Abstract:
A turbocharger for an internal combustion engine, particularly suitable for use in a work machine, is provided with a two stage turbine including a first turbine stage and a second turbine stage. A wastegate conduit is fluidly coupled with the two stage turbine. The wastegate conduit bypasses only a single one of the first turbine stage or second turbine stage. A valve is positioned in association with the wastegate conduit for controlling flow through the wastegate conduit. A compressor is coupled with and rotatably driven by the two stage turbine. The controllable wastegate conduit provides effective control of the power input to the compressor.

Description:
TECHNICAL FIELD 
     The present invention relates to a turbocharger for use in an internal combustion engine, and, more particularly, to a turbocharger including a multi-stage turbine. 
     BACKGROUND 
     An internal combustion engine may include one or more turbochargers for compressing a fluid which is supplied to one or more combustion chambers within corresponding combustion cylinders. Each turbocharger typically includes a turbine driven by exhaust gases of the engine and a compressor which is driven by the turbine. The compressor receives the fluid to be compressed and supplies the fluid to the combustion chambers. The fluid which is compressed by the compressor may be in the form of combustion air or a fuel/air mixture. 
     U.S. Pat. No. 3,044.683 (Woollenweber) discloses a fluid passage extending from the high pressure side of the compressor to the inlet side of a turbine. A spring loaded valve is disposed within the fluid passage and opens upon a high pressure condition within the compressor. The spring loaded valve thus merely acts to bypass some of the high pressure gas on an over pressure condition to the turbine of the turbocharger. 
     U.S. Pat. No. 5,724,813 (Fenelon et al.) assigned to the assignee of the present invention, discloses a turbocharger having a single stage compressor. A portion of the compressed gas from the single stage compressor may be recirculated to the outlet side of the turbine using controllably actuated valves. The control scheme utilizes only a single stage compressor. 
     U.S. Pat. No. 5,701,741 (Halsall) discloses a turbocharger having a single stage turbine driven by exhaust gas from an exhaust manifold. A bypass valve is fluidly connected at opposite ends with the inlet and outlet to the turbine. The valve may be actuated to bypass exhaust gas around the turbine. The rotational speed of the single stage compressor may thereby be adjusted. 
     Bypass systems as described above which bypass from the compressor to the turbine are primarily used to prevent a “surge” condition within the compressor, rather than adjust power inputs to the compressor. Bypass systems which bypass the entire turbine are used to control the power input to the compressor. Since the entire turbine is bypassed, however, the ability to control the power input to the compressor and thus the boost from the compressor is limited. That is, it may not be possible to selectively control the boost from the compressor over a relatively wide operating range. 
     The present invention is directed to overcoming one or more of the problems as set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a turbocharger for an internal combustion engine is provided with a two stage turbine including a first turbine stage and a second turbine stage. A wastegate conduit is fluidly coupled with the two stage turbine. The wastegate conduit bypasses only a single one of the first turbine stage or second turbine stage. A valve is positioned in association with the wastegate conduit for controlling flow through the wastegate conduit. A compressor is coupled with and rotatably driven by the two stage turbine. 
     In another aspect of the invention, a method of operating a turbocharger in an internal combustion engine is provided with the steps of: providing a two stage turbine including a first turbine stage and a second turbine stage; fluidly coupling a wastegate conduit with the two stage turbine so as to bypass only a single one of the first turbine stage and the second turbine stage; providing a compressor mechanically coupled with the two stage turbine; controlling a flow of exhaust gas through the wastegate conduit; and rotatably driving the compressor with the two stage turbine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an embodiment of a turbocharger of the present invention for use with an internal combustion engine. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawing, there is shown an embodiment of a turbocharger  10  for use with an internal combustion engine  12 . 
