Patent Publication Number: US-2010111692-A1

Title: Exhaust gas turbocharger for a motor vehicle

Description:
The present invention relates to an exhaust gas turbocharger for a motor vehicle with a shaft carrying a compressor wheel and a turbine wheel according to the preamble of Claim  1 . 
     Exhaust gas turbochargers are usually operated with a high rotational speed which as a rule requires hydrodynamic oil lubrication of the axial and radial bearings. Since with exhaust gas turbochargers a rotor is in contact with various media, namely with cold fresh air on a compressor side, an engine oil in the bearing region and a hot exhaust gas flow on a turbine side, effective sealing in the region of the rotating rotor, that is in the region of the shaft, to the neighbouring static components is indispensable. Here, the use of shaft sealing rings has established itself, which shaft sealing rings more preferably with a narrower design of the shaft sealing ring compared to a groove width, enforce a diversion of a media flow around the shaft sealing ring preloaded on the bearing housing. Since an axial gap between the shaft sealing ring and a circumferential groove turned into the shaft, that is into the rotor, is generally embodied very narrow, a major reduction of an oil leakage into the turbine or compressor housing and a severe reduction of an exhaust gas or a fresh air flow in the bearing housing (so-called blow-by) occurs. If in addition a plurality of shaft sealing rings are employed axially adjacent to one another the length of the gap is extended accordingly, as a result of which the sealing effect can be additionally intensified. Through ever increasing requirements in terms of emission reduction two and more shaft sealing rings are therefore increasingly employed both on a turbine as well as a compressor side. However, the assembly of the individual shaft sealing rings is often problematic because of a usually constant shaft diameter and time and again results in defectively assembled shaft sealing rings, which are not immediately discovered upon delivery to the customer and can then result in subsequent failures of the exhaust gas turbocharger. 
     The present invention therefore deals with the problem of stating an improved or at least an alternative embodiment for an exhaust gas turbocharger of the generic type which more preferably is characterized by simplified assembly. 
     According to the invention this problem is solved through the subject matter of the independent claim. Advantageous embodiments are the subject matter of the dependent claims. 
     The present invention is based on the general idea of designing an outer diameter of a rotor in an exhaust gas turbocharger, that is an outer diameter of a shaft carrying a turbine wheel and a compressor wheel adjacent to shaft-sided circumferential grooves, in which shaft sealing rings are held, differently in size, as a result of which on the one hand the assembly of the shaft sealing rings, more preferably with an outer diameter increasing in assembly direction of the shaft sealing rings can be facilitated and on the other hand a clearly improved sealing effect can be achieved. Here, the shaft carries a compressor wheel on the compressor side in the known manner and a turbine wheel on the turbine side and is mounted in the bearing housing of the exhaust gas turbocharger, wherein according to the invention at least two shaft sealing rings axially adjacent to each other are provided which are held in respective shaft-sided circumferential grooves. These at least two shaft sealing rings axially adjacent to each other can be provided on the turbine and/or compressor side, wherein at the same time an outer diameter of the shaft surrounding the shaft-sided circumferential grooves can be different in size so that more preferably a respective radial depth of two groove flanks belonging to a circumferential groove can be different in magnitude. The establishment of different size outer diameters in the region of the circumferential grooves according to the invention can be simply realised in terms of production through appropriate turning tools so that simplified assembly and increased tightness on the one hand can be achieved easily in terms of production and on the other hand cost-effectively. 
     This solution can be applied on the turbine side and/or the compressor side. With an advantageous further development of the solution according to the invention the outer diameter of the shaft increases in assembly direction of the shaft sealing rings in the region of the circumferential grooves in a sawtooth-like manner. Such an embodiment more preferably offers a major advantage when a pressure from the turbine side is higher than an oil pressure in the bearing housing, wherein through the sawtooth-like embodiment of the outer diameter of the shaft, a blow-by gas flow from the turbine side can be clearly reduced. 
     Further important features and advantages of the invention are obtained from the subclaims, from the drawings and from the respective figure description by means of the drawings. 
     It is to be understood that the features mentioned above and still to be explained in the following cannot only be used in the respective combinations stated but also in other combinations or by themselves, without leaving the context of the present invention. 
    
