In a two-stage supercharging electric-assist turbocharger, a first compressor wheel, a rotor of an electric motor, a second compressor wheel, and a turbine wheel are coaxially coupled to a same, common shaft member, in that order. A compressor housing is structured to define therein a communicating passage to accommodate the electric motor in the communicating passage. A first water jacket is formed in at least one rib integrally formed with an outer periphery of a motor housing and also serving as a radiating fin, for forced-cooling air flowing through the communicating passage. A second water jacket is formed in a motor housing for forced-cooling a stator of the electric motor. A third water jacket is formed in an intermediate housing constructing a part of the compressor housing for forced-cooling a control unit configured to control the electric motor.

TECHNICAL FIELD

The present invention relates to a turbocharger for an internal combustion engine, and specifically to a two-stage supercharging turbocharger having an electric-assist function by means of an electric motor.

BACKGROUND ART

One such two-stage supercharging electric-assist turbocharger has been disclosed in the following Patent document 1.

In the turbocharger disclosed in the Patent document 1, a two-stage supercharging compressor section is constructed by a first compressor stage and a second compressor stage arranged adjacent to each other. An electric motor is arranged on the side of the first compressor stage, whereas an exhaust-gas turbine section is arranged on the side of the second compressor stage, in a manner so as to sandwich the first and second compressor stages between the electric motor and the turbine section. The turbine section is driven by exhaust gas energy. The first compressor wheel, the second compressor wheel, the motor rotor, and the turbine wheel are integrally coupled to a same, common shaft.

A first flow passage is provided outside of a stator (stator coils) of the motor for introducing air through the first flow passage into the suction side of the first compressor stage. Also provided is an intercooler (i.e., a heat exchanger) installed in a second flow passage, through which the pressure side of the first compressor stage and the suction side of the second compressor stage are connected to each other.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

However, with the configuration of the prior-art turbocharger as disclosed in the above patent document 1, the air, which is introduced into the suction side of the first compressor stage, flows through the first flow passage arranged outside of the motor stator, and thus a motor-stator cooling effect can be expected by the air flow through the first flow passage. However, more of the quantity of heat removed from the motor stator is delivered to the first compressor stage. The quantity of heat, which is delivered to the first compressor stage, is an disadvantage in rising a boost pressure at the first compressor stage and also in rising a boost pressure at the second compressor stage subsequently to the first compressor stage.

Additionally, according to the configuration of the prior-art turbocharger, prior to introducing the air from the pressure side of the first compressor stage into the second compressor stage, the air discharged from the pressure side of the first compressor stage is cooled by means of the intercooler, which is disposed in a middle of the second flow passage and arranged outside of a body including a body part of the compressor section and a body part of the turbine section. Thus, the redundant flow passage system including the second flow passage as well as the first flow passage leads to a pressure loss. Also, the air discharged from the pressure side of the first compressor stage has more of the quantity of heat removed from the motor stator, and thus a comparatively large thermal load is applied to the intercooler. Therefore, the prior-art turbocharger system requires a comparatively large capacity of intercooler. Necessarily, the entire system configuration of the turbocharger including the intercooler has to be undesirably increased in size.

It is, therefore, in view of the previously-described drawbacks of the prior art, an object of the invention to provide a turbocharger configured to improve the performance to cool air suctioned into each compressor stage of a multistage compressor section as well as air introduced into a stator of an electric motor, and consequently to further increase a boost pressure.

According to one aspect of the invention, a turbocharger comprises a turbine wheel driven by exhaust gases exhausted from an internal combustion engine, a first compressor wheel and a second compressor wheel both coaxially connected to the turbine wheel through a shaft member, a main housing that accommodates therein the first and second compressor wheels and has a communicating passage defined inside of the main housing for sending air pressurized by the first compressor wheel to the second compressor wheel, an electric motor having a cylindrical motor housing and installed in the communicating passage and structured to share the shaft member common to the first and second compressor wheels and the turbine wheel as a rotation axis, a plurality of radiating fins provided between the main housing and the motor housing inside of the main housing and configured to partition the communicating passage into a plurality of regions in a circumferential direction, a control unit configured to control the electric motor, and a cooling mechanism structured to cool potential heat-generation portions inside of and outside of the main housing.

