WO2005108747A1

WO2005108747A1 - Turbocharger with reduced thermal inertia and method of producing the same

Info

Publication number: WO2005108747A1
Application number: PCT/EP2004/005102
Authority: WO
Inventors: Edouard Massart; Marylène Ruffinoni; Michel Millot
Original assignee: Honeywell International Inc.
Priority date: 2004-05-12
Filing date: 2004-05-12
Publication date: 2005-11-17
Also published as: EP1747350A1

Abstract

The present application provides a turbine housing unit for a turbocharger of an internal combustion engine having a reduced thermal inertia, and a method of producing the same. The turbine housing unit has an internal passage comprising an inlet (10), an outlet (14) and a volute (12) for receiving a turbine wheel and is produced by the following steps: preparing a plurality of metallic turbine housing pieces (2, 4), each having an inner wall surface that defines a part of the internal passage; applying a thermal insulating layer (8) on the inner wall surfaces; and joining the turbine housing pieces (2, 4) at respective parting lines (6) after applying the thermal insulating layer (8) to obtain an integral turbine housing unit.

Description

TURBOCHARGER WITH REDUCED THERMAL INERTIA AND METHOD OF PRODUCING THE SAME BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a turbocharger having a reduced thermal inertia and a method of producing the same. 2. Description of the Related Art Many internal combustion engines are equipped with turbochargers to improve engine efficiency. Typically, turbochargers consist of three principle components: a turbine, a compressor, and a housing unit. In operation, the turbine captures high-temperature gases coming from the engine exhaust manifold. These exhaust gases then are used to drive a compressor which, in term, pumps high pressure air into the engine inlet and combustion chambers. The effect of this process in an internal combustion engine is to increase the volume of air available for combustion. Because more air is available, a correspondingly greater amount of fuel can be consumed, or burnt, per cycle. In theory, the greater the fuel burnt, the greater the horsepower. Under the current state of the art, turbocharger turbine housing units and gas exhaust manifolds for gasoline and diesel engines are typically made of steel or cast iron. Steel and iron inherently possess a high thermal inertia, i.e. a high ability of conducting and storing heat. As a result, the captured gas heat energy dissipates and less energy is available to drive the turbocharger turbine, thereby reducing the performance and efficiency of the turbine. In addition, it takes considerable time for the turbine housing unit to match the temperature of the captured gas when there is a substantial change of exhaust gas temperature. In particular, the heat energy loss may delay the activation of an exhaust gas catalytic converter disposed downstream of the turbocharger turbine in the exhaust line of the internal combustion engine at a cold start of the engine. The delay of the full operation of the exhaust gas catalytic converter increases the emission levels as monitored at the exhaust tail pipe. One method to minimize this emission is through reducing the thermal inertia of the turbine housing unit. One approach to reducing the thermal inertia of the turbine housing unit is to replace the steel or iron material of the turbine housing unit with a material having a low thermal inertia such as a carbon-carbon composite material as disclosed in US 5 810 556 A. Another approach is to apply a thermal insulation to the outside of the turbine housing unit to prevent radiation of heat. It goes without saying that the latter approach is only a partial solution to the problem of reducing the thermal inertia of the turbine housing unit. Still further, a ceramic layer could be applied to the inner walls of the turbine housing unit which are in contact with the exhaust gas. However, when applying such a ceramic layer to the inner wall of a turbine housing unit, even a skilled person encounters, as discussed in DE 102 32 754 Al, several difficulties. For example, there is a large difference between the coefficients of thermal expansion of the ceramic layer and the steel or iron material of the turbine housing unit. As a result, there is a high risk that the ceramic layer comes off and damages the turbocharger or other parts of the exhaust system such as the exhaust gas catalytic converter when the turbocharger is subjected to vibrations and shock in operation. In view of the above drawbacks, DE 102 32 754 Al abandons the use of ceramic layers on the inner walls of the turbine housing unit. Instead of a ceramic layer, there is proposed a double wall structure for the turbocharger housing unit which is filled with a compressed thermal insulating powder containing microporous silica. Although this approach reduces the thermal inertia of the turbine housing unit as compared with the approach of arranging a thermal insulator on the outside of the turbine housing unit, heat energy still dissipates into the inner wall of the double wall structure. SUMMARY OF THE INVENTION It is an object of this invention to further reduce the thermal inertia of a turbine housing unit in a turbocharger for internal combustion engines by applying a thermal insulating layer on inner wall surfaces of the turbine housing unit that come into contact with the exhaust gas . According to the present invention, the foregoing and additional objects are attained by a method of producing a turbine housing unit according to claim 1 and by a turbine housing