valve body rotates assuming that the recessed portion is not formed on the outer peripheral portion of the valve body.