PATENT ABSTRACT
A method of effecting multi-stage supercharging in internal combustion machines comprising a supercharging system, wherein fresh air is first routed through a low-pressure compressor. At least a portion of the compressed air stream is compressed further in a high-pressure compressor, and the rest of the fresh air compressed in the low-pressure compressor is routed around the high-pressure compressor via a compressor bypass. All the fresh air is ultimately delivered to the internal combustion engine. The compressor bypass is opened or closed by a self-actuating valve in dependence on the differential pressure, the compressor bypass opening as soon as the pressure downstream from the high-pressure compressor is lower than the pressure upstream of the high-pressure compressor.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
   To increase the power of internal combustion engines, exhaust gas turbochargers are used. Due to the large operating range of internal combustion engines in passenger vehicles, the exhaust gas turbocharger must be regulated in order to achieve a set boost pressure. To this end, in a multistage supercharging system (i.e., one with exhaust gas turbochargers connected in series), the fresh air is compressed first in a low-pressure compressor of a first exhaust gas turbocharger and then in a high-pressure compressor of a second exhaust gas turbocharger. When large volumes of fresh air are present, the choke limit of the high-pressure compressor is exceeded. To keep the high-pressure compressor from functioning as a choke in this case, some of the fresh air can be diverted around the high-pressure compressor through a compressor bypass. When the volume of fresh air is below the choke limit of the high-pressure compressor, the compressor bypass is closed. 
   To keep the pressure build-up in the exhaust gas turbocharger from not lagging when the temperature of the exhaust gas is low and the volume of exhaust gas is very small, as is the case at low rpm, exhaust gas turbochargers of the kind currently used in internal combustion engines have a very low intrinsic mass and therefore respond even at low exhaust flow rates. The power limits of the exhaust gas turbocharger can be broadened for example by regulated two-stage supercharging, as known from Bosch,  Kraftfahrttechnisches Taschenbuch [Automotive Handbook ], 23 rd  Edition, Vieweg, 1999, pages 445-446. In regulated two-stage supercharging, two exhaust gas turbochargers of different sizes are connected in series. The stream of exhaust gas first flows into an exhaust manifold. From there, the exhaust gas stream is expanded via a high-pressure turbine. When large volumes of exhaust are present, as at high rpm, a portion of the mass flow of the exhaust gas can be diverted around the high-pressure turbine through a bypass. The entire exhaust gas mass flow is then utilized by a low-pressure turbine downstream of the high-pressure turbine. The mass flow of aspirated fresh air is first precompressed by a low-pressure stage and then compressed further in the high-pressure stage. Ideally, the fresh air mass flow is intercooled between the low-pressure stage and the high-pressure stage. 
   At low engine rpm, i.e., low exhaust gas mass flow rates, the bypass circumventing the high-pressure turbine remains completely closed and the entire exhaust gas mass flow is expanded via the high-pressure turbine. This produces a very rapid and high build-up of boost pressure. As the rpm increases, the expansion work is continuously shifted to the low-pressure turbine by virtue of a corresponding increase in the cross section of the bypass. Thus, regulated two-stage supercharging permits infinitely variable adjustment to engine demands on the turbine and compressor side. Due to the decreasing flow of exhaust gas through the high-pressure turbine, the compressor power of the high-pressure compressor also decreases. When large fresh air mass flows are present, the compression is done by the low-pressure compressor alone. Fresh air does flow through the high-pressure compressor, but the pressure before and after the high-pressure compressor is the same. As soon as the choke limit of the high-pressure compressor is exceeded, that is, once the stream of fresh air flowing through the high-pressure compressor exceeds the volume flow that the high-pressure compressor can handle without pressure loss, the high-pressure compressor acts as a choke and the pressure of the fresh air decreases as it flows through the high-pressure compressor. To keep the choke limit from being exceeded, when fresh air mass flow rates are high, a portion of the fresh air is diverted around the high-pressure compressor through a compressor bypass. The compressor bypass contains a valve that closes or opens the bypass. This valve is currently controlled by means of an external control unit. 
