Abstract:
A guide vane of a turbo-compressor has variable turbine geometry, in particular for a motor vehicle. The guide vane has a profile with a lower side, an upper side and a front edge. The guide vane is characterized by a nose which extends along the front edge of the vane, from the front edge of the vane to the upper side of the vane and forms a low pressure on the upper side of the vane when waste gas impacts the guide vane. A guide vane configuration, a turbo-compressor, a motor vehicle and a method for operating this type of turbo-compressor are further disclosed.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a guide blade for a turbocharger. The present invention also relates to a guide blade arrangement, a turbocharger, a motor vehicle and a method for operating a turbocharger of said type. 
     DE 10 2007 018 618 A1 describes the generally known design of a turbocharger for increasing the power of an internal combustion engine of a motor vehicle, said turbocharger being composed substantially of a radial turbine, with a turbine wheel which is driven by the exhaust-gas flow of the internal combustion engine, and of a radial compressor, which is arranged in the intake tract of the internal combustion engine and serves for compressing fresh air, said radial compressor having a compressor wheel which is connected in a rotationally conjoint manner to the turbine wheel by a rotor shaft. 
     The rotational speed of the radial compressor and therefore the so-called charge pressure is predefined by the rotational speed of the turbine or, more precisely, by means of the exhaust-gas mass flow flowing through the turbine. The turbine is usually dimensioned for a medium rotational speed and a medium power range of the internal combustion engine. In this way, by virtue of the fact that the rotating parts of the turbocharger have a sufficiently low mass moment of inertia, fast response behavior of the turbocharger and therefore fast implementation of an acceleration demanded by the vehicle driver is attained. Secondly, the turbine is operated with high efficiency in a medium rotational speed and power range of the internal combustion engine. A problem with said configuration, however, is the full-load range of the internal combustion engine. In the full-load range, the rotational speed of the turbine can increase to such an extent that the bearing arrangement, which is already subjected to high rotational speed loading, of the rotor shaft is damaged, or the admissible charge pressure of the internal combustion engine is exceeded. This may result in severe damage to or even destruction of the internal combustion engine. 
     One option for controlling the rotational speed of the turbocharger and therefore the charge pressure of the compressor is the use of a turbocharger with a so-called variable turbine geometry (VTG). U.S. Pat No. 6,709,232 describes a VTG turbocharger of said type. A VTG turbocharger has a guide blade ring which radially surrounds the turbine wheel. The guide blades are fastened with the rotary axles thereof to a carrier ring. On the rear side of the carrier ring, the rotary axles of the guide blades have a guide journal which engages into an adjusting ring. All the guide blades are rotated simultaneously by means of the adjusting ring. The adjusting ring is moved either by means of an electric actuating motor or by means of a vacuum capsule. The direction and the flow speed of the exhaust gas impinging on the turbine wheel blade arrangement is controlled by means of the angle of incidence of the guide blades. A shallow angle of incidence of the guide blade results in a reduced inlet cross section for the exhaust gas. However, in order that the same exhaust-gas mass flow can pass into the turbine per unit of time, the flow speed of the exhaust gas must increase. Furthermore, the angle at which the exhaust-gas mass flow impinges on the turbine blade arrangement is greater when the guide blades are at a shallow angle than when the guide blades are set at a steep angle. Therefore, for the same exhaust-gas quantity, a flat angle of incidence of the guide blades leads to a higher turbine rotational speed than a steep angle of incidence. In this way, in the case of a VTG turbocharger, it is possible by means of the angle of incidence of the guide blades both to realize a high charge pressure very quickly, for example in the event of acceleration of a motor vehicle from a standstill, and also to realize a reduced charge pressure, for example during full-load operation of the internal combustion engine at a constant high speed. VTG turbochargers are very widely used in particular in diesel engines. 
