Turbocharger bearing assembly and lubrication thereof

A turbocharger bearing housing defines a bore in which the turbocharger shaft is mounted for rotation in journal bearings. Lubricating oil is delivered to the journal bearings and a residue is retained in reservoirs in which the journal bearings are located. Two fluid retaining members are disposed around the shaft, each forming a wall of the reservoir and being sealed to the bearing housing. The lubricating fluid reservoir is provided between the fluid retaining members to a depth that at least partially immerses the journal bearings even when the oil supply is interrupted. This ensures that there is sufficient oil to lubricate the bearings at engine start-up, thereby reducing the risk of wear.

The present invention relates to a bearing assembly of a turbocharger for an internal combustion engine and, in particular, to the lubrication of the same.

Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.

The turbocharger shaft is conventionally supported for rotation by journal bearings in a bore in a central bearing housing connected between the turbine and compressor wheel housing. In automotive heavy duty diesel engine applications these are generally in the form of a pair of fully floating bearings or rolling element bearings that are retained in position relative to the shaft by circlips or the like. Axial forces imparted to the shaft by the compressor or turbine are resisted by an axial thrust bearing that is typically in the form of a thin disc disposed around the shaft and supported on one side by a thrust collar and on the other by the bearing housing and/or other components. The thrust bearing has a central bore for receiving a thrust collar that is mounted on the shaft for rotation therewith immediately adjacent a radial step defined thereon.

The turbocharger shaft and bearing assembly rotate at very high speeds and effective lubrication is imperative to avoid premature failure through wear or seizure. Lubricating oil is supplied to the bearing assembly from the engine oil system via an oil inlet in the bearing housing. Oil is distributed via galleries and passages in the bearing housing to circumferential holes in the outer races of journal bearings. When the engine is in operation the oil is supplied under pressure to the rotating bearing assemblies and the oil penetrates through the circumferential holes to an interface between the inner part of the bearings and the shaft. Similarly, the oil is supplied from the galleries and passages to the periphery of the thrust bearing from where is penetrates through a radially extending passage in the disc to the interface between it and the thrust collar. The oil drains from the bearing assembly bore between the thrust bearing and collar and at the end of the bearing housing bore adjacent to the turbine housing.

When the internal combustion engine is started there is a delay before the oil arrives at the bearing assembly. This delay can be increased during cold conditions as the oil has a higher viscosity than when the engine has been running for some time. The lack of lubricating oil at the bearing surfaces during this delay can result in excessive wear even in the relatively short period concerned.

It is important to provide an effective sealing arrangement at each end of the rotating shaft to prevent oil leakage from the central bearing housing into the compressor or turbine housing during use and to prevent leakage of the high gas pressures from the compressor and turbine housings into the bearing housing. This is typically provided by one or more ring seals (often known as “piston” ring seals) disposed between the shaft and the bearing housing at each end. The seals are typically disposed in respective grooves in the shaft and are each arranged with radial and axial clearances relative to the respective groove wall so as to allow the passage of gas in small volumes across the seals but to choke the flow so to accommodate the pressure differential between the relatively high pressure regions in the compressor and turbine housings and the relatively low pressure area in the bearing housing. The seals are designed to limit the flow of gas between the bearing housing and the compressor and turbine housings. At the same time the pressure difference across these end seals also serves to restrict the possibility of oil leakage out of the bearing housing into the compressor or turbine housings. When the turbocharger shaft is not rotating, for example, when the engine is switched off, the pressure differential across the seal is negligible and oil that is already present in the region of the seal can leak past the end seals through the axial and radial clearances. This is particularly so if the turbocharger is tilted with respect to the horizontal (which it might be, for example, if the vehicle is parked on an incline).

It is an object of the present invention, amongst others, to provide for a turbocharger bearing assembly with an improved lubrication supply.

According to a first aspect of the present invention there is provided a turbocharger comprising: a turbocharger shaft for rotation about an axis and for supporting a compressor wheel at first end and a turbine wheel at a second end; a bearing housing having a wall defining a bore in which the shaft is received with a substantially annular clearance, the shaft being supported for rotation in the bore by at least one journal bearing housed within the annular clearance; a first gas seal between the shaft and the bearing housing proximate the first end of the shaft; a second gas seal between the shaft and the bearing housing proximate the second end of the shaft; at least one fluid passage for delivering lubricating fluid from a fluid source to the at least one journal bearing; at least two fluid retaining members disposed around the shaft and each having an outer portion that is sealed to the wall that defines the bore; and the at least one journal bearing being disposed at an axial position between fluid retaining members so that, in use, a volume of lubricating fluid is retained in at least one lubricating fluid reservoir defined between the fluid retaining members and the wall to a depth that at least partially immerses the journal bearing when the turbocharger shaft is not rotating

