Abstract:
A turbocharger having a variable geometry turbine intake incorporating a mobile cylindrical piston ( 70 ) for varying the area of the induction nozzle in the turbine ( 18 ). Blades ( 90 ) mounted on the piston for controlling the flow in the nozzle penetrate through a slotted heat shield ( 92 ) having a central opening wherein the rear disc of the turbine wheel is embedded to provide a smooth aerodynamic flow in the turbine vanes. A shield ( 100 ) engaged between the heat screen and a central housing of the turbocharger prevents the gas from the rear disc cavity from recycling into the cavity housing the blades further enhancing the aerodynamic flow. An axial actuating device ( 77 ) is secured for operating the piston via a shaft coupled by a cross ( 72 ) to the piston and coupled to an actuating hub ( 118 ) in the actuating device by quick connect unthreaded connection ( 122 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to turbochargers with variable geometry. More particularly a turbocharger is provided having a turbine intake with a variable nozzle with sliding blades, with the blades entering via a heat screen of slotted sheet metal suspended in the housing of the turbine and having the turbine wheel embedded therein in order to provide an aerodynamic surface directing the flow of exhaust gas in a turbine wheel with a substantially complete rear disc, an aerodynamic shield and an uncoupled axial actuating device to facilitate mounting. 
     2. Description of the Related Art 
     High-output turbochargers use variable-geometry systems for the turbine nozzle intakes in order to increase the performance and aerodynamic yield. Variable-geometry systems for turbochargers have typically been of two types: a type with rotating blades and a type with a piston. The rotating blade type exemplified in U.S. Pat. No. 5,947,681, entitled PRESSURE BALANCED DUAL AXLE VARIABLE NOZZLE TURBOCHARGER provides a plurality of individual blades placed in the intake nozzle of the turbine, which can turn in order to reduce or increase the area of the nozzle and the flow volume. The piston type, which is exemplified in U.S. Pat. Nos. 5,214,920 and 5,231,831 both entitled TURBOCHARGER APPARATUS, and U.S. Pat. No. 5,441,383 entitled VARIABLE EXHAUST DRIVEN TURBOCHARGERS uses a piston or a cylindrical wall which can be displaced concentric to the axis of rotation of the turbine in order to reduce the intake area of the nozzle. In most cases the variable-geometry turbocharger of the piston type includes blades with a leading edge which is fixed with respect to the flow of air, which are either mounted on the piston or on a stationary nozzle wall facing the piston and which enter into slots in the opposite surface during displacement of the piston. 
     In variable-geometry, piston-type turbochargers of the prior art the challenge has been to maximise the aerodynamic performance balanced by the tolerancing of the contact surfaces, principally of the blades and the reception slots which are subjected to an extreme temperature variation and to mechanical stress, as well as to provide a means for actuating the piston according to a configuration which can be easily manufactured. 
     SUMMARY OF THE INVENTION 
     A turbocharger incorporating the present invention has a casing having a turbine housing receiving exhaust gas from an exhaust head of an internal combustion engine at an intake and having an exhaust outlet, a compressor housing having an air intake and a first volute, and a central housing between the turbine housing and the compressor housing. A turbine wheel is mounted in the turbine housing to extract the energy from the exhaust gas. The turbine wheel is connected to a shaft which extends from the turbine housing through a shaft bore in the central housing and the turbine wheel has a substantially complete rear disc and multiple vanes. A bearing mounted in the shaft bore of the central housing supports the shaft for rotational movement and a vane wheel is connected to the shaft facing the turbine wheel and enclosed in the compressor housing. 