unit according to claim 13. As mentioned above, a skilled person faces several problems when applying a thermal insulating layer such as a ceramic layer on the inner wall surfaces of the turbine housing unit. Among others, there is the problem that the thermal insulating layer has to be rather thin to compensate for the vibrations and shock in operation, thereby reducing the thermal inertia of the turbine housing unit only to a small extent. If, on the other hand, the thermal insulating layer is made thicker to reduce the thermal inertia of the turbine housing unit to the full extent, thermal stress at the interface between the thermal insulating layer and the metallic material of the turbine housing unit becomes excessively high, so that the thermal insulating layer is likely to come off or spall. According to the present invention, the above problem is alleviated to a large extent by dividing the turbine housing unit into a plurality of separate turbine housing pieces, each turbine housing piece having an inner wall surface that defines a part of the internal passage of the finished turbine housing unit. It is easier to apply the thermal insulating layer to the inner wall surfaces of the individual turbine housing pieces than to the inner wall surfaces of a finished turbine housing unit. For example, it is difficult to deposit a thermal insulating material on the inside of an internal passage having a scroll configuration such as the volute for receiving the turbine wheel. Even if one succeeded in depositing the thermal insulating material with a sufficient thickness, thickness and bonding strength of the thermal insulating layer would certainly differ depending on the accessibility of the respective location of deposition. According to a first aspect of the present invention, the above problem is solved by providing a method of producing a turbine housing unit for a turbocharger of an internal combustion engine, said turbine housing unit having an internal passage comprising an inlet and an outlet and a volute for receiving a turbine wheel, said method comprising: preparing a plurality of metallic turbine housing pieces, each turbine housing piece having an inner wall surface that defines a part of said internal passage; applying a thermal insulating layer on said inner wall surfaces; and joining the turbine housing pieces at respective parting lines after applying the thermal insulating layer to obtain an integral turbine housing unit. When applying the thermal insulating layer on the inner wall surfaces of the individual turbine housing pieces, it is possible to further optimize the process parameters as compared with the case of applying a thermal insulating layer to the inner wall surfaces of a finished turbine housing unit. As result, a thermal insulating layer having a greater and/or more controlled thickness and/or a higher bonding strength can be achieved. According to a second aspect of the present invention, the above problem is solved by providing a turbine housing unit for a turbocharger of an internal combustion engine, having an internal passage comprising an inlet and an outlet and a volute for receiving a turbine wheel, said turbine housing unit further comprising: a plurality of metallic turbine housing pieces, each turbine housing piece having an inner wall surface that defines a part of said internal passage, and said turbine housing pieces being joined at respective parting lines; and a thermal insulating layer on said inner wall surfaces. The above turbine housing unit is readily discernible from a conventional turbine housing unit having a one-piece configuration in that it has a multiple-piece configuration with the individual pieces being joined at the parting lines. Since the turbine housing unit may be produced by the method according to the first aspect of the invention, the thermal insulating layer may have a greater and/or more controlled thickness and/or a higher bonding strength. Preferably, the plurality of turbine housing pieces includes two turbine housing pieces that are joined at a parting line which divides the volute in an axial direction of the volute. In this case it is easier to apply the thermal insulating layer on the inner surface walls of the volute, which has a complicated scroll configuration. Further, at the inlet the internal passage of the turbine housing unit there may be provided a waste gate for bypassing the turbine wheel to be received in the volute and for waste-gating excess exhaust gas to the exhaust gas system downstream of the turbocharger. If the turbine housing unit is divided into appropriate pieces, it is quite easy to apply the thermal insulating layer on the inner wall surfaces of the waste gate. Preferably, the turbine housing pieces are prepared by casting. In this case each turbine housing piece can be cast separately, or the turbine housing pieces are prepared by casting a one-piece turbine housing and then cutting the turbine housing into pieces. The turbine housing pieces are preferably made of steel or iron. The thermal insulating layer is preferably a thermal barrier coating including a ceramic top layer and a metallic bond coat. Among such thermal barrier coatings, a ceramic top layer of stabilized zirconia and an MCrAlY bond coat (where M is selected from a group of cobalt, nickel, and iron) may be used. Such thermal barrier coatings have a high spalling resistance and are known as protective coatings which insulate the blades and vanes of aircraft engines or gas turbine engines (see, for example, US 6 395 343 Bl) . The bond coat and the ceramic top layer