Additionally, the cooling mechanism comprises a first cooling part placed in the radiating fins for cooling the air in the communicating passage by cooling water supplied to the first cooling part, a second cooling part placed in the motor housing for cooling a stator of the electric motor by cooling water supplied to the second cooling part, and a third cooling part having a cooling structure that cools the control unit by cooling water supplied to the third cooling part.

Preferably, the control unit is fixedly positioned on an outer periphery of the main housing or the control unit is positioned adjacent to the main housing as an external control unit separated from the main housing.

In the case that the control unit is fixedly positioned on the outer periphery of the main housing, preferably, the first cooling part is a first water jacket formed in at least one of the plurality of radiating fins, the second cooling part is a second water jacket formed in the motor housing, and the third cooling part is a third water jacket formed in a part of the main housing.

To ensure the high-efficiency cooling performance, preferably, a distribution means (a cooling-water distributor) is further provided in a cooling-water supply system of the cooling mechanism, for switching between a supply mode and a shut-off mode of cooling water supplied through the distribution means to each of the first, second, and third cooling parts, depending on an operating condition of a vehicle on which the internal combustion engine is mounted.

Alternatively, a distribution means (a cooling-water distributor), which is provided in a cooling-water supply system of the cooling mechanism, may be structured to predetermine a flow rate of cooling water to be distributed to each of the first, second, and third cooling parts, depending on a cooling load of each of the first, second, and third cooling parts.

Furthermore, to improve the cooling effect for cooling the air in the communicating passage, preferably, the radiating fins are configured to protrude radially outwards from an outer periphery of the motor housing as viewed in an axial direction of the shaft member, and continuously formed over an entire axial length of the electric motor from a first axial end of the electric motor facing the first compressor wheel to a second axial end of the electric motor facing the second compressor wheel.

In a similar manner to the above, to improve the cooling effect for cooling the electric motor, preferably, in the electric motor, the air prevailing in an inlet of the communicating passage is introduced through a first axial end of the motor housing facing the first compressor wheel into the motor housing, and the introduced air is exhausted from a second axial end of the motor housing facing the second compressor wheel into an outlet of the communicating passage.

Effects of the Invention

According to the invention, the electric motor is installed in the main housing, in which the communicating passage is defined, and arranged between the first-stage compressor wheel (the first compressor wheel) and the second-stage compressor wheel (the second compressor wheel). Additionally, the first cooling part, which has a structure that cools the air in the communicating passage, and the second cooling part, which has a structure that cools the stator of the electric motor, are provided independently of each other. Hence, there is a less tendency for the air suctioned or introduced into the second-stage compressor wheel as well as the air suctioned or introduced into the first-stage compressor wheel to be thermally affected by heat generated by the electric motor. In particular, the combined provision of the first cooling part and the second cooling part, which are appropriately placed independently of each other, are advantageous with respect to the air cooling performance of air suctioned into the second-stage compressor wheel and the motor-stator cooling performance. Therefore, it is possible to greatly improve the boost pressure.

Additionally, the third cooling part, which has a structure that cools the control unit for electric-motor control, is placed independently of the first and second cooling parts, and thus a motor-control function of the control unit can be maintained stably. Also, there is a less tendency for the air suctioned or introduced into each of the compressor wheels to be thermally affected by heat generated by the control unit.

Furthermore, in the case that the control unit is fixedly positioned on the outer periphery of the main housing, the first, second, and third cooling parts are eventually built in the main housing. Hence, an external cooler (or an external heat exchanger), separated from the main housing, is unnecessary. This is very advantageous with respect to downsizing and space-saving around the turbocharger.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The detailed description of embodiments of a turbocharger according to the invention will be given with reference to the accompanying drawings.FIG. 1is a partially cutaway view in perspective of a turbocharger1of the embodiment.FIG. 2is a longitudinal cross-sectional view of the turbocharger1of the embodiment shown inFIG. 1.FIG. 3is a lateral cross-sectional view taken along the line A-A ofFIG. 2.FIG. 4is a schematic perspective view illustrating the water cooling system of the turbocharger1shown inFIGS. 1-3.FIG. 5is a left-hand side view illustrating the water cooling system shown inFIG. 4.