     A substantially cylindrical piston is concentric to the turbine wheel and can be displaced parallel to an axis of rotation of the turbine wheel. A plurality of blades extend substantially parallel to the axis of rotation from a first end of the piston in the proximity of the rear disc. A heat screen is engaged at its external circumference between the turbine housing and the central housing and extends radially inwards towards the axis of rotation. The rear disc of the turbine wheel is embedded in the heat screen for the smooth flow of exhaust gas in the vanes. The heat screen also has a plurality of slots receiving the blades. An actuating device is provided to displace the piston from a first position in which the first end is in the proximity of the heat screen to a second position in which the first end is remote from the heat screen. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The details and features of the present invention will be better understood in relation to the detailed description and the drawings in which: 
     FIG. 1 is an elevation in transverse cross-section of a turbocharger using an embodiment of the invention; 
     FIG. 2 is a top view of a first embodiment of the heat screen; 
     FIG. 3 is a top view of a second embodiment of the heat screen; 
     FIG. 4 is an elevation in transverse cross-section of an embodiment of the invention with an aerodynamic shield in association with the heat screen; 
     FIG. 5 is an exploded view of the actuating device; 
     FIG. 6 is a detailed view of the quick-connect connection between the swivel pipe connector and the diaphragm assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawings FIG. 1 shows an embodiment of the invention for a turbocharger  10  which includes a turbine housing  12 , a central housing  14  and a compressor housing  16 . A turbine wheel  18  is connected by the shaft  20  to an impeller  22 . The turbine wheel converts the energy of the exhaust gas of an internal combustion engine provided with an exhaust head (not shown) with a volute  24  in the turbine housing. The exhaust gas is expanded through the turbine and exits from the turbine housing via the outlet  26 . 
     The compressor housing includes an intake  28  and an outlet volute  30 . A rear plate  32  is connected by bolts  34  to the compressor housing. The rear plate is, in turn, fixed to the central housing by means of bolts (not shown). A first annular seal  36  is engaged between the rear plate and the compressor housing and a second annular seal  38  is engaged between the rear plate and the central housing. Bolts  40  and fixing washers  42  connect the turbine housing to the central housing. 
     Journal bearings  50  mounted in the shaft bore  52  of the central housing support the shaft during rotation. A clamping collar  54  mounted on the shaft adjacent to the impeller engages an abutment bearing  56  forced between the central housing and the rear plate in the illustrated embodiment. A sleeve  58  is engaged between the clamping collar and the impeller. A rotational seal  60 . such as a piston segment, provides a seal between the sleeve and the rear plate. A circlip  62  urges the journal bearing into the bore and a nut  64  urges the impeller and the bearing components onto the shaft. 
     The variable-geometry mechanism of the present invention includes a substantially cylindrical piston  70  moving into the turbine housing which is concentrically aligned on the rotational axis of the turbine. The piston can be displaced longitudinally by means of a cross-piece  72 , having three branches in the illustrated embodiment, being attached to the piston and being attached to an actuating shaft  74 . The actuating shaft enters a bushing  76  extending through the turbine housing and is connected to an actuating device  77 . In the illustrated embodiment the actuating device is mounted on projections on the turbine housing using a support  78  and bolts  80 . 
     The piston slides in the turbine housing by means of a low-friction attached piece  82 . A cylindrical seal  84  is inserted between the piston and the attached piece. The piston can be displaced from a closed position illustrated in FIG. 1, substantially reducing the area of the intake nozzle which extends from the volute  24  to the turbine. In the fully open position, a radial projection  86  on the piston enters a recess  88  which defines the course of the piston. 
     The blades  90  of the nozzle extend from the radial projection on the piston. When the piston is in the closed position the blades are housed in a recessed portion of the moulded piece of the central housing. A heat screen  92  is engaged between the turbine housing and the central housing. The screen is of a suitable shape to extend into the cavity of the turbine housing from the interface between the central housing and the turbine housing and to provide a wall inside the intake nozzle of the turbine. The turbine wheel includes a substantially complete rear disc and a central orifice  94  (best seen in FIGS. 2 and 3 as described hereinunder) in the screen receives the rear disc of the turbine wheel in an embedded manner in order to provide a substantially smooth aerodynamic trajectory from the outlet of the volute of the turbine housing to the turbine wheel. 
     FIG. 2 shows a first embodiment of the heat screen including closed slots  96  to receive the blades  90 . The circumference of the orifice  94 , in which the rear disc of the turbine wheel is embedded, is inside the portion of the profiles of the slots housing the rear edge of the blades. This embodiment provides an optimal aerodynamic profile but the production constraints and the tolerances between the slots and the blades may prevent effective use of this embodiment in some applications. 
