Abstract:
An exhaust gas installation is provided that comprises an exhaust gas evaporator mounted downstream of an internal combustion engine of a motor vehicle. The exhaust gas evaporator has a sandwich-type structure wherein exhaust gas planes and coolant planes are alternately directly adjacently arranged, providing a very compact while very efficient exhaust gas evaporator.

Description:
[0001]    This nonprovisional application is a continuation of International Application No. PCT/EP2008/010662, which was filed on Dec. 15, 2008, and which claims priority to German Patent Application No. DE 10 2007 060 523.6, which was filed in Germany on Dec. 13, 2007, and which are both herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a device for exchanging heat and to a motor vehicle with a device of this type. 
         [0004]    2. Description of the Background Art 
         [0005]    Thermal energy recovery from exhaust gases of an internal combustion engine is gaining steadily in importance in the automotive sector as well. In this regard in particular, thermal energy recovery by means of an exhaust gas evaporator continues to be the main focus in order to hereby achieve an increase in efficiency with respect to the operation of an internal combustion engine. In an exhaust gas evaporator, heat is removed from the exhaust gas and supplied to a coolant or cooling agent, which is typically evaporated in so doing. The thermal energy removed from the exhaust gas can be used for a downstream Clausius-Rankine process. 
         [0006]    For example, DE 601 23 987 T2, which corresponds to U.S. Pat. No. 6,845,618. deals with this topic, in which a Rankine cycle system is described in relation to an internal combustion engine, in which a high-temperature and high-pressure vapor can be generated with use of an evaporator by means of the thermal energy from an exhaust gas of the internal combustion engine. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore an object of the present invention to provide a device for exchanging heat in an especially compact and efficient manner, particularly in regard to use in a motor vehicle. Accordingly, in an embodiment, one of a participating media is guided in a serpentine-like manner within one of the stacked layers in a plate heat exchanger. 
         [0008]    Because the present exhaust gas evaporator is designed with a so-called sandwich design, in which exhaust gas layers and coolant layers are arranged alternately directly side-by-side, the exhaust gas layers can come extensively into contact with the coolant layers, so that the thermal energy transfer from the exhaust gases to the coolant can occur especially rapidly and effectively. 
         [0009]    Based on the large available contact surface areas between an exhaust gas side and an evaporator side of the exhaust gas evaporator, it can in addition be made very compact. This is of particular advantage especially in automotive engineering, because here components of a motor vehicle are to take up as little space as possible and at the same time to be made very light. Thus, a very high-performance construction with respect to the interaction of the exhaust gas side and the evaporator side of the exhaust gas evaporator is advantageously provided by the sandwich design. 
         [0010]    According to an embodiment of the present invention, a first flow space has a first flow path for the first medium with flow path sections which can be flown through one after the other in opposite directions. The flow path sections can be separated from one another by a partition wall arranged between the at least two plates of the least one plate pair. 
         [0011]    In an embodiment, two flow path sections can be flown through directly one after another and can be connected to one another via a deflection section. The deflection section can be formed by a recess, for example, by an opening in the partition wall. According to another embodiment, the deflection section can be formed by a gap remaining between the partition wall and a lateral boundary of the first flow space, for example, a plate pair. 
         [0012]    Two or more than two partition walls can be formed together as a single piece. The two or more partition walls can be formed by an auxiliary plate arranged between the at least two plates of the at least one plate pair and formed especially as a corrugated sheet. 
         [0013]    In an embodiment, at least one flow path section can have one, two, or more than two flow channels which can be flown through parallel to one another. At least two of the flow channels of the at least one flow path section can be connected to one another via the deflection section. For operation of the device of the invention, it is possible to set a pressure loss, on the one hand, and a residence time of the first medium in the first flow space, on the other, by a predefined number of parallel-connected flow channels. 
         [0014]    The flow channels can be closed at their front ends by a boundary of the first flow space or by one or both plates of the plate pair. 
