DE10125989A1 - Exhaust gas cooling method and device - Google Patents

Abstract

The method for cooling vehicle exhaust gases flowing through a heat exchanger (10) comprises feeding atmospheric air (18a) at an acute angle through it. Independent claims are included for: (a) a heat exchanger for use in the method; and (b) an exhaust gas system containing the heat exchanger.

Description

The present invention relates to a method for exhaust gas cooling in motor vehicles, in particular in motor vehicles with a NO _x storage catalytic converter and / or another catalytic converter device. The invention also relates to an exhaust system, in particular an exhaust gas heat exchanger, for performing this method.

Exhaust gas catalysts usually have a relatively limited optimal thermal working range to ensure proper exhaust gas cleaning, which for example lies between about 200 ° C and 500 ° C for NO _x storage catalysts. Below this range, catalytic converters are not yet sufficiently catalytically active to be fully functional and to store the unwanted pollutants contained in the exhaust gas as desired and / or convert them into harmless substances, while above this range there is initially a very strong deactivation and associated with severe thermal aging eventually even catalyst destruction due to overheating. A less serious effect is, for example, that lean-gasoline engines often have to be operated with lambda = 1 at higher exhaust gas temperatures, although the combustion process would still allow lean operation associated with low fuel consumption. Since the catalyst temperature is essentially determined by the temperature of the exhaust gas flowing through, the control, in particular the limitation, of the exhaust gas temperature by means of engine measures and / or targeted exhaust gas cooling is of particular importance for the correct operation of exhaust gas catalysts.

Processes for exhaust gas cooling in the manifold area of are known from the prior art Exhaust systems in motor vehicles known in which an air duct from the Front section cooling air is brought up to the manifold by dynamic pressure. The The supply of cooling air can be switched via a flap. This procedure is however, especially for vehicles with front transverse engine only for concepts with existing manifold optimal.  

Exhaust systems are also known which are in the underbody area by a Enlargement of the surface of the exhaust pipes, such as. B. by forming Multi-pipe systems that enable increased heat dissipation. The disadvantage here is however, that the heat dissipation of such a multi-tube exhaust gas heat exchanger Demand, for example during a desulfation process, cannot be reduced. To reduce heat dissipation in such cases, it is known to be the hot Exhaust gas flow through a bypass line around the multi-tube heat exchanger or other used exhaust gas heat exchanger to conduct and thus the heat dissipation designed to be switchable on the exhaust side. However, the exhaust flaps required for this are quite expensive and with unknown operational reliability over the life of the vehicle. In addition, the heat dissipation can also be achieved with a switchable partition Heat exchanger in relation to the underbody are not completely prevented, since the air flowing out of the engine compartment, similar to a direct current Heat exchanger, automatically leads to a cooling effect on the exhaust system.

DE-OS-34 37 477 A1 describes a catalytic heat exchanger for cleaning the Exhaust gases from internal combustion engines described in the manner of a conventional cross or cross flow heat exchanger is built and not just one Cooling, but if necessary also heating the applied Enables catalyst coating. The heat exchanger consists of a block with a variety of catalyst-coated first flow channels for the purifying exhaust gas and one across these first flow channels extending plurality of second flow channels for a fluid Heat exchange medium, such as air or cooling water, too Heating purposes can be provided with electrical heating elements. The Flow channels are made up of alternating groups of one another first or second flow channels lying next to one another in layers arranged so that a good heat exchange is guaranteed. The heat output is here in a known manner about the amount of flow Heat exchange medium controllable.

The object of the present invention is to provide a simple, flexible and effective process for exhaust gas cooling, not only through higher Cooling performance is characterized as a conventional method but also one needs-based heat dissipation for optimal adjustment or control of the exhaust gas  or catalyst temperature. The task therefore also consists in Creation of a device for carrying out this method.

This object is achieved in that the inflowing Ambient air in an air-cooled air flow Exhaust gas heat exchanger at an acute angle of preferably about 20-70 °, in particular about 35-55 °, with respect to the exhaust gas flow direction or longitudinal direction is passed through the exhaust gas heat exchanger to thereby reduce the low Temperature level of the incoming air as optimal as possible for removal excess thermal energy and for cooling the exhaust gas as required use. Due to the inclination of the cooling channels selected according to the invention the longitudinal direction naturally results in a significantly better flow of the Exhaust gas heat exchanger with a despite a certain DC component Heat transfer correspondingly greater cooling capacity than with a purely rectangular one Arrangement according to the prior art.

