Exhaust gas turbocharger, in particular for a motor vehicle

Abstract

An exhaust gas turbocharger may include a turbine housing and a turbine wheel. The turbine wheel may include a first quantity of a plurality of moving blades. The turbine wheel may be rotatable relative to the turbine housing about a turbine wheel center of rotation and have a turbine wheel radius. A variable turbine geometry may include a blade bearing ring on which a second quantity of a plurality of guide blades are rotatably mounted in each case about a guide blade center of rotation. The plurality of guide blades may be adjustable between a closed position, in which a flow cross section between the guide blades for an exhaust gas to flow through is at a minimum, and an opened position, in which the flow cross section is at a maximum.

Claims

1. An exhaust gas turbocharger, comprising: a turbine housing; a turbine wheel including a first quantity of a plurality of moving blades, the turbine wheel being rotatable relative to the turbine housing about a turbine wheel centre of rotation and having a turbine wheel radius (R.sub.TR); a variable turbine geometry including a blade bearing ring and a second quantity of a plurality of guide blades disposed on the blade bearing ring, the plurality of guide blades respectively rotatably mounted about a guide blade centre of rotation, wherein the plurality of guide blades are adjustable between a closed position where a flow cross section between the guide blades for an exhaust gas to flow through is at a minimum and an opened position where the flow cross section is at a maximum; wherein each of the plurality of guide blades in a longitudinal profile includes a first profile nose facing away from the turbine wheel centre of rotation and a second profile nose facing towards the turbine wheel centre of rotation, and a straight connecting line between the first profile nose and the second profile nose defining a profile chord; wherein a spacing (R.sub.TE) of the second profile nose from the turbine wheel centre of rotation in the opened position of the guide blades and the turbine wheel radius (R.sub.TR) satisfy the following relationship: 1.03≦R.sub.TE/R.sub.TR≦1.06; and wherein an angle (χ) formed as an apex point with respect to the turbine wheel centre of rotation between two adjacent guide blade centres of rotation and an opening angle (κ) of at least one of the plurality of moving blades in a longitudinal section satisfy the following relationship: 0.4≦χ/η≦2.4.

2. The exhaust gas turbocharger according to claim 1, wherein the spacing (R.sub.TE) and the turbine wheel radius (R.sub.TR) satisfy the following relationship:
1.04≦R.sub.TE/R.sub.TR≦1.06.

3. The exhaust gas turbocharger according to claim 1, wherein: the longitudinal profile of the respective guide blades includes a centre line, the centre line being divided by the guide blade centre of rotation into a first chord with a first chord length and a second chord with a second chord length and wherein the first chord is defined by a first connecting straight line of the guide blade centre of rotation with the first profile nose and the second chord is defined by a second connecting straight line of the guide blade centre of rotation with the second profile nose.

4. The exhaust gas turbocharger according to claim 3, wherein the plurality of guide blades are configured such that the exhaust gas entering the turbine housing strikes the guide blade at an inflow angle <4° relative to the first chord when the guide blades are in the closed position.

5. The exhaust gas turbocharger according to claim 3, wherein an angle (ξ.sub.2) between (i) a third connecting straight line connecting the turbine wheel centre of rotation and the second profile nose and (ii) the first chord lies in the following angle interval: 35°≦ξ.sub.2≦55°, when the guide blades are in the opened position, and 95°≦ξ.sub.2≦110°, when the guide blades are in the closed position.

6. The exhaust gas turbocharger according to claim 3, wherein a first angle (ξ.sub.1) between (i) a third connecting straight line connecting the turbine wheel centre of rotation and the second profile nose and (ii) the second chord with respect to a second angle (ξ.sub.2) between (i) a fourth connecting straight line connecting the turbine wheel centre of rotation and the second profile nose and (ii) the first chord satisfies at least one of the following relationships:
1.4≦ξ.sub.2/ξ.sub.1≦1.6, and
1.2≦ξ.sub.2/ξ.sub.1≦1.4.