Referring now toFIGS. 1-2, turbocharger1is a two-stage supercharging turbocharger in which air to be fed into an intake system of an internal combustion engine (not shown) mounted on a vehicle can be compressed or supercharged within a turbocharger housing2in two stages. That is to say, turbocharger1is configured such that a rightmost-end turbine wheel3, a leftmost-end first compressor wheel (a first compressor impeller)4, and an intermediate second compressor wheel (a second compressor impeller)5are coaxially installed onto an elongated cylindrical shaft member6common to the turbine wheel3, the first compressor wheel4, and the second compressor wheel5, for supercharging air by exhaust gas energy.

Turbocharger housing2is formed into a substantially cylindrical shape. The substantially cylindrical turbocharger housing2is mainly constructed by a right-hand side turbine housing7, a left-hand side compressor housing8serving as a main housing, and an intermediate housing9located between the turbine housing7and the compressor housing8. Shaft member6is formed into a stepped cylindrical shape. The stepped cylindrical shaft member6is accommodated in the turbocharger housing2.

Shaft member6is made of a metal material. Turbine wheel3is fixedly connected to the right-hand axial end of shaft member6. The first compressor wheel4is fixedly connected to the left-hand axial end of shaft member6. The second compressor wheel5is fixedly connected to the intermediate portion of shaft member6. Shaft member6is rotatably supported by bearings29at three different axial positions of shaft member6, adjacent to the turbine wheel3, the first compressor wheel4, and the second compressor wheel5, respectively.

Three members, namely, the turbine housing7, the compressor housing8, and the intermediate housing9are formed of cast products, for example, metal cast products.

As shown inFIG. 1, turbine housing7is formed into a substantially volute shape. Turbine wheel3is accommodated in the turbine housing7. Turbine wheel3is made of a metal material and formed as a metal cast product. Turbine wheel3has a plurality of turbine blades10.

Intermediate housing9serves to mainly support the shaft member6. Intermediate housing9also serves to couple the turbine housing7and the compressor housing8together in an axial direction of the shaft member6.

The first compressor wheel4, the second compressor wheel5, an electric motor11interposed between the first compressor wheel4and the second compressor wheel5, a first cylindrical member12interleaved between the first compressor wheel4and the electric motor11, and a second cylindrical member13interleaved between the second compressor wheel5and the electric motor11are all accommodated in the compressor housing8.

Hereupon, compressor housing8is mainly constructed by a first housing14that houses and covers the first compressor wheel4, a second housing15(an intermediate housing) that accommodates therein the electric motor11and the first cylindrical member12, and a third housing16that houses and covers the second cylindrical member13and the second compressor wheel5. Compressor housing8has a communicating passage17defined inside of the compressor housing8, for sending air pressurized (compressed) by the first compressor wheel4to the suction side of the second compressor wheel5. Communicating passage17is located between the first compressor wheel4and the second compressor wheel5along a longitudinal direction of the compressor housing8containing both the second housing15and the third housing16, and formed as a substantially cylindrical-hollow, comparatively large space defined between the outer periphery of shaft member6and the inner periphery of compressor housing8.

The first housing14serves to mainly accommodate the first compressor wheel4. The second housing15serves to mainly accommodate both the first cylindrical member12and the electric motor11in the communicating passage17. The second housing15also serves to couple the first housing14and the third housing16together in the axial direction of the shaft member6. On the other hand, the third housing16serves to mainly accommodate both the second compressor wheel5and the second cylindrical member13. As appreciated from the partially cutaway view ofFIG. 1, in a similar manner to the turbine housing7, the third housing16is formed into a substantially volute shape. The third housing16also serves to couple the second housing15and the intermediate housing9together in in the axial direction of the shaft member6.

