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Electric motor assisted turbocharger

Electric motor assisted turbocharger description/claims

This invention relates to an electric motor assisted turbocharger, and in particular to a turbocharger for an internal combustion engine.

Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation, of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing.

In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted, an annular inlet passageway arranged around the turbine chamber, an inlet arranged around the inlet passageway, and an outlet passageway extending form the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine chamber and rotates the turbine wheel.

Under steady state conditions of engine speed and load a conventional turbocharger can supply the required amount of air to the engine for efficient combustion. However, there are other conditions, such as at engine start up or transient conditions such as a sudden requirement for a high load from the engine, in which the energy in the exhaust gas is not sufficient to enable the turbocharger to deliver the required air supply to the engine quickly enough. Modern engines are designed to reduce engine fuelling in such circumstances to avoid high levels of exhaust emissions through incomplete combustion, and accordingly engine response suffers during such transient or other conditions.

It is known to address the above problem by providing a turbocharger with an integral electric motor to assist rotation of the compressor to improve the response of the turbocharger and thus the engine performance. An example of such an electric motor assisted turbocharger is disclosed in U.S. Pat. No. 5,604,045. The electric motor is essentially a synchronous motor located within the turbocharger bearing housing, and comprising a magnetic rotor assembly mounted to the turbocharger shaft surrounded by a fixed stator comprising field coils wound on magnetically permeable pole pieces. The operation of the synchronous motor is essentially conventional in that the field coils are energised with an AC supply to create a rotating magnetic field around the shaft which couples with the magnetic field of the magnetic rotor. The motor may be energised whenever the turbocharger requires power assistance to ensure optimum air supply to the engine.

With the above known form of electric motor assisted turbocharger, a synchronous motor comprising a magnetic rotor is used to avoid the need for commutation. A disadvantage of the synchronous motor is that relatively complicated control electronics are required as the excitation frequency of the stator coils must always be matched to the rotational speed of the turbocharger, so that both a variable frequency control signal and means for monitoring the speed of the turbocharger are required. See for instance the control system disclosed in PCT patent application WO98/16728.

It is an object of the present invention to obviate or mitigate the above disadvantages.

According to a first aspect of the present invention there is provided a turbocharger comprising:

a turbine wheel and a compressor wheel mounted to a turbocharger shat;

an electric induction motor for assisting rotation of the compressor wheel in predetermined circumstances, the motor comprising a fixed stator having motor field coils which generate a rotating magnetic field when energised by an AC control signal which induces electric current flow in a rotor to generate a rotor magnetic field which interacts with the stator magnetic field producing torque in the rotor;

wherein the electric current flow induced in the rotor which generates said rotor magnetic field comprises eddy currents.

Since induced eddy currents are relied upon to produce the rotor magnetic field, the rotor may have an advantageously simple structure and may for instance comprise a unitary solid member. Since the motor is an induction motor commutation is not required. Moreover, since the motor is asynchronous there is no requirement to provide a variable AC control frequency in order to vary the rotational speed of the motor. These and other advantages of the present invention are described in more detail below.

The present invention also provides an AC induction motor comprising a stator having motor field coils which generate a rotating magnetic field when energised by an AC control signal which induces electric current flow in a rotor to generate a rotor magnetic field which interacts with the stator magnetic field to rotate said rotor about an axis, the electric flow induced in the rotor which generates said rotor magnetic field comprising eddy currents, and wherein the field coils are arranged in an annular array adjacent first and second axial surfaces of the rotor and the maximum diameter of the rotor is smaller than the outer diameter of the annular coil array such that eddy currents are induced in the rotor which flow from one axial side of the rotor to the other across the peripheral edge of the rotor.

This particular rotor and stator coil configuration) enhances the efficiency of the motor in the manner described below. This improved motor may be used in applications other than the power assistance of turbochargers.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-section through a conventional turbocharger schematically illustrating the major components;

FIGS. 2a and 2b schematically illustrate two alternative configurations for the motor assisted turbocharger according to the present invention;

FIG. 3 is a cross-section through the turbine and bearing housings of a turbocharger including an electric motor in accordance with the present invention; and

FIGS. 4a, 4b, 5a and 5b schematically illustrate motor assemblies in which exemplify a further aspect of the present invention.

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• Utility Patent

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