Turbochargers are widely used on internal combustion engines and, in the past, have been particularly used with large diesel engines, especially for highway trucks and marine applications. 
More recently, in addition to use in connection with large diesel engines, turbochargers have become popular for use in connection with smaller, passenger car power plants.  The use of a turbocharger in passenger car applications permits selection of a power plant that develops the same amount of horsepower from a smaller, lower mass engine.  Using a lower mass engine has the desired effect of decreasing the overall weight of the car, increasing sporty performance, and enhancing fuel economy.  Moreover, use of a turbocharger permits more complete combustion of the fuel delivered to the engine, thereby reducing the overall emissions of the engine, which contributes to the highly desirable goal of a cleaner environment. 
The design and function of turbochargers are described in detail in the prior art, for example, U.S. Pat.  Nos.  4,705,463, 5,399,064, and 6,164,931, the disclosures of which are incorporated herein by reference. 
Turbocharger units typically include a turbine operatively connected to the engine exhaust manifold, a compressor operatively connected to the engine air intake manifold, and a shaft connecting the turbine and compressor so that rotation of the turbine wheel causes rotation of the compressor impeller.  The turbine is driven to rotate by the exhaust gas flowing in the exhaust manifold.  The compressor impeller is driven to rotate by the turbine, and, as it rotates, it increases the air mass flow rate, airflow density and air pressure delivered to the engine cylinders. 
As the use of turbochargers finds greater acceptance in passenger car applications, three design criteria have moved to the forefront.  First, the market demands that all components of the power plant of either a passenger car or truck, including the turbocharger, must provide reliable operation for a much longer period than was demanded in the past.  That is, while it may have been acceptable in the past to require a major engine overhaul after 80,000-100,000 miles for passenger cars, it is now necessary to design engine components for reliable operation in excess of 200,000 miles of operation.  It is now necessary to design engine components in trucks for reliable operation in excess of 1,000,000 miles of operation.  This means that extra care must be taken to ensure proper fabrication and cooperation of all supporting devices. 
The second design criterion that has moved to the forefront is that the power plant must meet or exceed very strict requirements in the area of minimized NOx and particulate matter emissions.  Third, with the mass production of turbochargers, it is highly desirable to design a turbocharger that meets the above criteria and is comprised of a minimum number of parts.  Further, those parts should be easy to manufacture and easy to assemble, in order to provide a cost effective and reliable turbocharger. 
Turbocharger efficiency over a broad range of operating conditions is enhanced if the flow of motive gas to the turbine wheel can be controlled, such as by making the vanes pivotable so as to alter the geometry of the passages therebetween.  The design of the mechanism used to effect pivoting of the vanes is critical to prevent binding of the vanes.  Other considerations include the cost of manufacture of parts and the labor involved in assembly of such systems. 
Additionally, the design of the vane is critical to both the efficiency of the gas delivery to the turbine, as well as the reliability of the variable geometry assembly.  While movement of the vanes allows for control of the gas delivery, it also adds the problem of leakage past the moveable vanes.  Additionally, due to the extreme environment that the moveable vanes are placed in, the structure of the vanes, especially where pivotally connected via vane posts and the like, must be sound to avoid failure. 
In U.S. Pat.  No.  6,679,052 to Arnold, the Applicant attempted to improve efficiency of the air delivery to the turbine wheel by providing a vane having a convex portion adjacent the leading edge and a concave portion adjacent the vane trailing edge.  As shown in FIG.  1, the Arnold vane 106 has an outer surface 108, an inner surface 110, a leading edge 112, a trailing edge 114, an actuation tab 116, and a post hole 118.  The leading edge 112 is characterized by having a larger radius of curvature such that an adjacent portion of its outer surface 108 is located a greater distance from the actuation tab 116, thereby increasing the airfoil thickness of the vane adjacent the leading edge.  The inner surface 110 has a shape that is defined by two differently shaped sections.  Moving from the leading edge 112, the inner surface has a convex-shaped portion 120 that is defined by a radius of curvature that is greater than that of the leading edge to contour or blend the leading edge into the inner surface.  The convex-shaped portion 120 extends from the leading edge 116 to just past the tab 116.  Moving from the convex-shaped portion 102, the inner surface has a concave-shaped portion 122 that extends to the vane trailing edge 122. 