   A sequence valve for sequential supercharging using two exhaust gas turbochargers is known from  ATZ Automobiltechnische Zeitschrift  88 (1986), page 268. At low rpm, the sequence valve initially causes the fresh air to bypass one of the two compressors. The second compressor is not tied in until higher rotational speeds are reached. For this purpose, the bypass is made to accommodate a displacement body, the upstream and downstream sides of which are both subjected to a pressure force in the closed state. As long as the pressure force on the downstream side is greater than that on the upstream side, the valve is closed. As soon as the pressure on the upstream and downstream sides is equal, the valve opens. Since the entire displacement body is moved each time, a relatively large mass must be moved in order to open and close the bypass. This makes for relatively slow opening of the valve. 
   An internal combustion engine provided with supercharging in a high-pressure turbine and a low-pressure stage that is larger than the high-pressure stage is disclosed in DE A [Published German Patent Application] 195 14 572. To obtain the most lag-free supercharging possible, the high-pressure turbine and the low-pressure turbine are initially connected in series. At about 50 to 60% of the rated rpm, the exhaust gas is diverted completely around the high-pressure turbine through a bypass. This consequently simultaneously shuts off the high-pressure compressor, which is driven by the high-pressure turbine and is connected in series to a low-pressure compressor driven by the low-pressure turbine. In this case, the high-pressure compressor is circumvented via a boost-air line that contains a check valve to prevent boost air from flowing back through the boost-air line while the high-pressure compressor is operating. However, DE A 195 14 572 gives no indication of whether the check valve is self-actuating or is controlled from the outside. Nor does the document disclose the differential pressure at which the check valve closes the boost air line. 
   SUMMARY OF THE INVENTION 
   In a method of effecting multistage supercharging in an internal combustion engine comprising a supercharging system, fresh air is first routed through a low-pressure compressor. At least a portion of the compressed air stream is compressed further in a high-pressure compressor. The rest of the fresh air compressed in the low-pressure compressor is routed around the high-pressure compressor through a compressor bypass. The compressor bypass is opened or closed by a self-actuating valve in dependence on the differential pressure at the high-pressure compressor, the self-actuating valve being adjusted in the compressor bypass in such a way that it opens as soon as the pressure after the high-pressure compressor is lower than the pressure before the high-pressure compressor. All the fresh air is ultimately delivered to the internal combustion engine. 
   The fact that the self-actuating valve does not open until the pressure downstream from the high-pressure compressor is lower than the pressure upstream of the high-pressure compressor ensures that the entire air stream is routed through the high-pressure compressor as long as the latter is helping to compress the air. Only when the high-pressure compressor is no longer contributing to air compression, but on the contrary is actually acting as a choke—which occurs as soon as air is being forced through the compressor without the compressor being driven adequately—is the compressor bypass opened to the boost air. 
   In a preferred embodiment, the self-actuating valve opens when there is a pressure difference of less than 100 mbar. One advantage of the low pressure differential at which the compressor bypass opens is that it prevents kickback or pulsation of the valve body in response to pressure fluctuations. When pressure fluctuations occur, the pressure difference in the high-pressure compressor usually does not fall below 100 mbar, and thus when there are small pressure differences of less than 100 mbar the self-actuating valve remains open even in the presence of pressure fluctuations. A further advantage of the low opening pressure is the ability to utilize both the high-pressure and the low-pressure stages in an optimum manner. This prevents unnecessary pressure losses. 
   The self-actuating valve comprises, for example, a displacement body accommodated in a valve housing. The displacement body is divided into at least two displacement parts, of which a first displacement part, facing the upstream side, closes or opens the compressor bypass. 
   In a further embodiment, the self-actuating valve is a flap valve with a return spring. 
   The advantage of a self-actuating valve for opening or closing the compressor bypass is that there is no need for a valve actuator. It is also unnecessary to implement a valve control function in the control unit. 
   In a preferred embodiment, the fresh air is cooled in a first heat exchanger downstream of the low-pressure compressor. The fresh air is preferably cooled in a second heat exchanger before entering the internal combustion engine. 
