WO2006126993A1

WO2006126993A1 - Turbocharger compressor having improved erosion-corrosion resistance

Info

Publication number: WO2006126993A1
Application number: PCT/US2005/018018
Authority: WO
Inventors: Phillippe M. Renaud; Marc J. Wilson
Original assignee: Honeywell International Inc.
Priority date: 2005-05-24
Filing date: 2005-05-24
Publication date: 2006-11-30

Abstract

In a preferred embodiment, the invention is directed to a long-route EGR system wherein exhaust gases are recirculated through the compressor stage so as to lower the temperatures produced in the engine cylinders and to thereby reduce the formation of NOx. According to the invention, the EGR system includes a turbocharger having an aluminum compressor wheel that is plated with an erosion-corrosion-resistant coating such as electroless nickel, electroless nickel polytetrafluoroethylene, anodization, anodization with polytetrafluoroethylene, or combinations thereof. The erosion-corrosion-resistant coating may provide a protective barrier against the acidic conditions of the exhaust gas and against particulate matter in the exhaust gas. As a result, the erosion-corrosion-resistant coating may help improve the expected life time of compressor components that are used in a long-route EGR. Furthermore, the coating may also be applied to other surfaces exposed to recirculated exhaust gases, such as the compresor housing and a backplate.

Description

TURBOCHARGER COMPRESSOR HAVING IMPROVED EROSION-CORROSION

RESISTANCE

BACKGROUND OF THE INVENTION

[0001] The invention relates to turbochargers having exhaust gas recirculation upstream of the compressor stage.

[0002] Turbochargers may be used to increase an internal combustion engine's horsepower by compressing the air flowing into the intake of the engine. Compressing the air entering the engine permits a greater amount of air to enter each cylinder per stroke. As a result, a turbocharged engine produces more power than the same engine without the charging. [0003] In some cases, the turbocharger may use the exhaust flow from the engine to spin a turbine, which in turn spins a compressor. Turbochargers include a turbine wheel and a compressor wheel that are mounted on a common shaft. Exhaust gases passing through the turbine wheel cause the turbine wheel to rotate the compressor wheel so as to compress ambient air, which is supplied to the intake manifold of the engine. To improve the efficiency and reduce the inertia of the rotating parts, the compressor wheel may be made of a lightweight material, such as aluminum. This permits the turbine and compressor to accelerate more quickly, and start providing boost earlier.

[0004] In recent years, there has been increasing pressure in the form of governmental legislation to reduce internal combustion engine emissions, such as NOx and particulate matter (PM). Oxides of nitrogen (NOx) may be formed when temperatures in the combustion chamber are about 2500° F or hotter. At these elevated temperatures, the nitrogen and oxygen in the combustion chamber may chemically combine to form nitrous oxides.

[0005] Exhaust gas recirculation (EGR) is a method that has been used to reduce the level of NOx in exhaust gases. In EGR systems, some of the exhaust gases that would otherwise be discharged into environment are recirculated into the intake stream. The recirculated exhaust gases have already combusted and have a significantly lower oxygen content, so they do not burn again when they are recirculated. The exhaust gases may displace some of the normal intake charge. As a result, the combustion process may be cooler by several hundred degrees so that NOx formation may be reduced. [0006] In some turbocharger applications, the EGR may route a portion of the exhaust gases to the inlet of the compressor, where it may be mixed with ambient air that is to be compressed (so-called "low-pressure" or "long-route" EGR). The exhaust gases may include corrosive components and particulate matter that may deleteriously affect the aluminum components of the turbocharger such as the compressor wheel and housing. In particular, these exhaust gas substances can lead to erosion and/or corrosion of the compressor components, especially of the compressor wheel which may be rotating at very high speeds. As a result, the components may prematurely fail. To overcome this problem, some turbocharger manufacturers have developed compressor stage components, such as the compressor wheel, made of titanium alloy. However, the use of titanium may not be desirable for several reasons. First, titanium is substantially more expensive than aluminum and is more difficult to work with, thus increasing the costs of producing the turbocharger unit. Second, titanium is heavier than aluminum and thus increases the rotational inertia of the compressor wheel. As a result, the turbocharger may be less responsive than an otherwise equivalent unit employing an aluminum wheel. [0007] U.S. Patent Nos. 6,508,240 and 6,606,983 to Bedwell et al. describe a piston and cylinder liner for an internal combustion engine, wherein the piston and cylinder liner are ferrous and have an electroless nickel coating for providing resistance to corrosion from organic acids and other corrosive products of combustion present in exhaust gases that are recirculated to the engine intake. The usage environment and conditions experienced by Bedwell's piston and cylinder liner are substantially different from the conditions experienced by turbocharger compressor components. For example, high-speed impingement of particulate matter onto surfaces, as occurs in turbocharger compressors in long-route EGR systems, is not even addressed in the Bedwell patents. The piston and cylinder liner also do not have thin, sharp edges prone to erosion from high-speed impingement of exhaust gas particulate matter, such as occurs in compressor blades of turbochargers in long-route EGR systems. [0008] Thus, there still exists a need for turbocharger compressor components having improved corrosion and erosion resistance while avoiding the disadvantages associated with titanium-based components. BRIEF SUMMARY OF THE INVENTION