The Applicant in Arnold felt that the enlarged and upwardly oriented leading edge and the shape of the inner surface of this vane would operate to provide improved aerodynamic effect.  However, the Arnold vane still suffered from the drawback of leakage between the vane and the adjacent components (the upstream and downstream nozzle rings which are not shown.)  While the Arnold vane 106 had a curved surface in a longitudinal direction of the vane inner surface 110, it had a flat surface in a traverse direction.  Such a flat surface along the inner surface 110 in a traverse direction can provide a substantially uniform pressure profile along the traverse direction and promotes leakage along the side edges of the vane between the vane and the adjacent components such as the nozzle rings between which the vane is sandwiched. 
The Applicant in Arnold provided yet another embodiment of a vane that was again intended to provide improved aerodynamic effects and improve efficiency.  This other embodiment is shown in FIG.  2 and is a vane 124 having an outer surface 126, an inner surface 128, a leading edge 130, a trailing edge 132, an actuation tab 134, and a post hole 136.  The leading edge 130 is characterized by having a somewhat smaller radius of curvature, and the inner surface 128 comprises three differently shaped sections.  Moving from the leading edge 130, the inner surface 128 has a downwardly canted generally planar section 138 that extends away from the vane leading edge adjacent the tab 134 at an angle of approximately 45 degrees.  The canted section 138 extends for less than about ¼ the total distance along the inner surface and is transitioned to a convex section 140.  The convex section is defined by a radius of curvature that is generally less than that used to define the arc of the outer surface 126.  The convex section 140 extends along the inner surface to about the mid point of the vane and defines a point of maximum airfoil thickness for the vane. 
Similar to the other Arnold embodiment, vane 124 still suffered from the drawback of leakage between the vane and the adjacent components (the upstream and downstream nozzle rings which are not shown).  While the Arnold vane 124 had multiple curved surfaces in a longitudinal direction of the vane inner surface 128, it had a flat surface in a traverse direction.  Such a flat surface along the inner surface 128 in a traverse direction can provide a uniform pressure profile in the traverse direction and promotes leakage along the side edges of the vane between the vane and the adjacent components such as the nozzle rings between which the vane is sandwiched. 
FIG.  17 shows a perspective view of a vane S, which corresponds to the vane illustrated in FIG.  1 of EP-A-1 422 385.  As can be seen from FIG.  17, the prior art vane S fastened on the shaft W has a sealing flange F on the vane mounting ring side, said flange being designed to be substantially circular and concentric with the axis of the vane shaft, in order to reduce the leakage flow between the vane shaft and hole in the vane mounting ring and to protect the hole from the ingress of particles. 
In order to ensure the mechanical adjustment function of the vane S, an axial gap between the vane S and the vane mounting ring and also the second wall, such as, for example, the disk, is required.  However, the leakage flow occurring through this axial gap has a negative impact on the efficiency of the turbocharger, in particular when there are small quantities of exhaust gas.  In order to keep the leakage flow losses as small as possible, on the one hand the axial gap has to be designed to be as small as possible and, on the other hand, the highest possible throttling action has to be achieved in the gap. 
As can be seen from the cross-sectional view of FIG.  18 of the appended drawing, the only contribution made by the prior art turbocharger to reducing the leakage flow is provided only by an axial length section of the vane, based on its total length, which corresponds substantially to the diameter of the sealing flange F.
Thus, there is a need for a vane that improves sealing in a turbocharger, such as a variable geometry turbocharger.  There is a further need for such a vane that is reliable and cost-effective.  There is yet a further need for such a vane that facilitates assembly of the turbocharger.