         [0015]    A first deflection section can be arranged with respect to a second flow channel at a first partition wall of a first flow channel at a first front end of the first flow channel and a second deflection section can be arranged with respect to a third flow channel, different from the second flow channel at a second partition wall of the first flow channel at a second front end, lying opposite to the first front end of the first flow channel. 
         [0016]    The flow channels together with the deflection channels can form a single serpentine-like meandering flow path through the first flow space. 
         [0017]    The first and the second flow space can be flown through in different main flow directions. 
         [0018]    The second flow space can have a larger flow cross section than a flow path section of the flow path in the first flow space, particularly a larger flow cross section than the first flow space. This type of embodiment is designed particularly for operation with a liquid, optionally evaporating first medium and a gaseous second medium. 
         [0019]    The device of the invention can be used in a motor vehicle with a combustion engine and an exhaust gas line and is used for exchanging heat between a coolant, particularly of a cooling circuit of the combustion engine, and the exhaust gas or between a cooling agent of a cooling circuit of an air conditioning system and the exhaust gas, whereby the coolant or the cooling agent is evaporated particularly in the device. The exhaust gas in this case can be the second medium. In this case, the first flow channels are arranged essentially vertical, for example, essentially perpendicular to a base of the motor vehicle. 
         [0020]    The term “exhaust gas system” can be understood here to be any component through which exhaust gases of an internal combustion engine are conducted after leaving the internal combustion engine. The term “exhaust gas system” thus also comprises components of an exhaust gas recirculation system. In particular, the exhaust gas evaporator described herein may be integrated into an exhaust gas recirculation system of this type. 
         [0021]    The term “coolant” can describe any vaporizable working medium by means of which thermal energy can be taken up in a sufficient amount and transported. Water in particular, which can also be present as water vapor, is especially highly suitable for this purpose. 
         [0022]    The term “sandwich design” is largely self-explanatory, it being clear, particularly in connection with the exhaust gas evaporator described herein, that exhaust gas layers are arranged alternately with coolant layers in or at the exhaust gas evaporator. The designation “plate design” is also often used for the term “sandwich design.” 
         [0023]    It is therefore also advantageous when on the exhaust gas side more than one exhaust gas layer and/or on the coolant side more than one coolant layer are provided, because heat exchange between the exhaust gas and the coolant can be realized much more effectively, particularly with several exhaust gas and/or coolant layers. The coolant layers can be connected parallel in particular, so that it is assured that all coolant layers can be supplied with coolant independently of one another. It is also possible, however, that one or more coolant layers are connected to one another in series. 
         [0024]    In this case, the exhaust gas layers and the coolant layers can abut directly with their respective broadsides or the exhaust gas layers and the coolant layers are arranged separated from one another only by a highly heat-conducting partitioning device. Each coolant layer can be enclosed on both sides by an exhaust gas layer in each case, so that the coolant layers are always warmed or heated from two sides. 
         [0025]    So that the exhaust gases, on the one hand, in the exhaust gas layer and the coolant, on the other, in the coolant layer can be conducted through the exhaust gas evaporator, an embodiment provides that the exhaust gas evaporator on the exhaust gas side can have an exhaust gas guiding device and/or on the evaporator side a coolant guiding device, which are separated spatially from one another. 
         [0026]    The coolants hereby can be conducted along and in the coolant layer, when several coolant channels, running parallel to one another, such as flow channels, are arranged in each coolant layer. Hereby, especially long, narrow coolant channels can be provided advantageously, in which the coolant can heat up rapidly. 
         [0027]    Merely owing to the described sandwich design, in which exhaust gas layers and coolant layers may be arranged directly side-by-side, a high performance with respect to the exhaust gas evaporator can be achieved with only a small space being necessary. Because in the present case additional exhaust gas channels or coolant channels can be provided in the individual layers of the exhaust gas evaporator, a high performance or improvement of performance can be achieved even with very narrowly predefined space constraints. 