[0009] The present invention addresses the above needs and achieves other advantages by providing, in one embodiment, a turbocharger having an aluminum compressor wheel coated with an erosion-corrosion-resistant coating. By "erosion-corrosion-resistant coating" is meant a coating that increases the resistance of the coated component to erosion caused by abrasion from particulate matter and/or to corrosive attack from acidic and/or alkaline substances. In some embodiments, the erosion-corrosion-resistant coating may comprise electroless nickel, electroless nickel polytetrafluoroethylene, or combinations thereof. In some other embodiments, the erosion-corrosion-resistant coating may comprise anodization, anodization with polytetrafluoroethylene, or combinations thereof. The erosion-corrosion resistant coating helps to improve the expected operating life of an aluminum compressor wheel in long-route EGR environments. In another alternative embodiment, an inner surface of the compressor housing and any surfaces of the backplate that may come into contact with exhaust gases may also include an erosion-corrosion-resistant coating. The erosion-corrosion-resistant coating may be used to provide a protective barrier for the aluminum components that may be present in the compressor stage.

[0010] In one embodiment, the invention is directed to a low pressure EGR system wherein exhaust gases are recirculated through the compressor stage so as to lower the temperatures produced in the engine cylinders and to thereby reduce the formation of NOx. In one embodiment, the EGR system includes a turbocharger having a compressor wheel that is plated with an erosion-corrosion-resistant coating such as electroless nickel or electroless nickel polytetrafluoroethylene. The erosion-corrosion-resistant coating may provide a protective barrier against the acidic/alkali conditions of the exhaust gas and against particulate matter in the exhaust gas. As a result, the erosion-corrosion-resistant coating may help improve the expected operating life of compressor components that are used in a long-route EGR turbocharger system. [0011] Thus, the invention can overcome the above-noted drawbacks associated with the prior art by allowing the manufacture of components from aluminum so as to retain its advantages, while substantially improving the durability of the components in the erosive- corrosive EGR environment. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWlNG(S) [0012] 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: [0013] FIG. 1 is a schematic illustration of an exemplary EGR system in combination with a turbocharger;

[0014] FIG. 2 is a cross-sectional view of a turbocharger having an erosion-corrosion- resistant coating in accordance with one embodiment of the invention; and [0015] FIG. 3 is a Pourbaix diagram that graphically illustrates the stability of aluminum oxide.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which some but not all embodiments of the invention are shown.

Indeed, the invention 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.

[0017] With reference to FIG. 1, an exemplary exhaust gas recirculation (EGR) system in combination with a turbocharger device is illustrated and broadly designated as reference number

10. In the illustrated embodiment, the system 10 comprises a low-pressure EGR (also known as

"long route" EGR) wherein recirculated exhaust gases are introduced into the inlet of the compressor so that the exhaust gases pass through the compressor along with the ambient air drawn in through the inlet.

[0018] The system 10 may include a combustion engine 12 having a plurality of combustion chambers or cylinders 14. Each combustion chamber may have one or more intake valves (not shown) and one or more exhaust valves (not shown) that may each be arranged so that it is in fluidic communication with intake and exhaust manifolds 20, 22, respectively.

