Patent Application
20080271449
The present disclosure relates to turbochargers having a sliding piston in the turbine nozzle for regulating exhaust gas flow into the turbine.
An exhaust gas-driven turbocharger is a device used in conjunction with an internal combustion engine for increasing the power output of the engine by compressing the air that is delivered to the engine's air intake to be mixed with fuel and burned in the engine. A turbocharger comprises a compressor wheel mounted on one end of a shaft in a compressor housing and a turbine wheel mounted on the other end of the shaft in a turbine housing. Typically the turbine housing is formed separately from the compressor housing, and there is a center housing connected between the turbine and compressor housings for containing bearings for the shaft. The turbine housing defines a generally annular chamber that surrounds the turbine wheel and that receives exhaust gas from the engine. The turbine assembly includes a nozzle that leads from the chamber into the turbine wheel. The exhaust gas flows from the chamber through the nozzle to the turbine wheel and the turbine wheel is driven by the exhaust gas. The turbine thus extracts power from the exhaust gas and drives the compressor. The compressor receives ambient air through an inlet of the compressor housing and the air is compressed by the compressor wheel and is then discharged from the housing to the engine air intake.
One of the challenges in boosting engine performance with a turbocharger is achieving a desired amount of engine power output throughout the entire operating range of the engine. It has been found that this objective is often not readily attainable with a fixed-geometry turbocharger, and hence variable-geometry turbochargers have been developed with the objective of providing a greater degree of control over the amount of boost provided by the turbocharger. One type of variable-geometry turbocharger employs a sliding piston in the turbine nozzle. The piston is slidably mounted in the turbine housing and is connected to a mechanism that translates the piston axially back and forth. Changing the position of the piston has the effect of changing the effective flow area through the turbine nozzle, and thus the flow of exhaust gas to the turbine wheel can be regulated by controlling the piston position. In this manner, the power output of the turbine can be regulated, which allows engine power output to be controlled to a greater extent than is generally possible with a fixed-geometry turbocharger.
Typically the sliding piston mechanism also includes vanes that are either attached to an end of the piston or to a fixed wall of the turbine nozzle. When the piston is fully closed, there is still an opening between the end of the piston and the fixed wall of the nozzle, and the vanes typically extend fully across this opening. However, when the piston begins to open, in some such piston mechanisms a vane-free gap begins to develop either between the end of the piston and the ends of the vanes (when the vanes are mounted on the fixed nozzle wall) or between the ends of the vanes and the nozzle wall (when the vanes are mounted on the piston). This is undesirable because at the moment the gap begins to develop, the flow of exhaust gas around the vane ends and through the vane-free gap has poor aerodynamics, which adversely impacts turbine efficiency. The flow rate into the turbine also tends to change quite abruptly with small changes in piston position during this initial opening movement of the piston, which makes it difficult to control the turbine with accuracy during this transition.
In order to try to overcome such disadvantages, it has been proposed to include slots either in the piston end or in the nozzle wall for the vanes to extend into. In this manner, the vanes can be made long enough so that even when the piston is fully open, the vanes extend fully across the nozzle opening. However, this has its own drawbacks. Because the exhaust gas flowing through the nozzle is very hot, the piston, vanes, and nozzle wall are all subject to dimensional changes caused by thermal growth and contraction as the gas temperature changes. Accordingly, in order to prevent the vanes from binding in the slots at all operating conditions, it is necessary to provide large tolerances. Therefore, there are substantial gaps between the vanes and the edges of the slots that receive the vanes, and the exhaust gas can leak through these gaps. This not only partially defeats the purpose of the vanes, but when the slots are in the fixed nozzle wall they can allow hot exhaust gas to migrate into the center housing where the gas can heat up the bearings, which is highly undesirable.