     In the simplest case, as a guide blade profile, use is made of profiles with a straight profile central line and a symmetrical thickness distribution. The regulability of such guide blade profiles is good. However, the thermodynamic and fluid-dynamic efficiency of such profiles is limited, in particular in the starting range of the internal combustion engine. For this reason, to optimize thermodynamic and fluid-dynamic efficiency in VTG turbocharger technology, use is made of a wide variety of guide blade profile variants, for example with continuously curved profile central lines, profile central lines which are curved in sections, profiles with an asymmetrical thickness distribution, profiles with an S bend, etc. Depending on the design, these profile variants however have various disadvantages with regard to their regulability. 
     As a result of the impingement of exhaust-gas flow on the guide blades, there is a certain pressure distribution across the guide blade surface, which pressure distribution, depending on the angular position of the guide blades, exerts a moment on the guide blades which has an opening or closing action. Since mechanical systems are always afflicted with play to a certain extent, a non-defined angular position of the blades arises in the angular region of this moment reversal. This non-defined angular position must be avoided from a regulating technology aspect. 
     Furthermore, a closing moment caused by the flow impinging on the guide blades can lead to a self-boosting effect when the guide blades are nearly closed. That is to say, if the blades are already nearly closed, the flow speed of the exhaust gas increases owing to the reduced flow cross section. This in turn causes the closing moment to increase. In the worst case, the actuator for adjusting the angle of incidence of the guide blades can then no longer impart the force required for opening the guide blades, and the turbocharger rotational speed increases in an uncontrolled manner. Furthermore, the emergency running characteristics of a closing guide blade geometry of said type are extremely poor, for example because, in the event of a failure of the actuator, the turbine rotational speed and therefore the charge pressure on the compressor side increase in an uncontrolled manner. 
     The disadvantages just mentioned should therefore be eliminated to the greatest possible extent. 
     Against this background, it is the object of the present invention to provide an improved guide blade. 
     BRIEF SUMMARY OF THE INVENTION 
     Said object is achieved according to the invention by means of a guide blade having the features of patent claim  1  and/or by means of a guide blade arrangement having the features of patent claim  12  and/or by means of a turbocharger having the features of patent claim  13  and/or by means of a motor vehicle having the features of patent claim  14  and/or by means of a method having the features of patent claim  15 . 
     Accordingly, the following is provided: 
     A guide blade of a turbocharger which is equipped with a variable turbine geometry, in particular for a motor vehicle, which guide blade has a profile with a blade underside, a blade top side and a blade leading edge, wherein a nose is provided which extends along the blade leading edge, which nose extends from the blade leading edge toward the blade top side and forms a negative pressure on the blade top side when exhaust gas impinges on the guide blade. 
     A guide blade arrangement for a turbocharger with adjustable turbine geometry, having: a multiplicity of guide blades according to the invention, a receptacle device for rotatably receiving the guide blades, wherein the guide blades are arranged in a circular configuration in the receptacle and wherein axes of rotation of the guide blades are arranged parallel to one another, an adjusting device for the uniform adjustment of an angle of incidence of the guide blades, wherein the adjusting device is designed as an adjusting ring, an actuator for adjusting the adjusting ring, and a coupling for connecting the actuator to the adjusting ring. 
     A turbocharger, in particular for a motor vehicle, having a guide blade arrangement according to the invention, which turbocharger has: a turbine housing, a turbine wheel which has a turbine blade arrangement and which is arranged in the turbine housing, a compressor housing, a compressor wheel which is arranged in the compressor housing, and a rotor shaft which connects the turbine wheel to the compressor wheel in a rotationally conjoint manner, wherein an angle at which an exhaust-gas impinges on the turbine blade arrangement can be adjusted by adjusting the angle of incidence of the guide blades. 
     A motor vehicle having a turbocharger of said type. 
     A method for operating a turbocharger of this type which has a multiplicity of guide blades according to the invention, wherein the multiplicity of guide blades are impinged on by a flow of exhaust gas in such a way that the exhaust gas impinges on a respective guide blade only in the region of the nose, as a result of which a negative pressure is formed in the region of the nose of each guide blade. 
     The idea on which the present invention is based is now inter alia that of providing a nose on the blade leading edge of the guide blade, which nose extends along the blade leading edge. Owing to the nose, a negative pressure is formed on the blade top side when a flow impinges on the blade leading edge. Said negative pressure results in a force which acts away from the blade top side. 