[0019] Products of combustion produced in combustion chambers 14, hereinafter termed

"exhaust gases", are exhausted to exhaust manifold 22 and may pass via one or more exhaust ducts 24 to a turbocharger 25. The turbocharger 25 includes a turbine 30 which operatively drives a compressor 35 in a known manner. After being expanded in the turbine 30, the exhaust gases may then be exhausted to atmosphere 40.

[0020] In some embodiments the exhaust gases may pass through an exhaust soot or particulate filter (not shown) before being exhausted to atmosphere. An exhaust particulate filter may remove soot and other solid particulate matter from the exhaust gases and may optionally be coated with an oxidation catalyst to oxidize unburned fuel components such as unburned hydrocarbons (HC) and CO.

[0021] A portion of the exhaust gases may be vented into the EGR system through EGR line 45 located downstream of the turbine 30. In some embodiments, the EGR line 45 may include an EGR flow valve 50 and an EGR cooler 55. EGR flow valve 50 may be controlled so that the flow rate of exhaust gases entering line 45 can range from zero up to a predetermined maximum flow rate gas. The exhaust gas may then flow into an EGR mixer 65 that mixes the exhaust gases with ambient air prior to entering the inlet of the compressor 35. In some embodiments, a portion of the exhaust gas may be passed directly to the EGR mixer without passing though the turbine (not shown).

[0022] The air supplied to intake manifold 20 comprises mostly compressed ambient air drawn through an air inlet 70 upstream of compressor 35. For consistent terminology, the mixture of ambient air drawn into the system through air inlet 70 and the exhaust gases delivered through line 45 is referred to as "combustion air." In some embodiments, the ambient air may pass through an air filter (not shown).

[0023] The combustion air then flows through air duct 80 and passes through compressor 35 of turbocharger 25 where it is compressed. The compressed combustion air is supplied to the intake manifold 20. Compression of the combustion air causes the combustion air to become heated. Accordingly, the compressed, heated combustion air may be cooled in an after-cooler or intercooler 90 (hereinafter termed intercooler) before passing to intake manifold 20. In some embodiments, the EGR system may include an electronic control unit (not shown) and one or more sensors (not shown) that may be used to monitor and control the system. [0024] The invention may comprise a variety of different turbocharger configurations and designs including free floating, wastegated, variable geometry, or electrically-assisted turbochargers. Centrifugal compressor wheels in general are well known in the art for use in turbochargers and the like, wherein the compressor wheel comprises an aerodynamically contoured array of impeller blades supported on a central wheel hub section which is mounted in turn onto a rotatable shaft for rotation therewith. In the context of a turbocharger, by way of example, the wheel hub section may include a central axial bore through which the shaft extends, and a threaded nut may be fastened over the shaft at a nose end of the compressor wheel to hold the hub section tightly against a shaft shoulder or other diametrically enlarged structure such as a thrust bearing unit on the shaft. The rotatable turbocharger shaft drives the compressor wheel in a direction such that the contoured blades axially draw in air that is compressed and is discharged radially outwardly at an elevated pressure into a volute-shaped chamber of a compressor housing. The pressurized air is then supplied from the housing to the air intake manifold of a combustion engine for mixture and combustion with fuel, all in a well known manner. [0025] With reference to FIG. 2, an exemplary turbocharger that may be used in the practice of the present invention is illustrated and broadly designated as reference number 100. In the illustrated embodiment, the compressor wheel is depicted as having a boreless hub. It should be recognized that the exact configuration of the compressor wheel or turbine wheel is not critical to the invention provided that the turbocharger may be capable of delivering a mixed charge of combustion air to the intake manifold.