The present disclosure concerns a turbocharger having a sliding piston, which substantially avoids the drawbacks of prior turbochargers noted above. The turbocharger includes a set of fixed vanes mounted on a fixed first wall of the turbine nozzle and projecting axially toward an opposite second wall of the nozzle, and a set of moving vanes mounted on the end of the piston and projecting in an opposite axial direction toward the first wall of the nozzle. The two sets of vanes are circumferentially staggered relative to each other and overlap each other in closed and partially open positions of the piston. When the piston is fully open, however, the two sets of vanes no longer overlap, such that there is a vane-free space between the respective ends of the two sets of vanes. This arrangement can substantially improve on the smoothness of the flow rate change through the nozzle as the piston is moved from its closed position toward its open position or vice versa. Furthermore, the necessity of providing slots in the nozzle wall or piston is avoided, thereby substantially eliminating the possibility of exhaust gas leaking through such slots and possibly heating up the bearings.
In one embodiment, when the piston is in the closed position, both sets of vanes extend substantially fully across the open axial extent of the nozzle (i.e., the two sets completely overlap each other) to provide a low-area, high-guidance flow path for the exhaust gas. During the piston stroke, the flow area changes substantially linearly with piston position, thereby assuring that no sudden change in turbine flow characteristic will occur.
Alternatively, it is also possible that when the piston is closed, the two sets of vanes do not completely overlap each other.
In accordance with one embodiment, there are equal numbers of fixed and moving vanes, and each moving vane is approximately midway, along a circumferential direction, between two fixed vanes. In one embodiment, all of the fixed and moving vanes are substantially identical to one another in outer contour and axial length.
In one embodiment, the fixed vanes are mounted on a heat shield that is formed separately from the center housing and turbine housing. The heat shield is captured between the center and turbine housings. Alternatively, the fixed vanes can be mounted on a different component, such as a piece separate from the heat shield, the piece being captured between the heat shield and the turbine housing. Various other mounting schemes can also be used.
In accordance with one embodiment, the turbocharger also includes an anti-rotation device that prevents the piston from rotating about its axis by any significant amount, while allowing the piston to translate axially.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
The present inventions now will be described more fully hereinafter with reference to the accompanying drawings in which some but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
A turbocharger 20 in accordance with one embodiment of the invention is shown in
A heat shield 32 is disposed between the center housing 22 and turbine housing 38. The heat shield comprises a first wall of the turbine nozzle 43; an opposite second wall 45 of the nozzle is formed by the turbine housing 38. The heat shield 32 supports a set of circumferentially spaced fixed vanes 34 that extend axially from the heat shield partway across the axial extent of the nozzle 43 toward the second wall 45.
The turbine housing 38 defines a generally cylindrical bore 44 whose diameter generally corresponds to a radially innermost extent of the chamber 42. The turbine wheel 40 resides in an upstream end of the bore 44 and the turbine wheel's rotational axis is substantially coaxial with the bore. The term “upstream” in this context refers to the direction of exhaust gas flow through the bore 44, as the exhaust gas in the chamber 42 flows into the turbine wheel 40 and is then turned to flow generally axially (left to right in
In one embodiment, the turbine wheel can be a “splittered” turbine wheel (not shown) in which there are full-length blades alternating with partial-length blades. An example of such a splittered turbine wheel is described in published PCT application WO 2004/074642 A1 to Lombard et al. entitled “Turbine Having Variable Throat”, published on Sep. 2, 2004, the entire disclosure of which is hereby incorporated herein by reference. The full-length blades have a greater length in the axial direction than do the partial-length blades. More particularly, the full-length blades are positioned such that they span substantially the full axial extent of the nozzle 43 when the piston is in the fully open position as in
The turbocharger includes a sliding piston assembly 50 that resides in the bore 44 of the turbine housing. The piston assembly comprises a tubular carrier 52 whose outer diameter is slightly smaller than the diameter of the turbine housing bore 44 such that the carrier 52 can be slid axially into the bore 44 from its downstream end (i.e., slid right to left in
The piston assembly 50 further comprises a piston 62 of tubular form. The piston is coaxially disposed within the central bore of the carrier 52 and is slidable relative to the carrier in the axial direction. The piston is axially slidable between a closed position as shown in
The carrier 52 can have an axial split (not shown) extending the length of the carrier. The split enables the carrier to expand and contract in diameter in response to thermal effects or other causes. The carrier advantageously has an inner diameter only slightly greater than the outer diameter of the piston 62, such that a very small gap exists between the carrier and piston. Accordingly, leakage flow through the gap is minimized. Because the carrier can expand and contract in diameter, there is no need to make the gap large to facilitate assembly or to accommodate dimensional changes during operation. The ability of the carrier to expand also means that binding of the piston is avoided.