    
     
       Preferred exemplary embodiment of the invention are shown in the drawings and are explained in more detail in the following description, wherein same reference characters refer to same or similar or functionally same components. 
       Here it shows, in each case schematically, 
         FIG. 1  a sectional view through an exhaust gas turbocharger according to the invention in the region of a mounting with steadily changing outer diameter of the shaft, 
         FIG. 2  a view as in  FIG. 1 , however with an outer diameter of the shaft changing step-like in the region of the shaft sealing rings, 
         FIG. 3  likewise a view as in  FIG. 1 , however with an outer diameter of the shaft  3  changing sawtooth-like. 
         FIG. 4  a possible assembly of the shaft sealing rings on a shaft of an exhaust gas turbocharger according to  FIG. 1 , 
         FIG. 5  a sectional view through an exhaust gas turbocharger according to the prior art upon assembly of a shaft sealing ring, 
         FIG. 6  a sectional view through an exhaust gas turbocharger according to the invention upon the assembly of a shaft sealing ring, 
         FIG. 7  a sectional view through an exhaust gas turbocharger according to the invention with a further embodiment. 
     
    
    
     According to  FIGS. 1 to 3  exhaust gas turbocharger  1  according to the invention comprises a shaft  2  carrying a compressor wheel and a turbine wheel which is mounted in a bearing housing  3 . In the region of the mounting of the shaft  2  in the bearing housing  3  at least two shaft sealing rings  4  and  4 ′ axially spaced from each other are provided here, which are held in corresponding, shaft-sided circumferential grooves  5 ,  5 ′. According to the invention, an outer diameter D 1 , D 2 , D 3  and D 4  of the shaft  2  surrounding the shaft-sided circumferential grooves  5 ,  5 ′ is different in size. This also becomes particularly clear by the respective drawn-in overlap lengths U 1 , U 2 , U 3  and U 4  of the shaft sealing rings  4 ,  4 ′ in the corresponding circumferential grooves  5 ,  5 ′. 
     Looking at  FIG. 1  it is evident that the outer diameter D of the shaft  2  in assembly direction  6  of the shaft sealing rings  4 ,  4 ′ steadily increases. Obviously a step-like enlargement of the outer diameters D of the shaft  2  is also conceivable here, as is shown for example according to  FIG. 2 . Alternatively to this, the outer diameter D can also increase sawtooth-like in the region of the circumferential grooves  5 ,  5 ′ in assembly direction  6 , as is for example shown according to  FIG. 3 . The two drawn shaft sealing rings  4 ,  4 ′ are to be understood purely exemplarily so that obviously more than two shaft sealing rings  4 ,  4 ′ can also be arranged, each of which is held in a corresponding shaft-sided circumferential groove  5 ,  5 ′. It is likewise conceivable that such shaft sealing rings  4 ,  4 ′ are arranged on the compressor side and/or the turbine side. 
     Through the shaft tapering against the assembly direction  6 , clearly simplified assembly of the shaft sealing rings  4 ,  4 ′ can be achieved so that the exhaust gas turbocharger  1  according to the invention is easier to assemble and more preferably producible with less scrap. Preferentially the outer diameter D of the shaft  2  tapers towards a bearing side. Through the different overlap lengths U between the shaft sealing rings  4 ,  4 ′ and the circumferential grooves  5 ,  5 ′ an overall sealing section consisting of the individual overlap lengths U 1 , U 2 , U 3  and U 4  can be increased wherein according to the case shown in  FIG. 2  the following applies: U 1 &lt;U 2 &lt;U 3 &lt;U 4 . A shaft sealing ring  4 ,  4 ′ provided for this is designed in such a manner that the maximum stress occurs during the assembly of the shaft sealing ring  4 ,  4 ′ above the outer diameter D 3 . The increase of the outer diameter D is preferentially so selected here that a gap r approximately corresponds to a gap t. Through an enlargement of the overlap lengths U 3  and U 4  an abutting ring area would also increase at the shaft sealing ring  4 ′, which in turn results in a reduction of a surface pressure. This is necessary since the shaft sealing ring  4 ′ is subjected to higher thermal load and thus the permissible material stresses are also lower than at normal temperatures. 