[0026] In the illustrated embodiment, the turbocharger 100 includes a compressor stage 112 comprising a compressor wheel 113. The compressor wheel may comprise a centrifugal compressor wheel of the type having a central hub 122 extending along a rotational axis between a relatively small diameter nose 123 at one end to a significantly larger wheel or tip diameter 124 at an opposite end. In one embodiment, the compressor wheel may include a back disk 125 which is defined as a contoured surface at the larger diameter of the wheel 113, facing axially away from the nose 123, wherein the back disk 125 may be designed to extend axially from the plane of maximum wheel diameter as shown in FIG. 2. In some embodiments, the turbocharger may include an EGR pump (not shown here) that may be incorporated into the existing rotor group of the turbocharger by adding impeller vanes to the backside (back disc) of the compressor wheel. In this embodiment, the EGR pump drives and pressurizes the exhaust gas from the EGR cooler to a mixer which combines the recirculated exhaust flow with the charge air to create a substantially homogeneous flow stream for introduction into the intake manifold. Turbocharger units having integral EGR pumps are described in U.S. Patent No. 6,145,313, the disclosure of which is hereby incorporated herein by reference. [0027] The central hub 122 of the compressor wheel 113 supports an array of aerodynamically contoured impeller blades 126 which extend smoothly with complex curvatures between the nose 123 and the tip diameter 124.

[0028] The compressor wheel 113 may be formed from a relatively lightweight, relatively low inertia material such as an aluminum alloy. An example of a suitable alloy is C354. In some embodiments, the compressor wheel and its associated impeller blades may be formed from a monolithic piece or body of aluminum alloy that can be cast, forged, and/or machined to have a desired configuration.

[0029] The illustrated compressor stage 112 is shown within the turbocharger 100 of generally conventional overall geometry and operation. In particular, by way of brief explanation and as viewed in FIG. 2, turbocharger 100 includes a turbine wheel 127 mounted within a turbine housing 128 adapted for flow-through passage of exhaust gases from an engine (not shown). The turbine housing 128 may include a radial exhaust gas inlet (not shown) feeding a volute 104. Exhaust gas flowing through the turbine exits at turbine outlet 106. The exhaust gases rotatably drive the turbine wheel 127 for correspondingly rotating the turbocharger shaft 116 supported by suitable bearings 130 within a so-called center housing 132. Lubrication ports 134 and related flow passages within the center housing 132 may be provided for circulating lubricating oil to the bearings 130, with the oil flow and bearing design accommodating relatively high speed shaft rotation. The turbocharger shaft 116 extends in turn through a backplate 120 for connection to the compressor wheel 113 to rotatably drive the compressor wheel within compressor housing 136. During such wheel rotation, the compressor wheel assembly 112 draws in the combustion air through an axial inlet 138 (combustion air inlet) and discharges the air radially into a volute chamber 140 (combustion air outlet) at elevated pressure. Such pressurized air is supplied in turn from the compressor housing 136 to the intake manifold or the like of a combustion engine (not shown) for admixture and combustion with fuel. This provision of charged air to the engine may result in a substantial increase in mass flow through the engine to correspondingly permit engine operation at increased performance levels. [0030] In one alternative embodiment, the compressor wheel may be made of a light weight material, such as an aluminum alloy. The light weight material permits manufacture of compressor wheels having reduced rotating inertia compared to wheels of heavier materials such as titanium, such that the wheels are more responsive during accelerations and decelerations of the turbocharger. Compressor wheels made of these lightweight materials may operate at very high rotational speeds and thus be subject to undesirable fatigue and failure. This may especially be true in a long-route EGR environment where exhaust gas is recirculated through the compressor thereby increasing material temperature. In some embodiments, the compressor housing 136 and the backplate 120 may also be formed from a lightweight material, such as an aluminum alloy.

[0031] It can be seen from FIG. 1 that a portion of the exhaust gas is mixed with ambient air and then introduced into the compressor. As discussed above, the exhaust gas may comprise one or more corrosive compositions and/or particulate matter that may adversely affect the components of the compressor assembly by eroding and/or corroding the components. The corrosive composition(s) may include acidic and corrosive elements having a pH below about 4.0. Exhaust gases from diesel engines may include gaseous pollutants such as unburned hydrocarbons (HC), nitrogen oxides (NOx), carbon monoxide (CO), as well as particulate matter, which may comprise both a dry, solid carbonaceous fraction and a soluble organic fraction ("SOF"). The SOF may be present in the exhaust gas as either a vapor phase or a liquid phase, or both, depending on the temperature of the exhaust and can also be adsorbed onto the solid carbonaceous fraction. Additionally, the exhaust gas may contain other particles, such as zinc and phosphate (resulting from lubricating oils), calcium, magnesium and silicates (resulting from engine coolant) and iron (resulting from engine wear). Combined together they form exhaust ash. Metallic elements from the EGR cooler (copper) may also be found as particles. As used herein, the term "particulate matter" means complete particulate matter including all solid particles and SOF emitted from the engine in its exhaust gas.