The arrangement ensures that there is at least one reservoir of lubricating fluid for each journal bearing after the fluid supply has been interrupted for some time as a result of an internal combustion engine to which the turbocharger may be connected not being used. Thus even when the internal combustion engine is started after an extended period of it being switched off, the lubricating fluid reservoir maintains lubrication of the journal bearing(s) during and after the start-up process until fresh fluid is delivered from the engine. In this context reference to “in use” refers to the turbocharger being supplied with lubricating fluid but not specifically to the instance where the shaft is rotating during operation of the turbocharger. In practice during operation of the turbocharger the volume of fluid in the reservoir may well vary in view of the turbulent conditions.

There may be provided at least one fluid drain in the bearing housing for draining lubricating fluid away from the bore. This may be provided in the wall. The at least one drain may be disposed outside the at least one lubricating reservoir. The at least one drain may comprise a drain that is disposed axially outboard of the at least two fluid retaining members. A radial clearance between the oil retaining members and the shaft may allow lubricating fluid to flow out of the at least one lubricating fluid reservoir to the at least one drain. The wall may have an opening that is disposed so as to allow excess lubricant to egress the reservoir by overflowing the wall and passing through the opening.

The wall may extend substantially axially between the at least two fluid retaining members.

The fluid retaining members may be disposed between the second seal and the compressor wheel and may be axially spaced from the second seal. The first seal may comprise a sealing ring or a plurality of axially spaced sealing rings. The second seal may comprise a sealing ring or a plurality of axially spaced sealing rings.

At least one of the fluid retaining members may be integral with the first seal. The fluid retaining members may be disposed between the first and the second seals.

The fluid retaining members need not extend all the way around the shaft as fluid may be retained to a sufficient depth by members that extend only part-way around the shaft.

The fluid retaining members are preferably fixed relative to the bearing housing.

At least one of the fluid retaining members may be substantially annular or partially annular. There may be more than one journal bearing provided along the length of the shaft and the fluid retaining members may be configured to provide a single reservoir for both journal bearings or separate reservoirs. There may be provided a third fluid retaining member disposed between the first and second journal bearings so as to provide first and second reservoirs, one for each journal bearing. A first reservoir is defined between first and third fluid retaining members and a second reservoir is defined between second and third fluid retaining members.

A first of the fluid retaining members may be provided by an axial thrust bearing acting between the shaft and a thrust face of the bearing housing so as to resist axial thrust forces of the shaft. The axial thrust bearing may comprise a sealing element disposed in an interface between a surface of the bearing and the thrust face of the bearing housing. A thrust collar fixed relative to the shaft may be provided, the thrust bearing being positioned to bear against the collar. The thrust face may have a fluid relief passage offset by a predetermined distance from the shaft axis, whereby fluid above a certain level in the reservoir is able to egress via the relief passage. The fluid relief passage may take any convenient form.

At least one of the fluid retaining members may be integrally formed with the bearing housing and it may be in the form of a substantially annular rib integrally formed with the bearing housing and extending towards the shaft. Two such retaining members may be provided, each to one side of a respective journal bearing.

At least one of the fluid retaining members may be seated in a groove defined in the bearing housing. The outer portion of such a fluid retaining member may be resiliently flexible such that it expands or deforms radially into said groove.

The outer portion of at least one of the fluid retaining members comprises an elastomeric element for sealing against a surface of the bearing housing and may further comprise a circlip bonded to the elastomeric member. Alternatively, it may further comprise a body with an outer surface for supporting the elastomeric element.

At least one of the fluid retaining members may comprise an annular body with a central aperture in which said shaft is received, the body having a substantially radially extending portion and a substantially axially extending portion that forms said outer portion, an outer surface of the axially extending portion being arranged to seal against a surface of the bearing housing.

At least one of the fluid retaining members may be disposed immediately adjacent to the at least one journal bearing so as to retain the bearing in an axial direction.

The at least one lubricating fluid reservoir may be substantially in the form of a partial cylinder arranged around the shaft.

There may be provided a turbine wheel at one end of a shaft for rotation therewith and/or a compressor wheel mounted to the other end of the shaft for rotation about said axis.

The thrust bearing may have an oil passage therethrough and may communicate with an oil supply passage in the bearing housing.

