Exhaust gas turbocharger for an internal-combustion engine
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to an exhaust gas turbocharger for an internal-combustion engine of the type having a radial turbine with exhaust gas supplied in the radial direction to drive the turbine blades. 
     On the basis of German Pat. No. (DE-PS) 31 35 107, an exhaust gas turbocharger of this type is known, the turbine of which has a bypass. The bypass comprises a shutoff device that can be actuated by means of an actuating drive and influenced by the detected operating parameters of the internal-combustion engine. However, in this case, it is a disadvantage that the efficiency of the exhaust gas turbocharger in the lower speed range is still unsatisfactory. In the case of today&#39;s small radial turbine housings, a flow occurs against the blades of the turbine wheel that is high in losses, especially in the lower speed range. This flow results in an extreme thrust flow and negatively influences the consumption behavior of the internal-combustion engine because of the flat flow against the blades. 
     It is an objective of the invention to improve the efficiency of a radial turbine in the lower speed range. 
     According to the invention, this objective is achieved by providing an exhaust gas duct opening in the axial direction of the turbine wheel at the turbine housing on the side of the turbine wheel shaft. 
     Further features of preferred embodiments of the invention involve the structural arrangement of separate exhaust gas ducts formed in the turbine housing and control devices for controlling the relative exhaust flow between the exhaust gas ducts as a function of the turbine speed and/or changing air output pressure. 
     The radial turbine arrangement constructed according to the invention has the further advantage that the blades of the turbine wheel of the radial turbine of the exhaust gas turbocharger are designed for two different operational ranges. Thus the blades have a physical bend which, in the case of a radial flow feeding in the upper speed range of the exhaust gas turbocharger, permits a flow against the blades that has few thrust losses. In a lower speed range of the exhaust gas turbocharger, the feeding of the flow takes place approximately semiaxially to the blades with improved operation at these speeds. On the basis of the blade bend along the axial extension, a flow against the blades is also obtained in this lower speed range that has few thrust losses so that the nonsteady behavior in the lower speed range of the exhaust gas turbocharger is significantly improved. 
    
    
     Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustrating only, an embodiment in accordance with the present invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional schematic view of a radial turbine driving a compressor constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 1a is a longitudinal sectional schematic view of an alternate preferred embodiment of the present invention; 
     FIG. 2 is a cross-sectional view through the radial turbine housing taken along section I--I of FIG. 1; 
     FIG. 3 is a cross-sectional view through a blade of the radial turbine wheel taken along section II--II of the FIG. 1; and 
     FIG. 4 is a detail of the radial turbine housing that is marked with &#34;x&#34; in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     An internal-combustion engine shown in diagram form in FIG. 1 comprises an intake system 2, an exhaust gas system 3 and an exhaust gas turbocharger 4. The exhaust gas turbocharger 4 has a compressor 5 and a turbine arrangement 6 with a radial turbine wheel 7. The radial turbine wheel 7 is surrounded by a radial turbine housing 8 having spiral-shaped gas ducts 11, 13. Upstream of the spiral-shaped gas ducts 11, 13, a guiding device 10 is arranged in the radial turbine housing 8. Guiding device 10 is controllably influenced by an actuating drive 9 of a control unit 17. By means of a sensor 15, the speed of the radial turbine wheel 7 can be determined, which is fed to the control unit via a signal-transmitting line 16. A plurality of pivotal flaps 14 are arranged in the nozzle area 18 of the spiral-shaped gas duct 13, shown as detail &#34;x&#34; in FIG. 4. The gas duct 11 opens out at a front side 12 of the radial turbine housing 8 on the side of the turbine wheel shaft. The area of the mouth is developed to be in the shape of a circular or ring shaped nozzle 26 (see FIG. 4). A plurality of pivotal flaps 20 are arranged upstream of the area of the mouth of the gas duct 11. 
     The radial turbine housing 8, shown in FIG. 2 in its cross-section, comprises the spiral-shaped gas ducts 11, 13 with the guiding device 10 arranged upstream of the spiral-shaped gas duct 11. A guiding body 21 of the guiding device 10 is fastened on a shaft 22 which is pivotally supported in the respective adjacent walls 8a, 8b of the gas ducts 11, 13. A hub 24 extends around a rotational axis 23 of the shaft 22 of the radial turbine wheel 7. Surrounding ring-shaped nozzle 26 is located in the mouth area of the gas duct 11 between the hub 24 and a wall 25 which separates the spiral-shaped gas ducts 11, 13. The circular nozzle cross-section can be made into channels by means of a plurality of uniform and evenly distributed segments 27. 
     As shown in FIG. 3, against which the flow is directed, the blades 28 of the radial turbine wheel have a concave surface along the axial extension. The bend of the blades 28 in the axial longitudinal direction, as well as the axial longitudinal extension from a downstream section (a) and an upstream section (b) of the blades are asymmetrical. In the case of the illustrated embodiment, the length (a) is larger than the length (b). 
     When the internal-combustion engine is operated in the lower speed range, where the exhaust gas mass flow is low, the actuating drive 9 have moved the guiding body 21 of the guiding device 10 into the position shown in FIG. 1. The exhaust gas flow of the internal-combustion engine is then supplied to the surrounding nozzle 26 via the gas duct 11. In the nozzle 26 area, the exhaust gas flow can be influenced by the plurality of pivotal flaps 20 upstream of same so as to act upon the blades of the radial turbine wheel 7 approximately semi-axially, as indicated by the arrow in FIG. 3. Because of the approximately thrust-free flow against the blades of the radial turbine wheel 7, the response behavior of the charged internal-combustion engine is improved, and a reduction of the fuel consumption is achieved because of the increased overall efficiency of the exhaust gas turbocharger. 
     When the speed of the radial turbine wheel determined by the sensor 15 exceeds an indicated preselected speed, the control unit 17 actuates the actuating drive to block the gas duct 11 by means of the guiding body 21 of the guiding device 10. The exhaust gas flow of the internal-combustion engine through the gas duct 13, which is influenced by the plurality of pivotal flaps 14, is then in a known manner supplied radially to the blades of the radial turbine. 
     The control unit 17 may also be influenced by the charge pressure in the intake system determined by a sensor instead of by the speed of the radial turbine wheel. This is shown in FIG. 1a. 
     Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only, and is not to be taken by way of limitation. The spirit and scope of the present invention are to be limited only by the terms of the appended claims.