Spring regulated variable flow electric water pump

1. A variable flow electric water pump for use in an engine coolant system of a motor vehicle, the electric water pump comprising:

a pump housing defining a fluid chamber, a motor chamber, a fluid inlet and a discharge port providing a flowpath for coolant flowing through said fluid chamber, and an interface established between said fluid inlet and said fluid chamber defining a flange surface;

an electric motor disposed in said motor chamber of said pump housing and including a stationary stator assembly and a rotor unit having a rotor shaft supported for rotation about a longitudinal axis and extending into said fluid chamber;

an impeller fixed to said rotor shaft for rotation in said fluid chamber and operable to pump coolant from said fluid inlet to said discharge port, said impeller having a rim surface aligned with said flange surface of said pump housing; and

a biasing arrangement for normally locating said rotor unit in a first position that is axially offset relative to said stator assembly for locating said impeller in a retracted position within said fluid chamber to provide a low flow characteristic between said fluid inlet and said discharge port when said impeller is driven by said rotor shaft at a low rotor-speed, said biasing arrangement being configured to exert a preload on said rotor unit,

wherein a first clearance gap is established between said rim surface of said impeller and said flange surface of said pump housing when said impeller is located in its retracted position, said fluid inlet and said discharge port being fluidly connected through said first clearance gap, said first clearance gap being configured to decrease the coolant flow rate between said fluid inlet and said discharge port,

wherein rotation of said impeller at a high rotor speed causes said rotor unit to overcome said preload and move to a second position axially aligned with said stator assembly for causing said impeller to move from its retracted position into an extended position within said flow chamber to provide a high flow characteristic between said fluid inlet and said discharge port,

wherein a second clearance gap is established between said flange surface of said pump housing and said rim surface of said impeller when said impeller is located in its extended position, said second clearance gap configured to increase the coolant flow rate between said fluid inlet and said discharge port, and

wherein said first clearance gap is larger than said second clearance gap.

2. The electric water pump of claim 1 , wherein said biasing arrangement is a mechanical biasing arrangement including a biasing member configured to exert said preload on said rotor unit.

3. The electric water pump of claim 2 , wherein said biasing member is a coil spring disposed between a portion of said pump housing and said rotor unit.

4. The electric water pump of claim 1 , wherein said biasing arrangement is a magnetic biasing arrangement including a plurality of magnets extending axially outwardly from said rotor unit and operable to align the center of a magnetic field associated with said rotor unit with the center of a magnetic field associated with said stator assembly for locating said rotor unit in its first position.

5. The electric water pump of claim 1 , wherein said rotor shaft is axially moveable relative to said pump housing and has a first end slideably and rotatably supported by a first guide bushing and a second end slideably and rotatably supported by a second guide bushing.

6. The electric water pump of claim 1 , wherein a pressure differential established across said impeller in response to increasing rotor speed is operable to cause said impeller to move from its retracted position into its extended position, and wherein such axial movement of said impeller causes concurrent axial movement of said rotor unit relative to said stator assembly from its first position into its second position.

7. The electric water pump of claim 1 , wherein a pressure differential established across said impeller in response to increasing rotor unit speed is operable to cause said impeller to move from its retracted position into its extended position, and wherein such axial movement of said impeller causes concurrent axial movement of said rotor unit relative to said stator assembly from its first position into its second position.

8. The electric water pump of claim 1 , wherein said first clearance gap and said second clearance gap are established between said rim surface of said impeller and said flange surface of said pump housing in a direction of said longitudinal axis.