     It is therefore possible according to the invention to provide a guide blade which, over the entire operating range of a turbocharger with variable turbine geometry, is acted on by a force with a uniform direction of action. Said force generates a moment about an axis of rotation of the guide blade in the direction for opening the variable turbine geometry. In this way, an actuator of smaller dimensions can be used for adjusting the angle of incidence of the guide blades, as a result of which the turbocharger can be produced more cheaply overall. Since the force has the same direction of action over the entire operating range, the regulating characteristics of the turbocharger are significantly improved, because during operation of the turbocharger, there is no angular range with an undefined angle of incidence of the guide blades. 
     Furthermore, by means of the guide blades according to the invention, the emergency running characteristics of the turbocharger are also improved because, in the event of failure of the actuator, the guide blades automatically move, under the action of the force acting on the blade top side, in the direction for opening the variable turbine geometry. 
     Advantageous embodiments and refinements of the present invention will emerge from the further subclaims and from the description in conjunction with the figures of the drawing. 
     In a typical embodiment of the present invention, the nose forms a step-like cross-sectional widening of the profile. It is ensured in this way that the desired negative pressure is formed on the blade top side when the guide blade is impinged on by a flow of exhaust gas. 
     In a preferred embodiment of the present invention, the profile has a profile central line which defines a profile basic shape of the guide blade. Here, the profile central line runs from a first curvature central point of a first head radius in the region of the blade leading edge to a second curvature central point of an end radius in the region of a blade trailing edge situated opposite the blade leading edge. In this way, the basic shape of the profile of the guide blade can be produced with little expenditure, as a result of which the production costs of the guide blade according to the invention can be further reduced. 
     In a likewise preferred embodiment of the present invention, in the region of the nose, a third curvature central point of a second head radius is provided spaced apart from the profile central line. The third curvature central point is provided such that the blade leading edge is formed by the first and the second head radii. Here, the second head radius is greater than the first head radius. By means of the arrangement of the curvature central points, it is ensured that the shape of the nose can be defined using simple geometrical shapes, as a result of which the production costs of the guide blade according to the invention can likewise be reduced. 
     In a further preferred embodiment of the present invention, in the region of the step-like cross-sectional widening, the profile runs over the second head radius approximately perpendicular to the profile central line and merges into the profile basic shape. This ensures as fast as possible a transition from the large profile thickness of the nose to the smaller profile thickness of the profile basic shape. This also yields the greatest possible negative pressure on the blade top side, as a result of which a force acting away from the blade top side can be generated even at low exhaust-gas flow speeds. This increases the range of application of the guide blade according to the invention. 
     In a likewise preferred embodiment of the present invention, the third curvature central point is provided between the profile central line and the blade top side. The nose is provided in a front third of the profile with respect to the blade leading edge. This results in the greatest possible torque about the axis of rotation of the guide blade, as a result of which automatic opening of the variable turbine geometry is ensured even in the case of a low exhaust-gas mass flow. 
     In a further preferred embodiment of the present invention, the profile central line is a straight line or has a continuous curvature. In this way, the profile basic shape can be produced by means of simple, geometrically representable profile central lines, as a result of which the guide blades according to the invention can be produced more simply, and production costs are reduced. 
     In a likewise preferred embodiment of the present invention, an end edge of the nose is of sharp-edged design. This additionally improves the effect of the nose with regard to the generation of a negative pressure on the blade top side, as a result of which the exertion of an opening moment on the guide blade can be attained even at low flow speeds. 
     In a further preferred embodiment of the present invention, the nose has an extent, in the longitudinal direction of the profile proceeding from the blade leading edge, of up to 30% to 50%, preferably 30%, of a length of the profile. It is ensured in this way that the nose always generates an opening moment, and not a closing moment, about the axis of rotation of the guide blade. 
     The embodiments and refinements specified above may—where expedient—be combined with one another in any desired way. 