The ambient air is preferably in the direction of the motor vehicle Underbody area passed through the exhaust gas heat exchanger.

It is preferably an exhaust gas heat exchanger with several transverse to Exhaust gas flow direction alternately arranged side by side exhaust gas and Cooling channels used, the cooling channels in particular several in each Exhaust gas flow direction include individual channels arranged one behind the other. This Exhaust and cooling channels are formed by layers of smooth or corrugated sheet, the fins of the exhaust gas or cooling channels are preferably used to increase turbulence are provided with indentations or infiltration. Preferably, too Exhaust gas outlet side of the exhaust gas heat exchanger used as an air outlet, while the Cooling ducts in the front surface that is flown by the exhaust gas or the exhaust gas inlet side be closed to the heat exchange on this side as possible minimize.

To improve the flow or flow behavior, the Air flow here preferably through an optionally switchable, contour-superior air guiding device, in particular air baffles, on the Exhaust gas heat exchanger controlled, preferably at about the same angle as that  Exhaust gas channels is inclined with respect to the longitudinal axis of the exhaust gas heat exchanger and the Contour of the exhaust gas heat exchanger in the exhaust gas flow direction on the air inlet side increasingly stronger and less and less towered on the air outlet side.

The inflow is used to increase the effectiveness and to increase the cooling capacity of the exhaust gas heat exchanger with cooling air preferably by a control device controlled. In particular, at least a first air or Louver flap device used on the air inlet and / or air outlet side, which are at a certain distance and inclination with respect to the Exhaust gas heat exchanger is arranged to be as open as possible strong cooling air flow against the exhaust gas heat exchanger and possibly also against to be able to direct other parts of the assigned exhaust system. To improve the In this case, the flow behavior becomes particularly opposite to the tendency Cooling channels selected. The open position is preferably the rest position the air or louvre flap device.

To control the air flow, improve the flow behavior and If necessary, increasing the ground clearance will increase the distance and / or the inclination at least one air or louvre flap device with respect to the Exhaust gas heat exchanger set as required. To ensure optimal Ground clearance in critical driving situations, this change is preferred by a simple height adjustment at the rear pivot point of an assigned Air or louvre flap carrier.

In addition or as an alternative to the described air or Louvered flap solution can provide the flow of cooling air to the exhaust gas heat exchanger also adjusted as required by targeted air supply via an air duct changed and the heat dissipation, for example, by changing the air supply can be controlled accordingly by means of a simple air flap device. The Air is preferably supplied from the area of the highest dynamic pressure in the Front end area of an assigned motor vehicle, for example below one Shock absorber, where appropriate in addition to the exhaust gas heat exchanger Areas of the exhaust system, such as. B. a downstream catalyst device, targeted be cooled.  

To achieve optimal cooling, the air or louvre flaps of the air or lamella flap devices preferably by means of an associated flap Control device selectively controlled individually or in groups as required. The setting is made continuously, but there can also be a change between 2 end positions or between several intermediate positions. The Control takes place as required depending on the operating conditions an assigned internal combustion engine and / or an assigned Catalytic converter device of the motor vehicle, the driving speed and / or the Vehicle acceleration and / or the catalyst temperature and / or characteristic or Control data of any catalyst heating measures used as control parameters be, so that in connection with suitable motor measures always a optimal exhaust gas and catalyst temperature is guaranteed.

In the exhaust gas cooling method according to the invention, the heat dissipation is thus fresh air side and not, as in the prior art, designed to be switchable on the exhaust gas side, so that the above problems of known methods with one when not needed reducible heat dissipation or with exhaust flaps are eliminated. By relocating the exhaust gas cooling in the vehicle underbody area is the inventive The process is also much more flexible and versatile than one Conventional cooling in the manifold area.