[0032] The EGR line 45 may introduce the EGR gas upstream of compressor 35. As a result, corrosive compositions and particulate matter may come into contact with surfaces inside of the compressor housing. Such materials could adversely impact the operation of or shorten the life of compressor 35. In some embodiments, it may be desirable to include an exhaust particulate filter to help remove the undesirable matter from the exhaust gas before it is introduced into the compressor. Unfortunately, it may not always be feasible to remove all such materials, or in some cases the filter may fail. These exhaust gas materials can lead to substantial erosion- corrosion of turbocharger compressor components, particularly of the compressor wheel. Particulate matter impinging on the thin leading edges of compressor blades at high speeds can relatively quickly erode the blades and degrade the integrity of the wheel if it is formed of a relatively erosion-prone material such as aluminum and is not protected by an effective erosion- corrosion-resistant coating.

[0033] Aluminum is a thermodynamically reactive metal that forms a strong oxide film protecting the aluminum from further atmospheric corrosion. With reference to Fig. 3, the stability of the oxide film is expressed by the Pourbaix diagram. The Pourbaix diagram illustrates that the oxide film has complete protection in the pH range from about 4 to 8.5. Outside of these limitations, aluminum can corrode in acid or alkali environments. Aluminum may also be vulnerable to mechanical erosion. In the context of the invention, mechanical erosion refers to the "progressive loss of original material from a solid surface due to mechanical interaction between the surface and a fluid, multicomponent fluid, or impinging liquid, or solid particles." Mechanical erosion may be influenced by the impact velocity of the particle, angle of impact, particle size, shape, material, ambient temperatures and combinations thereof. [0034] It is evident from the foregoing discussion that the introduction of such undesirable materials in the intake air may adversely affect the expected life of the compressor components. This may be especially true for the compressor wheel operating at high speeds, as previously noted. For example, at the very high rotating speeds to which the compressor wheel may be subjected, it is absolutely necessary that the rotating assembly be dynamically balanced. If one or more portions of the compressor wheel corrode and/or erode at a greater rate than the remaining portions, the compressor wheel may become unbalanced, which may result in component failure.

[0035] In one alternative embodiment, the corrosion and erosion resistance of the lightweight compressor components may be improved by providing a protective coating on the surface of the components. In this regard, FIG. 2 illustrates the surface 114 of the compressor wheel 113, the inner surface 137 of the compressor housing 136, and a surface 121 of the backplate 120 that may include an erosion-corrosion-resistant coating. The coating may be used to provide a protective barrier for compressor components that are exposed to exhaust gases that are flowing through the compressor stage. In one alternative embodiment, the compressor wheel may comprise an aluminum alloy having a protective coating on its surface. In another alternative embodiment, the compressor wheel may comprise an aluminum alloy that has been anodized to improve the corrosion and abrasion resistance of the aluminum. In some embodiments, the housing and back plate may also comprise an aluminum alloy having a protective coating and/or that has been anodized to improve its corrosion and abrasion resistance. [0036] In one alternative embodiment, the protective coating may comprise an electroless nickel coating that may be plated onto the surfaces of the compressor components. The electroless nickel coating may have good uniformity, corrosion resistance, wear and abrasion resistance, solderability, high hardness, excellent adhesion, and low coefficient of friction. As discussed above, a coating of uniform thickness may be particularly desirable because of the need to dynamically balance the compressor wheel and to maintain the proper aerodynamic contour of the wheel.

[0037] In general, electroless nickel plating may be characterized as the selective reduction of nickel ions at the surface of a catalytic substrate that is immersed in an aqueous bath of the nickel ions. Electroless bath compositions typically contain an aqueous solution of metal ions to be deposited, catalysts, one or more reducing agents, one or more complexing agents and bath stabilizers, all of which are tailored to specific metal ion concentration, temperature and pH range. In electroless nickel depositing, use is made of a chemical reducing agent, thus avoiding the need to employ an electrical current as required in conventional electroplating operations. In an electroless plating process, metal ions are reduced to metal through the action of chemical reducing agents serving as electron donors. The nickel ions are electronic acceptors, which react with the electron donors to form nickel metal that becomes deposited on the substrate. The catalyst is simply the surface provided to the bath, which serves to accelerate the electroless chemical reaction to allow oxidation and reduction of the nickel ion to metal. [0038] It is believed that the improved erosion resistance of the electroless nickel coating may be due to the low porosity of the coating. Generally, electroless nickel coatings may have a lower porosity and a more uniform thickness in comparison to an equivalent electroplated nickel. In order to achieve the desired erosion resistance, the pretreatment and plating conditions must achieve good adhesion and continuity.