     The present invention will be explained in more detail below on the basis of the exemplary embodiments illustrated in the schematic figures of the drawing, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a schematic sectional view of an embodiment of a guide blade according to the invention; 
         FIG. 2  shows a schematic, enlarged sectional view of the exemplary embodiment of the guide blade according to the invention illustrated in  FIG. 1 ; 
         FIG. 3  shows a schematic view of an exemplary embodiment of a guide blade arrangement according to the invention; and 
         FIG. 4  shows a schematic view of an exemplary embodiment of an exhaust-gas turbocharger according to the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In the figures of the drawing—unless stated otherwise—identical components, elements and features have been denoted by the same reference numerals. 
       FIG. 1  shows a schematic sectional view of an exemplary embodiment of a guide blade according to the invention. 
       FIG. 1  firstly shows a guide blade  1  with a blade underside  2  and a blade top side  4 . The blade underside  2  and the blade top side  4  form, together with a blade leading edge  9  and a blade trailing edge  12 , the boundary of the profile  3  of the guide blade  1 . The blade leading edge  9  constitutes an incident-flow edge of the profile  3 . The guide blade  1  has a profile central line  5  of a profile basic shape  6 . The profile basic shape  6  has a symmetrical thickness distribution. The profile central line  5  runs from a first curvature central point  7  of a first head radius  8  of the blade leading edge  9  to a second curvature central point  10  of an end radius  11  of the blade trailing edge  12 . The profile central line  5  is formed here by a multiplicity of curvature central points of a multiplicity of circles laid tangentially on the profile basic shape  6 . The profile central line  5  preferably has a continuous curvature. Alternatively, the profile central line  5  may also be formed by a straight line or by any desired other linear two-dimensional form. The profile  3  of the guide blade  1  furthermore has a step-like cross-sectional widening  13  in the form of a nose  13  which extends along the blade leading edge  9 . The nose  13  runs from the blade leading edge  9 , which is formed as the incident-flow edge, in the direction of the blade top side  4  to the blade trailing edge  12 . The nose  13  is defined by a second head radius  15  at the blade leading edge  9 . Here, the second head radius  15  is preferably greater than the first head radius  8 . The curvature central point of the second head radius  15  does not lie on the profile central line  5 . That is to say, in the region of the blade leading edge  9 , the profile  3  of the guide blade deviates from the symmetrical thickness distribution of the profile basic shape  6 . In a longitudinal direction  17  of the guide blade  1 , the step-like cross-sectional widening  13  in the form of the nose  13  runs from the blade leading edge  9  as far as at most half way along the profile  3 . The step-like cross-sectional widening  13  however preferably ends in the front third of the profile  3  with respect to the blade leading edge  9 . The guide blade  1  furthermore has an axis of rotation  41  which is preferably arranged in a front third of the guide blade  1  in the longitudinal direction  17 . 
     When a flow of exhaust gas from an internal combustion engine impinges on the guide blade  1  at the blade leading edge  9  formed as the incident-flow edge, a negative pressure  16  is formed on the blade top side  4  as a result of the nose  13 . A force  40  resulting from said negative pressure  16  acts away from the blade top surface  4  and generates a torque  42  about the axis of rotation  41 . 
       FIG. 2  illustrates a schematic, enlarged sectional view of the exemplary embodiment of the guide blade according to the invention illustrated in  FIG. 1 . 
       FIG. 2  shows, in an enlarged view, the blade leading edge  9  of the guide blade  1 . The first head radius  8  with the first curvature central point  7  and the second head radius  15  with a third curvature central point  14  are illustrated, for clarity, as solid circles. The third curvature central point  14  is not arranged on the profile central line  5 . The third curvature central point is preferably arranged between the profile central line  5  and the blade top side  4 . With respect to the longitudinal direction  17  of the profile  3 , the third curvature central point  14  is preferably arranged in the front third of the profile  3  in relation to the blade leading edge  9 . The shape of the nose  13  is defined substantially by the second head radius  15 . Since the third curvature central point  14  does not lie on the profile central line  5 , the course of the profile  3  in the region of the blade leading edge  9  is defined both by the first head radius  8  and also by the second head radius  15 . Proceeding from the blade underside  2 , an outer contour of the profile  3  runs, via a portion  18 , over the first head radius  8  and the second head radius  15  back to the blade top side  4 . The portion  18  is in this case preferably formed perpendicular to the profile central line  5 . The transition  19  from the second head radius  15  to the perpendicular portion  18  is preferably of sharp-edged design. 