By applying a suitable catalyst coating on the surfaces of the exhaust gas heat exchanger that come into contact with the exhaust gas, it can also be used as a catalyst, in particular as a NO _x storage catalyst, the short distance between the catalyst coating and the cooling air flow and the resulting good thermal contact extremely effective temperature control of the catalyst temperature with only a minimal response time or time delay when changing the cooling parameters.

A suitable one for carrying out this exhaust gas cooling method according to the invention Exhaust gas heat exchanger includes exhaust gas and cooling channels, the cooling channels according to the invention at an acute angle of preferably about 20-70 °, in particular about 35-55 °, with respect to the usually continuous in the longitudinal direction extend exhaust ducts. Several exhaust and cooling channels are here transversely to the exhaust gas flow direction alternately arranged side by side, the  Exhaust channels preferably each have a plurality of in the exhaust gas flow direction include individual channels arranged one behind the other. With a preferred one Embodiment are the exhaust and cooling channels from layers smooth and corrugated Sheet formed, the lamellae to increase turbulence with indentations or Infiltration can be provided. The cooling channel openings are preferred here closed on the exhaust gas inlet side of the exhaust gas heat exchanger, while the Exhaust gas outlet side is simultaneously designed as a cooling air outlet.

The exhaust gas heat exchanger preferably comprises at least one with a contour Cooling air guide device, such as suitable air baffles, which for Improvement of the inflow and flow behavior, preferably below about the same angle α as the cooling channels are inclined with respect to the exhaust channels. The Cooling air guide device, which may also be switchable, preferably overhangs the contour of the exhaust gas heat exchanger in Exhaust gas flow direction on the cooling air intake side increasingly stronger as it the contour on the cooling air outlet side is less and less dominated.

In a special embodiment, the exhaust-side surface of the cooling channels can also be a suitable catalyst coating of a known type, such as. B. include a conventional three-way coating, an oxidation coating or a NO _x storage catalyst coating, so that the exhaust gas heat exchanger according to the invention also has corresponding catalyst properties.

An exhaust gas system for carrying out the described method according to the invention comprises a catalyst device, in particular a NO _x storage catalytic converter, an upstream exhaust gas heat exchanger of the type described and a control device for controlling the air flow to the exhaust gas heat exchanger as required.

The control device preferably comprises at least one continuously adjustable first air flap device, but in particular at least one adjacent to the exhaust gas heat exchanger at a certain distance and one certain angles vertically and / or inclinably arranged Louver flap device for generating the strongest possible transverse or  Inclined flow, the lamellae of which can be selected individually or in groups are controllable.

As an alternative or in addition to this, the control device can also be implemented by a second air or louvre flap device for continuously controllable air duct targeted supply of cooling air, the inlet preferably in the area highest dynamic pressure in the front end area of an assigned motor vehicle, for example, is arranged below a bumper.

Further details, features and advantages of the present invention will become apparent not only from the associated claims - for themselves and / or in combination - but also more preferred according to the invention from the following description Exemplary embodiments in conjunction with the associated drawings. In the Drawings in which the same components are provided with the same reference symbols, show in a schematic representation

Figure 1a shows a longitudinal section through an inventive exhaust gas heat exchanger with a view from the right.

Figure 1b shows a cross section through the exhaust gas heat exchanger according to Figure 1a with a view from the front..;

FIG. 2 shows the exemplary embodiment according to FIG. 1 with cooling air guide plates and controllable lamella flap devices for controlling the cooling air flow as required and for improving the cooling capacity; and

Fig. 3 shows the embodiment of FIG. 2 with a targeted air supply through a controllable air duct to optimize the cooling performance.

1 a shows an exhaust gas heat exchanger 10 according to the invention with a front exhaust gas inlet side 10 a (shown on the left in the drawing), an opposite rear exhaust gas outlet side 10 b and a plurality of parallel exhaust gas channels 12 for the exhaust gas extending continuously between these two sides 10 a and 10 b an associated internal combustion engine (not shown), in particular a direct injector, of an associated motor vehicle (also not shown). Incoming hot exhaust gas, which is symbolized by an arrow 14 for clarification, thus enters the exhaust gas heat exchanger 10 from the front on the exhaust gas inlet side 10 a, passes it in the longitudinal direction through the exhaust gas channels 12 , and according to the following explanations, it meets requirements as required by a thermal contact with cooling air is cooled, and then interacts with a correspondingly lower temperature level making desired on the exhaust gas outlet side rear 10 b again. From here it can then be fed to a downstream NO _x storage catalytic converter (not shown) and / or another catalytic converter device for proper exhaust gas purification.

