Water-lubricated shaft assembly for turbochargers

Abstract

Shaft arrangement for a turbocharger with - a rotatably mounted turbocharger shaft (11) and - at least one bearing arrangement (12, 22) for rotatably supporting the turbocharger shaft (11), wherein the at least one bearing arrangement (12, 22) comprises water lubrication, wherein at least one bearing element (13, 14, 23, 24, 25) of the bearing arrangement (12, 22) is coated at least sectionally with an amorphous carbon coating, characterized in that the bearing arrangement (12, 22) comprises a rolling bearing (22) with the following bearing elements: an outer ring (24) and an inner ring (23) rotatable relative to the outer ring (24), and a number of rolling elements (25) arranged between the inner ring (23) and the outer ring (24), wherein the outer ring or the inner ring and / or at least one of the rolling elements is coated with the amorphous carbon coating.

Description

[0001] The invention relates to a shaft arrangement for a turbocharger according to the preamble of patent claim 1 and to an exhaust gas turbocharger according to the preamble of patent claim 8.

[0002] Exhaust gas turbochargers are used, among other things, in modern drive concepts for vehicles with internal combustion engines to increase their efficiency. Exhaust gas turbochargers typically comprise a rotatably mounted turbocharger shaft, which is connected to a turbine wheel for driving the turbocharger shaft at a first end in the exhaust tract and a compressor wheel for compressing the intake air at a second, opposite end in the intake tract.

[0003] Typically, the turbocharger shaft is lubricated with lubricating oil, for example, from the vehicle's engine oil circuit. In addition to its lubrication function, the oil also serves to cool the turbocharger. Plain bearings are typically used to support the rotating turbocharger shaft, although rolling bearings are also used in very specific applications.

[0004] The disadvantage here is that the oil used to lubricate the turbocharger shaft can enter the intake tract and / or the exhaust tract in the axial direction of the turbocharger shaft, since the lubricated bearings cannot be designed to be completely leak-tight due to their function. If oil enters the intake tract, this leads to contamination (sooting) of the charge air duct and the intercooler downstream in the direction of flow, as well as to the formation of deposits that endanger the operational reliability of the engine, which is also located downstream.

[0005] Additionally, oil particles, soot, or carbon particles can be generated from the exhaust turbocharger into the engine's combustion chamber, causing unwanted pre-ignition and so-called "super-knocking." These particles can lead to engine damage if they occur frequently. Such particles are formed primarily due to the reheating of hot engine and exhaust system components after the engine is shut down, resulting in a lack of further flow through the oil supply lines in the exhaust turbocharger and the oil supply lines.

[0006] However, oil leaking from the turbocharger during normal vehicle operation can cause significant damage not only in the intake tract but also in the exhaust system. There is a risk that downstream components of the exhaust system, such as a catalytic converter or particulate filter, could be damaged as a result of additional exothermic reactions or the buildup of contaminants.

[0007] In these cases, new and increasingly used vehicle operating strategies, such as so-called coasting mode, have a reinforcing effect. In this operating mode, the engine's exhaust valves are deactivated, stopping the exhaust gas mass flow through the turbocharger and thus the turbocharger itself. If the lubricating oil pressure is maintained, there is a risk that the sealing effect of the turbocharger will be insufficient, and the oil will be forced into the intake or exhaust tract even more intensively.

[0008] It is therefore known to provide exhaust gas turbochargers with water lubrication for the bearings of the exhaust gas turbocharger shaft. For example, EP 0 270 921 A2 describes a drive system with an internal combustion engine and a coupled exhaust gas turbocharger that has water-lubricated plain bearings.

[0009] The so-called hydrodynamic plain bearings used here are lubricated using water as a lubricant, so that when the shaft rotates, water is carried as a lubricating film between a sliding surface of the plain bearing and a corresponding surface of the shaft. A relationship between the relative speed of the two surfaces and the resulting frictional force is illustrated, for example, in the so-called "Stribeck curve."

[0010] In simplified terms, the Stribeck curve can be divided into four areas: the static friction area, the boundary friction area, the mixed friction area and the fluid friction area.

