Patent Publication Number: US-2007101714-A1

Title: Exhaust gas turbocharger for an internal combustion engine and method of operating an exhaust gas turbocharger

Description:
This is a Continuation-In-Part Application of pending international patent application PCT/EP2005/003097 filed Mar. 23, 2005, and claiming the priority of German patent application 10 2004 026 796.0 filed Jun. 2, 2004. 
    
    
     BACKGROUND OF THE INVENTION  
      The invention relates to an exhaust gas turbocharger for an internal combustion engine and a method for operating an exhaust gas turbocharger including a turbine and a compressor with a common shaft and an electric machine connected thereto via a disengageable clutch.  
      Exhaust gas turbochargers are used both in spark-ignition and auto-ignition internal combustion engines to increase the cylinder charge. Increasing the cylinder charge both increases the engine power and also increases the combustion air ratio, and thus reduces the formation of soot in the lower and intermediate load and rotational speed ranges of auto-ignition internal combustion engines. It can also result in a reduction of nitrogen oxide emissions, depending on the combustion temperature.  
      Exhaust gas turbochargers generally comprise two turbo-machines which are coupled by means of a shaft, a turbine, to which the expanding exhaust gas mass flow of the internal combustion engine is applied and a compressor which is driven by the turbine via the shaft and compresses intake air. Since turbo-machines have an operating behavior different from internal combustion engines, exhaust gas turbochargers and/or their peripherals have to be designed in such a way that sufficient air is made available by the exhaust gas turbocharger both in the low and in the upper load and rotational speed ranges in order to achieve the desired operating behavior of the internal combustion engine.  
      When there is a sudden increase in the load and/or rotational speed of the internal combustion engine, the exhaust gas turbocharger reacts in a delayed manner because of its mass inertia. This delayed response behavior is known as “turbo lag” and is distinguished by the fact that the exhaust gas turbocharger of the internal combustion engine supplies momentarily an amount of air which is insufficient for the corresponding engine operating point. In the non-steady state operating mode of the internal combustion engine the poor response behavior causes both insufficient acceleration and high fuel consumption, which could be reduced by eliminating the poor response behavior.  
      If the exhaust gas turbocharger is configured for the rated power point of the internal combustion engine, it is generally too large for rapid response in the lower and intermediate load and rotational speed ranges and, because of its mass inertia, provides results in an unsatisfactory operating behavior of the internal combustion engine in terms of engine torque, agility and consumption. There are different approaches for improving the response behavior of the exhaust gas turbocharger in the aforesaid range.  
      One of the approaches in this regasd is to couple the exhaust gas turbocharger to an electric machine. The electric machine is rigidly connected to the exhaust gas turbocharger and accelerates it when necessary. The necessary power levels are approximately 1-2 kW for a four cylinder engine, for example. With such a high power consumption, current motor vehicle onboard power systems are at their power limit. A large part of the energy is required to accelerate the electric machine itself. The electric machine&#39;s rotor which is connected to the exhaust gas turbocharger substantially reduces the dynamics of the exhaust gas turbocharger in the unsupported operating range owing to the mass inertia of its rotor.  
      JP 57 059025 A discloses an exhaust gas turbocharger comprising a compressor and a turbine, the compressor being connected to the turbine via a shaft in a rotationally fixed manner. The exhaust gas turbocharger includes an electric machine which can be connected to the exhaust gas turbocharger via a clutch, the exhaust gas turbocharger being able to be driven at least temporarily by a disk-shaped flywheel, said disk-shaped flywheel being able to be connected to the exhaust gas turbocharger via the clutch. The disk-shaped flywheel is connected to the exhaust gas turbocharger by dry friction.  
      EP 0 420 666 B1 discloses a method for an exhaust gas turbocharger comprising a compressor and a turbine and also comprising a shaft which connects the compressor and the turbine to one another in a rotationally fixed manner. An electric machine can be connected to the exhaust gas turbocharger via a clutch. At a rotational speed n ATL  of the exhaust gas turbocharger which is higher than a rated rotational speed n konts  of the flywheel, the electric machine for driving the flywheel is not active but rather absorbs excess energy which is present at the exhaust gas turbocharger in the mode of operation of the electric machine as a generator, and feeds the excess energy, for example, into a motor vehicle onboard power system, the drive of the flywheel being maintained by means of the exhaust gas turbocharger.  