7. The exhaust gas turbocharger according to claim 1, wherein a ratio of a flow area (A.sub.TR) between two moving blades with respect to an inlet area (A.sub.LS) between two guide blades obeys the following relationship:
0.36≦A.sub.LS/A.sub.TR≦3.82.

8. The exhaust gas turbocharger according to claim 1, wherein a ratio of a height (h.sub.TR) of one of the plurality of moving blades with respect to a height (h.sub.LS) of one of the plurality of guide blades obeys the following relationship:
0.8≦h.sub.LS/h.sub.TR≦1.2.

9. The exhaust gas turbocharger according to claim 1, wherein a ratio of a diameter (D.sub.TR) of at least one of the plurality of moving blades with respect to a height (h.sub.TR) of the at least one of the plurality of moving blades obeys the following relationship:
0.1≦h.sub.TR/D.sub.TR≦0.2.

10. The exhaust gas turbocharger according to claim 1, wherein the angle (χ) and the opening angle (κ) further satisfy the following relationship:
0.6≦χ/κ≦1.7.

11. The exhaust gas turbocharger according to claim 1, wherein a length (S.sub.2) of a second connecting line between two adjacent second profile noses in the opened state of the guide blades and an inlet width (S.sub.3) between two adjacent moving blades obey the following relationship:
0.92≦S.sub.2/S.sub.3≦1.25.

12. The exhaust gas turbocharger according to claim 3, wherein the first chord length and the second chord length in the longitudinal profile of at least one of the plurality of guide blades has the following relationship: 0.5≦L.sub.1/L.sub.2≦1.0, wherein L.sub.1 is the first chord length and L.sub.2 is the second chord length.

13. An exhaust gas turbocharger for an internal combustion engine, comprising: a turbine housing and a turbine wheel disposed therein rotatable relative to the turbine housing about a turbine wheel centre of rotation, the turbine wheel defining a turbine wheel radius (R.sub.TR) and including a plurality of moving blades; a variable turbine geometry including a blade bearing ring and a plurality of guide blades rotatably mounted about a guide blade centre of rotation on the blade bearing ring, the plurality of guide blades adjustable between a closed position where a flow cross-section between the respective guide blades for an exhaust gas flow is at a minimum, and an opened position where the flow cross-section is at a maximum; the plurality of guide blades respectively having in a longitudinal profile a first profile nose facing away from the turbine wheel centre of rotation, a second profile nose facing towards the turbine wheel centre of rotation, and a profile chord defined by a first connecting line between the first profile nose and the second profile nose; wherein a spacing (R.sub.TE) of the second profile nose from the turbine wheel centre of rotation in the opened position of the plurality of guide blades and the turbine wheel radius (R.sub.TR) satisfy the following relationship: 1.03≦R.sub.TE/R.sub.TR≦1.06; and wherein a length (S.sub.2) of a second connecting line between two adjacent second profile noses in the opened position of the guide blades and an inlet width (S.sub.3) between two adjacent moving blades satisfy the following relationship: 0.45≦S.sub.2/S.sub.3≦3.2.

14. The exhaust gas turbocharger according to claim 13, wherein the length (S.sub.2) and the inlet width (S.sub.3) further satisfy the following relationship: 0.65≦S.sub.2/S.sub.3≦1.7.

