Patent ID: 8151777

Claim:
An intake pipe structure of an internal combustion engine, comprising: a joint pipe unit for communicating an intake passage for introducing air into said internal combustion engine with a return passage for returning blow-by gas from said internal combustion engine to said intake passage, said joint pipe unit being constituted by an intake pipe portion forming said intake passage therein, and a return pipe portion forming said return passage therein and including a first end which is connected with said intake pipe portion and which has an opening opened to said intake pipe portion, said return pipe portion having an inner diameter and an inner peripheral surface, a center axis of said intake pipe portion near said first end and a center axis of said return pipe portion lying on a common plane, said inner peripheral surface of said intake pipe portion having a first surface section spaced from said first end of said return pipe portion, and extending smoothly and continuously without annular steps or other configurations of a ledge, within the range of a first length which is set from said first end and is larger than said inner diameter, said inner peripheral surface having a second surface section extending from said first surface section to said first end of said return pipe portion, within the range of a second length which is set from said first end of said return pipe portion and which is smaller than said first length, said second surface section having a curved surface portion gradually enlarged and curved at the downstream side of said return pipe portion to smoothly continue the surface of said first surface section to an inner peripheral surface of said intake pipe portion, thereby enlarging said opening at the downstream side in such a manner that said opening is in the form of a non-circular shape, said curved surface portion having a maximum extent of enlargement towards an expansion direction, so that said opening has a maximum extent of enlargement in said expansion direction, said expansion direction does not lie on said common plane, rather said expansion direction angularly deviates from the center axis of said intake pipe portion.