The exhaust gas channels 12 are provided on their surface in contact with the exhaust gas with a NO _x storage catalytic converter coating, so that the exhaust gas heat exchanger 10 also acts as a NO _x storage catalytic converter. However, it is also possible, for example, to apply only a pure oxidation coating, such as palladium and / or platinum, or a three-way catalyst coating, such as palladium and / or platinum and / or rhodium, which give the exhaust gas heat exchanger 10 corresponding catalyst properties. However, the exhaust gas heat exchanger 10 can of course also be designed as a pure heat exchanger without additional catalyst properties.

A plurality of continuous cooling channels 16 extend obliquely from bottom to top through the exhaust gas heat exchanger 10 at an acute angle α of approximately 50 ° with respect to the longitudinal direction or exhaust gas flow direction. On the exhaust gas inlet side 10 a, however, the openings of these cooling channels 16 are closed, so that the cooling air exchange with the surroundings is essentially restricted to the top and bottom 10 c and 10 d of the exhaust gas heat exchanger 10 . Due to the backward inclined cooling channels 16 , the bottom 10 c serves as a cooling air inlet, while the top 10 d serves as a cooling air outlet. The exhaust gas outlet side 10 c is also designed as a cooling air outlet, since the cooling channels 16 opening there are not closed. It thus serves both as an exhaust gas outlet and as a cooling air outlet.

The ambient air flowing during the journey, essentially from the underbody area, which is symbolized by arrows 18 a for clarification, thus flows on the underside or cooling air inlet side 10 c of the exhaust gas heat exchanger 10 into the cooling channels 16 inclined to the rear, through which they are inclined from below is directed upward through the exhaust gas heat exchanger 10 to the rear, where it receives heat through the contact with the adjacent exhaust channels 12 of heat energy from the exhaust gas flowing through this and thereby cools it. The air heated in this way, with respect to its entry point, is displaced slightly to the rear, essentially on the upper side or cooling air outlet side 10 d of the exhaust gas heat exchanger 10 (with regard to the smaller part of the cooling air flowing through the exhaust gas outlet side 10 b that flows through the exhaust gas heat exchanger 10 , reference is made to the description here referred to Fig. 2), which is symbolized by arrows 18 b. When it emerges, it mixes with the ambient air flowing around the exhaust gas heat exchanger 10 and in this case releases the thermal energy absorbed by the exhaust gas back into the environment.

In the cross section shown in FIG. 1b through the exhaust gas heat exchanger 10 with a view in the exhaust gas flow direction, it can be seen that the exhaust and cooling channels 12 and 16 are alternately arranged next to one another in the transverse direction. The six illustrated exhaust gas channels 12 , which each extend over the entire height of the exhaust gas heat exchanger 10 , are each delimited by two spaced layers of smooth sheet metal, while the adjacent seven cooling channels 16 consist of intermediate layers of corrugated sheet metal, each of which has a large number of Form individual channels arranged one behind the other. To increase the turbulence and optimize the heat transfer, the fins are provided with indentations or infiltration.

Due to the cooling channels 16 which are inclined backwards at an acute angle α with respect to the longitudinal direction of the exhaust gas heat exchanger 10 according to the invention, the inflow of the ambient air 18 a flowing essentially from the longitudinal direction is markedly favored in comparison with conventional exhaust gas heat exchangers with their right-angled arrangement of the cooling channels with respect to the exhaust gas channels that there is a correspondingly greater cooling air throughput, which in turn is in turn associated with a significantly better cooling capacity than in the prior art. An additional improvement of this cooling capacity can be achieved by the exemplary embodiment according to the invention shown in FIG. 2 with an additional guide device 24 for the cooling air flow and an additional control device 26 for the cooling air flow 18 a, which enables a stepless control of the cooling capacity as required.