      Furthermore, EP 0 345 991 B1 discloses an exhaust gas turbocharger for an internal combustion engine. The exhaust gas turbocharger has an exhaust gas turbine and a compressor. The turbine and the compressor are connected to one another via a shaft in a rotationally fixed manner. An electric machine can be connected to the exhaust gas turbocharger via a clutch. Furthermore, the exhaust gas turbocharger includes an electric machine which can be connected to the turbocharger via a clutch.  
      The exhaust gas turbocharger includes a generator which can be operated by the internal combustion engine via a clutch located between the generator and the internal combustion engine. The electric energy produced in the process is supplied to the rotating electric machine which then operates as an electric motor and drives the exhaust gas turbocharger. When the exhaust gas turbocharger is driven which results in an increase of the rotational speed of the exhaust gas turbocharger, the compressor is operated in a characteristic diagram range in which it supplies the internal combustion engine with quantities of air adapted to the engine operating points. In this process, the generator is connected to the crankshaft of the internal combustion engine via a clutch so that an increased torque occurs at the crankshaft of the internal combustion engine. As a result, the fuel consumption is increased while the effective average pressure of the internal combustion engine remains the same.  
      It is the object of the present invention to connect an electric machine to an exhaust gas turbocharger in such a way that the response time of the exhaust gas turbocharger is reduced. Also, little installation space should be required and energy requirements should low. Furthermore the transient response behavior of the exhaust gas turbocharger is to be improved and excess energy of the exhaust gas turbocharger should be utilized.  
     SUMMARY OF THE INVENTION  
      In an exhaust gas turbocharger for an internal combustion engine comprising a compressor and a turbine interconnected by a shaft in a rotationally fixed manner, and an electric machine which can be connected to the exhaust gas turbocharger via a clutch, the exhaust gas turbocharger can be driven at least temporarily by a disk-shaped flywheel rotatably supported on the shaft and being operable selectively by the turbine and by an electro-dynamic structure for improving the response behavior of the exhaust gas turbocharger.  
      In this way, the power requirement for accelerating the exhaust gas turbocharger does not have to be met by an electric machine since the energy necessary to accelerate the exhaust gas turbocharger is transmitted to the exhaust gas turbocharger with a high power density by the rotational energy of the flywheel. Where necessary, the connection between the flywheel and the exhaust gas turbocharger is established or eliminated by means of the clutch. Furthermore, the flywheel can be driven by an electric machine. The electric machine compensates for the frictional losses occurring at the flywheel. Where necessary, it can accelerate the flywheel or generate energy. The power demand which is incurred for compensating the frictional losses or for accelerating the flywheel is low so that the load on the onboard power system is negligible. The clutch is composed of a disk which is connected in a rotationally fixed manner to a shaft of the exhaust gas turbocharger, a pole structure, a yoke and a coil, an air gap preventing friction between the disk connected to the exhaust gas turbocharger and the pole structure.  
      In a particular embodiment, the flywheel comprises the pole structure for increasing the effective flywheel. In addition, the pole structure is part of the clutch via which the exhaust gas turbocharger can be coupled to the flywheel or the electric machine.  
      In a further embodiment, the pole structure has at least two disks for a functionally reliable clutch.  
      In a further embodiment, the disks of the pole structure are constructed in an annular shape for reasons of weight. If the exhaust gas turbocharger is accelerated by the flywheel a large flywheel is desired. However, the flywheel has to be accelerated itself before it can accelerate the exhaust gas turbocharger. In contrast, in that process, a small mass is desired. For this reason, an annular shape like that of the pole structure, is the shape which is most advantageous in terms of weight.  
      In a further embodiment, a disk which is connected to the shaft of the exhaust gas turbocharger in a rotationally fixed manner as a component of the clutch is arranged between the disks of the pole structure.  