are preferably applied by plasma spraying. As explained above, it is difficult to deposit a material on the inside of an internal passage having a complicated and long configuration such as the volute. This is all the more true for plasma spraying. The metallic bond coat and the ceramic top layer exhibit a sufficient bonding strength only if, in the plasma spraying process, the molten particles have an appropriate temperature when they reach the location of impact, and if the molten particles have an angle of impact that is not excessively acute. Further, the turbine housing pieces are preferably joined at the parting lines thereof by welding. These and further objects, features and advantages of the invention will become apparent from the hereinafter following detailed description of presently preferred embodiments taken in conjunction with the figures of the drawing. BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a cross-sectional view of a turbine housing unit having a two-piece configuration according to one embodiment of the invention; FIGS. 2 and 3 are perspective views of the turbine housing pieces shown in FIG. 1; and FIGS. 4 and 5 are perspective views of a turbine housing unit having a three-piece configuration according to another embodiment of the invention. DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the drawings, wherein like numerals of reference designate like elements throughout, it will be seen that in FIG. 1 there is provided a two-piece turbine housing unit for a turbocharger of an internal combustion engine. The turbine housing unit has an internal passage comprising an inlet 10, an outlet 14, and a volute 12 having a single scroll configuration for receiving a turbine wheel. If installed in an exhaust system of an internal combustion engine, the internal passage guides exhaust gas discharged from the internal combustion engine from the inlet 10 to the turbine wheel in the volute 12 prior to discharge through the outlet 14. The internal passage further comprises a waste gate 16 at the inlet 10 which communicates the inlet 10 with the outlet 14 to bypass the turbine wheel and to waste- gate excess exhaust gas to the outlet 14. The turbine housing unit is constituted of first and second turbine housing pieces 2 and 4 which are welded at a parting line 6 which divides the volute 12 into two parts in an axial direction of the volute 12. As best seen in FIGS. 2 and 3, the first turbine housing piece 2 defines half of the inlet 10 and the volute 12, the main part of the waste gate 16, and the outlet 14. The second turbine housing piece 4 defines the other half of the inlet 10 and the volute 12, and a part of the waste gate 16 that opens to the inlet 10. As designated by the thick continuous line 8 in FIG. 1, the inner wall surfaces of the outlet 14 and the volute 12 are covered with a thermal insulating layer. Although not shown, the inner wall surfaces of the inlet 10 and the waste gate 16 are covered by the thermal insulating layer 8 as well. The turbine housing pieces 2, 4 are made of cast iron. For example, HK30 can be used, a Fe-Cr-Ni alloy consisting of 0.25-0.35 wt% C, 0.75-1.75 wt% Si, 23-27 wt% Cr, 19-22 wt% Ni, 1.2-1.5 wt% Nb, balance Fe and unavoidable impurities such as Mn, P, S, Mo. The thermal insulating layer 8 is a plasma-sprayed thermal barrier coating including a ceramic top layer of yttria stabilized zirconia and an MCrAlY bond coat. For spraying the top layer, the powder Metco 204C-NS™ may be used, containing 8% Y₂0₃, balance Zr0₂ and having spheroidal particles with a size of -125 +46 μm. For spraying the bond coat, the NiCrAlY powder Amdry 962™ may be used, containing 22% Ni, 10% Cr, 1% Al and less than 1% Y and having spheroidal particles with a size- of -106 +56 μm. The thickness of the bond coat is 50 to 150 μm, while the thickness of the ceramic layer may vary in the 100 to 500 μm range. There may be interposed a sub-micron thick alumina scale on the bond coat which improves the bonding of the ceramic top layer to the bond coat. Further, the ceramic top has an interconnected network of subcritical microcracks with micron-width opening displacements, which reduce the effective modulus (increase compliance) of the stabilized zirconia layer in the plane of the coating. Increased compliance provided by the microcracks enhances coating durability by eliminating or minimizing stresses associated with thermal gradient and cast iron/zirconia thermal expansion mismatch strains in the stabilized zirconia layer. For further details on the properties of such thermal barrier coatings, it is referred to US 6 395 343 Bl, which has been mentioned before. The ceramic top layer has a bond strength as high as 50 MPa which is considered to be robust in the operation of a turbocharger. The turbine housing unit according to the above embodiment is prepared as follows. First, the two turbine housing pieces 2, 4 are prepared separately by casting iron. The cast pieces are heat-treated, machined at the connections formed on the inlet 10 and the outlet 16 and at the parting line 6, and finally washed to obtain the first and second turbine housing pieces 2, 4 shown in FIGS. 2 and 3, respectively. Before coating, the machined surfaces of the first and second turbine housing pieces 2, 4 are masked with high temperature tapes or a specific tool. First, the bond coat is applied to the inner wall surfaces that define the respective parts of the internal passage by plasma-spraying NiCrAlY particles. Then, the bond coat is heat-treated to create a chemical bonding between the bond coat and the substrate material.