The thrust bearing assembly may comprise inner and outer concentric members, said inner member being fixed to the shaft for rotation therewith and said outer member being fixed to the bearing housing. The outer member may have an oil passage therethrough for delivering oil from a supply passage in the housing to the annular clearance between the inner and outer members of the thrust assembly. The oil passage in the outer member may have a side port for communication with said oil supply passage in the bearing housing.

The inner member of the thrust bearing assembly may be a bush having a radially outward extending flange that abuts said outer member.

The shaft may be stepped and the inner member may abut said step.

The lubricating fluid may be engine oil.

According to a second aspect of the present invention there is provided a turbocharger as defined above in combination with an internal combustion engine, an exhaust gas path from the internal combustion engine for directing exhaust gas to the turbine, an air inlet path for directing air from the compressor wheel to an inlet manifold of the internal combustion engine, the internal combustion engine having a lubricating fluid reservoir that is in fluid communication the at least one fluid passage, wherein the at least one lubricating fluid reservoir is retained between the fluid retaining members to a depth that at least partially immerses the journal bearing when no exhaust gas flows from the engine to the turbine.

In this condition the engine may be stopped.

Referring toFIG. 1, the illustrated turbocharger comprises a turbine1joined to a compressor2via a central bearing housing3. The turbine1comprises a turbine wheel4rotating within a turbine housing5. Similarly, the compressor2comprises a compressor wheel6that rotates within a compressor housing7. The turbine wheel4and compressor wheel6are mounted on opposite ends of a common turbocharger shaft8that extends through the central bearing housing3.

In use, the turbine wheel4is rotated by the passage of exhaust gas passing over it from an inlet9, which is connected to the outlet manifold of the internal combustion engine, to an outlet10. This in turn rotates the compressor wheel6that draws intake air through a compressor inlet11and delivers boost air to the inlet manifold of an internal combustion engine via an outlet volute12.

The turbocharger shaft8rotates on fully floating journal bearings13aand13bhoused towards the turbine end and compressor end respectively of the bearing housing3. Oil is fed to the bearings under pressure from the oil system of the engine via an oil inlet14, gallery14aand passages14b. Each journal bearing13a,13bis retained in place by circlips (not shown inFIG. 1) and is provided with circumferentially spaced radial holes16for oil to pass to the turbocharger shaft8. The oil drains out of the bearings13a,13band returns to the engine sump.

End gas seals (not shown inFIG. 1, but shown inFIGS. 2 and 3) S are provided between the shaft8and the bearing housing at the compressor and turbine ends of the bearing housing, as is well known. These serve to maintain the pressure differential between the bearing housing and the compressor and turbine housings by preventing significant gas leakage between the two and this restricts the risk of oil leakage out of the bearing housing past the seals S.

Referring now toFIGS. 1,2and3(inFIGS. 2 and 3the view is reversed with respect toFIG. 1so that the compressor is to the left and the turbine to the right, although neither are shown), the turbocharger shaft8has a rotation axis A (FIGS. 2 and 3only) and is stepped to form two portions: a first portion8aof a first diameter that supports the turbine wheel4and both journal bearings13a,13b; and a second portion8bof a second diameter, less than said first diameter, that supports a thrust bearing assembly17and an oil slinger18of conventional configuration. The thrust bearing assembly17flanks the journal bearing13bat the compressor end and comprises a thrust collar20in the form of a bush fixed concentrically on the shaft8so that it rotates therewith and a radially outboard thrust bearing21in the form of a washer that is fixed to the bearing housing3such that it does not rotate. The collar20abuts against the step Stbetween the two portions8a,8bof the turbocharger shaft8and has a radially outward extending flange22. The thrust bearing21is concentrically disposed over the collar20, to one side of the flange22and is penetrated by an internal radially extending oil passage (hidden). In operation, an axial force acting on the shaft is resisted by the thrust bearing assembly17and in particular by the flange22of the collar abutting the thrust bearing21. Oil is delivered into the oil passage through a side port (not shown) in the bearing21that interfaces with the gallery14ain the bearing housing3. Thus pressurised oil from the bearing housing supply14,14a, and14bis carried through the thrust bearing21via its passage so as to lubricate the abutting surfaces of thrust bearing assembly17.

The oil slinger18operates, as is well known, to direct excess oil away from the compressor end seal S.