         [0028]    It is advantageous accordingly when to conduct the exhaust gases several exhaust gas channels, running parallel to one another, are arranged in the exhaust gas layer as well. For example, these exhaust gas channels can run linearly through the exhaust gas evaporator with respect to their front ends from an exhaust gas evaporator inlet side to an exhaust gas evaporator outlet side. The exhaust gas channels are opened in each case at their front ends, so that the exhaust gases can flow into the exhaust gas channels via openings in the front ends and flow out again. In this case, preferably a plurality of exhaust gas channels are arranged side-by-side in the exhaust gas layer, so that several exhaust gas channels are arranged between a first side region and a second side region. Thus, the exhaust gases can be conducted over a wide area in the plurality of exhaust gas channels in a first main flow direction through the exhaust gas evaporator. 
         [0029]    The exhaust gas evaporator in this case can be constructed especially simply, when the coolant channels on the evaporator side are arranged with a similar or even identical orientation as the exhaust gas channels on the exhaust gas side. 
         [0030]    However, so that the coolant can take up thermal energy from the exhaust gases especially effectively, it is advantageous when the coolant can stay for a sufficiently long time in the exhaust gas evaporator. On the one hand this can be realized, for example, in that the coolant passes through the exhaust gas evaporator with a lower flow velocity. On the other hand, the exhaust gas evaporator can be made longer. An embodiment provides that the coolant in the exhaust gas evaporator in a coolant layer can cover an especially long stretch through the exhaust gas evaporator. Such a long stretch in a coolant layer can be realized in an especially simple structural manner when the coolant channels are spatially connected to one another. By means of the spatial connection, the coolant can flow from one coolant channel to another coolant channel and thereby stay for an especially long time in the exhaust gas evaporator. 
         [0031]    In this example connection, the coolant channels can be closed at their front ends. As a result, it is not necessary that openings at the front ends, for example, of two coolant channels directly next to one another and/or corresponding to one another must be connected to one another by suitable tubing. Rather, suitable connecting openings between two coolant channels can be provided in a common partition wall. 
         [0032]    Thus, an embodiment also provides that a first connecting opening to a second coolant channel can be arranged on a first partition wall of a first coolant channel at the first front end of the first coolant channel and a second connecting opening to another coolant channel is arranged on a second partition wall of the first coolant channel at a second front end of the first coolant channel. As a result, all coolant channels of a coolant layer can be combined into a meandering coolant stretch. Basically, such connecting openings can be provided on each partition wall. Cooling channels can also be connected in parallel, in that the connecting openings are provided in a suitable manner on the partition walls and/or at the front ends. 
         [0033]    To be able to provide the longest coolant stretch possible in one of the coolant layers, it is therefore advantageous when the coolant channels together form a single meandering coolant stretch through the exhaust gas evaporator. 
         [0034]    It is advantageous, further, if the exhaust gas evaporator has a coolant stretch and an exhaust gas stretch, whereby the coolant stretch is arranged with a different orientation in the exhaust gas evaporator than the exhaust gas stretch. As a result, the exhaust gases and the coolant can flow through the exhaust gas evaporator, for example, in a crossflow. It is clear that the exhaust gases and the coolant could also flow in a counterflow to one another in suitably selected channels. 
         [0035]    In this example connection, an object of the invention is also achieved by a method for operating an internal combustion engine of a motor vehicle, in which exhaust gases of the internal combustion engine are conducted by means of an exhaust gas unit into the environment and thermal energy is removed from the exhaust gases beforehand by means of vaporizable coolants, and in which the exhaust gases within an exhaust gas evaporator are conducted in a first main flow direction and the coolant in a main flow direction opposite to the first main flow direction through the exhaust gas evaporator, whereby the coolant is conducted through the exhaust gas evaporator in sections transverse to the main flow directions. The exhaust gases and the coolant in this case are moved not only in counterflow to one another through the exhaust gas evaporator, but also in crossflow, as a result of which the coolant in particular remains for an especially long time in the exhaust gas evaporator and in so doing, can become warmed or heated especially well. 