Thereafter, yttria stabilized zirconia particles are applied to the bond coat by plasma-spraying to form the ceramic top layer with the above-mentioned interconnected network of subcritical microcracks. The plasma-spraying is performed in the direction indicated by the hatched arrows in FIGS. 2 and 3. For the second turbine housing piece 4 the bond coat and the ceramic top layer may be sprayed from only one side (i.e. from the side facing the inner wall surfaces of the inlet 10, the volute and the opening of the waste gate 16) , whereas for the first turbine housing piece 2 the bond coat and the ceramic top layer are preferably sprayed from two sides (i.e. from the side facing the inner wall surfaces of the inlet 10 and the volute 12, and from the side facing the inner wall surface of the outlet 14) . The turbine housing pieces are configured such that there is sufficient access to the inner wall surfaces for the plasma-spray gun to cover the entire internal passage with the bond coat and the ceramic top layer. Finally, the coated turbine housing pieces 2, 4 are joined at the parting line 6, for example, by automatic laser welding. Usually, the ceramic top layer is sufficiently crack resistant for withstanding the welding process without damage. However, in order to minimize the risk of damage, one can consider masking the edges of the turbine housing pieces 2, 4 at the parting line 6 so that the ceramic top layer is deposited sufficiently far from the welding line. The above method of producing the turbine housing unit has the advantage that there is good access to the inner wall surfaces that define the internal passage of the turbine housing unit. Thanks to the good accessibility, the process parameters for applying the bond coat and, above all, the ceramic top layer can be optimized to achieve a thermal barrier coating having a sufficient thickness and a good bonding strength. Further, since the entire inner wall surface of the internal passage is coated with the thermal insulating ceramic layer, the thermal inertia of the turbine housing unit can be reduced to a large extent, thus increasing the performance and efficiency of the turbocharger and accelerating the activation of an exhaust gas catalytic converter disposed downstream of the turbocharger at a cold start of the internal combustion engine. The above embodiment can be modified as shown in FIGS. 4 and 5. FIGS. 4 and 5 show a perspective view of turbine housing pieces of a three-piece turbine housing unit according to another embodiment of the invention. This embodiment differs from the one discussed above only in that the first turbine housing piece is sub-divided into two sub-pieces 2a, 2b along the longitudinal axis of the outlet 14. Each sub-piece 2a, 2b defines a part of the inlet 10 and the volute 12, and one half of the waste gate 16. Dividing the first turbine housing piece into the two sub-pieces 2a, 2b shown in FIGS. 4 and 5 improves the accessibility to the outlet 14 and the waste gate 16, so that the quality of the thermal insulating layer can be further improved. Again, the spraying direction is indicated by hatched arrows. As a matter of course, the turbine housing unit can be divided into four pieces or more, if need be. Apart from that, the invention can be modified as follows. The turbine housing unit may have an internal passage with a configuration different from the one shown in the drawings. For example, the waste port 16 can be omitted. Further, the volute 12 can have a twin scroll configuration to which sufficient access could be achieved by dividing the turbine housing unit at parting lines which run in the radial direction of each scroll. Instead of casting the turbine housing pieces separately, it is also possible to prepare a conventional one-piece turbine housing and then cutting the turbine houses into the desired number of turbine housing pieces. Further, the turbine housing pieces can be made of a metal other than cast iron, including steel. Still further, the thermal insulating layer is not limited to a thermal barrier coating as described above. It is possible to use other bond coats and other ceramic top layers, or to apply the ceramic layer directly on the inner wall surfaces of the turbine housing pieces. Moreover, the thermal insulating layer need not be applied by plasma spraying. Other coating processes such as EB-PVD are appropriate as well. Besides, a thermal insulating material other than ceramic could be used, including glass and mineral. Still further, the risk of damage to the thermal insulating layer involved with the welding process can be reduced not only by masking the turbine housing pieces at the parting lines but also by selectively removing the applied thermal insulating layer in the vicinity of the edges of the turbine housing pieces at the parting lines before welding. Alternatively, the machined surfaces of the parting lines can have a stepped configuration so as to make the welding stop in the turbine housing pieces before reaching the thermal insulating layer. Finally, the turbine housing pieces can be joined by methods other than welding, including mechanical connections such as bolds, V-bands, a threaded connection, or the like. Apart from the above modifications, various other modifications and alterations will be apparent to those skilled in the art. Accordingly, this description of the invention should be considered exemplary, not as limiting the scope of the invention set forth in the following claims .