      In a further embodiment, the disks of the pole structure include a toothed structure with teeth and tooth gaps, the teeth of one disk lying opposite the tooth gaps of the other disk. The toothed structure and in particular the positioning of the teeth and of the tooth gaps opposite one another, are necessary to the design of a functionally reliable clutch, since by virtue of this design an induced magnetic flux can be divided in the disk which is positioned between the two disks of the pole structure, and is deflected and exerts a torque on the disk by virtue of the deflection.  
      In a further embodiment, the two disks of the pole structure are held together by means of a non-magnetic strap. Owing to the centrifugal forces occurring during a rotational movement, the two disks can be deformed. A functionally reliable clutch could not be ensured without a strap. The non-magnetic strap holds the two disks together even at high rotational speeds in such a way that the two disks are spaced apart from one another in parallel. This ensures a functionally reliable clutch.  
      In a further embodiment, for reasons of weight and installation space the flywheel is composed of a rotor of the electric machine, a disk, a tubular element and the pole structure.  
      In a further embodiment, the pole structure is connected in a rotationally fixed manner to the rotor of the electric machine via the disk and the tubular element, both to increase the effective flywheel and to increase the rotational speed of the flywheel.  
      In a further embodiment, the clutch is arranged between the compressor and the turbine of the exhaust gas turbocharger in order to protect the electric machine against high temperatures and the compressor against the ingress of oil.  
      In a further embodiment, the clutch is an eddy current clutch or a hysteresis clutch. This provides for wear-free operation and good electrical actuation properties.  
      In still another embodiment, the flywheel is held as far as possible at a minimum rotational speed which corresponds to a rated rotational speed, by means of the electric machine or by means of the exhaust gas turbocharger, in order to ensure sufficient rotational energy of the flywheel for the acceleration of the exhaust gas turbocharger.  
      In the method according to the invention for operating the exhaust gas turbocharger, when a rotational speed of the exhaust gas turbocharger is higher than a rated rotational speed of the flywheel, the electric machine is not active in order to drive the flywheel but rather said electric machine absorbs the excess energy of the exhaust gas turbocharger in its operating mode as a generator and feeds the acquired energy, for example, into a motor vehicle onboard power system, while the flywheel is driven by the exhaust gas turbocharger. In order to accelerate the exhaust gas turbocharger in the operating ranges in which the rotational speed of the exhaust gas turbocharger is lower than the rated rotational speed of the flywheel, the electric machine is used to accelerate the flywheel only if the rotational speed of the flywheel drops below its rated rotational speed, in order to ensure sufficient rotational energy of the flywheel at a later time.  
      In one development of the method according to the invention, in operating ranges in which the rotational speed of the exhaust gas turbocharger corresponds at least to the rated rotational speed of the flywheel, the flywheel is accelerated by the exhaust gas turbocharger with the clutch closed so that the electric machine can be switched off as an energy saving measure.  
      In a further embodiment of the method according to the invention, the exhaust gas turbocharger is driven by the flywheel in operating ranges in which the rotational speeds of the exhaust gas turbocharger are lower than the rotational speed of the flywheel.  
      The invention will become more readily apparent from the following description of particular embodiments thereof on the basis of the accompanying: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematically simplified sectional illustration of an exhaust gas turbocharger according to the invention,  
       FIG. 2  is an exploded illustration of the exhaust gas turbocharger according to the invention,  
       FIG. 3  is a perspective detail view of a pole structure of the exhaust gas turbocharger, and  
       FIG. 4  is a developed view of the pole structure showing magnetic flux lines occurring during operation when the coil is energized. 
    
    
     DESCRIPTION OF A PARTICULAR EMBODIMENT  
       FIG. 1  illustrates an exhaust gas turbocharger  1  of an internal combustion engine, for example a spark ignition engine or a diesel engine. The internal combustion engine, which is preferably used in motor vehicles, has an intake section with, for example, inlet valves via which air is fed to a combustion chamber of the internal combustion engine. The air is used to burn fuel which is either added to the air outside the combustion chamber or inside the combustion chamber. The fuel/air mixture in the combustion chamber is subsequently burnt. The burning of the fuel/air mixture produces exhaust gas which passes from the combustion chamber into an exhaust section via, for example, outlet valves. Some of the exhaust gas energy can then be used to increase the air supply to the combustion chamber by means of the exhaust gas turbocharger  1  arranged in the air or gas flow circuit of the internal combustion engine.  