         [0036]    In an embodiment, both the exhaust gas channels and the coolant channels can be arranged differently in the exhaust gas evaporator. To reduce in particular the risk that a critical collection of liquid, particularly of water, can occur in one of the coolant channels, it is advantageous if the coolant channels are arranged oriented essentially vertical within the exhaust gas evaporator, particularly essentially vertical to a roadway surface. 
         [0037]    By means of the connecting openings, which can be arranged very close to the front end walls, it can be avoided, moreover, that collection pools for still not evaporated water arise on the bottom side of a coolant layer. In this way, the risk of a decline in performance of the exhaust gas evaporator, based on such water collection sites, are avoided. In an especially advantageous embodiment variant in this regard, it can be provided that in addition to the connecting openings also especially an inlet opening of the coolant layers is placed on the bottom side, so that it can be reliably assured that the coolant channels of a coolant layer can be initially supplied with coolant, particularly with water. In other words, before startup of an internal combustion engine coolant is ideally available in all coolant channels of the exhaust gas evaporator, so that uniform evaporation of the coolant in the coolant layers can be assured. 
         [0038]    As long as a critical water accumulation in one of the coolant channels or one of the coolant layers can be avoided, it is also possible to provide the coolant channels or the coolant layers deflected from a vertical orientation in the exhaust gas evaporator. A noncritical inclination angle of the exhaust gas evaporator to be set accordingly that still avoids the situation in which, for instance, an edge coolant channel and/or an edge coolant layer is critically flooded with water, but an opposite edge coolant channel and/or an opposite edge coolant layer is not, can be reduced as a precaution by more than 5°, ideally by about 10°, so that unfavorable inclined positions, for example, based on an inclined mounting of an internal combustion engine, an exhaust gas unit in a motor vehicle, and/or an unfavorable inclined position of the motor vehicle per se, can be prevented. 
         [0039]    The supplementary term “edge” can include coolant channels and/or coolant layers which are arranged outward on the exhaust gas evaporator compared with the other coolant channels or coolant layers. 
         [0040]    The previously mentioned inclination angle can be measured from a vertical plane. 
         [0041]    Thus, it can be especially assured that initially all coolant channels are supplied with a liquid coolant or with water. This reduces the risk that, for example, a coolant channel initially not supplied with water conveys the evaporating water alone. 
         [0042]    The channels of the exhaust gas evaporator can be made and designed variously. For example, the coolant channels can be made as tube bundles or with a plate design with separating webs. The exhaust gas evaporator is especially simple to manufacture in terms of construction, if coolant channels of a coolant layer are formed by a corrugated sheet folded multiple times in a plane. 
         [0043]    This type of corrugated sheet can form the channels described herein, for example, in conjunction with separating webs arranged parallel to the present layers, whereby the exhaust gas channels can also be realized especially simply by means of separating webs arranged on this type of corrugated sheet. 
         [0044]    In order to have the least possible flow losses within the channels, smooth channel walls can be provided in another embodiment. In particular, the dimensions of the cooling channels can be shaped almost without limitation by variously selected dimensioning of the channel side walls or the channel bottom walls. 
         [0045]    For example, a change in the channel width can entail a pressure loss and/or a change in the thermal energy transfer surface area. The width of the channels as well can affect the number of channels in an exhaust gas evaporator and/or the total distance of a coolant stretch of a coolant layer. 
         [0046]    The exhaust gas guiding device and the coolant guiding device can also be formed variously in terms of construction. The thermal energy can pass into the coolant especially well from the hot exhaust gases, if the exhaust gas guiding device is formed in an exhaust gas layer in the parallel flow and the coolant guiding device in a coolant layer in the serpentine flow. Because the flow in the exhaust gas guiding device is parallel, the exhaust gases can pass the exhaust gas evaporator, for example, with a higher velocity and noncritical back pressure, whereas the coolant because of the serpentine flow can stay for a sufficiently long time in the exhaust gas evaporator, so that it can take up the thermal energy especially effectively. 
         [0047]    It can be understood that depending on the application other advantageous designs can be used for the present exhaust gas evaporator. The flow guidance in the exhaust gas evaporators in particular can be a decisive criterion for an especially high efficiency. Moreover, the strength of an exhaust gas evaporator can be substantially affected with appropriately rigid channels. 