Claims

1. A method of producing a turbine housing unit for a turbocharger of an internal combustion engine, said turbine housing unit having an internal passage comprising an inlet (10), an outlet (14) and a volute (12) for receiving a turbine wheel, characterized by preparing a plurality of metallic turbine housing pieces (2, 4), each turbine housing piece having an inner wall surface that defines a part of said internal passage; applying a thermal insulating layer (8) on said inner wall surfaces; and joining the turbine housing pieces (2, 4) at respective parting lines (6) after applying the thermal insulating layer (8) to obtain an integral turbine housing unit.

2. A method according to claim 1, wherein said plurality of turbine housing pieces includes two turbine housing pieces (2, 4) to be joined at a parting line (6) which divides said volute (12) in an axial direction of the volute.

3. A method according to claim 1 or 2, wherein said internal passage further comprises a waste gate (16) at said inlet (10) .

4. A method according to claim 1, wherein said plurality of turbine housing pieces (2, 4) is prepared by casting each turbine housing piece separately.

5. A method according to claim 1, wherein said plurality of turbine housing pieces (2, 4) is prepared by casting a one-piece turbine housing and cutting said turbine housing into pieces.

6. A method according to claim 1, wherein said plurality of turbine housing pieces (2, 4) is made of steel or iron.

7. A method according to claim 1, wherein the thermal insulating layer (8) is a thermal barrier coating including a ceramic top layer and a metallic bond coat.

8. A method according to claim 7, wherein said ceramic top layer is stabilized zirconia.

9. A method according to claim 7, wherein said ceramic top layer is applied by plasma spraying.

10. A method according to claim 7, wherein said bond coat is an MCrAlY layer.

11. A method according to claim 7, wherein said bond coat is applied by vapor deposition.

12. A method according to claim 1, wherein said turbine housing pieces (2, 4) are joined at said parting lines

(6) by welding.

13. A turbine housing unit for a turbocharger of an internal combustion engine, having an internal passage comprising an inlet (10), an outlet (14) and a volute (12) for receiving a turbine wheel, characterized by a plurality of metallic turbine housing pieces (2, 4) , each turbine housing piece having an inner wall surface that defines a part of said internal passage, said turbine housing pieces being joined at respective parting lines (6); and a thermal insulating layer (8) on said inner wall surfaces.

14. A turbine housing unit according to claim 13, wherein said plurality of turbine housing pieces includes two turbine housing pieces (2, 4) being joined at a parting line (6) which divides said volute (12) in an axial direction of the volute.

15. A turbine housing unit to claim 13 or 14, wherein said internal passage further comprises a waste gate (16) at said inlet (10) .

16. A turbine housing unit according to claim 13, being made of steel or iron.

17. A turbine housing unit according to claim 13, wherein the thermal insulating layer (8) is a thermal barrier coating including a ceramic top layer and a metallic bond coat.

18. A turbine housing unit according to claim 17, wherein said ceramic top layer is stabilized zirconia.

19. A turbine housing unit according to claim 17, wherein said bond coat is an MCrAlY layer.

20. A turbine housing unit according to claim 13, wherein said turbine housing pieces (2, 4) are joined at said parting lines (6) by welding.