     Internal combustion engine  12  generally includes a plurality of combustion cylinders  14 , only three of which are shown for simplicity sake in the drawing. The particular number of combustion cylinders  14  within internal combustion engine  12  may vary, depending upon the particular application. Internal combustion engine  12  also includes an exhaust manifold  16  and an inlet manifold  18 . Inlet manifold  18  provides air or a fuel/air mixture to combustion cylinders  14 . Exhaust manifold  16  receives exhaust gas from combustion cylinders  14 . Exhaust manifold  16  and inlet manifold  18  are shown with a single part construction for simplicity sake in the drawing. However, it is to be understood that exhaust manifold  16  and/or inlet manifold  18  may be constructed as multiple-part manifolds, depending upon the particular application. 
     Turbocharger  10  includes a two stage turbine  20  and a two stage compressor  22 . Two stage turbine  20  is fluidly coupled with exhaust manifold  16  as indicated schematically by line  24 . Two stage turbine  20  includes a first stage in the form of a radial or mixed flow turbine wheel  26  and second stage in the form of an axial turbine  28 . Turbine wheel  26  and axial turbine  28  are each carried by a shaft  30  and rotatable about a longitudinal axis  32  of shaft  30 . More particularly, two stage turbine  20  includes a volute section  34  which receives exhaust gas from exhaust manifold  16  via line  24 . Volute section  34  may be in the form of a single volute as shown, or may be in the form of a split volute or other configuration, depending upon the particular application. Exhaust gas enters volute section  34  and impinges against a plurality of vanes  36  of turbine wheel  26 . Turbine wheel  26  is thus rotatably driven by exhaust gas from exhaust manifold  16 . 
     The exhaust gas flows in an axial direction away from turbine wheel  26  and impinges against a plurality of vanes  42  disposed radially around shaft  30  and between turbine wheel  26  and axial turbine  28 . Vanes  42  are controllably actuated, as indicated by double headed arrow  44  to adjust air flow rate and direction downstream from turbine wheel  26 . 
     The exhaust gas then flows to and impinges against a plurality of blades  46  of axial turbine  28  positioned radially around shaft  30 . The particular configuration and pitch angle of blades  46  may of course be dependent upon the particular application. The spent exhaust gas then flows to a muffler system (not shown) positioned downstream from turbocharger  10 , as indicated by directional arrow  48 . 
     Two stage compressor  22  includes a first compressor  50  and a second compressor  52 . First compressor  50  and second compressor  52  each include a compressor wheel  54  and  56 , respectively. Two stage compressor  22  receives combustion air as indicated by directional arrow  58 . First compressor wheel  54  and second compressor wheel  56  compress the combustion air in a series manner to provide a desired total compression ratio. Second compressor wheel  56  discharges the compressed combustion air into a volute section  60  which is fluidly coupled with inlet manifold  18  as indicated schematically by line  62 . Two stage compressor  22  thus provides compressed combustion air to inlet manifold  18 . 
     According to an aspect of the present invention, wastegate conduits  64  and  66  are fluidly coupled with two stage turbine  20 . Wastegate conduits  64  and  66  respectively bypass only a single one of first turbine stage (i.e., turbine wheel)  26  or second turbine stage (i.e., fan)  38 . In the embodiment shown, wastegate conduit  64  bypasses first turbine stage  26 , and wastegate conduit  66  bypasses second turbine stage  28 . 
     More particularly, first turbine stage  26  includes an inlet and an outlet which are respectively positioned upstream and downstream therefrom. Similarly, second turbine stage  28  includes an inlet and an outlet which are respectively positioned upstream and downstream therefrom. Wastegate conduit  64  has an inlet end fluidly coupled with the inlet of first turbine stage  26  and an outlet end fluidly coupled with the outlet of first turbine stage  26 . Wastegate conduit  66  has an inlet end fluidly coupled with the inlet of second turbine stage  28  and an outlet end fluidly coupled with the outlet of second turbine stage  28 . In the embodiment shown, the outlet end of wastegate conduit  64  and inlet end of wastegate conduit  66  are each fluidly coupled with a region between first turbine stage  26  and nozzle vanes  42 . However, it will also be appreciated that the outlet end of wastegate conduit  64  and/or the inlet end of wastegate conduit  66  may be fluidly coupled with the region between diverter vanes  42  and second turbine stage  28 . 