         [0048]    The efficiency in this case can proceed in two optimization directions. On the one hand, one wishes to achieve minimal pressure loss in that no deflections or internal structures are present within a stretch. On the other hand, the largest possible surface area is to be available for thermal energy transfer. It should be noted in addition for the pressure loss that the working medium greatly reduces its density with the change of the physical state, particularly from liquid to gaseous, and this can multiply the flow velocity. A specific optimum must be found therefore between pressure loss and heat output. 
         [0049]    Particularly, in exhaust gas evaporators, the strength, as already mentioned above, is another important topic, because the working medium, particularly a coolant, usually should be operated at working pressures above ambient pressure, in order to achieve a sufficiently good effectiveness in association with the exhaust gas evaporator. Therefore, the selected geometries of the employed components must also be able to easily absorb the compressive forces possibly arising because of the occurring working pressures. Thermal stresses, possibly caused by the temperature differences between the two working media, therefore the exhaust gases, on the one hand, and the coolant, on the other, must also be able to be absorbed. The selected sheet thickness of a corrugated sheet also has a direct effect on the strength, particularly when individual sheet regions of the exhaust gas evaporator are used as tie rods. Further, the sheet thickness may have an effect on the thermal conductivity. 
         [0050]    Another possibility of increasing efficiency is to provide turbulence-generating structures in the channels. This can be easily assured by the previously described structure of the present exhaust gas evaporator, particularly in view of a corrugated sheet folded multiply in a plane. 
         [0051]    The exhaust gas evaporator described here can be used advantageously in almost all motor vehicles, particularly also in commercial vehicles. 
         [0052]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
           [0054]      FIG. 1  shows schematically a view of a motor vehicle with an internal combustion engine and an exhaust gas unit with an exhaust gas evaporator; 
           [0055]      FIG. 2  schematically shows a perspective view of the exhaust gas evaporator of  FIG. 1 ; 
           [0056]      FIG. 3  schematically shows a partially cutaway view of the exhaust gas evaporator of  FIGS. 1 and 2 ; 
           [0057]      FIG. 4  schematically shows a perspective view of a corrugated sheet of the exhaust gas evaporator in  FIGS. 1 to 3  for realizing a first coolant layer; and 
           [0058]      FIG. 5  shows a perspective view of an alternative corrugated sheet. 
       
    
    
     DETAILED DESCRIPTION 
       [0059]    The motor vehicle  1  shown in  FIG. 1  comprises an internal combustion engine  2  with a downstream exhaust gas unit  3 , in which in this exemplary embodiment an exhaust gas evaporator  5 , a catalyst  6 , a central silencer  7 , and a rear silencer  8  are arranged in an exhaust gas line  4 . Vehicle  1  stands with four wheels  9  (identified here only by way of example) on a road base  10 , which lies in the plane of the paper according to the illustration in  FIG. 1 . 
         [0060]    Exhaust gas evaporator  5  is shown in detail schematically in  FIGS. 2 to 4 , whereby particularly in  FIG. 2  the sandwich design  11  of exhaust gas evaporator  5  can be clearly seen with its many exhaust gas layers  12  (identified here only by way of example) and with its many coolant layers  13  (also identified here only by way of example). Exhaust gas layers  12  are hereby formed somewhat thicker with respect to their thickness  14  than the narrower coolant layers  13 , so that exhaust gases can pass through exhaust gas layers  12  more rapidly. Advantageously, in the sandwich design  11 , selected here, the two outer layers are exhaust gas layers  12 , so that it is assured that all coolant layers  13  are surrounded on both sides by exhaust gas layers  12 . As a result, the coolant in coolant layers  13  can be heated especially rapidly. 
         [0061]    Both coolant layers  13  and exhaust gas layers  12  are arranged in a vertical orientation  15  in exhaust gas evaporator  5 , whereby the bottom side  16  of exhaust gas evaporator  5  faces the road base  10 . According to the sandwich design  11  of the present exhaust gas evaporator  5 , a coolant layer  13  follows an exhaust gas layer  12 . 