   The low-pressure compressor used to compress the fresh air is preferably actuated by a low-pressure turbine and the high-pressure compressor by a high-pressure turbine. In this case, the low-pressure turbine and the high-pressure turbine are preferably driven by the stream of exhaust gas leaving the engine. A portion of the exhaust gas can be diverted around the high-pressure turbine through a high-pressure bypass and the low-pressure turbine through a low-pressure bypass. Valves are preferably disposed in both the high-pressure bypass and the low-pressure bypass in order to adjust the volume of exhaust gas circumventing the high-pressure and low-pressure turbines, respectively. 
   Particularly when the low-pressure compressor and the high-pressure compressor are handling high flow rates of the kind that occur at high rotational speeds, the volume flow in the high-pressure compressor can exceed the delivery limit of the compressor. This causes the high-pressure compressor to act as a choke, and the pressure after the high-pressure compressor to be lower than the pressure before the high-pressure compressor. To keep the volume flow in the high-pressure compressor from becoming greater than the delivery capacity of the high-pressure compressor, a portion of the fresh air can be routed around the high-pressure compressor through the compressor bypass. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a method flow diagram of a two-stage supercharging system; 
       FIG. 2  illustrates an embodiment of a self-actuating valve with a displacement body accommodated therein; and 
       FIG. 3  shows the valve of  FIG. 2  with the compressor bypass closed. 
   

   Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates embodiments of the invention, in several forms, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a flow diagram of a two-stage supercharging system. Fresh air  1  is delivered to a low-pressure compressor  2 . In low-pressure compressor  2 , the air is compressed to a pressure above the ambient pressure. Since the air heats as it is being compressed, in the embodiment shown in  FIG. 1  the low-pressure compressor is followed by a first heat exchanger  3  in which the fresh air  1  compressed by low-pressure compressor  2  is cooled. The heat exchanger  3  can also be omitted, however. 
   The fresh air  1  compressed in low-pressure compressor  2  is delivered to a high-pressure compressor  4 . In high-pressure compressor  4 , the air compressed in low-pressure compressor  2  is compressed further. After the fresh air  1  has been compressed in high-pressure compressor  4 , the fresh air  1  is delivered to an internal combustion engine  8 . Internal combustion engine  8  can be operated on either a self-ignition or a spark-ignition principle. The fresh air  1  is preferably cooled in a second heat exchanger  7  before entering the internal combustion engine  8 . 
   To keep the volume flow through high-pressure compressor  4  from becoming greater than the maximum possible delivery capacity, a compressor bypass  5  branches off in front of high-pressure compressor  4 . Compressor bypass  5  is closed by a self-actuating valve  6 . Self-actuating valve  6  is implemented, for example, as a check valve that opens as soon as the pressure downstream from the self-actuating valve in the direction of flow becomes lower than the pressure upstream of the self-actuating valve  6 . This phenomenon occurs when the volume flow in high-pressure compressor  4  is greater than its maximum delivery capacity. In that case, high-pressure compressor  4  acts as a choke. 
   High-pressure compressor  4  is preferably driven via a first shaft  45  by a high-pressure turbine  9 , which is driven by an exhaust gas stream  15  emitted by the internal combustion engine  8 . At low engine speeds, that is, at low mass flows of exhaust gas, high-pressure bypass  10  remains completely closed and the entire mass flow of exhaust gas is expanded via high-pressure turbine  9 . This results in a faster and higher build-up of boost pressure. As the rotational speed of the internal combustion engine  8  increases and the mass flow of exhaust gas therefore also increases, the cross section of high-pressure bypass  10  is continuously enlarged by the opening of a first exhaust gas regulating valve  11 . Enlarging the cross section of high-pressure bypass  10  reduces the portion of the exhaust gas mass flow acting on high-pressure turbine  9 . The power transmitted from high-pressure turbine  9  to high-pressure compressor  4  can be reduced in this way. As the compressor power decreases, fresh air  1  passes through high-pressure compressor  4  without being compressed further. A pressure ratio of Π=1 becomes established, i.e., the pressure upstream of the high-pressure compressor and downstream from the high-pressure compressor is the same. As soon as the fresh air stream delivered to high-pressure compressor  4  is greater than the maximum delivery capacity of high-pressure compressor  4 , high-pressure compressor  4  acts as a choke and the pressure decreases. As soon as the pressure downstream from high-pressure compressor  4  becomes lower than the pressure upstream of high-pressure compressor  4 , self-actuating valve  6  opens compressor bypass  5 . This causes a portion of the fresh air  1  to flow through compressor bypass  5 , and the volume flow through high-pressure compressor  4  adjusts so that the pressure upstream of and downstream from high-pressure compressor  4  is the same. 