The first compressor wheel4is made of a metal material and formed as a metal cast product. The first compressor wheel4has a plurality of first compressor blades18. The second compressor wheel5is made of a metal material and formed as a metal cast product. The second compressor wheel5has a plurality of second compressor blades19.

As shown inFIGS. 2-3, electric motor11is mainly constructed by a cylindrical motor housing (a motor case)20, a stator21including stator coils (stator windings) fixed to the inner peripheral side of motor housing20, and a rotor22fixed to the shaft member6and having a plurality of permanent magnets. Electric motor11is structured to share the shaft member6common to all the first compressor wheel4, the second compressor wheel5, and the turbine wheel3as a rotation axis of the motor. For instance, when sufficient supercharging action cannot be achieved owing to a shortage of rotation of turbine wheel3, electric motor11comes into operation. Hence, a shortage of rotation of turbine wheel3can be supplemented by driving the turbine wheel3with the electric motor11in operation. Electric motor11also serves as a generator, for instance depending on a vehicle operating condition, thereby enabling energy regeneration. Electric motor11is arranged to partially occupy a specified portion of the comparatively large space that provides the communicating passage17defined between the shaft member6and the compressor housing8, the specified portion corresponding to an annular space adjacent to the outer periphery of shaft member6and concentric to the axis of shaft member6.

Motor housing20is made of a metal material. As shown inFIGS. 2-3, motor housing20has a plurality of radiating fins23formed integral with the outer periphery of the cylindrical motor housing20, and a plurality of ribs24formed integral with the outer periphery of the cylindrical motor housing20. Each of radiating fins23is formed as an axially elongated radiating fin continuously extending over the entire axial length of motor housing20. Each of ribs24is formed as an axially elongated rib continuously extending over the entire axial length of motor housing20. Each of ribs24also serves as a radiating fin, but formed as a comparatively thick-walled rib as compared to the thickness of each of radiating fins23. As shown inFIG. 3, the plurality of radiating fins23and the plurality of ribs24are formed to protrude radially outwards from the outer periphery of the cylindrical motor housing20, as viewed in the axial direction of shaft member6. The plurality of ribs24and the plurality of radiating fins23are mixed together, such that the plurality of ribs24are circumferentially equidistant-spaced apart from each other and that each of the ribs24is interposed between two adjacent radiating fins23,23. Top lands (top surfaces) of the plurality of radiating fins23and ribs24are kept in abutted-engagement with the inner peripheral surface of the second housing15. Hence, with the plurality of radiating fins23and ribs24existing on the outer periphery of motor housing20, an outer annular space of the previously-discussed comparatively large space that provides the communicating passage17, remaining on the outer peripheral side of the cylindrical motor housing20, is partitioned and subdivided into a plurality of segmental spaces in the circumferential direction.

In the shown embodiment, the circumferentially equidistant-spaced four ribs24of motor housing20are fixedly connected to the second housing15with bolts (not shown) in the radial direction of motor housing20. The first cylindrical member12is fixedly connected to a first axial end of motor housing20, facing the first compressor wheel4, with bolts (not shown). In a similar manner, the second cylindrical member13is fixedly connected to a second axial end of motor housing20, facing the second compressor wheel5, with bolts (not shown).

As shown inFIGS. 1-2, the first cylindrical member12is made of a metal material and formed as a metal cast product. The first cylindrical member12has a plurality of rectifying fins25formed on the outer periphery of the first cylindrical member12. The plurality of rectifying fins25are provided to rectify the flow of pressurized air on the pressure side of the first compressor wheel4, and introduce the rectified air into the plurality of segmental spaces of the communicating passage17, defined between the motor housing20and the second housing15. A first axial end of the first cylindrical member12, facing the first compressor wheel4, is slightly spaced apart from the backface of the first compressor wheel4with a slight clearance space. A second axial end of the first cylindrical member12, facing apart from the first compressor wheel4, is fixedly connected to the first axial end of motor housing20, facing the first compressor wheel4, with bolts (not shown).