[0011] If there is no relative movement of the turbocharger shaft with respect to the plain bearing, what is known as static friction prevails between the two surfaces (static friction zone). If slow relative movement occurs, the contact points initially separate (boundary friction zone) until relative movement begins. With increasing speed, friction initially decreases significantly with speed within the mixed friction zone. After a complete lubricating film has formed, friction increases again in the so-called "fluid friction zone" with further increasing speed.

[0012] In particular, within the boundary friction and mixed friction range, a significant proportion of the wear phenomena on the respective components occurs, which leads to premature failure of the exhaust gas turbocharger and reduces the efficiency of the exhaust gas turbocharger.

[0013] Since the engine is continuously running under current operating strategies, and thus a constant exhaust flow ensures a constant rotation of the turbocharger shaft, fluid friction is of primary importance for optimal turbocharger design. Operation in the mixed friction range, boundary friction range, or static friction range, in contrast, occurs very rarely.

[0014] However, this strategy cannot be maintained due to the new operating strategies of modern vehicles described above. Since no exhaust gas is available to drive the turbocharger shaft during engine shutdown, such as during coasting, the movement of the turbocharger shaft is reduced or comes to a complete standstill. To resume operation, the turbocharger shaft must be set in motion from a standstill. Consequently, with this type of operating strategy, the shaft assembly passes through the particularly wear-prone limit and mixed friction regions far more frequently, which can be expected to result in greater wear and an increased probability of failure.

[0015] Document DE 10 2009 059 660 A1 relates to a shaft, in particular a rotor in an exhaust gas turbocharger, with at least one bearing area. To increase service life, document DE 10 2009 059 660 A1 proposes providing the shaft, at least in part in the bearing area, with a DLC coating that reduces friction and wear, and / or providing the shaft with at least one bulge in the bearing area, which is particularly convex.

[0016] Document EP 0 270 921 A2 relates to a drive system with an internal combustion engine and a coupled exhaust gas turbocharger. To simultaneously achieve effective lubrication and cooling of the exhaust gas turbocharger using simple structural means, document EP 0 270 921 A2 proposes that the exhaust gas turbocharger have water-lubricated plain bearings.

[0017] Document DE 102 47 200 A1 relates to a method for simultaneously machining one or more semiconductor wafers on both sides by removing material between two rotating work disks while supplying a liquid containing abrasives or colloids. The semiconductor wafers are moved along a path by one or more carrier disks. To achieve cost advantages over a longer service life of the carrier disks, document DE 102 47 200 A1 proposes providing the carrier disk, at least in partial areas of the front and back, with a coating that is chemically inert, largely abrasion-resistant, and low-friction under operating conditions.

[0018] The object of the invention is therefore, on the one hand, to prevent contamination of the intake and exhaust tract and, on the other hand, to reduce wear even under changing operating conditions and, at the same time, to enable even more efficient operation of the exhaust gas turbocharger.

[0019] This object is achieved by means of a shaft arrangement for a turbocharger having the features of patent claim 1 and an exhaust gas turbocharger having the features of patent claim 9. Further embodiments emerge from the respective dependent patent claims.

[0020] Accordingly, a shaft arrangement for a turbocharger, in particular for a turbocharger of a motor vehicle, is described, comprising a rotatably mounted turbocharger shaft and at least one bearing arrangement for rotatably mounting the turbocharger shaft, wherein the at least one bearing arrangement comprises water lubrication, and at least one bearing element of the bearing arrangement is coated at least in sections with an amorphous carbon coating, wherein the bearing arrangement comprises a rolling bearing with the following bearing elements: an outer ring and an inner ring rotatable relative to the outer ring, and a number of rolling elements arranged between the inner ring and the outer ring, wherein the outer ring or the inner ring and / or at least one of the rolling elements is coated with the amorphous carbon coating.

[0021] This means that the turbocharger shaft is rotatably mounted in one or more bearing assemblies that incorporate a water lubrication system. The water lubrication system can, for example, be coupled to an engine cooling circuit for exchanging cooling water as a lubricant for the water lubrication system. Alternatively, a separate circuit can be provided for the lubricant intended for water lubrication.

[0022] Water-based lubricants, such as distilled water, cooling water, and / or other water-based solutions with suitable additives, can be used as lubricants for water lubrication. These additives can include, in particular, suitable refrigerant and / or antifreeze additives.