   The opening pressure of self-actuating valve  6  is preferably set at a value Δp&lt;100 mbar. This means that the self-actuating valve opens as soon as the pressure downstream from self-actuating valve  6  is lower than the pressure upstream of self-actuating valve  6  by the opening pressure difference. 
   Low-pressure compressor  2  is preferably driven via a second shaft  46  by a low-pressure turbine  12 . The exhaust gas stream can be diverted around low-pressure turbine  12  through a low-pressure bypass  13  that can be opened or closed by a second exhaust gas regulating valve  14 . 
   First exhaust gas regulating valve  11  and second exhaust gas regulating valve  14  are preferably controlled by an external control unit. 
     FIG. 2  depicts an opened self-actuating valve. 
   Self-actuating valve  6  comprises a valve housing  16  with a displacement body  17  accommodated therein. Displacement body  17  is divided into a first displacement part  18  and a second displacement part  19 . First displacement part  18  faces the upstream side, i.e., the side of displacement body  17  first impinged on by the flow of fresh air. The inflow direction is indicated by the arrow marked with reference numeral  20 . 
   Formed between first displacement part  18  and second displacement part  19  is a cavity  21  in which a resilient element  22  is accommodated. Resilient element  22  is preferably a spiral spring implemented as a pressure spring. 
   Resilient element  22  bears with one end against an inner face  23  of first displacement part  18  and with the second end against a shoulder  24  of a bushing  25  that is connected to second displacement part  19 . 
   The flow is incident on first displacement part  18  at an upstream side  26  that is opposite inner face  23  of first displacement part  18 . The air striking this upstream side  26  exerts a pressure force on first displacement part  18 . As long as the pressure force on the upstream side  26  of first displacement part  18  is greater than the biasing force of resilient element  22  exerted on the inner face  23  of first displacement part  18 , compressor bypass  5  is open. As the pressure of the air in compressor bypass  5  decreases, so does the pressure force on the upstream side  26  of first displacement part  18 . As soon as the pressure force on upstream side  26  is lower than the biasing force of resilient element  22 , first displacement part  18  moves against the inflow direction  20  of the air and is placed, with a sealing element  27 , in a closure seat  28  (cf.  FIG. 3 ). 
   As the pressure of the air in compressor bypass  5  increases, so does the pressure force on upstream side  26 . As soon as the pressure force on upstream side  26  is greater than the biasing force of resilient element  22 , first displacement part  18  lifts out of its closure seat  28  and opens compressor bypass  5 . The fresh air then flows around displacement body  17  into a throat  29  formed between displacement body  17  and valve housing  16 . 
   Second displacement part  19  is held in valve housing  16  by bridges  30 . Said bridges  30  have, for example, a rectangular, triangular, circular or teardrop-shaped cross section, or any other cross section known to those skilled in the art. In a preferred embodiment, the cross section of the bridges  30  is configured as teardrop-shaped, the fresh air being incident on the semicircular end of the teardrop-shaped bridge  30 . The bridge  30  is thereby configured in a particularly flow-promoting manner in the inflow direction. 
   Fashioned in second displacement part  19  is a bore  31  in which a guide pin  32  is movably received. First displacement part  18  is fixed to guide pin  32  at the end facing upstream side  26 . The connection of guide pin  32  to first displacement part  18  can be made in a force-locking or a form-locking manner. Thus, the connection can be effected, for example, by shrinking, as a press fit, as a screw connection, as a glued joint or as a welded joint. Guide pin  32  is preferably connected to first displacement part  18  by shrinking. 
   The length of guide pin  32  is so calculated that when compressor bypass  5  is closed, a large enough segment of guide pin  32  remains accommodated in bore  31  so that it cannot tilt or drop out of bore  31 . In order for first displacement part  18  to be moved in inflow direction  20  or against inflow direction  20 , bore  31  is preferably oriented parallel to inflow direction  20 . A bushing  33  in which guide pin  32  is guided is preferably accommodated in bore  31 . Bushing  33  is preferably made of a static-friction-reducing material, for example PTFE, to improve the sliding properties of guide pin  32 . 