The thrust bearing21is designed to bear against a substantially radially extending wall portion21aof the bearing housing which is shown in more detail inFIG. 6. This is a view looking in the direction from left to right inFIG. 3. The wall portion21adefines a generally annular recess21band is penetrated by a central bore25for receipt of the shaft8. Three peripheral lobes26, outside of the recess21b, define apertures27by which the thrust bearing21is fixed to the wall portion21aof the bearing housing3by suitable fixing screws or the like. A lower part of the bearing21is designed to seal against the wall portion21aat a sealing interface indicated at28inFIGS. 2 and 3. Above the interface, the side edge of the wall portion21aat the recess21ahas an opening29extending from the periphery towards the central bore25at a level above the central axis of the bore. This opening29affords the lubricating oil an exit whereby if the level of oil in the bearing housing exceeds this depth it can overflow through the opening29and egress to drain. The opening29is positioned at a predetermined distance from the central axis of the shaft (and the central aperture of the bearing), the distance affecting the depth of oil retained at that axial location.

FIG. 2shows a prior art representation of the bearing assembly and the oil lubrication system alongside an embodiment of the present invention inFIG. 3for ease of comparison. The oil is depicted in the form it has been allowed to settle, as it would between uses of the turbocharger. It will be appreciated that during use the oil will flow with be turbulent around the shaft. In both embodiments the bearings13a,13bare retained in place relative to the axis A of the shaft8by circlips30in a bore31defined between the shaft8and the housing3. The bore31is defined by a wall31aand there is a generally annular clearance between external surface of the shaft8and the wall31ain which the bearings13a,13bare received. In the prior art embodiment ofFIG. 2, two circlips are provided for each bearing whereas inFIG. 3only one circlip per bearing is shown. When the oil supply is interrupted (e.g. the engine is stopped), in the prior art embodiment ofFIG. 2the oil gradually drains from the bearing area at each end of the shaft8. At the compressor end it drains between the thrust bearing21and collar and then via the interface28. At the turbine end it drains to the oil pan at the location indicated by reference number32. This leaves a small residue of oil in a recess in the wall31between the bearings as indicated at33. It will be evident that this oil does not come into contact with the bearings13a,13band does not therefore provide any lubrication on start-up of the engine.

In the embodiment of the present invention ofFIG. 3, the shaft8is fitted with a pair of annular oil retaining members40,40′ that serve to retain the oil in the bottom of the annular clearance between the shaft8and the wall31adefining the housing bore31. In this embodiment, the annular member40,40′ in each case is a metal ring comprising a body41defining a central aperture42and a peripheral projection43extending in a generally axial direction and pressed into the housing bore31such that an outer surface44of the projection43bears against a surface45of the wall31ain the bearing housing in a sealing relationship. One example of a ring of this type is a “Welch” type plug. The wall surface45is the same surface against which an outer surface of the bearings13a,13bare respectively supported. The wall31aextends axially between the bearings13,13aand in a region between the retaining members40,40′, the wall31ahas openings31bon each side as depicted inFIG. 7. This allows excess lubricating fluid to flow over the sides of the wall to drain. However, the sides of the wall31ain this area are still designed to maintain the reservoir between the annular members40,40′ at a depth that immerses the bearings13a,13b.

A first of such annular members40is disposed between the floating bearing13aand the turbine wheel (not shown inFIG. 3), immediately adjacent to an axially outer edge of the bearing13a. In this position it serves not only to retain the lubricating oil in the bore31but also to retain the bearing13ain position. A second annular member40′ is disposed between the first and second floating bearings13a,13bso as to maintain a first oil reservoir50around the shaft8between the two annular members40,40′ and the wall31afor the first bearing13aand a second oil reservoir51between it, the thrust bearing assembly17and the wall31afor the second journal bearing13b. The annular fluid retaining members40,40′ thus serve as a dam to maintain the first and second oil reservoirs50,51in that they allow oil to accumulate at the bottom of the bore31to a level coincident with the sides of wall31aand/or the edge of the aperture42in the body41of the annular member40,40′ (as shown inFIG. 3) from where it can seep gradually through the clearance between the annular member body41and the shaft8to drain. However, even when the engine is switched off the reservoirs50,51are maintained to be of a minimum depth that ensures there is sufficient oil present during start-up in order cover at least part of the floating bearings13a,13band the thrust bearing assembly17so as to provide lubrication.

If the turbocharger is intended for operation in a generally horizontal condition only the first annular member40axially outboard of the floating bearing13ais required to maintain a single oil reservoir that extends axially over both journal bearings13aand13bto the thrust bearing assembly17. An example of this is shown inFIG. 1. Alternatively, if the turbocharger is likely to encounter inclination in installation or use then the presence of the two annular members40,40′ is beneficial as illustrated inFIGS. 4 and 5. InFIG. 4, the turbocharger shaft is inclined to the horizontal at a positive angle denoted by Ø, whereas inFIG. 5the shaft is inclined by a negative angle denoted by α. It can be seen in these figures that the oil reservoirs50,51are maintained to a sufficient degree even during significant inclination.