FIG. 2 shows the exhaust gas heat exchanger 10 according to FIG. 1 as part of an exhaust system 20 , which is mounted in the underbody area of a motor vehicle, which is only represented by the contour 22 of a heat shield plate, and a NO _x - (not shown) connected downstream of the exhaust gas heat exchanger 10 . Includes storage catalyst and / or another catalyst device. The already closed for inflowing cooling air front side or exhaust gas inlet side 10 a of the exhaust gas heat exchanger 10 is through which there widening on the nature of larger dimensions of the exhaust gas heat exchanger 10, exhaust system 20 again additionally against inflowing ambient air 18 a shielded, so that on this side of the exhaust heat exchanger 10 of the heat exchange with the exhaust gas 14 flowing through is minimized. On the other hand, on the rear side or exhaust gas outlet side 10 b, the open cooling channels 16 ending there open into the exhaust system 20 , so that the cooling air 18 b exiting there mixes with the exhaust gas 14 exiting through the exhaust gas channels 12 also opening there and mixes with it through the exhaust system 20 of the downstream catalyst device.

To improve the flow-through and flow-around behavior and the associated optimization of heat dissipation, the exhaust gas heat exchanger 10 is provided according to the invention with a large number of protruding air baffle plates 24 arranged one behind the other in the exhaust gas flow direction, each on the side of the cooling channels on the outside of the exhaust gas heat exchanger 10 facing away from the incoming exhaust gas are attached and extend parallel to the exhaust channels 16 across them. The air baffles 24 are inclined at approximately the same angle α of approximately 50 ° as the cooling channels 16 with respect to the longitudinal direction of the exhaust gas heat exchanger 10 , so that the ambient air 18 a flowing in the longitudinal direction during the travel is detected and obliquely from bottom to top along the exhaust gas heat exchanger 10 is passed, which is naturally associated with a corresponding cooling of the exhaust gas heat exchanger 10 and the exhaust gas 14 flowing through it. For better detection and guidance of the incoming air 18 a, the air baffles 24 are designed in such a way that they initially do not protrude the contour of the exhaust gas heat exchanger 10 in the exhaust gas flow direction on the underside or cooling air inlet side 10 c, and then increasingly protrude, while on the top side or cooling air outlet side 10 d increasingly less strong and finally no longer protrude at a certain end area.

Below and above the exhaust gas heat exchanger 10 , a lamella flap 26 a and 26 b is arranged, each comprising a plurality of individually controllable and continuously adjustable individual fins 28 a and 28 b and extending over the entire length and width of the exhaust gas heat exchanger 10 . The lamella flaps 26 a, 26 b are inclined in the opposite direction to the air baffles 24 with respect to the longitudinal axis of the exhaust gas heat exchanger 10 or the horizontal to improve their functioning. In order to change the inflow behavior in a targeted manner and to ensure optimum ground clearance in critical driving situations, the lamella flap 26 a is designed to be adjustable in height and thus also incline at the rear articulation point of a lamella flap carrier (not shown). Corresponding to this, the upper lamella flap 26 b can also be designed to be adjustable in height and inclination.

To increase the ground clearance, however, one of the two air flap devices 26 a and 26 b can optionally be omitted. In addition, the exhaust gas heat exchanger according to the invention could be incorporated rotated 10 for this purpose, optionally by 90 °, and it could be the lamellar flaps 26 are disposed laterally to the exhaust gas heat exchanger 10, so that it no longer obliquely from bottom to top but rather at a certain angle α in the horizontal direction of the cooling air is flowed through. If necessary, additional or differently designed air or lamella flaps 26 can also be used and suitably arranged in order to achieve a sufficient inflow 18 a of the exhaust gas heat exchanger 10 or also a need-based partitioning against this inflow.

In addition to the louvre flaps 26 (or possibly also as an alternative to this), at least one further comparable louvre flap 26 or another air flap device can be arranged adjacent to the downstream NO _x storage catalytic converter or another catalytic converter device and by means of targeted control of the air flow to supplement, direct on-demand catalyst cooling can be used.

