Patent Application
20230390896
The present disclosure relates to turbochargers in which a turbine of the turbocharger is driven by exhaust gas from an internal combustion engine (ICE). The invention relates more particularly to turbine housings defining a twin scroll in which a divider partitions an annular volute into two meridionally divided scrolls each fed by a separate exhaust system from the ICE.
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 air intake of the engine 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 yet another center housing connected between the turbine and compressor housings for containing bearings for the shaft. The turbine housing defines a generally annular volute that surrounds the turbine wheel and that receives exhaust gas from an engine. The turbine assembly includes a nozzle that leads from the volute into the turbine wheel. The exhaust gas flows from the volute 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.
In multiple-piston reciprocating engines, it is known to design the exhaust system in such a manner as to take advantage of the pressure pulsation that occurs in the exhaust stream. In particular, it is known to employ what is known as “pulse separation” wherein the cylinders of the engine are divided into a plurality of subgroups, and the pulses from each subgroup of cylinders are substantially isolated from those of the other subgroups by having independent exhaust passages for each subgroup. To take best advantage of pulse separation, it is desired to minimize the communication or “cross talk” between the separate groups of cylinders. Accordingly, in the case of a turbocharged engine, it is advantageous to maintain separate exhaust passages all the way into the turbine of the turbocharger. Thus, the turbine housing into which the exhaust gases are fed is typically divided into a plurality of substantially separate parts.
In a meridionally divided turbine housing, the scroll or volute that surrounds the turbine wheel and receives the exhaust gases is divided into a plurality of passages in the meridional plane such that each passage occupies substantially a full circumference and the passages succeed each other in the axial direction.
The present disclosure particularly concerns processes for reducing the surface roughness of internal surfaces of a meridionally divided turbine housing. Surface friction losses on the internal wetted surfaces of the turbine housing volute have a significant impact on aerodynamic performance of the turbine. Turbine housings are generally cast in sand molds, and are used as-cast, without any post-processing to improve the surface finish. This is not ideal, however, because the as-cast surface roughness achieved in sand casting is typically about Ra 25 (average roughness of 980 μin). The application of mold coatings can reduce this value somewhat, but generally it still is not possible to achieve a level of roughness that would significantly reduce friction losses.
There are known processes for polishing or honing internal surfaces of parts. Abrasive flow machining (AFM), also known as extrude honing, entails pressurizing a thick abrasive fluid or paste and causing it to flow through the internal chamber or passage of a part to smooth the inner surfaces of the chamber. There remain needs in the art of AFM processes as applied to meridionally divided turbine housings.
The present disclosure relates to AFM processes and fixtures used therein, for treating the inner surfaces of a meridionally divided turbine housing. The turbine housing comprises an inlet pipe partitioned by a center wall into a first inlet conduit and a second inlet conduit for conducting two separate fluid streams into the turbine housing. A volute connected to the inlet pipe extends from the inlet pipe circumferentially about a center axis of the turbine housing, and an annular nozzle passage connects to the volute and extends radially inwardly therefrom with respect to the center axis. The turbine housing includes an axial bore connected to the nozzle passage and extending axially to a discharge opening, and further includes a divider disposed within the volute so as to divide the volute into a first scroll and a second scroll, the first scroll being connected to the first inlet conduit and the second scroll being connected to the second inlet conduit. In accordance with an embodiment of the invention, an abrasive flow machining process comprises the steps of:
In one embodiment, the first portion of the second angular sector is adjacent the first angular sector. The first angular sector suitably can occupy between 30° and 60° of the circumference.
The throttling step can comprise gradually reducing the degree of flow restriction within the first portion of the second angular sector with increasing circumferential distance from the first angular sector. Additionally or alternatively, throttling of the flow within the second portion of the second angular sector can be uniform over the second portion.
The blocking and throttling steps advantageously can be accomplished by a fixture installed in the axial bore of the turbine housing. The fixture regulates the flow of abrasive medium through the nozzle of the turbine.