The height of the reservoir50,51is initially determined by the height of the sides of the wall31aand the upper surface of the oil covers the surface of the shaft8nearest the bottom of the wall31a. Over time, when the shaft8has not been rotated for some time, some of the oil gradually drains through the clearance between the shaft and the members40,40′ but the reservoirs50,51are still maintained to immerse the bearings13a,13b. The dotted line inFIG. 7indicates the minimum surface level of the reservoirs. Ideally the retained oil is of sufficient volume that it extends up the side of the bearings13a,13b. The wall31acould be configured to extend the width of the reservoir in the direction represented by double-headed arrow X inFIG. 7.

It will be appreciated that the annular fluid retaining members40,40′ can take many different forms to achieve the desired aim. Examples are shown inFIGS. 8 to 10. InFIG. 8the annular member60differs from that described above in that the axially extending portion61is flared outwards at one end (as indicated at62) and is resilient. In use it can be inserted into the housing bore31such that the flared portion62is initially radially inwards deformed or deflected whilst it is moved axially along the bore31to its intended position whereupon the flared portion62can spring radially outwards into an annular groove63formed in the bearing housing3. This helps to locate the annular member60in the correct axial position along the shaft8. In another alternative shown inFIG. 9the annular member70comprises a circlip71to which is bonded an elastomeric ring (e.g. rubber)72. The circlip71acts to retain the annular member70(and therefore the adjacent bearing11or12) in place whereas the ring72provides the seal against the bearing housing3. In the embodiment ofFIG. 10the annular member80is a metal ring81having an O-ring seal82bonded or otherwise fitted to a peripheral groove or recess in the ring.

FIG. 11shows an alternative embodiment of the bearing housing whereby the annular members90,90′ are defined integrally with the bearing housing3. The housing bore31is machined so as to define annular ribs90,90′ extending radially inwards towards the shaft8. A first annular rib90is provided is defined axially outboard of the first bearing13aagain immediately adjacent thereto and the second annular rib90′ is provided axially in board of the second bearing13band is immediately adjacent thereto. In both cases the annular ribs90,90′ serve as bearing retainers. In operation, a first oil reservoir forms at the bottom of the housing bore31in the region denoted by reference number95between the ribs90,90′ and a second reservoir forms at the bottom of the housing bore31between the rib90′ and the thrust bearing assembly17as denoted by reference number96to a level that coincides with the radial inner edge of the rib90′. In both cases the reservoirs serve to assure a predetermined oil depth for contact with part of the bearings13a,13band which is sufficient to ensure effective lubrication.

In all the above embodiments, the thrust bearing assembly17serves as an oil-retaining member by sealing one end of a reservoir. It is to be understood that in other embodiments of the invention, the other designs of annular oil retaining members may be used at one end of the reservoir rather that the thrust bearing. For example an annular oil retaining member of the kind represented at40,40′,60,70,80or90,90′ may be used axially outboard of the bearing13bto maintain reservoir51,96instead of using the thrust bearing for this purpose.

In embodiments where the thrust bearing assembly17is used to retain oil it is desirable for the oil not to leak at the interface28(FIG. 3) between the thrust bearing21and the surface of the housing3at the bottom of the bore31. Various sealing arrangements may be provided to ensure that significant leakage is prevented at pressure up to a certain limit such as, for example, 0.1 bar. Example seals include: an annular or partially annular O-ring seals; a suitably shaped thin gasket seal which may be punched from sheet material (corrugated features may be provided to ensure better sealing); a moulded beaded seal of composite material or otherwise; a machined surface projecting from the thrust face of the housing for embedding into the softer material (e.g. bronze) of the thrust bearing; and metal ring seal of C-shape. Any seal should have good thermal conductivity to assist in the dissipation of heat away from the bearing when oil is not present.

It will be appreciated that the oil dam arrangements described above allow oil to be retained in a reservoir around the bearings between operations of the turbocharger (i.e. between switch off and start-up of the engine) so that there is sufficient oil to lubricate the bearings on start up.

It is to be appreciated that numerous modifications to the above-described embodiments may be made without departing from the scope of the invention as defined in the appended claims. For example, it will be understood that the precise shape and configuration of the components that make up the bearing assembly may vary. Moreover, it is to be understood that the different configurations of annular oil retaining members (including the thrust bearing assembly) may be used in any suitable combination.