Turbine housing and variable geometry turbocharger

1 . A turbine housing for accommodating a turbine rotor, comprising:

a scroll passage forming part which forms a scroll passage extending along a circumferential direction around an axis of the turbine rotor;

a nozzle passage forming part which forms a nozzle passage for directing exhaust gas from the scroll passage to the turbine rotor disposed on an inner peripheral side of the scroll passage, the nozzle passage forming part having a shroud-side passage wall surface and a hub-side passage wall surface which define the nozzle passage; and

a variable nozzle unit for adjusting a flow of the exhaust gas in the nozzle passage, the variable nozzle unit including at least one nozzle vane disposed in the nozzle passage,

wherein the scroll passage forming part includes a scroll passage wall surface connecting an outer peripheral end of the scroll passage and an outer peripheral end of the hub-side passage wall surface,

wherein, in a cross-section along the axis passing through a tongue portion of the scroll passage forming part, the scroll passage wall surface includes:

a first arc portion connected at an outer peripheral end to the outer peripheral end of the scroll passage and extending from the outer peripheral end inward in a radial direction of the turbine rotor; and

a cliff portion connected at one end to an inner peripheral end of the first arc portion and connected at another end to the outer peripheral end of the hub-side passage wall surface, the cliff portion including a second arc portion connected to the inner peripheral end of the first arc portion and forming an inflection point between the first arc portion and the second arc portion,

wherein the inflection point is located offset frontward from the outer peripheral end of the hub-side passage wall surface in an axial direction of the turbine rotor and outward from the outer peripheral end of the hub-side passage wall surface in the radial direction, and

wherein, when a length in the axial direction between the one end and the another end of the cliff portion is defined as X, the length X gradually decreases toward a downstream side of the scroll passage.

2 . The turbine housing according to claim 1 ,

wherein the cliff portion further includes an inclined portion extending from an inner peripheral end of the second arc portion rearward which is opposite to frontward in the axial direction and inward in the radial direction, the inclined portion being configured such that a distance from the axis of the turbine rotor to the inclined portion gradually decreases rearward in the axial direction.

3 . The turbine housing according to claim 1 ,

wherein the cliff portion further includes:

an extension portion extending from an inner peripheral end of the second arc portion along the axial direction rearward which is opposite to frontward in the axial direction; and

a third arc portion extending from a rear end of the extension portion rearward in the axial direction and inward in the radial direction.

4 . The turbine housing according to claim 1 ,

wherein, when a length in the axial direction between the one end and the another end of the cliff portion is defined as X, a maximum value of the length X of the cliff portion is defined as Xmax, and a length of the nozzle passage in the axial direction at the outer peripheral end of the hub-side passage wall surface is defined as L, the maximum value Xmax satisfies a condition of 0.75×L≤Xmax≤1.25×L.

5 . The turbine housing according to claim 1 ,

wherein, when a length in the radial direction between the one end and the another end of the cliff portion is defined as Y, the length Y gradually decreases toward a downstream side of the scroll passage.

6 . The turbine housing according to claim 1 ,

wherein, when a length in the radial direction between the one end and the another end of the cliff portion is defined as Y, the length Y is constant in a predetermined circumferential range.

7 . The turbine housing according to claim 1 ,

wherein, when a length in the radial direction between the one end and the another end of the cliff portion is defined as Y, a maximum value of the length Y of the cliff portion is defined as Ymax, and a maximum length of the scroll passage in the radial direction in a cross-section along the axis passing through the tongue portion of the scroll passage forming part is defined as Dmax, the maximum value Ymax satisfies a condition of 0≤Ymax≤0.7×Dmax.

8 . The turbine housing according to claim 1 ,

wherein, when an angular position θ is defined such that, with the angular position of the tongue portion in the circumferential direction of the turbine housing being 0°, the angle gradually increases from the tongue portion toward a downstream side of the scroll passage,

an angular position θmax of a downstream end of the cliff portion satisfies a condition of 120°≤θmax≤180°.

9 . The turbine housing according to claim 1 , further comprising:

a first gas introduction port for directing a gas to the scroll passage;

a second gas introduction port for directing a gas to the scroll passage, the second gas introduction port being disposed outward of the first gas introduction port in the radial direction; and

a confluence passage forming part which forms a confluence passage where the gas directed into the turbine housing through the first gas introduction port and the gas directed into the turbine housing through the second gas introduction port joins, the confluence passage communicating with an upstream end of the scroll passage.

10 . The turbine housing according to claim 9 ,

wherein the confluence passage forming part includes a contraction portion configured such that a flow-path area of the confluence passage gradually decreases toward a downstream side of the confluence passage,

wherein, in a cross-section perpendicular to the axis, the contraction portions includes:

an inner wall surface which forms the confluence passage, where a distance from the axis to the inner wall surface gradually increases toward a downstream side of the confluence passage; and

an outer wall surface which is formed father away from the axis than the inner wall surface and forms the confluence passage between the outer wall surface and the inner wall surface, where a distance from the axis to the outer wall surface gradually decreases toward a downstream side of the confluence passage.

11 . A variable geometry turbocharger, comprising:

the turbine housing according to claim 1 ; and

a turbine rotor rotatably accommodated in the turbine housing.