         [0062]    The coolant, which in this exemplary embodiment is water or in the heated state water vapor  17  (see  FIG. 3 ), reaches a coolant channel  19  via an inlet opening  18  (see  FIG. 4 ) according to a main flow direction  20 . The coolant meanders in coolant layers  13  through exhaust gas evaporator  5  and hereby takes up more and more thermal energy from the exhaust gases, which flow essentially linearly through exhaust gas layers  12  according to the main flow direction  21 . 
         [0063]    Whereas the coolant flows along a coolant stretch  22  meandering through coolant layer  13 , it reaches in each case other coolant channels  25  (identified here only by way of example) of coolant layers  13  via connecting openings  23  (identified here only by way of example) through individual partition walls  24  (identified here only by way of example) and thus snakes along the main flow direction  20 . All coolant channels  19  and  25  are essentially parallel to one another and arranged essentially in the vertical orientation  15  in the respective coolant layer  13 . In this regard, cooling channels  19  and/or  25  are flown through either in a first side flow direction  26  or in a second side flow direction  27 , which run transverse to the two main flow directions  20  and  21 . 
         [0064]    A coolant guiding device  28 , as it can provide several cooling channels  19  and/or  25  in one of the coolant layers  13  of exhaust gas evaporator  5 , can include a corrugated sheet  29  with a flat fin geometry  30 . By means of corrugated sheet  29 , the coolant guiding device  28  is provided especially simply in terms of construction. It is understood that depending on how the flat fin geometry  30  is selected with respect to a fin width  31  and/or a fin height  32 , the total length of the coolant stretch  22  and the number the coolant channels  19 ,  25  can be varied. In this case, the fin height  32  determines in particular the coolant channel height and the fin width  31  the coolant channel width, both of which are not explicitly illustrated, because they result essentially from the fin height  32  or the fin width  31 . 
         [0065]    Coolant channels  19 ,  25  are closed at their front ends  33 ,  33 A (not shown here, but identified by way of example), so that the coolant can flow only via connecting openings  23  from a coolant channel  19  into the other coolant channels  25 , until the coolant again leaves coolant layer  13  via an outlet opening  34  of the coolant guiding device  28 . Thus, by means of connecting openings  23 , a deflection of the coolant is achieved along the coolant stretch  22  within coolant layer  13 . 
         [0066]    In the specific exemplary embodiment according to  FIG. 4 , therefore a first connecting opening  23 A to a second coolant channel  19 B is arranged at a first partition wall  24 A of a first coolant channel  19 A at the first front end  33  of the first coolant channel  19 A and a second connecting opening  23 B to another coolant channel  19 C is arranged at a second partition wall  24 B of the first coolant channel  19 A at a second front end  33 A of the first coolant channel  19 A. 
         [0067]    An exhaust gas guiding device is not shown in the present case, because it essentially has structurally linearly formed exhaust gas channels, whose front ends are not closed, so that the exhaust gases can flow over them into the exhaust gas channels and also flow out again of the exhaust gas channels. The exhaust gas guiding device can also be made of a corrugated sheet, but without the previously described connecting openings  23 . Because several exhaust gas channels are connected parallel to the exhaust gas guiding device, the exhaust gas guiding device in this exemplary embodiment is designed as multiflow. In contrast to this, coolant channels  19 ,  25  are connected in series to coolant guiding device  28 , because the coolant flows sequentially through all coolant channels  19 ,  25 . Thus, coolant guiding device  28  is constructed as single-flow in this exemplary embodiment. 
         [0068]    A partition base (not shown here) is arranged between the exhaust gas guiding device and coolant guiding device  28 , to separate spatially in this way the specific exhaust gas layers  12  and coolant layer  13 , particularly the exhaust gas channels and the coolant channels  19 ,  25 , from one another. In particular, based on the combination selected here of the present corrugated sheet  29 , the partition base, and the closed front ends  33 ,  33 A, exhaust gas evaporator  5  gains a very high strength in an especially advantageous manner in connection with the sandwich design  11 . 