     FIG. 3 shows a second embodiment of the heat screen which provides an open profile at the rear edge of the slots, adjacent to the central orifice in order to reduce, to some extent, the tolerance requirements of these slots. The profile of the slots extends along and beyond the external surface of the vanes substantially as far as the diameter of the rear disc of the turbine wheel but the profile along the internal surface of the blade ends by leaving an orifice, generally designated  98 , joining the slot to the central orifice. In both embodiments, the fact of embedding the eccentricity of the hub of the turbine wheel and the rear disc in the central orifice minimises the clearance and the blade-free space between the rear edges of the blades and the diameter of the peak of the turbine blades of the turbine wheel. 
     FIG. 4 shows an aerodynamic shield  100  engaged between the heat screen and the central housing. The shield prevents the recirculation of exhaust gas leaks from the cavity of the rear disc of the turbine wheel into the recess in the central housing which houses the blades when the piston is in the closed position. Preventing the recirculation from the cavity of the rear disc encourages a smooth flow from the intake nozzle into the vanes of the turbine wheel. In the illustrated embodiment the shield is fixed between the heat screen and the central housing by the action of a spring comparable to a domed elastic washer. 
     The system for actuation of the piston in the embodiment illustrated in the drawings is a pneumatic actuating device  77  having a casing base  102  fixed to a support  78  as illustrated in FIG.  1 . As shown in FIG.  1  and in more detail in the exploded view of FIG. 5 a diaphragm  104  is engaged between the casing base and a cover  106 . A spring plate  108  in combination with the cover urges a spring  110  for the purpose of restoring the force on the diaphragm. The cover is held in position by an actuating device cap  112  which contains a vacuum intake  114  for actuation purposes. A seal  116  is provided between the cap and the cover. 
     An actuating hub  118  is fixed to the diaphragm by an elastic washer  120  which also acts as a plate for the spring  110 . The actuating hub is connected to the shaft by a quick-connect connection  122  which will be described in more detail hereinunder. A small-diameter centring connection  124  on the shaft enters a counter bore  126  in the hub. 
     As shown in FIG. 1 the linear displacement of the shaft is utilised by diametral extensions  130  on the shaft which slidingly enter a bore  132  in the turbine housing. In some embodiments a deflector  134  is mounted around the shaft in order to divert leaks of gas via the bushing  76  and get them away from the actuating device. 
     The quick-connect connection illustrated in detail in FIGS. 5 and 6 is made of sheet metal shaped in to a substantially cylindrical shape with a longitudinal slot  140 . Oppositely positioned cut-outs  142  in the wall of the cylinder provide clearance for tongues  144  which are lowered into the cylinder. A first set of tongues is adapted to engage in a slot  146  in the shaft while a second set of tongues is adapted to engage in a slot  148  in the actuating hub. The elasticity of the sheet metal tongues and of the slotted cylinder permits insertion of the shaft and of the hub into the quick-connect connection by the instant engagement of the tongues in the slots, thus overcoming the need for any threaded connection between the shaft and the hub. The alignment connection and the core in the hub maintain the axial alignment of the assembly. 
     An additional advantage of the quick-connect configuration is the possibility of removing the actuating device from the turbocharger without significant disassembly and—even more importantly—in most mounting configurations, without removing the turbocharger from the vehicle. The support  78  is loosened from the turbine housing and a compression tool is used to remove the tongues from the slots and to permit the shaft and/or the hub to be withdrawn from the quick-connect connection. 
     An alternative embodiment for the quick-connect connection is a star-shaped spring washer which is wedged in a cut-out in the actuating hub. By inserting the shaft into the hub, the star-shaped spring washer engages a shoulder on the shaft. A circumferential edge of the cut-out is crimped to fix the star-shaped spring washer in the cut-out. The second embodiment allows for reduced length over the overall coupling. 
     Having described the invention in detail as required by the law of industrial property, those skilled in the art will see modifications and substitutions which can be made to the specific embodiments disclosed herein. Such modifications and substitutions are within the scope and intention of the present invention as defined in the following claims.