15. An exhaust gas turbocharger for an internal combustion engine, comprising: a turbine housing and a turbine wheel disposed therein rotatable relative to the turbine housing about a turbine wheel centre of rotation, the turbine wheel defining a turbine wheel radius (R.sub.TR) and including a plurality of moving blades; a variable turbine geometry including a blade bearing ring and a plurality of guide blades rotatably mounted about a guide blade centre of rotation on the blade bearing ring, the plurality of guide blades adjustable between a closed position where a flow cross-section between the respective guide blades for an exhaust gas flow is at a minimum, and an opened position where the flow cross-section is at a maximum; the plurality of guide blades respectively having in a longitudinal profile a first profile nose facing away from the turbine wheel centre of rotation, a second profile nose facing towards the turbine wheel centre of rotation, and a profile chord defined by a connecting line between the first profile nose and the second profile nose; wherein a spacing (R.sub.TE) of the second profile nose from the turbine wheel centre of rotation in the opened position of the plurality of guide blades and the turbine wheel radius (R.sub.TR) satisfy the following relationship: 1.03≦R.sub.TE/R.sub.TR≦1.06; and wherein the first profile nose in the longitudinal profile of at least one guide blade of the plurality of guide blades defines an origin of a Cartesian coordinate system with an X-coordinate extending along the profile chord and a Y-coordinate extending orthogonally to the X-coordinate, the X-coordinate and the Y-coordinate of the following points being defined in the Cartesian coordinate system: x.sub.p, y.sub.p: Cartesian coordinates of the guide blade centre of rotation, x.sub.1, y.sub.1: a low point of a profile bottom side having a convex shape, x.sub.2, y.sub.2: a height of the profile bottom side having a concave shape, x.sub.3, y.sub.3: a height of a profile top side having a convex shape, x.sub.4, y.sub.4: a high point of a centre line of the longitudinal profile, x.sub.5, y.sub.5: an intersection of the profile bottom side having the convex shape with the profile chord, x.sub.6, y.sub.6: an intersection of the profile bottom side having the concave shape with the profile chord; and wherein the at least one guide blade in the longitudinal profile includes the following relationship:
0≦y.sub.p/y.sub.4≦2;
0≦y.sub.p/y.sub.1≦5; and
0≦y.sub.2/y.sub.p≦0.7.

16. The exhaust gas turbocharger according to claim 15, wherein the at least one guide blade in the longitudinal profile further includes the following relationship:
0.3 L.sub.Profile chord<x.sub.p<0.5 L.sub.Profile chord; wherein L.sub.Profile chord is a length of the profile chord.

17. The exhaust gas turbocharger according to claim 15, wherein the at least one guide blade in the longitudinal profile further includes at least one of the following relationships:
0≦y.sub.p/y.sub.3≦1; and
0≦y.sub.3/y.sub.1≦5.

18. The exhaust gas turbocharger according to claim 15, wherein the at least one guide blade in the longitudinal profile further includes the following relationship:
0≦|y.sub.1|/x.sub.1≦1.5.

19. The exhaust gas turbocharger according to claim 15, wherein the at least one guide blade in the longitudinal profile further includes one of the following relationships:
0.8≦(x.sub.p−x.sub.1)/x.sub.p; and
0.3≧(x.sub.p−x.sub.1)/x.sub.p.

20. The exhaust gas turbocharger according to claim 15, wherein the at least one guide blade in the longitudinal profile further includes the following relationships:
0.7≦(x.sub.p−x.sub.3)/x.sub.p≦0.7;b
1.5≦(x.sub.p−x.sub.5)/x.sub.p≦1.5;
0.7≦(x.sub.p−x.sub.4)/x.sub.p≦0.7;
1.7≦(x.sub.p−x.sub.2)/x.sub.p≦1.7;
1.5≦(x.sub.2−x.sub.5)/(x.sub.6−x.sub.2)≦1.5; and
1.5≦(x.sub.6−x.sub.2)/(x.sub.2−x.sub.5)≦1.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) It shows, in each case schematically

(2) FIG. 1a a rough schematic representation of an exhaust gas turbocharger according to the invention with variable turbine geometry in a part view,

(3) FIG. 1b the variable turbine geometry of FIG. 1a in a detail view,

(4) FIG. 2 a guide blade of the variable turbine geometry in a longitudinal profile,

(5) FIG. 3 the longitudinal profile of FIG. 2 with respective construction circles defining a guide blade.

DETAILED DESCRIPTION

(6) In FIG. 1a, an exhaust gas turbocharger according to the invention is shown in a rough schematic manner in a part view and marked with the reference character 1. The exhaust gas turbocharger 1 comprises a turbine housing 2 with a turbine wheel 3 comprising a first number of moving blades 4, which in the FIG. 1 are only shown in a rough schematic manner. The turbine wheel 3 is rotatable about a turbine wheel centre of rotation D relative to the turbine housing 2.