Fig. 2, the open rest or default position shows the two slats flaps 26, in which the individual lamellae of the lamellar flaps 26 are fully open 28, all so that the inflowing in the longitudinal direction, and again by arrows 18 a symbolized ambient air from the inclined blades 28 a of the lower Lamella flap 26 a is detected and its flow direction is changed such that it is passed obliquely from bottom to top through the cooling air channels 16 while the exhaust gas 14 flowing through the exhaust gas channels 12 cools, in order to finally exit the top side or cooling air outlet side 10 d of the exhaust gas heat exchanger 10 again. Part of the incoming relatively cool ambient air 18 a, however, is also directed essentially parallel to the air baffles 24 along the outside of the exhaust gas heat exchanger 10 , which causes an additional noticeable cooling of the exhaust gas heat exchanger 10 and the exhaust gas 14 flowing through it and together with that described cooling effect due to the cooling air flow leads to the desired reduction or at least limitation of the exhaust gas temperature and thus also the catalyst temperature.

Depending on the operating conditions of the (not shown) assigned internal combustion engine of the motor vehicle, such as. B. the driving speed and / or the vehicle acceleration, and / or the operating conditions of the downstream catalyst device, such as. B. the catalyst temperature or the characteristic or control data of any catalyst heating measures, the slat flap 26 to reduce cooling, to reduce air resistance or to attenuate noise emissions by selective control of the individual slats 28 or individual slat groups by means of an assigned (not shown) flap Control device completely or partially closed. The control is stepless, but it is also possible to switch between two end positions and / or to switch between certain additional intermediate positions. In an (not shown) complete closure of the slats 26, the flaps impinging on the lamellae flaps 26 inflowing air is substantially completely bypasses the exhaust heat exchanger 10 and a corresponding cooling of the exhaust gas heat exchanger at certain operating conditions by specifically avoided.

A further increase in the cooling capacity can be achieved by the exemplary embodiment according to the invention shown in FIG. 3, in which the air 18 a used for cooling is directed to the lower lamella flap 26 a by an additional air duct 30 . The inlet 32 of the air duct 30 is located in the area of the highest dynamic pressure in the front end area 34 below a bumper of the motor vehicle. It is provided with an infinitely adjustable switching flap 36 for the targeted infinitely variable control of the air flow. In addition, the contour of a front wheel 38 is also shown. The air duct 30 can not only be used in addition, as in the present case, but also as an alternative to the pure air or louvre flap solution according to FIG. 2.

The idea according to the invention was exemplified above on the basis of a special embodiment of an exhaust gas heat exchanger 10 according to the invention and an associated cooling air guiding or control device 16 , 24 or 26 , 30 , 36 for the targeted control of the inflow, throughflow and flow behavior with cooling air. However, it is of course also possible to use differently designed exhaust gas heat exchangers 10 with differently designed or inclined cooling channels 16 and / or cooling air guide devices 24 for carrying out the method according to the invention. To optimize the method it is of course also possible to use differently designed suitable control devices 26 , 30 , 36 for the targeted control of the flow behavior. As has already been mentioned, a downstream catalytic converter device of the exhaust system 20 can also be cooled directly by means of the ambient air and the cooling capacity and thus the catalytic converter temperature can be controlled as required by correspondingly assigned cooling air guiding or control devices.

Claims (49)