     Each wastegate conduit  64  and  66  includes a controllably actuatable valve  68  associated therewith. Valves  66  and  68  may be of conventional design, and may be configured to fully open or close, or be adjusted to an intermediate position between the full opened and closed positions. 
     Controller  70  is electrically coupled with each valve  68  via lines  72  and  74 , respectively. Controller  70  is also electrically coupled with one or more sensors  76  via an associated line  78  and receives an input signal therefrom. Sensor  76  senses an operating parameter associated with operation of turbocharger  10  and/or internal combustion engine  12  used to controllably actuate valves  66  and  68 . 
     INDUSTRIAL APPLICABILITY 
     During use, internal combustion engine  12  operates in known manner using, e.g., the diesel principle of operation. Exhaust gas is transported from exhaust manifold  16  to volute section  34  of two stage turbine  20  via line  24 . The exhaust gas impinges upon vanes  36  of turbine wheel  26  and rotatably drives turbine wheel  26 . The exhaust gas flows downstream from turbine wheel  26  to diverter vanes  42 . Vanes  42  may be controllably actuated, such as using controller  70 , to control the flow rate and/or flow direction of the exhaust gas. The exhaust gas then flows to second turbine stage or axial turbine  28 . The exhaust gas impinges against blades  46  of axial turbine  28  to assist in the rotational driving of two stage turbine  20 . The spent exhaust gas is then discharged to a muffler system, as indicated by arrow  48 . 
     Rotation of turbine wheel  26  and axial turbine  28  in turn causes rotation of shaft  30  which drives first compressor wheel  54  and second compressor wheel  56  of two stage compressor  22 . Combustion air or a fuel/air mixture is drawn into first compressor  50 , as indicated by arrow  58 . The combustion air or fuel/air mixture is compressed in a series manner within two stage compressor  22  using first compressor wheel  54  and second compressor wheel  56 . The compressed combustion air or fuel/air mixture is discharged from volute section  60  of second compressor  52  to inlet manifold  18  via line  62 . 
     Sensor  76  senses one or more operating parameters associated with internal combustion engine  12  and/or turbocharger  10  used to adjust the output power and/or rotational speed of shaft  30  within two stage turbine  20 . For example, it may be desirable to control the power level or boost of two stage compressor  22 . The boost of two stage compressor  22  is primarily dependent upon the rotational speed of shaft  30 . Under certain operating conditions, more or less boost from two stage compressor  22  may be desirable. By controlling valves  68  associated with wastegates conduit  64  and  66 , the boost of two stage compressor  22  can in turn be controlled. Controller  70  controllably actuates a selected valve  68  to open wastegate conduit  64  or  66 , or both simultaneously. 
     In a preferred method of operation, the plurality of nozzle vanes  42  disposed radially around shaft  30  are controllably positioned to control the air flow rate and/or air flow direction between turbine wheel  26  and axial turbine  28 . By controllably positioning vanes  42 , the boost of two stage compressor  22  can be controlled to some extent. However, it is not always possible to control the boost of two stage compressor  22 , depending upon the particular operating conditions of internal combustion engine  12 . Under such circumstances, nozzle vanes  42  are first adjusted and thereafter valves  68  are controllably actuated to open wastegate conduit  64  and/or  66 . 
     In contrast with conventional wastegate designs which bypass an entire two stage turbine from the inlet of the first turbine stage to the outlet of the second turbine stage, wastegate conduits  64  and  66  bypass only a single turbine stage within two stage compressor  20 . It is therefore possible to more precisely control the boost of two stage compressor  22  since only a portion of two stage turbine  20  is bypassed. For example, the pressure ratio at the inlet and outlet of turbine wheel  26  likely is different than the pressure ratio at the inlet and outlet of axial turbine  28 . By utilizing the known pressure ratios of turbine wheel  26  and/or axial turbine  28 , the rotational speed of shaft  30  may be more closely controlled. This in turn results in improved control of the boost from two stage compressor  22 . 
     Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.