Axial turbine for exhaust gas turbochargers
FIELD OF THE INVENTION 
     The present invention concerns axial turbines generally, and more particularly, axial turbines for exhaust gas turbochargers having turbine inlet ducting which rotates with the turbine. 
     BACKGROUND OF THE INVENTION 
     In turbochargers for internal combustion engines, it is aerodynamically advantageous to first let the engine exhaust gas flow inwards through an entry volute or through guide vanes over the complete periphery while imparting the necessary swirl and subsequently, after deflection, through the axial turbine. 
     An axial turbine having a radial flow turbine guide apparatus of the type mentioned above is shown in the European patent application No. 0 093 462 published Nov. 9, 1983 (FIG. 2). 
     In this device, the inner wall of the gas deflection duct located between the turbine guide apparatus and the turbine rotor blades is fixed and immovable. Because of gas friction on this wall, a loss of swirl occurs. Due to the high peripheral gas velocity, which increases in the radially inward direction, this inner wall of the gas deflection duct causes relatively high frictional losses. The isentropic efficiency of the turbine is reduced due to this by an additive 2% to 5% approximately. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The object of the invention is, therefore, to produce an axial turbine in which the loss of swirl in the gas deflection duct is reduced to a minimum and in which, consequently, a good efficiency is achieved. 
     This object is obtained by an axial turbine constructed in accordance with the present invention, wherein the engine exhaust gas, after it has been accelerated at the inlet volute or the turbine guide vanes by a peripheral component, is supplied to the turbine rotor blades via an exhaust gas deflection duct, which duct includes a deflection collar rigidly attached to the turbocharger shaft. By such an arrangement an improvement in efficiency is obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing, two preferred embodiments of the subject invention are shown in a simplified representation, wherein: 
     FIG. 1 is a partially sectional side-view of an axial turbine of an exhaust gas turbocharger constructed in accordance with a first preferred embodiment of the present invention; and 
     FIG. 2 is a modification of the arrangement of FIG. 1. 
     The same parts are provided with the same reference numbers in both figures. The flow directions of the working medium are indicated by arrows. Parts of the axial turbine which are not essential to the invention, such as the turbine exhaust duct, consoles and fastening elements, are omitted. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the turbocharger axis is indicated by 1. The axial turbine shown, which has radial gas inlet flow, is connected via the turbine casing 7 to an exhaust pipe (not shown) of a supercharged diesel engine. The turbocharger shaft 2 is supported in the turbine casing 7 by means of the shaft bearing 10 and it carries a turbine disc 3 provided with the turbine rotor blades 4. Radial flow turbine guide vanes 5 are located in the gas upstream of the axial flow turbine rotor blades 4 in the annular deflection duct 7a. A sealing air duct 8 and an air bypass duct 9 are also located in the turbine casing 7. 
     In accordance with the invention, the inner wall of the rotationally symmetrical exhaust gas deflection duct 7b is designed as a deflection collar 6 rotating with the turbocharger shaft 2. This deflection collar 6 is rigidly connected by means of screws 12 to the turbocharger shaft 2. The outer diameter of the rotating deflection collar 6 is larger than the diameter of the turbine disc 3 and can, at maximum, be equal to the outer diameter of the turbine rotor. An element for contactless sealing of the exhaust gas deflection duct is provided between the rotating deflection collar 6 and the casing 7. This element consists of two labyrinth seals 11, 11&#39;, which are located on a cylindrical concentric surface of the deflection collar 6 opening inwards. A sealing air duct 8 located in the turbine casing 7 is connected to a radial gap 15 located between the labyrinth seal 11 facing towards the turbine and the labyrinth seal 11&#39; facing away from the turbine. An air by-pass duct 9 located in the turbine casing 7 is connected to an air space 13. 
     The method of operation of the axial turbine for exhaust gas turbochargers is described below. 
     The engine exhaust gas flows through the exhaust gas duct 7a, through the ring of guide vanes 5 and the exhaust gas deflection duct 7b to the turbine rotor blades 4, in which it expands with a release of power, and is finally expelled to atmosphere through an exhaust pipe, which is not shown. The engine exhaust gas, which arrives in a mainly radial direction, is tangentially accelerated in the turbine guide vanes 5. Due to this, a swirl acting in the rotational direction of the turbine appears. 
     Since the inner wall of the exhaust gas deflection duct 7b rotates with the turbocharger shaft 2, the relative velocity between the tangential gas velocity and the rotating wall is substantially less, in this zone, than in the case of axial turbines without a rotating deflection collar. The resulting gain in turbine efficiency, due to the decreased friction, is approximately an additive 1.5 to 3%. 
     The sealing air supply through the sealing air duct 8 serves to cool the turbocharger shaft 2 and the turbine disc 3 and prevents the leakage of exhaust gas from the exhaust gas deflection duct 7b through the air space 13 to the shaft bearing 10 and its surroundings. 
     A frictional braking force occurs in the air space 13 on the side of the deflection collar 6 facing away from the gas flow, but this force is relatively small. The resultant axial force acting on the turbocharger shaft 2 depends, among other things, on the pressure distribution on the two sides of the deflection collar 6. Since the labyrinth seals 11 are located radially well out, this resultant axial force is greatly reduced and corresponds approximately to that of a radial turbine. Due to the flow losses in the labyrinth seal 11, the air pressure in the air space 13 behind the deflection collar 6 is reduced to approximately ambient pressure. Due to this, the axial force on the turbocharger shaft becomes small. With this arrangement, the sealing air consumption is somewhat greater than that in arrangements without rotating deflection collars 6. 
     In the embodiment shown in FIG. 2, the element for contactless sealing of the deflection duct 7b consists of a labyrinth seal 11 located concentrically in a plane normal to the axis. The labyrinth seal 11 is located on the outer diameter of the rotating deflection collar 6. The small exhaust gas quantity flowing from the exhaust gas deflection duct 7b inwards through the labyrinth seal 11 is led away into the by-pass duct 9 together with the sealing air flowing radially outwards from within. The sealing air consumption in this arrangement is smaller than that in the case of arrangements not having rotating deflection collars 6. This sealing air consumption is mainly determined by the cooling necessary for the deflection collar. A very small quantity of engine exhaust gas is lost here through the labyrinth seal 11. This loss of gas is also negligible because of the low gas density. One main advantage of this arrangement is that the axial force on the turbocharger shaft practically disappears.