1. A method for cooling motor vehicle exhaust gases in an air-cooled exhaust gas heat exchanger ( 10 ) through which the exhaust gases flow, characterized in that incoming ambient air ( 18 a) is passed through the exhaust gas heat exchanger ( 10 ) at an acute angle α with respect to the exhaust gas flow direction. 2. The method according to claim 1, characterized, that the angle α is 20-70 °. 3. The method according to claim 2, characterized, that the angle α is 35-55 °. 4. The method according to any one of the preceding claims, characterized in that the ambient air ( 18 a) in the direction of the motor vehicle underbody area ( 22 ) is passed through the exhaust gas heat exchanger ( 10 ). 5. The method according to any one of the preceding claims, characterized in that an exhaust gas heat exchanger ( 10 ) is used with a plurality of exhaust and cooling channels ( 12 and 16 ) arranged alternately next to one another transversely to the exhaust gas flow direction. 6. The method according to claim 5, characterized in that the cooling channels ( 16 ) each comprise a plurality of individual channels arranged one behind the other in the exhaust gas flow direction. 7. The method according to claim 6, characterized in that the exhaust gas and cooling channels ( 12 and 16 ) are formed by layers of smooth or corrugated sheet ( 12 a or 12 b). 8. The method according to claim 7, characterized in that the fins of the exhaust gas and / or cooling channels ( 12 or 16 ) are provided with indentations or infiltration to increase turbulence. 9. The method according to any one of the preceding claims, characterized in that the exhaust gas outlet side ( 10 c) of the exhaust gas heat exchanger ( 10 ) is used as an air outlet. 10. The method according to any one of the preceding claims, characterized in that the air flow and / or the air flow around the exhaust heat exchanger (10) has a contour superior air is controlled guiding means (24) to the exhaust gas heat exchanger (10) by at least. 11. The method according to claim 10, characterized in that the at least one air guide device ( 24 ) in the exhaust gas flow direction on the air inlet side ( 10 c) increasingly and on the air outlet side ( 10 d) less and less overhangs the contour of the exhaust gas heat exchanger ( 10 ) , 12. The method according to claim 10 or 11, characterized in that guide plates are used as at least one air guide device ( 24 ). 13. The method according to any one of the preceding claims, characterized in that the flow against the exhaust gas heat exchanger ( 10 ) with air flow ( 18 a) is controlled by a control device ( 26 , 30 , 36 ) 14. The method according to claim 13, characterized in that as a control device (26, 30, 36) at least a first air or lamellar flap means (26 a, 26 b) on the air inlet and / or air outlet side (10 or d c 10) is used, which is arranged at a certain distance and a certain inclination with respect to the exhaust gas heat exchanger ( 10 ). 15. The method according to claim 14, characterized in that the open position of the first air or lamella flap device ( 26 ) is selected as the rest position. 16. The method according to claim 15, characterized in that the at least one first air or louvre flap device ( 26 ) is inclined in opposite directions to the cooling channels ( 16 ). 17. The method according to any one of claims 14 to 16, characterized in that the distance and / or the inclination of the at least one first air or lamella flap device ( 26 ) with respect to the exhaust gas heat exchanger ( 10 ) is changed if necessary. 18. The method according to claim 17, characterized in that the distance or the inclination of the at least one first air or lamella flap device ( 26 ) is set at the rear articulation point of an associated lamella flap carrier. 19. The method according to any one of the preceding claims, characterized in that the flow of exhaust gas heat exchanger ( 10 ) with ambient air ( 18 a) is controlled by targeted air supply via at least one air duct ( 30 ). 20. The method according to claim 21, characterized in that the air is supplied through the at least one air duct ( 30 ) from the dynamic pressure region in the front end ( 34 ) of the motor vehicle. 21. The method according to claim 19 or 20, characterized in that the air flow through the at least one air duct ( 30 ) is controlled by a second air or lamella flap device ( 36 ). 22. The method according to any one of claims 14 to 21, characterized in that the settings of the air or louvre flap devices ( 26 , 36 ) are steplessly controlled. 23. The method according to any one of claims 14 to 22, characterized in that the air or louver flaps ( 28 ) of the air or louver flap devices ( 26 , 36 ) are selectively controlled individually or in groups. 