Exhaust promotion device, and exhaust system improvement method for internal combustion engine

An exhaust promotion device for an internal combustion engine for improving an exhaust efficiency of a combustion gas discharged from the internal combustion engine, the exhaust promotion device comprising:

• a casing provided in an exhaust system of the internal combustion engine
• an exhaust flow control cylinder concentrically provided in the casing so as to form an annular space portion between an inner surface of the casing and the exhaust flow control cylinder;
• and
• a cooling cylinder coaxial with the casing and provided at an end portion of the casing on an exhaust downstream side at an interval with respect to the exhaust flow control cylinder in an exhaust direction, wherein
• the exhaust flow control cylinder has a first expansion portion having an approximate truncated cone cross section that gradually expands in diameter toward a downstream side of an exhaust flow, and an acceleration portion continuously provided with the first expansion portion and having an approximate truncated cone cross section that gradually contracts in diameter toward the downstream side of the exhaust flow,
• a diversion hole through which the acceleration portion and the annular space portion communicate is provided in a connecting portion of the acceleration portion to the first expansion portion,
• the annular space portion is a second expansion portion whose capacity gradually increases toward the downstream side of the exhaust flow, and has, at an end portion thereof on the downstream side of the exhaust flow, a throttle facing an end portion of the acceleration portion on the downstream side of the exhaust flow, and
• the capacity of the annular space portion is more than 1.8 times and less than 2.6 times a capacity of the acceleration portion,
• on the exhaust downstream side of the casing, a downstream spherical surface portion is provided with a spherically expanded diameter from an end of the exhaust downstream side toward an exhaust upstream side, and the downstream spherical surface portion is continuous with an inner surface of the throttle,
• the throttle is located downstream from a discharge port of the acceleration portion, and a distance between the throttle and the discharge port is set to a value that maximizes a suction function by negative pressure generated near the throttle.