[0039] The electroless nickel coating may have a thickness that ranges from about 5 micrometers up to about 30 micrometers. In some embodiments, the thickness of the coating may be from about 6 to 16 micrometers.

[0040] In another alternative embodiment, the protective coating may comprise microscopic beads of polytetraflouethylene (PTFE) that are co-deposited with the electroless nickel. In one alternative embodiment, the electroless nickel coating may include up to about 20% polytetraflouethylene by weight, based on the total weight of the coating. [0041] In yet another embodiment, the aluminum compressor components may be anodized to improve oxidation and abrasion resistance. Anodizing is a known process and generally involves an electrochemical process that forms a protective oxide barrier on the outer surface of the aluminum part. The part to be anodized may be cleaned, and then immersed in an acid bath. The aluminum provides a positive pole, or anode, in the acid bath. A current is then applied and oxidation of the aluminum may occur. In some embodiments, the anodized aluminum part may be treated with a sealant. The anodized coating may help improve the corrosion and erosion resistance of the part. In some embodiments, the anodized aluminum may further include an electroless nickel or electroless nickel PTFE coating.

[0042] Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which the invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is 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.

Claims

THAT WHICH IS CLAIMED:

1. An exhaust gas-driven turbocharger for an internal combustion engine, the turbocharger comprising: a turbine housing including at least one exhaust gas inlet and at least one exhaust gas outlet; a turbine wheel connected to a rotatable shaft and driven by exhaust gas received from the engine through the exhaust gas inlet; a compressor housing having at least one air inlet and at least one charge air outlet; and a compressor wheel disposed in the compressor housing and attached to the rotatable shaft, the compressor wheel comprising an aluminum alloy coated with an erosion- corrosion-resistant coating.

2. The exhaust driven turbocharger according to Claim 1, wherein the erosion- corrosion -resistant coating comprises electroless nickel, electroless nickel polytetrafluoroethylene, or combinations thereof.

3. The exhaust driven turbocharger according to Claim 1, wherein the erosion- corrosion-resistant coating comprises anodization, anodization with polytetrafluoroethylene, or combinations thereof.

4. The exhaust driven turbocharger according to Claim 1, wherein the erosion- corrosion-resistant coating is from about 5 to 50 micrometers thick.

5. The exhaust driven turbocharger according to Claim 1, wherein the erosion- corrosion-resistant coating comprises electroless nickel plating having from about 5% to 25% by weight polytetrafluoroethylene, based on the total weight of the coating.

6. The exhaust driven turbocharger according to Claim 1, wherein the erosion- corrosion-resistant coating covers all surfaces of the compressor wheel that are exposed to the exhaust gases.

7. The exhaust driven turbocharger according to Claim 1, wherein an inner surface of the compressor housing includes an erosion-corrosion-resistant coating covering all surfaces of the housing that are exposed to the exhaust gases, the erosion-corrosion-resistant coating comprising electroless nickel, electroless nickel polytetrafluoroethylene, anodization, anodization with polytetrafluoroethylene, or combinations thereof.

8. The exhaust driven turbocharger according to Claim 1, further comprising a backplate located between the compressor wheel and the center housing, the back plate having a surface in contact the exhaust gases that is plated with an erosion-corrosion-resistant coating comprising electroless nickel, electroless nickel polytetrafluoroethylene, anodization, anodization with polytetrafluoroethylene, or combinations thereof.