24. The method according to any one of claims 13 to 23, characterized, that the controller depending on the operating conditions of a assigned internal combustion engine and / or an assigned Catalytic converter device of the motor vehicle takes place, the driving speed and / or vehicle acceleration and / or catalyst temperature  and / or identification or control data of possible catalyst heating measures as Control parameters are used. 25. The method according to any one of the preceding claims, characterized in that the exhaust gas heat exchanger ( 10 ) is also used as an exhaust gas catalytic converter by applying a suitable catalyst coating to surfaces which come into contact with the exhaust gas. 26. Exhaust gas heat exchanger ( 10 ) for motor vehicles with exhaust gas and cooling channels, characterized in that the cooling channels ( 16 ) extend at an acute angle α with respect to the exhaust gas channels ( 12 ). 27. Exhaust gas heat exchanger ( 10 ) according to claim 26, characterized in that the angle α is 20-70 °. 28. Exhaust gas heat exchanger ( 10 ) according to claim 27, characterized in that the angle α is 35-55 °. 29. Exhaust gas heat exchanger ( 10 ) according to one of claims 25 to 28, characterized in that the cooling channels ( 16 ) extend obliquely from bottom to top. 30. Exhaust gas heat exchanger ( 10 ) according to one of claims 26 to 29, characterized in that the exhaust gas and cooling channels ( 12 and 16 ) are arranged alternately next to one another transversely to the exhaust gas flow direction. 31. Exhaust gas heat exchanger ( 10 ) according to one of claims 26 to 30, characterized in that the cooling channels ( 16 ) comprise a plurality of individual channels arranged one behind the other in the exhaust gas flow direction. 32. Exhaust gas heat exchanger ( 10 ) according to one of claims 26 to 31, characterized in that the exhaust gas and cooling channels ( 12 and 16 ) are formed from layers of smooth or corrugated sheet. 33. exhaust gas heat exchanger ( 10 ) according to claim 32, characterized in that the fins of the exhaust gas and / or cooling channels ( 12 or 16 ) are provided with indentations or infiltration to increase turbulence. 34. Exhaust gas heat exchanger ( 10 ) according to one of claims 26 to 33, characterized in that the exhaust gas outlet side ( 10 b) of the exhaust gas heat exchanger ( 10 ) is designed as a cooling air outlet. 35. exhaust gas heat exchanger ( 10 ) according to any one of claims 26 to 34, characterized by at least one cross-contour cooling air guide device ( 24 ). 36. exhaust gas heat exchanger ( 10 ) according to claim 35, characterized in that the at least one cross-contour cooling air guide device ( 24 ) the contour of the exhaust gas heat exchanger ( 10 ) in the exhaust gas flow direction on the cooling air inlet side ( 10 c) increasingly stronger and on the cooling air outlet side ( 10 d) towered less and less. 37. Exhaust gas heat exchanger ( 10 ) according to claim 35 or 36, characterized in that the at least one cooling air guide device ( 24 ) comprises cooling air guide plates. 38. exhaust gas heat exchanger ( 10 ) according to claim 37, characterized in that the cooling air guide plates ( 24 ) are inclined at the same angle α as the cooling channels ( 16 ) with respect to the exhaust gas channels ( 12 ). 39. exhaust gas heat exchanger ( 10 ) according to any one of claims 26 to 38, characterized in that the exhaust gas-side surface of the exhaust gas channels ( 12 ) comprises a catalyst coating. 40. Exhaust system for motor vehicles with
a catalyst device;
an upstream exhaust gas heat exchanger ( 10 ) according to any one of claims 26-39; and
a control device ( 26 , 30 , 36 ) for controlling the air flow ( 18 a) of the exhaust gas heat exchanger ( 10 ). 41. Exhaust system according to claim 40, characterized in that the catalyst device comprises a NO _x storage catalyst. 42. Exhaust system according to claim 40 or 41, characterized in that the control device (26, 30, 36) at least (10) includes an exhaust gas heat exchanger adjacent to the disposed first air or lamellar flap means (26). 43. Exhaust system according to claim 42, characterized in that the at least one first air or lamella flap device ( 26 ) is inclined in the opposite direction to the cooling channels ( 16 ) of the exhaust gas heat exchanger ( 10 ). 44. Exhaust system according to claim 42 or 43, characterized in that the distance and / or the inclination of the at least one air or lamella flap device ( 26 ) with respect to the exhaust gas heat exchanger ( 10 ) are adjustable. 45. Exhaust system according to one of claims 40 to 44, characterized in that the control device ( 26 , 30 , 36 ) comprises at least one air duct ( 30 ) for the targeted supply of cooling air. 46. Exhaust system according to claim 45, characterized in that the inlet ( 32 ) of the air duct ( 30 ) is arranged in the dynamic pressure region in the front end ( 34 ) of an associated motor vehicle. 47. Exhaust system according to claim 45 or 46, characterized in that the air duct ( 30 ) comprises a second air or lamella flap device ( 36 ) for controlling the air flow. 48. Exhaust system according to one of claims 42 to 47, characterized in that the air or louvre flap devices ( 26 , 36 ) are infinitely adjustable. 49. Exhaust system according to one of claims 42 to 48, characterized in that the air or louvre flaps ( 28 ) of the air or louvre flap devices ( 26 , 36 ) are selectively controllable individually or in groups.