The first cylindrical member12has a plurality of first air introduction holes26(axial through holes) formed therein. The rightmost opening end (viewingFIG. 2) of the first air introduction hole26opens into the inside of the inner peripheral surface of motor housing20. The leftmost opening end (viewingFIG. 2) of the first air introduction hole26opens into the previously-noted slight clearance space at an axial position opposed to the backface of the first compressor wheel4. Also, the first cylindrical member12has a plurality of radially-extending second air introduction holes27formed therein. The upper opening end (viewingFIG. 2) of the second air introduction hole27opens through the outer peripheral surface of the first cylindrical member12. The lower opening end (viewingFIG. 2) of the second air introduction hole27is connected to an intermediate portion of the first air introduction hole26.

The second cylindrical member13is made of a metal material and formed as a metal cast product. The second cylindrical member13has a plurality of air exhaust holes28(axial through holes) formed therein. The rightmost opening end (viewingFIG. 2) of the air exhaust hole28communicates with the communicating passage17within the third housing16. The leftmost opening end (viewingFIG. 2) of the air exhaust hole28opens into the inside of the inner peripheral surface of motor housing20.

As appreciated from the longitudinal cross-section ofFIG. 2, with the first air introduction hole26and the second air introduction hole27formed in the first cylindrical member12, part of air, immediately after having been pressurized and compressed, is introduced from both the side of the backface of the first compressor wheel4and the rectifying fins25through the respective through holes26and27into the motor housing20. The air introduced into the motor housing20axially flows through apertures among the stator coils of stator21and apertures between the motor stator21and the motor rotor22toward the second compressor wheel5. Thereafter, the air is exhausted through the plurality of air exhaust holes28formed in the second cylindrical member13into the communicating passage17defined in the third housing16constructing a part of the compressor housing8(i.e., the main housing).

The fundamental structure of turbocharger1of the embodiment is as above. The function and operation of the turbocharger1having the fundamental structure as discussed above are hereunder described in detail.

When turbine wheel3is driven by exhaust gas energy, the first compressor wheel4and the second compressor wheel5, both coaxially installed onto the same shaft member6common to the turbine wheel3, rotate in concert with rotary motion of the turbine wheel3. At this time, by virtue of sucking action of the first compressor wheel4, outside air flows into the first compressor wheel4. Additionally, by virtue of sucking action of the second compressor wheel5, the air introduced into the communicating passage17constantly flows toward the suction side of the second compressor wheel5. Therefore, it is possible to more efficiently cool the electric motor11while preventing the introduced air from staying around the electric motor11, because of electric motor11installed in the communicating passage17between the first and second compressor wheels4and5. Consequently, it is possible to effectively suppress a deterioration in the driving efficiency of electric motor11, resulting from a temperature rise of electric motor11.

For instance, during electric-assist for a boost-pressure rise with the electric motor11acting on the common shaft member, a deterioration in the driving efficiency of electric motor11, which may occur owing to a temperature rise of electric motor11, can be suppressed, thereby avoiding a risk that a desired boost pressure cannot be ensured. Consequently it is possible to ensure a sufficient boost pressure, thereby maintaining a good vehicle driving state. Also, during electric power generation (i.e., during energy regeneration), a deterioration in the driving efficiency of electric motor11, which may occur owing to a temperature rise of electric motor11, can be suppressed, thereby avoiding a risk that a desired electric power generation efficiency cannot be gained. Surplus rotation of turbine wheel3, which is driven by exhaust gas energy, can be efficiently converted or regenerated to electric energy. This contributes to the improved fuel economy of the vehicle.

Additionally, radiating fins23are attached onto the outer periphery of motor housing20, and thus it is possible to efficiently transfer or radiate heat from the stator21fixed to the inner periphery of motor housing20to air flowing through the communicating passage17. As a result, it is possible to effectively suppress a temperature rise in stator21.

Also, part of air, immediately after having been pressurized and compressed by the first compressor wheel4, is introduced through the first air introduction hole26and the second air introduction hole27into the electric motor11. Hence, it is possible to directly cool (i) the stator21that generates heat and (ii) the rotor22whose temperature rises due to heat generated from the stator21, by the air introduced into the electric motor11. That is, it is possible to more efficiently cool from the inside and the outside of the electric motor11.