(7) The exhaust gas turbocharger 1 furthermore comprises a variable turbine geometry 5, which comprises a blade bearing ring which is not shown in the schematic representation of FIG. 1, on which a second number of guide blades 6 is rotatably mounted in each case about a guide blade centre of rotation P. The second number of guide blade 6 in this case is distinct from the first number of moving blades 4. In the example shown in FIG. 1a, the turbine wheel 3 exemplarily comprises twelve moving blades 4 and the variable turbine geometry 5 thirteen guide blades 6; obviously, in version another number of guide blades 6 and moving blades 4 respectively is also possible.

(8) For example, a variable turbine geometry 5 with eleven guide blades 6 and ten moving blades 4 is shown in a rough schematic manner for example in FIG. 1b. The guide blades 6 are adjustable between a closed position, in which a flow cross section between the guide blades 6 for exhaust gas to flow through is minimal and an opened position, in which this flow cross section is maximal.

(9) In the example of FIG. 1a, the turbine housing 2 has a volute-like geometry as well as an inlet opening 7 and an outlet opening 8. By means of the turbine wheel 3 a high-pressure region which is fluidically connected to the inlet opening 7 is separated from a low-pressure region which is fluidically connected to the outlet opening 8.

(10) For adjusting the guide blades 6 between the opened and the closed position, the variable turbine geometry 5 can comprise an adjusting element with a respective mounting which is not shown in the FIGS. 1a/b for the sake of clarity, wherein each guide blade 6 engages in such a mounting of the adjusting element via a respective adjusting lever. Obviously, other realisations for adjusting the guide blades 6 between the opened and the closed position or an intermediate position are also conceivable in versions.

(11) FIG. 2 now shows a guide blade 6 of the variable geometry 5 in a longitudinal section. The guide blade 6 in the longitudinal profile comprises a first profile nose 9 and a second profile nose 10. A profile chord 11 is defined by the connecting line between the two profile noses 9, 10.

(12) From FIG. 1b it is evident in turn that the spacing R.sub.TE of the second profile nose from the turbine wheel centre of rotation in the opened position of the guide blades and the radius of the turbine wheel R.sub.TR according to the invention satisfy the following relationship:
1.03≦R.sub.TE/R.sub.TR≦1.06.

(13) Such dimensioning of the variable turbine geometry 5 reduces undesirable excitation oscillations or oscillation loads on the guide blades 4 to a considerable degree which has a positive effect on the thermodynamic efficiency of the exhaust gas turbocharger 1. At the same time, the adjusting forces which are needed for moving the guide blades 4 are minimised. Similarly, the hysteresis behaviour of the variable turbine geometry 5 is minimised, as a result of which particularly good control behaviour can be achieved.

(14) Particularly advantageous with respect to the efficiency that can be achieved is a version in which the spacing R.sub.TE and the radius R.sub.TR satisfy the following relationship:
1.04≦R.sub.TE/R.sub.TR≦1.06, preferentially even 1.05≦R.sub.TE/R.sub.TR≦1.06.

(15) Again looking at the representation of FIG. 2 it is evident that in the longitudinal profile of the guide blade 6 its centre line 14 is subdivided by the guide blade centre of rotation P into a first chord 13a with chord length L.sub.1 and a second chord 13b with chord length L.sub.2. The first chord 13a in this case is defined by a connecting straight line of the guide blade centre of rotation P with the first profile nose 9 and the second chord 13b by a connecting straight line of the guide blade centre of rotation P with the second profile nose 10. In the example scenario of the figures, the guide blades 6 are now designed in such a manner that exhaust gas entering the turbine housing 2 strikes the guide blade 6 at an inflow angle α<4° relative to the first chord 13a when the guide blades 6 are in their closed position.