[0023] This makes it possible to lubricate the bearing assembly of the turbocharger shaft with a water-based lubricant instead of oil, thus reducing leakage problems. Even if some of the water-based lubricant from the bearing assembly enters the intake or exhaust tract of the turbocharger, contamination of the turbocharger is prevented—unlike with oil lubrication. In addition, water lubrication provides additional cooling for the turbocharger shaft and the entire exhaust turbocharger.

[0024] The provided at least partially amorphous carbon coating of the at least one bearing element of the bearing assembly is characterized by a particularly low coefficient of friction and is preferably arranged in those sections of the at least one bearing element that are in contact with another bearing element of the bearing assembly during rotation of the turbocharger shaft and that move relative to it. In other words, those sections that are subjected to a frictional force during operation due to the rotational movement of the turbocharger shaft are preferably coated. In this way, friction between these two bearing elements can be reduced.

[0025] The use of the amorphous carbon coating in conjunction with water lubrication has a beneficial effect on all areas of the "Stribeck curve" described above. Due to the amorphous carbon coating, both static friction compared to other bearing element coatings, such as ceramic with a polycrystalline lattice structure, and mixed friction of the bearing element are further reduced. Even when water lubrication has no or only a limited lubricating effect (i.e., at low turbocharger shaft speeds), friction and the associated force required to overcome the associated friction force can be significantly reduced. The result is significantly reduced wear on the bearing elements and the entire bearing assembly, as well as increased efficiency of the exhaust gas turbocharger. This is particularly advantageous for exhaust gas turbochargers with regular turbocharger shaft shutdowns.

[0026] In addition, the proposed combination can also have a positive impact on the fluid friction range. Shear losses, which represent the dominant loss components in this range, are significantly lower due to the comparatively low viscosity of the water-based lubricant compared to viscous oil, thus allowing for lower friction loss and more efficient operation even at higher turbocharger shaft speeds.

[0027] Due to the combination of water lubrication and special coating of one or more bearing elements, the shaft arrangement is particularly suitable for future exhaust gas turbochargers that are subject to highly fluctuating operating speeds, in particular regular downtimes due to a lack of exhaust gas.

[0028] In addition to improved friction properties, the combination of water lubrication and the described coating also offers the advantage of particularly advantageous emergency running properties for the turbocharger shaft. Should the water lubrication fail in an emergency, the amorphous carbon coating, as described above, ensures the lowest possible coefficient of friction.

[0029] The amorphous carbon coating is preferably a diamond-like carbon coating (DLC coating for short). The DLC coating is characterized, among other things, by its amorphous molecular structure and the associated particularly low coefficient of friction compared to other known bearing coatings.

[0030] Furthermore, the at least one bearing arrangement may comprise a radial bearing arrangement. In this case, the bearing arrangement is preferably designed as one or more radial bearings designed to support the turbocharger shaft.

[0031] According to the invention, the bearing assembly comprises a rolling bearing with the following bearing elements: an outer ring and an inner ring rotatable relative to the outer ring, as well as a number of rolling elements arranged between the inner ring and the outer ring. The bearing elements are arranged such that the outer ring rolls on the rolling elements, and these, in turn, roll on the inner ring. For this purpose, the outer ring and / or the inner ring each provide a running surface. The outer ring or the inner ring and / or at least one of the rolling elements is coated with the amorphous carbon coating.

[0032] Preferably, the amorphous carbon layer has a hardness of 1000 to 7000 HV (Vickers hardness).

[0033] According to a further embodiment, the amorphous carbon layer has an elastic modulus of 100 to 700 GPa.

[0034] Furthermore, an exhaust gas turbocharger with a water-lubricated shaft arrangement is proposed, wherein the shaft arrangement is designed according to the description.

[0035] The invention is illustrated below with reference to the single figure using an exemplary embodiment.

[0036] The figure shows, in a purely schematic representation, a shaft arrangement 10 for a turbocharger, in particular for a motor vehicle turbocharger, with a rotatably mounted turbocharger shaft 11 and two bearing assemblies 12, 22 for rotatably supporting the turbocharger shaft 11. The two bearing assemblies 12, 22 each include a water lubrication system. The water lubrication system is not shown in the figure for reasons of clarity. A turbine impeller 15 and a compressor impeller 16 are indicated at each of the two axial ends of the turbocharger shaft 11.