Furthermore, air can be pressurized and compressed in two stages by means of the first compressor wheel4and the second compressor wheel5. Therefore, it is possible to ensure a desired boost pressure, even under a comparatively low rotational speed of the shaft member6, thereby suppressing heat generated from the electric motor11. Additionally, the electric motor11can be downsized. Moreover, it is possible to ensure a desired boost pressure without so much increasing a supercharging ratio attained by the first-stage compressor wheel (i.e., the first compressor wheel4). Hence, a temperature rise in air compressed by the first compressor wheel4can be reduced, thus enabling the electric motor11to be efficiently cooled.

Furthermore, the two-stage supercharging turbocharger1is configured such that air is pressurized and compressed in two stages and that a shortage of rotation of turbine wheel3can be supplemented by electrically assisting the turbine wheel rotation through the use of the electric motor11. Hence, it is possible to realize or build a compact turbocharger system that enables a good supercharging efficiency from a low speed range, even through the use of the comparatively downsized electric motor11. Additionally, by working or operating the two-stage supercharging turbocharger1at comparatively low speeds, it is possible to suppress or reduce a centrifugal force as well as a torque. Thus, the mechanical strengths of bearings29that rotatably support the common shaft member6and the mechanical strength of electric motor11can be properly reduced. In other words, the overall size of the electric-assist system itself including at least the electric motor11and the bearings29can be appropriately reduced. This contributes to lower electric-assist system costs and simplified electric-assist system configuration.

In the meantime, owing to the aforementioned cooling action for the electric motor11, more of heat, removed from the electric motor11(stator21and rotor22) by air suctioned into the second compressor wheel5, is delivered to the second-stage compressor wheel (i.e., the second compressor wheel5). The heat removed from the electric motor11is a disadvantage in rising a boost pressure at the second compressor stage, that is, from the viewpoint of satisfactory pressurizing/compressing action of the second compressor wheel5.

For the reasons discussed above, in the shown embodiment, as shown inFIGS. 2-3, for the purpose of positively forced-cooling a control unit30itself in addition to forced-cooling for (i) the air flowing through the communicating passage17and forced-cooling for (ii) electric motor11(in particular, the motor stator21), while paying attention to the control unit30having an electronic circuit board and the like installed in a control-unit casing for controlling the electric motor11incorporated within the turbocharger1, and fixed and placed right above the second housing15constructing a part of the compressor housing8, a cooling mechanism31is further provided (seeFIGS. 4-5). By the way, inFIG. 1, which is a partially cutaway view of the turbocharger1, the control unit30is not shown. Also, the upper-half and the lower-half of the longitudinal cross-section ofFIG. 2are synthesized by two different cross-sections.

As shown inFIGS. 1-3, the air immediately after having been pressurized and compressed by the first compressor wheel4flows through the communicating passage17defined inside of the compressor housing8to the suction side of the second compressor wheel5, as discussed above. For the purpose of forced-cooling the air flowing through the communicating passage17, as a first cooling part of the cooling mechanism31, a first water jacket32into which cooling water (e.g., coolant) is supplied and through which the cooling water circulates, is formed in diametrically-opposing respective ribs, namely, the comparatively thick-walled upper rib24and the comparatively thick-walled lower rib24. These diametrically-opposing ribs24,24are configured to protrude radially outwards from the outer periphery of motor housing20and to radially cross the communicating passage17. As clearly shown inFIGS. 2 and 4, upper and lower first water jacket portions, constructing the first water jacket32, are formed to extend in the longitudinal direction of rib24(in other words, in the longitudinal direction of shaft member6). As appreciated from the perspective view ofFIG. 4, the upper and lower first water jacket portions, constructing the first water jacket32, are communicated with each other through respective circular-arc shaped communicating passages33,33at both ends of each of the upper and lower first water jacket portions in the longitudinal direction of rib24.