(16) FIG. 1b shows an angle ξ.sub.2 between a connecting straight line 16 connecting the turbine wheel centre of rotation D and to the second profile nose 10 and the first chord 13a. In the exemplary scenario, is in the angle interval 35°≦ξ.sub.2≦55°, in the case that the guide blades 6 are in the opened position and in the angle range 95°≦ξ.sub.2≦110°, in the case that the guide blades 6 are in the closed position. In addition, an angle ξ.sub.1 between the connecting straight line 16 connecting the turbine wheel centre of rotation D and the second profile nose 10 and the second chord 13b satisfies one of the two following relationships:
1.4≦ξ.sub.2/ξ.sub.1≦1.6, or 1.2≦ξ.sub.2/ξ.sub.1≦1.4.

(17) The angle X formed as apex with respect to the turbine wheel centre of rotation D between two adjacent guide blade centres of rotation P and the opening angle κ of a moving blade 6 in the longitudinal section obey the following relationship:
0.4≦χ/κ≦2.4. In a version, 0.6≦χ/κ≦1.7, even applies, and in a particularly preferred version 0.9≦χ/κ≦1.2.

(18) From FIG. 1b it is evident furthermore that the length S.sub.2 of the connecting line of two adjacent second profile noses 10 in the opened state of the guide blade 6 and the inlet width S.sub.3 between two adjacent moving blades 4 obey the following relationship: 0.45≦S.sub.2/S.sub.3≦3.2. In a version, 0.65≦S.sub.2/S.sub.3≦1.7, even applies, in a particularly preferred version 0.92≦S.sub.2/S.sub.3≦1.25. The ratio of a flow area A.sub.TR (not shown in the figures) between two moving blades 4 with respect to the inlet area between two guide blades 6 A.sub.LS (likewise not shown in the figures) obeys the following relationship: 0.36≦A.sub.LS/A.sub.TR≦3.82. In a version, 0.52≦A.sub.LS/A.sub.TR≦2.05, even applies. In a further version, even 0.74≦A.sub.LS/A.sub.TR≦1.5. Here, the inlet area A.sub.TR between two moving blades 4 is defined by the relationship A.sub.TR=h.sub.TR S.sub.3 and the inlet area A.sub.LS between two guide blades 6 by the relationship A.sub.LS=h.sub.LS S.sub.2. Here, h.sub.2 is the height of the guide blades 6 along their axis of rotation—in FIG. 1b, only the centre of rotation P is evident through which the axis of rotation runs—and h.sub.3 the height of the moving blade at the turbine wheel inlet, which in FIG. 1b has been exemplarily marked with the reference number 17 for a moving blade 4.

(19) Finally, the following relationship applies to the ratio of a height h.sub.TR of a moving blade 4 to the height h.sub.LS of a guide blade 6: 0.8≦h.sub.LS/h.sub.TR≦1.2. Again 0.9≦h.sub.LS/h.sub.TR≦1.1 applies in a version. The mentioned heights h.sub.TR, h.sub.LS in this case relate to a vertical direction H arranged orthogonally to the drawing direction of the figures. For the ratio of a diameter D.sub.TR of a moving blade 4 to the height h.sub.TR of the moving blade 4 the following relationship applies: 0.1≦h.sub.TR/D.sub.TR≦0.2. In a preferred version, 0.12≦h.sub.TR/D.sub.TR≦0.18, applies and in a further version even 0.13≦h.sub.TR/D.sub.TR≦0.16.

(20) In the example of the figures, an overlap of two adjacent guide blades 6 in the closed position and the length of a guide blade L.sub.LS furthermore applies:
0.05*L.sub.LS≦Δ≦0.4*L.sub.LS, preferentially 0.1*L.sub.LS≦Δ≦0.3*L.sub.LS, most preferentially 0.15*L.sub.LS≦Δ≦0.2*L.sub.LS.
Here, Δ of the overlap region of two adjacent guide blades 6—extends in their longitudinal profile—in their closed position, which consequently extends from a first profile nose 9 of a certain guide blade 6 as far as to the second profile nose 10 of the guide blade 6 that is adjacent to this guide blade 4.

