Oil drain structure for oil mist separator

1. An oil drain structure of an oil mist separator that is configured to separate oil mist from a blow-by gas of an internal combustion engine, and discharge the separated oil mist into an interior of the internal combustion engine while supplying the gas from which the oil mist is separated to an intake system, the oil drain structure comprising:

a drain pipe extending from the oil mist separator into the internal combustion engine, the drain pipe having an oil discharge hole extending through a lower end portion of the drain pipe along an axial direction of the drain pipe; and

a check valve configured to open and close the oil discharge hole, the check valve comprising

a valve body disposed below the lower end portion of the drain pipe, the valve body acting to open and close the oil discharge hole from a side of a lower end surface of the lower end portion of the drain pipe;

a valve head portion fitted into the drain pipe with a fine clearance as an orifice, the valve head portion being moveable in the axial direction of the drain pipe, the fine clearance being defined as a distance between the valve head portion and an inner wall surface of the drain pipe; and

a stem portion extending through the oil discharge hole to connect the valve body and the valve head portion with each other,

wherein the fine clearance is configured such that the separated oil mist is allowed to flow through the fine clearance even when the valve head portion is in a seated position,

wherein the fine clearance is constant, regardless of a vertical position of the valve head portion in the axial direction of the drain pipe, and

wherein the valve head portion is kept away from the inner wall surface of the drain pipe to maintain the fine clearance, regardless of the vertical position of the valve head portion in the axial direction of the drain pipe.

2. The oil drain structure of an oil mist separator as claimed in claim 1 , wherein the drain pipe has a constant cross-sectional shape at least in a range in which the valve head portion is moveable.

3. The oil drain structure of an oil mist separator as claimed in claim 1 , wherein a density of the valve head portion is lower than a density of oil.

4. The oil drain structure of an oil mist separator as claimed in claim 1 , wherein an auxiliary oil discharge passage is formed in a seat portion located at an upper end of the oil discharge hole, the auxiliary oil discharge passage being formed such that the oil is discharged in a state in which the valve head portion is in the seated position.

5. The oil drain structure of an oil mist separator as claimed in claim 1 , wherein the valve head portion is formed into a spherical shape.

6. The oil drain structure of an oil mist separator as claimed in claim 1 , wherein one of the valve head portion and the valve body of the check valve is made of an elastically deformable rubber and the other of the valve head portion and the valve body of the check valve is made of a synthetic resin, and the one of the valve head portion and the valve body of the check valve is inserted into the oil discharge hole while being deformed.

7. The oil drain structure of an oil mist separator as claimed in claim 1 , wherein a lower end surface of the check valve has a downwardly projecting conical surface or a downwardly projecting curved surface.

8. The oil drain structure of an oil mist separator as claimed in claim 1 , wherein the fine clearance is set to be equal to or smaller than one-tenth of a radius of the drain pipe.

9. The oil drain structure of an oil mist separator as claimed in claim 1 , wherein the tine clearance serves as the orifice such that the valve head portion is biased upward by a pressure difference between an upper side of the valve head portion and a lower side of the valve head portion to close the oil discharge hole via the valve body.