Turbine having variable throat

1. A turbine for a turbocharger, comprising:

a turbine wheel having an annular hub, a plurality of first turbine blades and a plurality of second turbine blades;

a turbine housing accommodating the turbine wheel and forming a passage for guiding a fluid flow to the turbine wheel; and

an annular member surrounding the turbine wheel and being movable in the axial direction to define a variable throat area through which the fluid flow passes from the passage to the turbine wheel;

wherein the plurality of first turbine blades each has a first length extending axially across the throat area, the plurality of second turbine blades each has a second length extending axially across the throat area, and the first length is shorter than the second length; and

wherein the annular member in a reduced-throat position forms a reduced throat area having an axial dimension smaller than the first length, and the annular member in an enlarged-throat position forms an enlarged throat area having an axial dimension greater than the first length, but smaller than the second length.

2. The turbine for a turbocharger according to claim 1 , wherein the first and second turbine blades are alternately arranged in the circumferential direction of the turbine wheel.

3. The turbine for a turbocharger according to any one of claim 1 or 2 , and further comprising variable nozzle vanes disposed within the passage.

4. The turbine for a turbocharger according to any one of claims 1 or 2 , and further comprising an actuator, wherein the actuator is configured to move the annular member in accordance with an operational state of the turbocharger.

5. The turbine for a turbocharger according to claim 4 , wherein the actuator is configured to increase the throat area when a rotational speed of the turbocharger increases.

6. The turbine for a turbocharger according to any one of claims 1 or 2 , and further comprising:

a nozzle including a plurality of vanes disposed within the passage; and

an actuator;

wherein the annular member is configured to move between a position close to the vanes and a position distant from the vanes;

wherein the actuator is configured to move the annular member to the position close to the vanes when it is positioned for a smaller throat area; and

wherein the actuator is configured to move the annular member to the position distant from the vanes when it is positioned for an enlarged throat area.

7. The turbine for a turbocharger according to claim 6 , and further comprising an actuator, wherein:

the actuator is configured to move the annular member away from the position close to the vanes as the rotational speed of the turbine wheel increases; and

the actuator is configured to move the annular member toward the position close to the vanes as the rotational speed of the turbine wheel decreases.

8. A turbine for a turbocharger, comprising:

a turbine housing;

a turbine wheel within the housing the turbine wheel defining an axial direction, and the turbine wheel having a plurality of turbine blades, wherein the housing forms a passage guiding fluid flow to an inlet of the turbine wheel;

an annular member surrounding the turbine wheel, the annular member being movable in the axial direction of the turbine wheel to vary an inlet area of the inlet of the turbine wheel; and

a nozzle disposed within the passage, the nozzle including a plurality of vanes; wherein:

the annular member moves between a position close to the vanes and a position distant from the vanes.

9. The turbine for a turbocharger according to claim 8 , and further comprising an actuator, wherein:

the actuator is configured to move the annular member away from the position close to the vanes as the rotational speed of the turbine wheel increases; and

the actuator is configured to move the annular member toward the position close to the vanes as the rotational speed of the turbine wheel decreases.