A fixture in accordance with one embodiment of the invention comprises:
In some embodiments, the first slot has a first axial slot width and the second slot has a second axial slot width, each of the first and second axial slot widths being non-uniform over said angular sector. In accordance with a particular embodiment, each of the first axial slot width and the second axial slot width progressively widens from a first circumferential location adjacent one circumferential side of the blocking area to a second circumferential location circumferentially spaced from the blocking area.
From the second circumferential location to a third circumferential location adjacent an opposite circumferential side of the blocking area, the first axial slot width can be substantially constant with changing circumferential location and the second axial slot width similarly can be substantially constant.
Processes and fixtures in accordance with embodiments of the invention can mitigate problems during AFM treatments of meridionally divided turbine housings, as further detailed below.
Having described the present disclosure in general terms, reference will now be made to the accompanying drawing(s), which are not necessarily drawn to scale, and wherein:
The present disclosure will now be described in fuller detail with reference to the above-described drawings, which depict some but not all embodiments of the invention(s) to which the present disclosure pertains. These inventions may be embodied in various forms, including forms not expressly described herein, and should not be construed as limited to the particular exemplary embodiments described herein. In the following description, like numbers refer to like elements throughout.
As noted above, AFM treatment of meridionally divided turbine housings for turbochargers is prone to certain problems, among which is the excessive erosion of the radially inner edge of the divider that divides the turbine volute into two separate scrolls. The divider is already a rather thin-walled member even before AFM processing, but it has been found that the AFM process can erode the divider until it becomes razor-thin at its radially inner edge. This presents a number of unacceptable risks: the razor-thin edge is a safety risk for workers handling the turbine housing, and the resulting weakening of the divider increases the risk of failure of the divider, which can lead to turbine wheel damage if pieces of the divider were to come loose and enter the wheel.
Accordingly, the inventors set out to mitigate the problem. Extensive trials were conducted in which different media, different process characteristics, and various masking technologies were tested, with no success.
In the course of the investigation, it was noted that divider erosion was particularly evident on the portion of the divider nearest the turbine housing inlet pipe, while erosion was less-pronounced at locations circumferentially far-removed from the inlet. The inventors of the present invention determined that non-uniform flow of the abrasive medium about the circumference of the turbine volute appeared to be a primary cause of this non-uniform wear. The inventors therefore began to seek a solution that would force the abrasive medium to flow more-uniformly about the volute via the use of some type of fixture. Computational fluid dynamics (CFD) modeling of the turbine housing with various fixture designs was undertaken to gain an understanding of the fluid dynamics involved, which led to the design of the fixture of the present invention. The fixture is mounted within the axial bore of the turbine housing, and during the AFM process it serves to regulate the flow of the abrasive medium throughout the volute. More specifically, the fixture defines slots for allowing the abrasive medium to pass through the nozzle into the axial bore, and the slots are configured to variably throttle the flow of medium depending on circumferential position around the circumference of the volute.
With reference now to
With reference to
It is also of key importance that the fixture 40 be properly oriented in a rotational sense. More particularly, with reference to
It will be noted in the drawings that the slots 52 and 54 do not have the same axial widths. In general, the first slot 52 is wider than the second slot 54 in the illustrated embodiment. The invention, however, is not limited to such an arrangement, and in some cases the slots widths can be equal.
In AFM process trials using the fixture substantially as illustrated in the drawings, uniformity of honing of the inner surfaces of the volute about the circumference was substantially improved compared to the results obtained without using the fixture. Furthermore, erosion of the volute divider was significantly reduced with the fixture.
Persons skilled in the art, on the basis of the present disclosure, will recognize that modifications and other embodiments of the inventions described herein can be made without departing from the inventive concepts described herein. Specific terms used herein are employed for explanatory purposes rather than purposes of limitation. Accordingly, the inventions are not to be limited to the specific embodiments disclosed, and modifications and other embodiments are intended to be included within the scope of the appended claims.