     The fact that the portion  18  is formed perpendicular to the profile central line  5  and the transition  19  is of sharp-edged design results in a particularly abrupt transition from the step-like cross-sectional widening  13  to the profile basic shape. In this way, a force acting away from the blade top side  4  is generated even at low incident-flow speeds at the blade leading edge  9 . 
       FIG. 3  illustrates a schematic view of an exemplary embodiment of a guide blade arrangement according to the invention. 
       FIG. 3  firstly shows a guide blade arrangement  20  with a receptacle  21  for receiving a multiplicity of guide blades  1 . For simplicity,  FIG. 3  shows only one guide blade  1 . The guide blades  1  have a bearing journal arranged in their axis of rotation  41 , which bearing journal is arranged on a circular line  25  of the receptacle  21 .  FIG. 3  also shows an actuator  22 , for example in the form of an electric actuator or a hydraulic cylinder. The actuator  22  is coupled to the guide blades  1  via a coupling  23  and an adjusting ring (not illustrated in  FIG. 3 ). To simplify the illustration, in  FIG. 3 , the coupling  23  is connected directly to the guide blade  1 . The actuator  22  is connected via a data line  26  to an engine controller  24 , for example of a motor vehicle. Illustrated centrally in the guide blade arrangement  20  is a turbine wheel  27 , with a turbine blade arrangement  28 , of a turbocharger. Here, the guide blades  1  radially surround the turbine wheel  27 . 
     The mode of operation of the guide blade arrangement  20  and of the guide blades  1  will be presented below. 
     All of the guide blades  1  can be pivoted about their axis of rotation  41  by means of the adjusting ring. Since the guide blades  1  radially surround the turbine wheel  27 , the flow cross section available for the exhaust gas flowing to the turbine wheel  27  can be varied through the adjustment of the angle of incidence of the guide blades  1 . The command to adjust the guide blade  1  in the direction  43 , that is to say the “closing” direction, or in the direction  44 , that is to say the “opening” direction, is imparted to the actuator  22  by means of the engine controller  24  as a function of the operating state of an internal combustion engine and a position of an accelerator pedal of the internal combustion engine. 
     When the guide blades  1  are closed, the flow cross section available for the exhaust gas is reduced. However, in order that the same exhaust-gas mass flow can flow through a reduced flow cross section, the flow speed increases. Furthermore, a closed position of the guide blades  1  yields a steep angle of impingement of the exhaust gas on the turbine blade arrangement  28 . As a result, the rotational speed of the turbine wheel  27 , and therefore the rotational speed of a compressor wheel of the turbocharger of the internal combustion engine, increases. As a result, the charge pressure and the power of the internal combustion engine increase. Said operating state of the guide blade arrangement  20  will arise for example during acceleration of a motor vehicle. 
     When the guide blades  1  pivot in the direction  44 , that is to say as the guide blades  1  open, the flow cross section available for the exhaust gas is enlarged. The flow speed of the exhaust gas decreases, and the incident-flow angle at which the exhaust gas impinges on the turbine blade arrangement  28  becomes shallower. The rotational speed of the turbine wheel  27  and therefore the rotational speed of the compressor wheel and the charge pressure of the internal combustion engine fall. This operating state arises for example during constant high-speed driving of a motor vehicle under full load. 