      The exhaust gas turbocharger  1  includes a turbine  3  which is provided downstream of the outlet valves in the exhaust section of the internal combustion engine and a compressor  2  which is disposed upstream of the inlet valves in the intake section of the internal combustion engine. The turbine  3  is driven by the exhaust gas of the internal combustion engine and drives the compressor  2  via a shaft  4 , so that air can be sucked in, and compressed by, the compressor  2 . The shaft  4  has a rotational axis  40 . The rotating components of the exhaust gas turbocharger  1 , such as the compressor  2 , turbine  3  and shaft  4 , are supported in a housing of the exhaust gas turbocharger  1  by means of bearings (not illustrated).  
      An electric machine  20 , a clutch  5 , which connects the electric machine  20  to the shaft  4  of the exhaust gas turbocharger  1 , and a flywheel  10 , which drives the exhaust gas turbocharger  1 , are arranged on the shaft  4  between the compressor  2  and the turbine  3 . The electric machine  20  is connected fixed in terms of rotation to the clutch  5 .  
      The electric machine  20  is composed of a cylindrical rotor  21  and stator  23  which surrounds the rotor  21 . The rotational axis  40  of the shaft  4  corresponds to a rotational axis  41  of the rotor  21 . A bearing  50 , for example a sliding bearing, is provided between the rotor  21  and the shaft  4  and permits the rotor  21  to rotate independently of the shaft  4 , at a rotational speed which differs from the rotational speed of the shaft  4 . The electric machine  20  is connected to a motor vehicle onboard power system  100  of the internal combustion engine.  
      The clutch  5  which is arranged at the compressor end comprises a first disk  11  which is connected fixed in terms of rotation to the shaft  4  of the exhaust gas turbocharger  1 , a pole structure  31  which bounds the first disk  11  peripherally in a prong-shaped form, a yoke  15  which surrounds the pole structure  31  and a coil  30  which is accommodated in the yoke  15 . The pole structure  31  can also be referred to as an element of the clutch  5  which rotates with it. Rotating parts of the clutch  5  are of disk-shaped design so that exclusively tensile stresses in the material can arise due to the centrifugal force. The shaft  4 , the clutch  5  and the rotor  21  have the same rotational axis  40 .  
       FIG. 2  shows an exploded illustration of the exhaust gas turbocharger  1  for the sake of further clarity. The yoke  15  which surrounds the pole structure  31  comprises two round, disk-shaped covers, a first cover  151  and a second cover  152 , the covers  151 ,  152  having a first collar  155 , and respectively a second collar  156 , which are arranged perpendicularly to a cover plane. A first round opening  153  and a second round opening  154  for accommodating the shaft  4  are formed in the center of the covers  151 ,  152 . The covers  151 ,  152  are in mirror-inverted positions with respect to one another so that a first end face  157  of the first collar  155  adjoins a second end face  158  of the second collar  156 . The end faces  157 ,  158  which point towards one another are permanently connected to one another after mounting, for example by welding or soldering.  
      The yoke  15  is embodied in two parts for reasons of mounting. It could also be embodied in such a way that the two openings  153  and  154  of the covers  151 ,  152  have a diameter in the order of magnitude of the diameter of the shaft  4  in order to accommodate the shaft  4  without friction. Likewise, bearings of the shaft  4  could also be integrated into the openings  153 ,  154  of the yoke  15 .  
      The yoke  15  accommodates the pole structure  31 . The pole structure  31  is of three-part design. A first part of the pole structure  31  forms a first annular disk  32  which has a toothed structure  44  and an external diameter D R1  and a cavity  37  (illustrated in more detail in  FIG. 1 ) with a diameter D I1 . A second part of the pole structure  31  (illustrated in  FIG. 2 ) forms a second annular disk  36  with an external diameter D R2  which also has the toothed structure  44 . The first disk  11  which is connected in a rotationally fixed manner to the shaft  4  is arranged between the first annular disk  32  and the second annular disk  36 .  