(21) As shown in FIG. 2, the guide blade 6 in the longitudinal profile can each have a profile bottom side 12a which in sections is formed in a convex manner and a profile top side 12b which is formed in a convex manner. The section of the profile bottom side 12a formed in a convex manner then has a low point P.sub.1. Likewise, the section of the profile bottom side 12a formed in a concave manner has a high point P.sub.2, the profile top side 12b a high point P.sub.3.

(22) From the representation of FIG. 2 it is also evident that the first profile nose 9 facing away from the turbine wheel 3 determines the original of a Cartesian coordinate system. An X-direction of this coordinate system is defined by the profile chord 11. Accordingly, a Y-direction of the coordinate system extends orthogonally to the X-direction away from the first profile nose 9. The spacing x.sub.p between first profile nose 9 and the guide blade centre of rotation P and the spacing x.sub.1 between first profile nose 9 and low point P.sub.1 in X-direction satisfy the following relationship: (x.sub.p−x.sub.1)/x.sub.p>0.8.

(23) Accordingly, the spacing x.sub.1 defined above and the spacing y.sub.1 between first profile nose 9 and the low point P.sub.1 satisfy the following relationship in Y-direction: y.sub.1/x.sub.1≦0.4.

(24) Looking now at the representation of FIG. 3, which shows the guide blade 6 analogously to FIG. 2 in a longitudinal profile it is evident that in the longitudinal profile of the guide blade 6 a centre line 14 is defined by a plurality of construction circles 15 between the profile top side 12b and the profile bottom side 12a. With respect to the radius r of the first construction circle K.sub.1 defining the first profile nose 9 the condition r/x.sub.p>0.08 or r/x.sub.p<0.045 applies.

(25) With respect to the X-coordinate x.sub.p of the guide blade centre of rotation P 0.03≦r/x.sub.p, preferentially 0.07≦r/x.sub.p, most preferentially 0.1≦r/x.sub.p applies in a version of the exemplary embodiment. In a version that is alternative to this,
r/x.sub.p≦0.4, preferentially r/x.sub.p≦0.38, most preferentially r/x.sub.p≦0.35 applies by contrast.

(26) In the longitudinal profile of the guide blade 6 shown in the example of FIG. 3 the following relationships apply to the diameter k.sub.1 of a first construction circle 15.sub.1 assigned to the first profile nose 9, for the diameter k.sub.2 of a first construction circle 15.sub.2 assigned to the second profile nose 10 and the construction circle 15.sub.max with maximum diameter k.sub.max:
1≦k.sub.max/k.sub.1≦20, and 1≦k.sub.max/k.sub.2≦10.

(27) In the Cartesian coordinate system show in the FIGS. 2 and 3 the following points are thus defined as already explained above, by the X and Y-coordinates: the Cartesian coordinates x.sub.p, y.sub.p of the guide blade centre of rotation P, the Cartesian coordinates x.sub.1, y.sub.1 of the low point P.sub.1 of the convex profile bottom side 12a, the Cartesian coordinates x.sub.2, y.sub.2 of the high point P.sub.2 of the concave profile bottom side 12a, the Cartesian coordinates x.sub.3, y.sub.3 of the high point P.sub.3 of the convex profile top side 12b.

(28) Furthermore, an intersection P.sub.5 of the convex profile bottom side 12a with the profile chord 11 is defined in the longitudinal profile of the guide blade 6 according to FIG. 2, which in the Cartesian coordinate system has the X and Y-coordinate x.sub.5, y.sub.5 respectively. Accordingly, an intersection P.sub.6 of the concave profile bottom side 12a with the profile chord 11 is also defined in the longitudinal profile of the guide blades 6, which in the Cartesian coordinate system has the X and Y-coordinate x.sub.6, y.sub.6 respectively. Through the Cartesian coordinates x.sub.4, y.sub.4, a high point P.sub.4 of the centre line 14 is defined.