     According to  FIG. 2  the following relationship generally applies for the overlap lengths U: U 1 &lt;U 2 =U 3 &lt;U 4 . Through a cylindrical design a particularly simple configuration of an assembly tool is possible here, wherein the function compared to  FIG. 1  is subject to only negligible change. 
     According to  FIG. 3  the outer diameter D between and in the region of the circumferential grooves  5 ,  5 ′ changes sawtooth-like, wherein it is characteristic that U 2 &lt;U 1  is approximately U 3 &lt; or approximately U 4  here. This embodiment is more preferably of advantage when the pressure from the turbine side is higher than an oil pressure in the bearing housing  3 . 
       FIG. 4  shows an assembly operation wherein it is clearly noticeable that through the invention the shaft sealing ring  4  either needs to be subjected to lesser preload than the shaft sealing ring  4 ′ or that the distance S 1  of the shaft sealing ring  4 ′ is greater than the distance S 2  of the shaft sealing ring  4  to the respective outer diameter of the shaft  2 . Both increase both the sturdiness as well as a safety during the assembly process. Obviously the shaft sealing rings  4 ,  4 ′ cannot only be employed on the turbine side but in the same manner also on the compressor side. As is additionally evident from  FIG. 1-4  a transition from a groove wall to a groove base of the circumferential grooves  5 ,  5 ′ is rounded, as a result of which a reduction of the notch effect and thus also a quality improvement can be achieved. 
     With the configuration of the shaft  2  according to the invention the assembly of the shaft sealing rings  4 ,  4 ′ can more preferably be facilitated since it is easier to pass the shaft sealing ring  4 ′ over the groove  5  of the shaft sealing ring  4 , which with constant outer diameter D in the past usually caused problems and time and again resulted in defectively installed shaft sealing rings  4 ,  4 ′. At the same time, a labyrinth length, determined from the sums of the overlap lengths U 1 -U 4 , can be extended and thus the sealing effect increased as a result. 
     From  FIG. 5 , which represents the prior art, the problem which is solved through the invention is clearly evident. With an exhaust gas turbocharger according to the prior art a joining tool  7  for the assembly of the shaft sealing rings  4  and  4 ′ has to be designed very thin in regions because of the always same diameter D of the shaft  2  which on the one hand does not make it sturdy and on the other hand relatively expensive. In addition, because of the small gap to the shaft  2  there is always the risk with a joining tool  7  of this type that the shaft is damaged during the assembly of the shaft sealing rings  4 ,  4 ′. In contrast with this, the joining tool  7 ′ according to  FIG. 6  can be constructed significantly thicker and thus sturdier and additionally allows a larger gap to the shaft  2  so that the risk of damaging the shaft  2  during the assembly of the shaft sealing rings  4 ,  4 ′ can be reduced. 
     According to  FIG. 7  an exhaust gas turbocharger  1  is shown wherein the shaft  2  is not directly mounted on the bearing housing  3  but indirectly on the bearing housing  3  via a bearing bush  8 . It likewise differs from the other views that the shaft  2  according to  FIG. 7  comprises a ring  9  carrying the circumferential grooves  5 ,  5 ′ which is permanently joined with the shaft  2 . In this case the circumferential grooves  5 ,  5 ′ are not directly arranged on the shaft  2  but on the ring  9 , wherein the ring  9  obviously can be formed as a separate component to the shaft  2  or as an integral part thereof. 
     In both detail views “X” and “Y” it is clearly visible that the bearing bush  8  has two chamfers  10 ,  10 ′ which facilitate assembly. The circumferential grooves  5 ,  5 ′ with the exhaust gas turbocharger according to  FIG. 7  are arranged radially offset relative to each other so that the circumferential groove  5  has a smaller diameter than the circumferential groove  5 ′, as a result of which the assembly can likewise be simplified.