In a similar manner, for the purpose of positively forced-cooling the stator21of electric motor11installed in the communicating passage17as well as means for cooling the stator21by the flow of air through the communicating passage17, as a second cooling part of the cooling mechanism31, a second water jacket34, into which cooling water is supplied and through which the cooling water circulates, is formed in the motor housing20. As shown inFIG. 4, the second water jacket34is formed as a continuous spirally-wound cooling-water passage by which the stator21is surrounded by a plurality of turns in the circumferential direction at regular intervals.

Furthermore, for the purpose of forced-cooling the control unit30, because the control unit30for controlling the electric motor11is fixedly positioned around the second housing15constructing a part of the compressor housing8, as a third cooling part of the cooling mechanism31, a third water jacket35, into which cooling water is supplied and through which the cooling water circulates, is formed in the second housing15. As shown inFIG. 4, the third water jacket35is formed as a continuous meanderingly-shaped cooling-water passage circumferentially meandering more than once within a specified circumferential section of the second housing15.

As shown inFIGS. 4-5, the inlets of the first, second, and third water jackets32,34, and35, all included in the cooling mechanism31, are connected to a common cooling-water supply pipe36through respective connecting pipes (branch pipes) branched from the common cooling-water supply pipe36. The outlets of the first, second, and third water jackets32,34, and35are merged into a common cooling-water exhaust pipe37. Actually, in order to provide a liquid-cooled forced circulation system that enables forced-circulation of cooling water, also provided are a heat exchanger (not shown) and a cooling-water pump (not shown) and the like in addition to the cooling-water supply pipe36and the cooling-water exhaust pipe37.

An electromagnetic directional control valve38(serving as cooling-water distribution means), such as an electromagnetic rotary multi-directional control valve, is disposed at the branch point (the branched portion) of the three branch pipes branched from the cooling-water supply pipe36. In the shown embodiment, directional control valve38is configured to enable mode-switching, depending on a vehicle operating condition, among (i) a first cooling-water distribution mode in which cooling water is supplied to any one of the first, second, and third water jackets32,34, and35, (ii) a second cooling-water distribution mode in which cooling water is supplied to any two of the first, second, and third water jackets32,34, and35, and (iii) a third cooling-water distribution mode in which cooling water is supplied to all of the first, second, and third water jackets32,34, and35.

As shown inFIGS. 4-5, three water-temperature sensors39,40, and41are mounted on the respective outlet sides (downstream sides) of the first, second, and third water jackets32,34, and35, and also a water-temperature sensor42is mounted on the control unit30. Output signals from these water temperature sensors39,40,41, and42are sent to a valve controller (not shown), which is configured to control an operating mode (an operating position) of directional control valve38.

More concretely, during an accelerating condition of the vehicle, the previously-noted third cooling-water distribution mode is selected for supplying cooling water to all of the first, second, and third water jackets32,34, and35. Hence, (i) the air flowing through the communicating passage17, (ii) the stator21of electric motor11, and (iii) the control unit30can be all forced-cooled by heat-exchange (heat-transfer) between the air flowing through the communicating passage17and the cooling water flowing through the first water jacket32, by heat-exchange between the motor stator21and the cooling water flowing through the second water jacket34, and by heat-exchange between the control unit30and the cooling water flowing through the third water jacket35. In contrast, when the vehicle operating condition is shifted from the vehicle accelerating condition to a constant-speed running state, the first cooling-water distribution mode is selected for supplying cooling water to only the first water jacket32. Hence, the air flowing through the communicating passage17can be forced-cooled.

Also, when the turbocharger1is driven by exhaust gas energy, the third cooling-water distribution mode is selected for supplying cooling water to all of the first, second, and third water jackets32,34, and35. Hence, the air flowing through the communicating passage17, the stator21of electric motor11, and the control unit30can be all forced-cooled by the three different water jackets32,34, and35, which are provided independently of each other. Furthermore, during a decelerating condition of the vehicle, the second cooling-water distribution mode is selected for supplying cooling water to the second water jacket34and the third water jacket35. Hence, the stator21of electric motor11and the control unit30can be forced-cooled. The previously-noted three different cooling-water distribution modes form the basis of cooling-water supply patterns for supplying cooling water to at least one of three different water jackets32,34, and35. Additionally, the degree of relative priority of these cooling-water supply patterns can be appropriately changed depending on latest up-to-date informational signals generated from water temperature sensors39,40,41, and42.