     Since a negative pressure is generated on the blade top side  4  of the guide blades  1  by the step-like cross-sectional widening in the form of a nose of the guide blade profile, a torque  42  acts in the “opening” direction of the guide blades  1 . In this way, it is achieved that the opening of the guide blades  1  is assisted by the torque  42  over the entire operating range of the turbocharger. The emergency running characteristics of a turbocharger equipped with such guide blades  1  according to the invention is thereby improved, because in the event of a failure of the actuator  22  or of the engine controller  24 , the rotational speed of the turbine wheel  27  is automatically reduced as a result of an opening of the guide blades  1 . Furthermore, since a negative pressure prevails on the blade top sides of the guide blades  1 , and therefore an opening torque  42  is generated, over the entire operating range of the turbocharger, it is achieved that, in contrast to known guide blades which generate a closing torque, a situation is prevented in which a so-called self-boosting closing effect arises when the guide blades are nearly closed. Said effect arises in particular in the case of guide blades which generate a closing torque. In the case of such guide blades, as a result of the increased flow speed when the guide blades are nearly closed, the closing torque increases to such an extent that the actuator can possibly no longer provide the force required for opening the guide blades. It is therefore possible with the guide blades  1  according to the invention and the guide blade arrangement  20  according to the invention for an opening torque on the guide blades  1  to be generated over the entire operating range of the turbocharger, as a result of which the emergency running characteristics of the turbocharger are significantly improved. Furthermore, the regulating behavior of the turbocharger is improved owing to the fact that the guide blades  1  have a defined angular position over the entire operating range of the turbocharger. 
       FIG. 4  shows a schematic view of an exemplary embodiment of an exhaust-gas turbocharger according to the invention. 
     An internal combustion engine  37  with four cylinders  38  is fluidically coupled to the turbine wheel  27 , which is situated in a turbine housing  30 , of a turbine  29  of an exhaust-gas turbocharger  39  via an exhaust line  35 . The turbine wheel  27  is connected in a rotationally conjoint manner to a compressor wheel  32  via a rotor shaft  34 . The compressor wheel  32  is arranged in a compressor housing  33  of a radial compressor  31  of the exhaust-gas turbocharger  39 . The compressor wheel  32  is fluidically coupled to the internal combustion engine  37  via an intake tract  36 . 
     During operation of the internal combustion engine  37  with the exhaust-gas turbocharger  39 , the internal combustion engine  37  provides exhaust gas to the turbine wheel  27  via the exhaust line  35 . The turbine wheel  27  lowers the enthalpy of the exhaust gas and converts the kinetic and thermal energy of the exhaust gas into rotational energy. The rotational energy is transmitted via the rotor shaft  34  to the compressor wheel  32 . The compressor wheel  32  sucks in fresh air, compresses it and conducts the compressed fresh air via the intake tract  36  to the internal combustion engine  37 . Since more oxygen is provided per unit of volume in the compressed air volume, more fuel can be burned in the internal combustion engine  37  per unit of air volume. The power output of the internal combustion engine  37  is hereby increased. 
     By means of the guide blades according to the invention or the guide blade arrangement according to the invention, it is possible for the exhaust-gas turbocharger  39  and the internal combustion engine  37  to be operated with increased safety and reliability. Furthermore, it is possible by means of the guide blades according to the invention for the actuator required for adjusting the guide blades to be of smaller dimensions than in known solutions, because the guide blades have a self-opening effect and therefore a smaller force is needed to adjust them. 
     Although the present invention has been described entirely on the basis of preferred exemplary embodiments, it is not restricted to these, but rather may be modified in a variety of ways. In particular, features of the individual exemplary embodiments mentioned above may be combined with one another in any desired way, if this is technically expedient. 
     In a preferred modification of the present invention, the guide blade  1  has at least one cross-sectional constriction. This likewise makes it possible for a negative pressure to be generated on the blade surface. Since the cross section of the profile  3  of the guide blade  1  is constricted and not widened in a step-like manner, the profile thickness of the profile  3  can be reduced. The spatial requirement of the guide blade  1  is reduced in this way. 
     In a further preferred modification of the present invention, the guide blade  1  has at least two step-like cross-sectional widenings  13 . 
     The materials, numerical values and dimensions specified are to be understood as examples and serve merely for explaining the embodiments and refinements of the present invention. 
     The specified guide blade, the specified guide blade arrangement and the specified turbocharger can be used particularly advantageously in the automotive field, and preferably in passenger motor vehicles, for example with diesel or applied-ignition engines, but may also be used in any other turbocharger applications if required.