(29) The following relationships apply to the extreme points P.sub.1, P.sub.2, P.sub.3, P.sub.4, for the intersections P.sub.5 and P.sub.6 defined above and to the guide blade centre of rotation P of the guide blade 6 in the longitudinal profile shown in FIG. 2 which is improved compared with conventional guide blades:
−0.7≦(x.sub.p−x.sub.3)/x.sub.p≦0.7,
−1.5≦(x.sub.p−x.sub.5)/x.sub.p≦1.5,
−0.7≦(x.sub.p−x.sub.4)/x.sub.p≦0.7,
−1.7≦(x.sub.p−x.sub.2)/x.sub.p≦1.7,
−2.0≦(x.sub.p−x.sub.6)/x.sub.p≦1.7,
−1.5≦(x.sub.2−x.sub.5)/(x.sub.6−x.sub.2)≦1.5,
−1.5≦(x.sub.6−x.sub.2)/(x.sub.2−x.sub.5)≦1.5.

(30) At the same time the following applies:
0≦y.sub.p/y.sub.4≦2;
0≦y.sub.p/y.sub.1≦5;
0≦y.sub.2/y.sub.p≦0.7;
0≦y.sub.3/y.sub.1≦5.

(31) For the position of the spacing x.sub.p of the guide blade centre of rotation P from the first profile nose 9 in X-direction the following applies:
0.3L.sub.Profile chord<x.sub.p<0.5L.sub.Profile chord, wherein L.sub.Profile chord is the length of the profile chord 11.

(32) At the same time, the non-equation 0≦y.sub.p/y.sub.3≦1 can apply to the Y-coordinate of the guide blade centre of rotation P relative to the Y-coordinate of the high point P.sub.3 of the convex profile top side 12b. According to a preferred version even 0.6≦y.sub.p/y.sub.3≦0.9, and according to a particularly preferred version 0.65≦y.sub.p/y.sub.3≦0.73.

(33) Furthermore, the following applies to the Cartesian coordinates x.sub.1, y.sub.1 of the first extreme point P.sub.1. According to a preferred version the following applies: 0≦y.sub.1/x.sub.1≦0.4, preferentially 0≦x.sub.1/y.sub.1≦0.3, particularly preferably even 0≦y.sub.1/x.sub.1≦0.2. However, alternatively to this, the following relationships can also apply: 0.80≦y.sub.1/x.sub.1≦1.5, in a preferred version 0.90≦y.sub.1/x.sub.1≦1.3, most preferentially 1.0≦y.sub.1/x.sub.1≦1.1.

(34) Furthermore, the relationship 0.8≦(x.sub.p−x.sub.1)/x.sub.p, preferentially 0.9≦(x.sub.p−x1)/x.sub.p, and most preferentially 0.99≦(x.sub.p−x.sub.1)/x.sub.p can apply to the X-coordinate x.sub.1 of the low point P.sub.1 and the X-coordinate x.sub.p of the guide blade centre of rotation P. In a version which is alternative thereto, the guide blade 6 by contrast satisfies the following conditions in the longitudinal profile:
(x.sub.p−x.sub.1)/x.sub.p≦0.3, preferentially(xp−x1)/x.sub.p≦0.2, most preferentially (x.sub.p−x.sub.1)/x.sub.p≦0.1.

(35) Looking at the longitudinal profile of FIG. 2 it is evident that the centre line 14 between profile bottom side 12a and profile top side 12b is subdivided by the guide blade centre of rotation P into the first chord 13a with chord length L.sub.1 and into the second chord 13b with chord length L.sub.2. The two chords 13a, 13b are connecting lines of the centre of rotation P with the first or second profile nose 9, 10. The relationship between L.sub.1 and L.sub.2 of the guide blade 6 in this case is 0.5≦L.sub.1/L.sub.2≦1.0. Preferentially, 0.6≦L.sub.1/L.sub.2≦1.0, most preferentially even 0.7≦L.sub.1/L.sub.2≦1 applies.