      The first annular disk  32  and the second annular disk  36  are held together at their circumference by a third part of the pole structure  31 , a non-magnetizable strap  38  in such a way that their disk faces are arranged parallel to one another. If the strap  38  were not present, centrifugal forces occurring during the operation of the clutch  5  would deform the two annular disks  32 ,  36  with the result that the coupling function of the clutch  5  could no longer be ensured. In order to prevent friction between the first disk  11  and the strap  38 , a radial depression  13  is provided in the strap  38  opposite the first disk  11 .  
      The external diameter D R1  of the first annular disk  32  and the external diameter D R2  of the second annular disk  36  correspond to the external diameter D S  of the first disk  11 . An internal diameter D Joch  of the yoke  15  is larger than an external diameter D Pol  of the pole structure  31  so that an annular space  18  remains in the yoke  15 . This annular space  18  which is present is provided to accommodate the coil  30 . The coil  30  which is accommodated in the yoke  15  serves to generate a magnetic field. For this purpose, the coil  30  is supplied with current by the motor vehicle onboard power system  100 .  
      Between the rotatable pole structure  31  and the yoke  15  as well as between the strap  38  which rotates with the pole structure  31  and the coil  30  there is an air gap  52  (illustrated in more detail in  FIG. 1 ). The air gap  52  prevents friction between the pole structure  31  and the yoke  15  or between the strap  38  and the coil  30 .  
      A connection of the electric machine  20  to the clutch  5  is realized by a second disk  35  which accommodates the shaft  4 , and a tubular element  34  which accommodates the shaft  4  and is connected in a rotationally fixed manner to the second disk  35 . One end of the tubular element  34  which faces the electric machine  20  is connected in a rotationally fixed manner to the rotor  21 . One end of the tubular element  34  which faces the clutch  5  is connected in a rotationally fixed manner to the second disk  35 . The second disk  35  is connected in a rotationally fixed manner to the first annular disk  32  in such a way that the first annular disk  32  accommodates the second disk  35  in its cavity  37 . The second disk  35  has an opening  49  for accommodating the shaft  4 .  
      The rotationally fixed accommodation of the second disk  35  in the cavity  37  of the first annular disk  32  can, for example, be effected by positive engagement. Likewise, the first annular disk  32  and the second disk  35  can also be embodied in one piece and the second disk  35  could then also have the toothed structure  44  corresponding to the first annular disk  32 . Although the toothed structure  44  on the second disk  35  would not have a function since there would be no toothed structure  44  on the second annular disk  36  lying opposite to it, this would be easier to manufacture in terms of fabrication technology than a disk with a crown gear which has the toothed structure  44 , and a face which is surrounded by the crown gear and does not have a toothed structure  44 .  
      An air gap  51  is formed between the first disk  11  and the pole structure  31 . The air gap  51  in the first instance prevents friction between the annular disks  32 ,  36  and the first disk  11  or between the strap  38  and the first disk  11 , and in the second instance serves as a carrier for magnetic flux  54  which is induced by the coil  30 . The pole structure  31  could also be of single part or two part design. In this context, the mounting possibilities of the first disk  11  which is arranged between the annular disks  32 ,  36  are to be noted.  
       FIG. 3  illustrates a detail of the pole structure  31  of the exhaust gas turbocharger  1 . The first and second annular disks  32 ,  36  have a toothed structure  44  with teeth  45  and tooth gaps  46  which are adjacent to the teeth  45  on their surfaces which respectively face the first disk  11 . The teeth  45  have a tooth height H Z  in the axial direction and a tooth length L Z  in the circumferential direction. The toothed structure  44  of the first and second annular disks  32 ,  36  is embodied in such a way that the teeth  45  of the first annular disk  32  lie opposite the tooth gaps  46  in the second annular disk  36 .  
       FIG. 4  shows a developed view of the pole structure  31  showing magnetic flux  54  occurring during operation when current is flowing through the coil  30  and magnetic poles  53 . The magnetic flux  54  is induced by the coil  30  (not illustrated in  FIG. 3 ) through which current flows. The magnetic poles  53  are formed in the teeth  45  of the first annular disk  32  and of the second annular disk  36 . Owing to the direction of flow of the magnetic flux  54 , the poles  53  can be divided into north poles and south poles, marked N and S, respectively, in  FIG. 4 . If the coil  30  does not have current flowing through it, no magnetic flux  54  is induced.  