9. A turbocharger for use with an engine having a low-pressure EGR system, said turbocharger comprising: a turbine housing including at least one exhaust gas inlet and at least one exhaust gas outlet; a turbine wheel driven by exhaust gas received through the exhaust gas inlet and connected to a rotatable shaft; a compressor housing having at least one air inlet for receiving combustion gases that comprise a mix of ambient air and recirculated exhaust gases, and at least one charge air outlet for supplying charged air to an intake manifold; a compressor wheel located in the compressor housing and attached to the rotatable shaft, the compressor wheel comprising an aluminum alloy having an erosion- corrosion-resistant coating covering a surface of the compressor wheel that is exposed to the exhaust gases, the erosion-resistant coating comprising electroless nickel, electroless nickel polytetrafluoroethylene, or combinations thereof.

10. The turbocharger according to Claim 9, wherein the erosion-corrosion-resistant coating is from about 5 to 50 micrometers thick.

11. The turbocharger according to Claim 9, wherein the erosion-corrosion-resistant coating comprises electroless nickel plating having from about 5% to 25% by weight polytetrafluoroethylene, based on the total weight of the coating.

12. The turbocharger according to Claim 9, further comprising a back plate located between the compressor wheel and the center housing, the backplate having a surface in contact with the exhaust gases that is plated with an electroless nickel coating, electroless nickel polytetrafluoroethylene coating, anodization, anodization with polytetrafluoroethylene coating or combinations thereof.

13. The turbocharger according to Claim 9, wherein the air inlet is in fluid communication with an air mixer.

14. A turbocharger and a long-route EGR system for an internal combustion engine, the turbocharger and a long-route EGR system comprising: a turbocharger having a compressor stage for supplying combustion air to an intake manifold of the engine and a turbine stage arranged to receive exhaust gases from an exhaust manifold of the engine and operatively connected to said compressor for driving said compressor; an EGR mixer located downstream from said turbine stage and in fluid communication with an air intake line whereby exhaust gases from said exhaust gases is mixed with ambient air to produce combustion air; a combustion air intake in fluid communication with the EGR mixer for delivering the combustion air to the compressor for pressurization; and a combustion air outlet in fluid communication with an intake manifold whereby the pressurized combustion air is discharged through the combustion air outlet and supplied to the intake manifold, and wherein the compressor includes a compressor wheel comprising an aluminum alloy and having an erosion-corrosion-resistant coating covering a surface of the compressor wheel that contacts the combustion air, the erosion-corrosion-resistant coating comprising electroless nickel, electroless nickel polytetrafluoroethylene, anodization, anodization with polytetrafluoroethylene or combinations thereof.

15. The turbocharger and a long-route EGR system according to Claim 14, further comprising an exhaust particulate filter downstream of and in fluid communication with an outlet of said turbine stage for removing particulate matter from the exhaust gases.

16. The turbocharger and a long-route EGR system according to Claim 14, further comprising an intake air filter upstream of the compressor for supplying filtered combustion air to said compressor stage.

17. The turbocharger and a long-route EGR system according to Claim 14, further comprising an intercooler disposed between the combustion air outlet and the intake manifold.

18. The turbocharger and a long-route EGR system according to Claim 14, wherein the compressor includes a compressor housing having an inner surface plated with an electroless nickel coating, electroless nickel polytetrafluoroethylene coating, anodization, anodization with polytetrafluoroethylene, or combinations thereof.

19. The turbocharger and a long-route EGR system according to Claim 14, wherein the compressor further comprises a backplate having a surface plated with an electroless nickel coating, electroless nickel polytetrafluoroethylene coating, anodization, anodization with polytetrafluoroethylene, or combinations thereof.

20. The turbocharger and a long-route EGR system according to Claim 14, wherein the erosion-corrosion-resistant coating is from about 5 to 50 micrometers thick.

21. The turbocharger according to Claim 14, wherein the erosion-corrosion-resistant coating comprises electroless nickel plating having from about 5% to 25% by weight polytetrafluoroethylene, based on the total weight of the coating.

22. A compressor wheel for use in a turbocharger and a long-route EGR system, comprising a central hub and a plurality of impeller blades extending outwardly therefrom, and wherein the compressor wheel comprises an aluminum alloy and an erosion-resistant coating covering a surface of the compressor wheel exposed to exhaust gases in the EGR system.

23. The compressor wheel according to Claim 22, wherein the erosion-resistant coating comprises electroless nickel, electroless nickel polytetrafluoroethylene, or combinations thereof.