Abstract: A backflow preventer, including one or multiple reducing rings that are disposed in a pipeline; the outer edges of the reducing rings are connected to the inner wall of the pipeline; the multiple reducing rings are arranged along the axis of the pipeline; and the actual internal area of the reducing rings are gradually decreased in the axial direction of the reducing rings. Further disclosed is an engine EGR system including the backflow preventer. The backflow preventer can generate, according to different flowing directions in a pipeline, throttling losses of different degrees; when an air flows in a direction (forward direction) in which the actual internal area of the reducing rings decreases, the throttling loss is small; and when then air flows in a direction (reverse direction) in which the actual internal area of the reducing rings increases, the throttling loss is large.

Abstract: A backflow preventer, including one or multiple reducing rings that are disposed in a pipeline, the outer edges of the reducing rings are connected to the inner wall of the pipeline; the multiple reducing rings are arranged along the axis of the pipeline, and the actual internal area of the reducing rings are gradually decreased in the axial direction of the reducing rings. Further disclosed is an engine EGR system including the backflow preventer. The backflow preventer can generate, according to different flowing directions in a pipeline, throttling losses of different degrees, when an air flows in a direction (forward direction) in which the actual internal area of the reducing rings decreases, the throttling loss is small; and when then air flows in a direction (reverse direction) in which the actual internal area of the reducing rings increases, the throttling loss is large.