      In  FIG. 4 , the north pole is formed in the first annular disk  32 , and the south pole in the second annular disk  36 . The first disk  11  which is positioned between the two annular disks  32 ,  36  is penetrated by the magnetic flux  54 . Owing to this penetration and the teeth which are located offset with respect to one another in the annular disks  32 ,  36 , a change occurs in the magnetization (remagnetization) of the first disk  11  when there is a rotational movement of the first disk  11  at a rotational speed which is different from a rotational speed of the annular disks  32 ,  36  of the pole structure  31 .  
      It is possible to realize a functional principle of hysteresis or of an eddy current in the clutch  5 . Whether the rotational movement or the rotational speed of the pole structure  31  corresponds or not is dependent on the functional principle used for the clutch  5 .  
      If the principle of hysteresis is used, the first disk  11  is composed of semihard material which has a pronounced hysteresis loop in the flux density B—field strength H—diagram, referred to for short as B-H diagram. The pole structure  31  is made of soft magnetic material, for example, iron. The teeth  45  of the pole structure  31  which are offset with respect to one another cause the magnetic flux  54  which penetrates each pole  53  to be divided into two parts and to pass through the first disk  11  partially in the tangential direction. In this context, the first disk  11  which is composed of the magnetically semihard material is magnetized. In an ideal case, the directions of the two partial fluxes emanating from a pole  53  will be offset by 180 degrees with respect to one another.  
      If the pole structure  31  rotates through, for example, one tooth length L Z , the location in the first disk  11  which has just been magnetized is penetrated in the other direction by the magnetic flux  54 . The first disk  11  is magnetized in the opposite direction. The work which is performed owing to the remagnetization corresponds to the area of a hysteresis loop and is referred to as remagnetization work.  
      The re-magnetization work generates a torque in the first disk  11  and an electromagnetic connection is produced between the pole structure  31  and the first disk  11 , as a result of which the connection of the exhaust gas turbocharger  1  is ultimately formed to the electric machine  20  via the clutch  5  and the first disk  11  with its rotationally fixed connection to the shaft  4 . The clutch  5  is then closed. In the case of the clutch  5  according to the principle of hysteresis, the first disk  11  and the pole structure  31  assume the same rotational speed.  
      If the eddy current principle is used, an electrically conductive material, for example iron, copper or aluminum, is to be used for the first disk  11 . When the first disk  11  is rotated, a locally induced magnetic field of the magnetic flux  54  is changed in terms of its strength and its direction. Due to the eddy currents which are locally induced due to changes in the magnetic field and are perpendicular to the magnetic field, magnetic fields are in turn generated which are directed in the opposite direction to the applied magnetic field. This produces a torque which gives rise to an electromagnetic connection between the pole structure  31  and the first disk  11 , as a result of which ultimately the connection of the exhaust gas turbocharger  1  to the electric machine  20  is formed via the clutch  5  and the first disk  11  with its rotationally fixed connection to the shaft. The clutch  5  is thus closed.  
      With an eddy current clutch, the torque which occurs is dependent on the relative rotational speed of the first disk  11  and of the pole structure  31 , that is to say an approximation of the rotational speed of the first disk  11  and of the pole structure  31  is not possible. The material used in eddy current clutches is advantageously more resistant to bursting than the material of hysteresis clutches.  
      According to both functional principles no magnetic flux  54  is produced in the pole structure  31  and no connection is formed between the electric machine  20  and the exhaust gas turbocharger  1  if current does not flow through the coil  30 . The clutch  5  is then opened.  
      For both types of clutch, the coil  30  and the stator  23  are arranged in a stationary fashion and the magnetic flux  54  is transmitted into the pole structure  31  via the air gap  52 . The first annular disk  32  which is connected to the rotor  21  via the tubular element  34  and the second disk  35  is held together with the second annular disk  36  by means of the strap  38  and said annular disks  32 ,  36  are subjected to pure tensile stress owing to the centrifugal force acting as a result of the rotation.  