According to the two-stage supercharging electric-assist turbocharger of the embodiment, the air flowing through the communicating passage17can be forced-cooled by cooling water flowing through the first water jacket32. Hence, there is a less tendency for the air suctioned into the second-stage compressor wheel as well as the air suctioned into the first-stage compressor wheel to be thermally affected by heat generated by the electric motor11. In particular, the first water jacket32(the first forced-cooling part) is advantageous with respect to the air cooling performance of air suctioned into the second-stage compressor wheel, thus greatly improving the boost pressure.

The stator21of electric motor11can be forced-cooled by cooling water flowing through the second water jacket34, and thus it is possible to greatly suppress a deterioration in the driving efficiency of electric motor11. Additionally, the control unit30for controlling the electric motor11can be forced-cooled by cooling water flowing through the third water jacket35, and thus an original motor-control function of the control unit30can be maintained stably. Also, there is a less tendency for the air suctioned into each of the compressor wheels4and5to be thermally affected by heat generated by the control unit30.

Furthermore, the first, second, and third water jackets32,34, and35are eventually built in the compressor housing8. Hence, an external cooler (or an external heat exchanger), separated from the main housing, is unnecessary. This is very advantageous with respect to downsizing and space-saving around the turbocharger1.

In the shown embodiment, as an example of installation location of a control unit for electric-motor control, the control unit30is fixedly positioned on the outer periphery of the second housing15constructing a part of the compressor housing8. The installation location of control unit30is not limited to the particular embodiment shown and described herein. In lieu thereof, the control unit30may be arranged adjacent to the compressor housing8as an external control unit separated from the compressor housing8.

Also, in the shown embodiment, the cooling-water distribution means is exemplified in an electromagnetic directional control valve38disposed at the branched portion of a cooling-water supply system of the cooling mechanism31. The cooling-water distribution means is not limited to the electromagnetic directional control valve38shown and described herein. Instead of using the previously-discussed directional control valve as a cooling-water distribution means, for the purpose of flow control, flow-constriction orifices (or flow-control valves) may be appropriately disposed in respective branch pipes branched from the cooling-water supply pipe36of the cooling-water supply system. For instance, with flow-constriction orifices whose area settings determine the controlled cooling-water flow rates through the respective orifices, a flow rate of cooling water to be distributed to each of the first, second, and third water jackets32,34, and35can be univocally predetermined depending on a cooling load (in other words, a thermal load) of each of the first, second, and third water jackets. Alternatively, with flow-control valves whose flow-rate settings determine the controlled cooling-water flow rates through the respective flow control valves, a flow rate of cooling water to be distributed to each of the first, second, and third water jackets32,34, and35can be univocally predetermined depending on a cooling load (in other words, a thermal load) of each of the first, second, and third water jackets. In a similar manner to the use of directional control valve38, through the use of the flow-constriction orifices (or the flow-control valves), the cooling efficiency of the entire cooling system can be improved. Also, the use of flow-constriction orifices, each having a simplified flow-rate control structure, is advantageous with respect to lower system installation time and costs, in other words, excellent mountability of the overall system on the vehicle.

Moreover, in the shown embodiment, as three different cooling parts, constructing the cooling mechanism31, the first, second, and third water jackets32,34, and35are exemplified. These three different cooling parts are not limited to such water-jacket type of heat exchangers. For instance, the first water jacket32may be replaced with small-size heat exchangers built in respective ribs24. In a similar manner, the third water jacket35may be replaced with a small-size heat exchanger partly built in the second housing15. Also, instead of integrally forming the third cooling part (i.e., the third water jacket35or the built-in heat exchanger) in the second housing15, the third cooling part for forced-cooling the control unit30may be arranged within the control unit30itself.

The entire contents of Japanese Patent Application No. 2016-095004 (filed Mar. 11, 2016) are incorporated herein by reference.

While the foregoing is a description of the preferred embodiments carried out the invention, it will be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the scope or spirit of this invention as defined by the following claims.