The carrier 52 includes a plurality of axially elongated apertures 60 through the side wall of the carrier. The turbocharger also includes a piston actuating linkage comprising a fork-shaped swing arm 70. The swing arm has a pair of arms 72 whose distal ends extend through two of the apertures 60 and engage the piston 62 at diametrically opposite locations of the piston. The swing arm is disposed adjacent the outer surface of the carrier and resides in a portion of the bore 44 that has an enlarged diameter. The swing arm is pivotable about a transverse axis so as to cause the piston to be advanced axially within the carrier 52.
As an alternative to having the piston actuating mechanism on the side of the piston as shown, it is possible to position the actuator behind the piston (to the right in
A set of moving vanes 54 is affixed to the end of the piston, and specifically to the flange portion 66. The moving vanes 54 extend in an opposite axial direction to that of the fixed vanes 34, toward the heat shield 32. As shown in
The fixed vanes 34 are circumferentially spaced apart about a 360° annulus and likewise the moving vanes 54 are circumferentially spaced about the 360° annulus. The moving vanes 54 are circumferentially staggered relative to the fixed vanes 34, and the fixed vanes 34 overlap with the moving vanes 54. The extent of the overlap depends on the position of the piston 62, as further described below.
In one embodiment as illustrated, there are equal numbers of fixed and moving vanes, and each moving vane 54 is approximately midway, along a circumferential direction, between two fixed vanes 34. This is best seen in
In one embodiment as illustrated, the fixed and moving vanes are substantially identical to one another in outer contour and vane axial length.
In one embodiment, the maximum axial travel of the piston 62 exceeds the axial length of the vanes, and therefore there is a vaneless gap between the fixed vanes 34 and the moving vanes 54 when the piston is fully open. At this position, the open axial extent of the nozzle has a first portion in which the exhaust gas flows between the parts of the fixed vanes 34, a second (middle) portion in which the exhaust gas flows through the vaneless gap between the fixed and moving vanes, and a third portion in which the exhaust gas flows between the parts of the moving vanes 54.
The second middle portion of the nozzle is free of vanes when the piston is fully open, and thus has a relatively high flow area. The ability of the piston to open far enough to develop this vaneless gap is a key feature of the present invention. The vaneless gap allows a higher maximum mass flow rate through the turbine nozzle, as required for some extreme operating conditions.
At partially open positions of the nozzle as in
When the piston is in the closed position as in
From this description of the illustrated embodiment, it will be recognized that there is never any vane-free gap in the closed and partially open positions of the piston. The flow area through the nozzle increases substantially linearly with piston position as the piston is opened. Additionally, the piston is able to move far enough to form a vaneless gap between the two sets of vanes. These features allow better control over the flow rate through the nozzle than in some prior turbocharger arrangements, particularly as the piston just begins to open from its closed position, and enable a high maximum mass flow rate when the piston is fully open as a result of the development of the vaneless gap between the two sets of vanes.
The fixed and moving vanes in accordance with the invention can be adapted to turbochargers of various configurations having sliding pistons. In some turbochargers, the piston may directly engage the bore of the turbine housing; in this case, an anti-rotation device (e.g., keys or splines) between the piston and turbine housing prevents the piston from rotating about its axis. In other turbochargers there may be an intervening carrier or sleeve in the turbine housing bore as in the illustrated embodiment, and the piston may directly engage the carrier or sleeve; in this case, anti-rotation devices are employed to prevent both the carrier and the piston from rotating. Additionally, in some turbochargers the fixed vanes may be mounted on a separate member such as a heat shield as in the illustrated embodiment. In other turbochargers the fixed vanes may be mounted directly on a portion of the center housing or on another component that is separate from the center housing and heat shield and is captured between the heat shield and turbine housing.
Thus, many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