         [0069]    It is understood that the described exhaust gas evaporator  5  represents only a first exemplary embodiment, but is not to be understood as limiting with respect to the invention. 
         [0070]      FIG. 5  shows an additional plate made as a corrugated sheet  41 , which is used in a device, not shown further, for the exchange of heat according to the present invention. Corrugated sheet  41  has partition walls  42 ,  42   a , which are formed as a single piece with one another and separate flow channels  43 ,  44 ,  45 ,  46 ,  47 ,  48 ,  49 ,  50  from each other. In this case, flow channels  43  and  45  form a first flow path section, flow channels  44  and  46  a second flow path section, flow channels  47  and  49  a third flow path section, and flow channels  48  and  50  a fourth flow path section. 
         [0071]    The first and third flow path sections in this case are flown through, for example, toward the viewer, whereas the second and the fourth flow path section are flown through away from the viewer. The first flow path section  43 ,  45  in this case is connected with the second flow path section  44 ,  46  via a deflection section formed by a recess  51 . The second flow path section  44 ,  46  is connected with the third flow path section  47 ,  49  via a deflection section, which is not shown. The third flow path section  47 ,  49  is connected in turn with the fourth flow path section  48 ,  50  via a deflection section formed by a recess  52 . Gaps forming the deflection sections result, due to recesses  51 ,  52 , between partition walls  42  and a side wall of the first flow space in which corrugated sheet  51  is arranged, said side wall which is not shown and closes the flow channels on its front end facing the viewer. 
         [0072]    Partition walls  42   a , in contrast, are connected to the side wall, so that the flow path sections are flown through in the mentioned sequence and alternately in the opposite flow directions. Thus, a single serpentine-like meandering flow path through the first flow space, which is formed by the series connection of the flow path sections, forms for the first medium. 
         [0073]    The object of the invention is achieved in particular also by an exhaust gas unit with an exhaust gas evaporator, which is mounted downstream of an internal combustion engine of a motor vehicle, whereby the exhaust gas evaporator has a sandwich design, in which exhaust gas layers and coolant layers are arranged alternately directly side-by-side, whereby the exhaust gas evaporator preferably has an exhaust gas guiding device on the exhaust gas side and a coolant guiding device on the evaporator side, which are separated spatially from one another, whereby preferably in each of the coolant layers several coolant channels running parallel to one another are arranged, which are connected particularly spatially one below the other, whereby the coolant channels are preferably closed at their front ends. 
         [0074]    Preferably, a first connecting opening to a second coolant channel is arranged on a first partition wall of a first coolant channel at a first front end of the first coolant channel and a second connecting opening to another coolant channel is arranged on a second partition wall of the first coolant channel at a second front end of the first coolant channel, whereby the coolant channels preferably together form a single meandering coolant stretch through the exhaust gas evaporator and/or are arranged essentially oriented vertically within the exhaust gas evaporator, particularly essentially vertical to a road surface, whereby the exhaust gas evaporator preferably has a coolant stretch and an exhaust gas stretch, whereby the coolant stretch is arranged with a different orientation in the exhaust gas evaporator than the exhaust gas stretch. 
         [0075]    Preferably, coolant channels of a coolant layer are formed by means of a corrugated sheet, folded multiply in the coolant layer, and/or the exhaust gas guiding device is formed as multiflow and the coolant guiding device as single-flow. 
         [0076]    The object of the invention is achieved in particular also by a method for operating an internal combustion engine of a motor vehicle, in which exhaust gases of the internal combustion engine are conducted by means of an exhaust gas unit into the environment and thermal energy is removed from the exhaust gases beforehand by means of vaporizable coolants, whereby the exhaust gases within an exhaust gas evaporator are conducted in a first main flow direction and the coolants in a main flow direction opposite to the first main flow direction through the exhaust gas evaporator, whereby the coolants are conducted through the exhaust gas evaporator transverse to the main flow directions in sections. 
         [0077]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.