[0037] The two bearing assemblies 12, 22 are each designed as a radial bearing assembly with a radial bearing. For example, the first bearing assembly 12 is designed as a plain bearing for the plain bearing support of the turbocharger shaft 11. A first bearing element 13 of the bearing assembly 12 has a surface facing a second bearing element 14, which is designed as a first sliding surface 13a and has an amorphous carbon coating for this purpose. A second bearing element 14 of the bearing assembly 12, which is rotationally fixedly assigned to the turbocharger shaft 11, has a surface facing the first bearing element 13, which is designed as a second sliding surface 14a and also has an amorphous carbon coating for this purpose. Of course, it is also possible to coat only one of the two sliding surfaces 13a, 14a with the amorphous carbon layer. A film of water lubrication forms between the two sliding surfaces when the turbocharger shaft 11 rotates.

[0038] In particular, the amorphous carbon layer is a diamond-like carbon coating (DLC coating).

[0039] As shown in the figure, the second bearing element 14, in particular its second sliding surface 14a, can be formed, preferably integrally, with the turbocharger shaft 11. For example, the turbocharger shaft 11 is coated with the amorphous carbon coating in the corresponding section. The turbocharger shaft itself represents the second bearing element 14.

[0040] The illustrated second bearing arrangement 22 is shown as a rolling bearing with the following bearing elements: an outer ring 24 and an inner ring 23 rotatable relative to the outer ring 24, as well as a number of rolling elements 25 arranged between the inner ring 23 and the outer ring 24. According to the invention, at least one of the bearing elements 23, 24, 25 is coated with the amorphous carbon coating. It is therefore possible, for example, to coat the outer ring 24 or the inner ring 23 accordingly. In this case, the surface or running surface facing and in contact with the rolling elements 25 is coated. Of course, both the outer ring 24 and the inner ring 23 can be coated accordingly. In addition or alternatively to the cases mentioned, one or more of the rolling elements 25 can be coated with the amorphous carbon coating.

[0041] Water lubrication is also provided for the second bearing arrangement 22 and is not shown for the purpose of clarity.

[0042] It is understood that the turbocharger shaft 11 can be supported not only with the combination of two bearing assemblies 12, 22, which are designed as plain bearings or roller bearings, as shown merely by way of example. Rather, the turbocharger shaft can also be supported with only one roller bearing and one or a plurality of plain and / or roller bearings in any combination.

Claims

[1] Shaft arrangement for a turbocharger with - a rotatably mounted turbocharger shaft (11) and - at least one bearing arrangement (12, 22) for rotatably supporting the turbocharger shaft (11), wherein the at least one bearing arrangement (12, 22) comprises water lubrication, wherein at least one bearing element (13, 14, 23, 24, 25) of the bearing arrangement (12, 22) is coated at least in sections with an amorphous carbon coating, characterized by that the bearing arrangement (12, 22) comprises a rolling bearing (22) with the following bearing elements: an outer ring (24) and an inner ring (23) rotatable relative to the outer ring (24), and a number of rolling elements (25) arranged between the inner ring (23) and the outer ring (24), wherein the outer ring or the inner ring and / or at least one of the rolling elements is coated with the amorphous carbon coating. [2] Shaft arrangement according to claim 1, characterized bythat the amorphous carbon layer is a diamond-like carbon coating. [3] Shaft arrangement according to claim 1 or 2, characterized by that the at least one bearing arrangement (12, 22) is a radial bearing arrangement. [4] Shaft arrangement according to one of claims 1 to 3, characterized by that the amorphous carbon layer has a hardness of 1000 to 7000 HV (Vickers hardness). [5] Shaft arrangement according to one of claims 1 to 4, characterized by that the amorphous carbon layer has an elastic modulus of 100 to 700 GPa. [6] Exhaust gas turbocharger with a water-lubricated shaft arrangement characterized by that the shaft arrangement (10) is designed according to one of claims 1 to 5.

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