   In the embodiment shown here, a chuck  34  that projects into cavity  21  is formed on second displacement part  19 . Said chuck  34  prolongs the bore  31  that receives guide pin  32 . The prolonged bore  31  increases the guide length of guide pin  32  in bore  31 , thereby ensuring that guide pin  32  will not tilt in bore  31 . 
   As on second displacement part  19 , a chuck  35  that projects into cavity  21  is also formed on first displacement part  18 . Fashioned in chuck  35  is a bore  36  that receives guide pin  32 . In bore  36 , guide pin  32  is connected to first displacement part  18  in a force-locking or form-locking manner. 
   Bushing  25  embraces chuck  34  on second displacement part  19 . Bushing  25  is preferably fastened force-lockingly to chuck  34  of second displacement part  19 , for example by means of a screw connection. The biasing force of resilient element  22  can be adjusted by means of the position of shoulder  24 , which, for example, can be moved toward or away from first displacement part  18  by a screwing movement of screwed-on bushing  25 . As the distance between shoulder  24  and the inner face  23  of first displacement part  18  decreases, the biasing force in resilient element  22  increases. As the biasing force of resilient element  22  increases, greater force and thus a higher pressure in compressor bypass  5  on the upstream side  26  of first displacement part  18  are needed to open compressor bypass  5 . 
   For installation purposes, valve housing  16  is preferably divided into a first housing part  37  and a second housing part  38 . Valve housing  16  is preferably divided at the position of accommodation of a retaining ring  39  to which the bridges  30  of second displacement part  19  are connected. Retaining ring  39  is received by a groove  40  in valve housing  16 . To permit assembly, one portion of groove  40  is formed in first housing part  37  and the other in second housing part  38 . Fitting the housing parts  37 ,  38  together causes the cross section of groove  40  to assume the same shape as the cross section of retaining ring  39 . After the assembly of housing parts  37 ,  28  [sic] and second displacement part  19  with retaining ring  39  mounted thereon via bridges  30 , the valve housing  16  is embraced by a band clamp  41 , for example a V-band clamp or any other band clamp known to those skilled in the art. 
   A groove  42  is formed in band clamp  41  and a collar  43  is formed on valve housing  16 . The cross section of collar  43  and the cross section of groove  42  are the same. So that housing parts  37 ,  38  can be joined to each other by means of band clamp  42 , the collar  43  is formed half in first housing part  37  and half in second housing part  38 . Thus, fitting the housing parts  37 ,  38  together produces collar  43 , whose cross section matches that of groove  43  in band clamp  41 . A durable connection of first housing part  37  to second housing part  38  is achieved by force- or form-locking connection with band clamp  41 . 
     FIG. 3  shows a closed self-actuating valve according to  FIG. 2 . As long as the pressure of the fresh air in the compressor bypass  5  is so low that the pressure force acting on upstream side  26  is lower than the biasing force of resilient element  22 , first displacement part  18  is positioned with sealing element  27  in closure seat  28 . As soon as the pressure of the fresh air in compressor bypass  5  is so great that the pressure force on upstream side  26  becomes greater than the biasing force of resilient element  22 , first displacement part  18  lifts out of closure seat  28  and thereby opens compressor bypass  5 . To keep resilient element  22  from bending while being compressed during the movement of first displacement part  18  toward second displacement part  19 , bushing  25  embraces resilient element  22 . An enlarged bearing surface of resilient element  22  on bushing  25  is obtained by the fact that bushing  25  protrudes beyond chuck  34 . Chuck  35  on first displacement part  18  is shaped so that it is received by bushing  25  and embraced by bushing  25  when compressor bypass  5  is open. When chuck  25  [sic] is inserted into bushing  25 , a cushion of air that damps the movement of first displacement part  18  forms inside bushing  25  between chuck  34  on second displacement part  19  and chuck  35  on first displacement part  18 . This prevents first displacement part  18  from striking sharply against second displacement part  19 , and thus any potential rebounding. 
   While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.