      The rotatable rotor  21  which is excited to permanent rotational movement by the electric machine  20  as required, the rotatable pole structure  31  and the parts which constitute the rotationally fixed connection between the rotor  21  and the pole structure  31 , these being the tubular element  34  and the second disk  35  which is connected in a rotationally fixed manner to the tubular element  34 , constitute the flywheel  10 . In order to increase the rotational speed of the exhaust gas turbocharger  1 , the flywheel  10  is connected to the exhaust gas turbocharger  1  via the clutch  5  when necessary.  
      In order to produce the rotational movement of the flywheel  10  with a rotational speed n kontS  of, for example, 100 000 l/min, a power of approximately 100 W has to be applied by the electric machine  20 , as a result of which, in contrast to the prior art, a significant reduction in the electric power demand to accelerate the exhaust gas turbocharger  1  is achieved. A further reduction in the power demand can be achieved by reducing, for example, the frictional losses in the bearings (not illustrated in more detail) and/or reducing the air resistance of the flywheel  10 . The reduction in the air resistance of the flywheel  10  can be achieved, for example by filling the toothed gaps  46  of the pole structure  31  with non-magnetizable material. The noise emissions can be kept low by filling the toothed gaps  46  with non-magnetizable material.  
      The inventive use of the rotor  21  and of the disk-shaped pole structure  31  as a flywheel  10  requires less drive power of the electric machine  20 , as a result of which the installation space required for the exhaust gas turbocharger  1  according to the invention is significantly reduced compared to previous designs.  
      While the internal combustion engine is operating in the idling range L leer  or a low partial load range L Teiln  or in the overrun conditions L Schub  at low rotational speeds n klein  the clutch  5  is opened and the exhaust gas turbocharger  1  is not coupled to the electric machine  20 . Owing to the low frictional losses and the large amount of rotational energy stored in the flywheel  10 , the flywheel  10  rotates at rotational speeds which are higher than a rated rotational speed n KontS  of the flywheel  10 . The flywheel  10  is not coupled to the electric machine  20  here, that is to say it rotates without energy being supplied by the electric machine  20 .  
      As soon as the speed of the flywheel  10  drops below its rated rotational speed n KontS , the electric machine  20  drives the flywheel  10 . The power to be applied by the electric machine  20  must just be sufficient to overcome bearing friction losses and air resistance.  
      While the internal combustion engine is operating with a high partial load L Teilh  and a low rotational speed n klein , the flywheel  10  is connected to the exhaust gas turbocharger  1  via the then closed clutch  5  and is driven at the corresponding rotational speed of the exhaust gas turbocharger  1  n ATL . The electric machine  20  is switched off in this case.  
      If the internal combustion engine is operating at a high partial load L Teilh  at high rotational speeds n gross  or at full load L Voll , the flywheel  10  is connected to the exhaust gas turbocharger  1  and is operated at the corresponding rotational speed n ATL  of the exhaust gas turbocharger  1 . The rotational speed n ATL  of the exhaust gas turbocharger  1  is higher than the continuous rated rotational speed n KontS  of the flywheel  10  to such an extent that energy is generated via the electric machine  20  and is fed, for example, into the motor vehicle onboard power system  100 .  
      If the internal combustion engine is in a power demand state, the clutch  5  is closed and the flywheel  10  accelerates the exhaust gas turbocharger  1 . In this context, the rated rotational speed n KontS  of the flywheel  10  can be reduced during the acceleration process until the electric machine  20  drives the flywheel  10  again so that the rated rotational speed n KontS  of the flywheel  10  is reached again. When the required exhaust gas turbocharger rotational speed n ATL  is reached, the flywheel  10  is decoupled from the exhaust gas turbocharger  1 .  
      Under motor-braking conditions of the internal combustion engine at high rotational engine speeds, the flywheel  10  which rotates freely is driven by the electric machine  20  as soon as its rotational speed n S  is below the rated rotational speed n KontS , so that the flywheel  10  remains at the rated rotational speed n KontS .